# Mods to a Blackface circuit?



## GuitarG. (Apr 1, 2010)

A while ago I converted my '71 Twin Reverb to Blackface specs. Re-wired the bias supply to fixed rather than that balance thing Fender had going on. Got rid of the resistors going to ground from pin 8 of the output tubes to straight wire, changed the resistors feeding the phase inverter from the 1meg to the 240k etc. The cap feeding the phase inverter was .01. After changing it too .001 (blackface spec) i notice the amp did not have as much bass as when it was silverface. With .001 it was around 120-140 Hertz. I changed that too a .003 value which brought the bass down too about the 60 Hz range. More like the silverface. I also added a small 3 way switch from the negative feedback resistor to go from the stock 820 ohm too 1500 ohm or cut out completely. Adds more diversity to the amp. I know "don't drill holes in the chassis", but i don't plan on selling the amp. I've had it too long, so I'm going with what my ears tell me even if it means a minor cosmetic change.
Under the filter pan there are the 5 filter caps. If i increase the middle cap, I believe its 40mF stock, too 80mF, how much is that going to impact the bass? Will it change the frequency or just add more punchiness or focus to the bass?
Also, what impact will lowering or raising the value of the slope resistor have on the sound of the amp?

Thanks,

GuitarG.


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## WCGill (Mar 27, 2009)

I'm thinking that the value of the power supply cap will be 20uf. Changing it to 80uf would likely be pointless. With the choke, the amp has ample filtering. Changing slope resistors values will change the voicing of the amp, lowering will result in more mids and a louder amp.


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## keeperofthegood (Apr 30, 2008)

If you have a cathode bypass of, say, 25uF and a coupling of say 0.01, and you want more bass, start by double those values, 50uF cathode, 0.022uf coupling. Going the other way blocks bass.

Have a read of this, the last bit pertains to the bass issue:


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## WCGill (Mar 27, 2009)

A 12AX7 with a 1.5k cathode resistor and a 25uf bypass cap has a -3db point of 10Hz and doubling the cap to 50uf gives you 5Hz. Your guitar can't produce either of these frequencies.


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## keeperofthegood (Apr 30, 2008)

LOL not to mention what the OT is able to produce too. Though, if I understand it, if you want a broader bandwidth with the OT, you need to lower the current through it.


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## WCGill (Mar 27, 2009)

I don't think OPT bandwidth is a function of current. Saturation will result in reduced bandwidth.


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## nonreverb (Sep 19, 2006)

WCGill said:


> I don't think OPT bandwidth is a function of current. Saturation will result in reduced bandwidth.


True, however I think keeps statement has more to do with current related to power dissipation in the transformer. As the wattage increases the design limits of the transformer are reached which produces saturation...and reduced bandwidth. Run 15 watts of signal into a 30 watt transformer and you'll get better bandwidth than if you run 30 watts into it.


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## keeperofthegood (Apr 30, 2008)

I didn't think to save the link, I have looked at so many manufactures and companies the past few weeks for PT and OT that much is becoming a blur. However, that is a statement as made by manufactures that current reduction increases bandwidth. The explanation given by nonreverb is good for why I think.

The OP of this thread did say though *"The cap feeding the phase inverter was .01. After changing it too .001 (blackface spec) i notice the amp did not have as much bass"* and the answer really is to put it back where it was, that if there had been not enough bass to begin with he should go from 0.01 to 0.022 and not lower to 0.001  The value of that coupling cap is mathed on the values of the prior stage plate load and the splitter stage input resistance, and drawing a value for a different set up wont bring you into that other setup unless those resistances are also altered.


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## GuitarG. (Apr 1, 2010)

Thanks for the replies. I was talking about two different caps entirely. The .01 cap that I was referring too is a ceramic disc cap that attaches to pin 2 of the PI. However, the stock resistors attaching to pin 2 and 7 are both 330k in a silverface circuit along with the .01 cap. As part of blackfacing the amp, the two 330k resistors need to be changed to 1meg resistors. If the .01 (silverface) cap is left in place it will become way too boomy after changing the resistors from 330k (silverface) too 1meg (blackface) values. So the stock values for a blackface, in this part of the circuit, is a .001 cap along with the two 1meg resistors. Ignore the resistor values i was reffering too in the OP. I was trying to go off memory without a schematic or the amp in front of me. I was out too lunch on that one. Too many numbers floating around in my head lately.
The difference i found soundwise is blackfacing the amp (blackface values) has less bass than when it was silverface. Like i said around 120-140 Hz range. Changing the .001 cap too .003 brings the bass too about the 60 hz range. More than adequate for playing that open low E. I suppose this would be similar to increasing the coupling caps for more low end.
The other cap i was reffering to is the middle cap of the five found under the cap pan (transformer side) for the main filtering. I believe the stock value is 40mF for this cap? Sorry, forgot to look at the schematic for that one. You may be right WCGill. Could be 20 uF and i was thinking 40 after doubling. Anyway it was once suggested too me that doubling this value (middle cap only) would increase bass. More focus? Or lowered frequency? I'll have to give that one a try while mabe going back to the .001 disc value on the PI.
And thanks for the info on the slope resistor. I may play around with that value also.


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## WCGill (Mar 27, 2009)

A well-designed transformer should not exhibit reduced bandwidth at it's rated power, nor should reducing the power increase the bandwidth of that transformer. It doesn't make sense.


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## nonreverb (Sep 19, 2006)

The point I was attempting to make (not very well) was that when the power exceeds spec, saturation is inevitable. Every transformer has an inherent performance spec. Once it's reached, the bandwidth sufferes. Particlarly where bass is concerned. In older designes like many Fenders, the spec on the transformers was underrated and compounded with that fact, primary windings were rarely balanced. This would contribute to a narrower bandwidth from the get go as they weren't exceptionally efficient. Add you're typical rock and roller to the mix with the amp pinned and you can hear the narrowed bandwidth every time he'she plugs in. A good example of a tranny with lots of potential bandwidth under high power conditions would be an SVT....it's also massive.


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## WCGill (Mar 27, 2009)

Nothing sounds better than a 5E3 pinned!

GuitarG, leave those PI resistor values alone, they're a much better match for the 12AT7 than the Blackface values. On this one, the CBS guys got it right, but if you're bound and determined to have the older values, it's your choice. The cap to the PI (pin 2) can be either .01, .001, or,..... Try clipping in a different power supply cap with the one already on the board before removing and replacing-careful-high voltages here! I'll bet you won't hear much difference, but worth a try. Best of luck.


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## GuitarG. (Apr 1, 2010)

If you add capacitance, generally speaking, more bass is going to come through. Increase resistance in specific parts of a circuit and you cut highs. Correct? Of course this depends on the part of the circuit you make changes too. I am tallking more specifically in the case of the PI.
In the case of converting from silverface to blackface specs the resistance on the PI was increased by about 3 times from 330k to 1 meg resistors. The capacitance was decreased by 10 times .01 to .001, thus less bass than the silverface. So changing the blackface .001 cap too .003 will reduce the capacitance by about 3 times from the silverface value of .01 and not 10 times, allowing more bass frequency through similar to the bass present in the silverface specs. That's my interpretation of what's happening here. I've already done these conversions WCGill. It sounds similar but has changed the character of the amp somewhat. It does'nt sound as "pushed" i guess as the silverface. You have too hear it. I hate trying to describe in words tone characteristics such as this.

Another couple of questions. Two things that I did not alter was where the opto-coupler is for the vibrato there is a 10meg resistor whith a .002 cap across it in the silverface amp. The blackface does not have this cap across the resistor. What is the purpose of that cap or how does it effect the vibrato?
And, on the reverb pedal jack there is a 220k resistor with a .002 cap going from the pedal jack to ground in the silverface. The blackface does not have that cap either. What is the purpose of that cap?

Thanks


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## WCGill (Mar 27, 2009)

The cap across the 10 meg resistor was a fix for vibrato ticking. The cap across the 220k resistor merely bleeds highs to ground and may help to stabilize the reverb return stage. I don't use either one in my builds, nor the 10 meg resistor nor the opto-coupler. It's not the best-sounding trem. The 330k and 1m resistors on the PI are grid reference resistors and their values are not overly critical. The plate resistors (47k), more so.


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## GuitarG. (Apr 1, 2010)

Thanks for the info guys. I'll probably just leave both caps in place then. I was just checking out your amps Bill. Cool looking amps. So, your vibrato is purely tube driven without the opto-coupler? I know many prefer that type of vibrato. 
One other question, can the voltage on the plates of the output tubes be decreased without causing stress or failure on any components? My Twin has the later blackface transformers. Plates are at about 470 volts.


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## WCGill (Mar 27, 2009)

Wall voltages are much higher now than when these amps were built and as a result B+ voltages are higher. Lowering them always will reduce stress but the only way is by changing the power transformer, or using a variac. Your plate voltage should be OK as long as you don't exceed recommended dissipation. Yes I use power-tube tremolo. It's quite different from the later Fender tremolo.


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## GuitarG. (Apr 1, 2010)

That's true. I know the power transformers can get a bit hot in these amps, partly due to current wall voltages? I may have too try it with a variac sometime. I know the outlets in my house measure consistantly at 117 volts. What was the old voltage, 110 volts?


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## keeperofthegood (Apr 30, 2008)

Voltages have ranged 110, 115, 117, 120, 121, 122, 125 sometimes you can see a 117 reading, but that is because you have a bad wire connection to the outlet or in the distribution panel etc. If you measure the voltage in at the main switch you will know if that is the case, only do so VERY CAREFULLY as sticking your hands on those wires can very easily be the last think you do after breakfast.

The power transformers will be all over the place too. You would need to read the manufacturers specifications on them. The value is usually stated as "ambient operating temperature" or "degrees elevation above ambient", and this temperature is dependent on the loading of the transformer. One made to give you 2 amps out, being run at 2 amps will be much warmer than one being only run at 1/2 amp.


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## WCGill (Mar 27, 2009)

Wow, that's low wall voltage these days, as I consistently see over 120 here. The old voltage is commonly put at 115. Thanks for the compliment GuitarG, much appreciated.


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## GuitarG. (Apr 1, 2010)

OK I guess you were right. I did a measurement at one of my outlets yesterday and it read 121 volts. A few other times I did get a reading of 117 volts. I see it can infact fluctuate.


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## GuitarG. (Apr 1, 2010)

Here's my conclusion on Blackfacing an early Silverface. As far as the sound goes they are pretty much identical. Almost the same circuit. Again we're talking "early" silverface compared to later blackface. In the case of my amp all i noticed is a bit of a decrease in bass. The feel is slightly different. The silverface seems to have a little more of a pushed or a certain focus thats slightly different than its blackface couterpart. Keep in mind this is just minor differences. I think the whole "blackface is better than silverface" thing is overhyped. Alot of that reputation probably stems from the later silverface amps with the linear transformers and the increased power, especially in the case of the twin. Those amps do sound different.Even those amps may sound good too some as stock and may sound better if mods are done right. I've heard a few that sound decent. I myself have not been inside one of these amps.
The only thing I would recommend doing, and probably the only thing i would change, if i have another early silverface, is changing the bias circuit so that it is complete fixed bias amp. And converting the output tube section too blackface specs. I would leave the preamp "as-is". Aside from specs I do like the sound of having the 1500 ohm NFB resistor more than stock.


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## GuitarsCanada (Dec 30, 2005)

GuitarG. said:


> Here's my conclusion on Blackfacing an early Silverface. As far as the sound goes they are pretty much identical. Almost the same circuit. Again we're talking "early" silverface compared to later blackface. In the case of my amp all i noticed is a bit of a decrease in bass. The feel is slightly different. The silverface seems to have a little more of a pushed or a certain focus thats slightly different than its blackface couterpart. Keep in mind this is just minor differences. I think the whole "blackface is better than silverface" thing is overhyped. Alot of that reputation probably stems from the later silverface amps with the linear transformers and the increased power, especially in the case of the twin. Those amps do sound different.Even those amps may sound good too some as stock and may sound better if mods are done right. I've heard a few that sound decent. I myself have not been inside one of these amps.
> The only thing I would recommend doing, and probably the only thing i would change, if i have another early silverface, is changing the bias circuit so that it is complete fixed bias amp. And converting the output tube section too blackface specs. I would leave the preamp "as-is". Aside from specs I do like the sound of having the 1500 ohm NFB resistor more than stock.


When did the Silverface amps go a little whacky? Would this be after say 1973 or so. When you say early Silverface, what years are we talking about?


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## GuitarG. (Apr 1, 2010)

GuitarsCanada said:


> When did the Silverface amps go a little whacky? Would this be after say 1973 or so. When you say early Silverface, what years are we talking about?


Around '73-'74. I don't know the circuitry of these amps that well. The transformers were changed. The wattage was bumbed up. A master volume added. This would have including many changes in the circuitry that im not really familiar with personally.
Silverface amps i refer too would be between '67-'72 or '73. The shumacher (not sure on the spelling) transformers on most of these amps are the same trannies used in the late blackface amps. Same # codes. So essentially the amps are identical with the main exception being the bias circuit with a few added components for stability against parasitic oscilation ie. the added caps on the output tubes etc. That's really the only thing i would change personally is the bias circuit and output section as far as blackfacing goes.
As far as the power transformer there are some twins I've seen with 2 holes (one for primaries and one for secondaries), and some where both primaries and secondaries share just one hole in the bell. Both one-holers and two-holers both have the same codes. After inquiring, both are still the same transformer only that Fender had used slightly different guages on the windings or a different wire. They went with whatever they had on hand at the time. Someone at Mercury Magnetics had given me that information, but both transformers (with the same codes) are the same whether one hole or two holes.


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## nonreverb (Sep 19, 2006)

GuitarG. said:


> Around '73-'74. I don't know the circuitry of these amps that well. The transformers were changed. The wattage was bumbed up. A master volume added. This would have including many changes in the circuitry that im not really familiar with personally.
> Silverface amps i refer too would be between '67-'72 or '73. The shumacher (not sure on the spelling) transformers on most of these amps are the same trannies used in the late blackface amps. Same # codes. So essentially the amps are identical with the main exception being the bias circuit with a few added components for stability against parasitic oscilation ie. the added caps on the output tubes etc. That's really the only thing i would change personally is the bias circuit and output section as far as blackfacing goes.
> As far as the power transformer there are some twins I've seen with 2 holes (one for primaries and one for secondaries), and some where both primaries and secondaries share just one hole in the bell. Both one-holers and two-holers both have the same codes. After inquiring, both are still the same transformer only that Fender had used slightly different guages on the windings or a different wire. They went with whatever they had on hand at the time. Someone at Mercury Magnetics had given me that information, but both transformers (with the same codes) are the same whether one hole or two holes.


Your right for most models. There are a couple that were abused early on however. The obvious one was the Bassman. There were many revisions done to it throughout the Blackface and Silverface era. Many to it's detriment.
Probably one of the most obvious changes from Blackface to Silverface was the introduction of the 5U4 rectifier to the amp lines. I generally revert SF amps I get back to 5AR4's when possible as it gives back a bit more plate voltage and alllows the tranny to run a wee bit cooler. It also results in a small increase in plate voltage which can affect the overall tone...usually for the better IMO.


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## GuitarG. (Apr 1, 2010)

nonreverb said:


> Your right for most models. There are a couple that were abused early on however. The obvious one was the Bassman. There were many revisions done to it throughout the Blackface and Silverface era. Many to it's detriment.
> Probably one of the most obvious changes from Blackface to Silverface was the introduction of the 5U4 rectifier to the amp lines. I generally revert SF amps I get back to 5AR4's when possible as it gives back a bit more plate voltage and alllows the tranny to run a wee bit cooler. It also results in a small increase in plate voltage which can affect the overall tone...usually for the better IMO.


True. Most of what I'm talking about is the Twin, Super Reverb amps with reference to the AB763 circuit. The Twin always had solid state rectification. 
Most do preffer the 5AR4 as opposed to the 5U4. The 5U4 deffinately has more sag. I had a silverface super reverb at one time with the 5U4 rectifier. That amp sounded amazing. I remember putting it for sale on consignment at a store. It sold after only a couple of days. I went to the store to pick up my money and found another silverface Super Reverb for sale there. It turns out that the person that bought mine put his up for sale. LOL Anyway, I always regretted selling that one. I would agree though, in most cases the 5AR4 is the better tube too use.


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## nonreverb (Sep 19, 2006)

GuitarG. said:


> True. Most of what I'm talking about is the Twin, Super Reverb amps with reference to the AB763 circuit. The Twin always had solid state rectification.
> Most do preffer the 5AR4 as opposed to the 5U4. The 5U4 deffinately has more sag. I had a silverface super reverb at one time with the 5U4 rectifier. That amp sounded amazing. I remember putting it for sale on consignment at a store. It sold after only a couple of days. I went to the store to pick up my money and found another silverface Super Reverb for sale there. It turns out that the person that bought mine put his up for sale. LOL Anyway, I always regretted selling that one. I would agree though, in most cases the 5AR4 is the better tube too use.


I have a SF Deluxe Reverb and when I bought it, that's the first thing I did.
I suspect that Fender changed from the 5AR4 to 5U4 on all it's tube rectified amps for cost or availability reasons as all the original 5AR4's I've seen in BF amps were European.


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## GuitarG. (Apr 1, 2010)

nonreverb said:


> I have a SF Deluxe Reverb and when I bought it, that's the first thing I did.
> I suspect that Fender changed from the 5AR4 to 5U4 on all it's tube rectified amps for cost or availability reasons as all the original 5AR4's I've seen in BF amps were European.


That's true most have the GZ34 european designation. Same tube as the 5AR4. I don't think there were many other changes to the Deluxe Reverb amps. Silverface Deluxes tend to go for as much money as blackface it seems.


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## nonreverb (Sep 19, 2006)

GuitarG. said:


> That's true most have the GZ34 european designation. Same tube as the 5AR4. I don't think there were many other changes to the Deluxe Reverb amps. Silverface Deluxes tend to go for as much money as blackface it seems.


Unfortunately, I wish that were true. There is some variation in pricing, but generally BF Deluxe reverbs are about double that of the typical SF Deluxe provided they're in mint original condition.. Ironic as they're almost the same amp.


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## GuitarG. (Apr 1, 2010)

nonreverb said:


> Unfortunately, I wish that were true. There is some variation in pricing, but generally BF Deluxe reverbs are about double that of the typical SF Deluxe provided they're in mint original condition.. Ironic as they're almost the same amp.


I suppose you are correct on that. Just shows that Blackface is better hype. I remember about 4 or 5 years ago there was alot of internet talk about silverface Deluxes being pretty much the same as the Blackfaces. Twins and Super Reverbs got left out of the picture somehow. This drove the price up for a while on silverface Deluxes to the point people selling them were asking almost as much for a silverface as a blackface Deluxe.
I guess it has dropped since then. For the majority of guitar players that don't know the circuits, "It has too be a blackface......".


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## parkhead (Aug 14, 2009)

I have not read all the replies but 

IMHO the cheesy vintage power supply and its small filters is 80% of the MOJO people are looking for in a vintage amp, particularly a Black Face Fender 
the next thing they do is recap the thing and go for beefier values looking for "vintage tone" with added bass 

in other words step away from the 80 mfd filters unless you want to join the attenuator crowd

there is some added MOJO to the actual Black Face amps 
the interstage caps are very musical the power supply is very soft and the layout and lead dress are tidy and tight 

these are all part of the sound 

However, anyone not GRABBING a silver face Deluxe instead of a reissue is missing the boat 

over the next 20 years the 20 watt amps will be the power level to own 
as PA gets more efficient and compact & personal transportation becomes a smart type car or transit 

p


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## nonreverb (Sep 19, 2006)

"in other words step away from the 80 mfd filters unless you want to join the attenuator crowd"

More importantly, 80uf is too big a value for the primary filter stage of a GZ34 or 5U4 rectifier...bad things can happen to them with too much capacitance...


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## keeperofthegood (Apr 30, 2008)

The way I have read and taken my understandings of filter caps is to look at them as energy reserves. Ways the power supply has to keep voltages "on spec", pushing an amp into overdrive, you do want that voltage to sag, to dip, to be used up before the need for it is done. Going smaller gives that 'empty sooner' effect.

Is that essentially right or wrong?

Oh and, on a Pi filter, capacitor input, the data sheets of the rectifiers will tell you up front the max safe capacitance. Ranges from 4uf to 20uf from the sheets I have looked at. This because of the current that it takes to charge up that first capacitor, exceed the tubes capability and you strip the cathode coating if I read it correctly. Following the choke, there is more liberty in the value for the capacitor, but then, that's where you get into that 'reserve' energy store that I have read about....


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## nonreverb (Sep 19, 2006)

keeperofthegood said:


> The way I have read and taken my understandings of filter caps is to look at them as energy reserves. Ways the power supply has to keep voltages "on spec", pushing an amp into overdrive, you do want that voltage to sag, to dip, to be used up before the need for it is done. Going smaller gives that 'empty sooner' effect.
> 
> Is that essentially right or wrong?
> 
> Oh and, on a Pi filter, capacitor input, the data sheets of the rectifiers will tell you up front the max safe capacitance. Ranges from 4uf to 20uf from the sheets I have looked at. This because of the current that it takes to charge up that first capacitor, exceed the tubes capability and you strip the cathode coating if I read it correctly. Following the choke, there is more liberty in the value for the capacitor, but then, that's where you get into that 'reserve' energy store that I have read about....


That's correct keeps. Worst case, with enough capactance, your typical 5Y3 or GZ34 built today could short out from the excessive current.


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## parkhead (Aug 14, 2009)

keeperofthegood said:


> The way I have read and taken my understandings of filter caps is to look at them as energy reserves. Ways the power supply has to keep voltages "on spec", pushing an amp into overdrive, you do want that voltage to sag, to dip, to be used up before the need for it is done. Going smaller gives that 'empty sooner' effect.
> 
> Is that essentially right or wrong?
> 
> Oh and, on a Pi filter, capacitor input, the data sheets of the rectifiers will tell you up front the max safe capacitance. Ranges from 4uf to 20uf from the sheets I have looked at. This because of the current that it takes to charge up that first capacitor, exceed the tubes capability and you strip the cathode coating if I read it correctly. Following the choke, there is more liberty in the value for the capacitor, but then, that's where you get into that 'reserve' energy store that I have read about....


yes there are also a secondary layers that most people do not think about... since all stages also ground through those caps they do contribute to the tone of the amp. 
If you use to big a supply cap you will not only have a larger reserve but also affect the ability of the attached stage to amplify low end ... sounds good, but i reality it is not 
Imagine replacing a typical .02 interstage coupling cap with say 2mfd or even .2mfd .. suddenly you would have horrible unmusical excessive bass
when Leo Fender filtered the tweed bassman Pre amp with an 8mfd filter he was intentionally limiting the low end his available speakers could not handle ..
The Fender resissue is filter 40mfd at that stage and does not hold a candle to an original tweed tonally 

Likewise there are time constant changes and intestage decoupling issues that are not obvious at first glance ...
for example what If I suggested that the power supply was actually a route for low frequency (subsonic) negative feedback... now look at it again in see how the stages interact 
now the amp is being played and as the filters reserve level changes voltages are fluctuating & the ability of the cap to block and decouple the feedback route is changing 
its an extremely dynamic situation with lots desirable sonic artifacts 

if you are aware of how these things affect tone there are some great tweeks to be found in subtle amp power supply changes 

I give Leo full credit since his true genius was tweaking his designs to perfection, how many pre cbs amps actually sounded bad ? Boxy, or congested 
he was dialing perfect eq blends with powersupply speaker transformer and tweaks in all those amps
The more the CBS guys changed suppliers or re tweaked the further they got from the sheer musicality Leo listened for

if you play a well maintained 64 deluxe reverb or tweed bassman the sheer perfection fo the musical fine tuning is astounding 

People often comment that the Bassman and the Early JTM45 have very different growl and attribute this to the transformers, however it is more likley the 
subtle filtering and choke changes Ken Bran intentionally made to the first marshalls ... 

again aspects of the circuit Marshall Ignored in their reissue programs 
those who know Pre 67 Marshalls don't have much interest in reissues unless they are planning on completely recapping the powersupply to the lower numbered specs 

with regard to the .001 & 1 meg vs the 330k & .01 specs the eq curve is the same if you do the math 
the 1megs simply drive the inverter with more signal 

the math you need is here (someone told me there would be no math ...well, they were lying) 

MetroAmp.com Forum &bull; View topic - Calculating RC time constants...and why you should.


P


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## GuitarG. (Apr 1, 2010)

All good points Parkhead. As far as the math you mention, because something is 10 times more the value does not mean you are going to get 10 times more or less bass. At least that's not what I necessarily meant on my earlier post. However, after listening I did notice a difference in the bass response which is why after going with the 1 meg resistors i ended up with a .003 cap intead of the .001 I initially put in. 

I'm wondering if Ken Bran actually made the changes to the early JTM45's because of the differences in transformers when likely trying an exact bassman circuit initially?


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## Wild Bill (May 3, 2006)

GuitarG. said:


> All good points Parkhead. As far as the math you mention, because something is 10 times more the value does not mean you are going to get 10 times more or less bass. At least that's not what I necessarily meant on my earlier post. However, after listening I did notice a difference in the bass response which is why after going with the 1 meg resistors i ended up with a .003 cap intead of the .001 I initially put in.
> 
> I'm wondering if Ken Bran actually made the changes to the early JTM45's because of the differences in transformers when likely trying an exact bassman circuit initially?


Not likely, GG! Output transformers are one of the most expensive devices in an amp. For years they were taken for granted as far as tone. It's only in the last 10-15 years that some hardworking guitar techs did a lot of experimenting with reverse engineering OTs to prove that they do indeed lend a lot to how the amp sounds!

That being said, remember that those first Marshalls were designed in the early 60's! Leo Fender didn't really start trying major non-hifi circuit tricks until the late 50's. Guitar amps started out as basically the same as hifi amps. Music at the time was jazz or country. Only a few artists like Muddy Waters were experimenting with an "electrified" guitar tone. Guitars and amps were pretty clean sounding back then. It was rock and roll that changed the world!

You're right about coupling caps being kinda "vague" about bass and values. The rule of thumb is that you have to change the value of a coupling cap by a factor of 10 to begin to even notice. That's because a coupling cap by itself is a very simple tone circuit. At the high impedances found at the input to tube grids that impedance and the capacitance forms a simple R/C circuit. When R is in zillions the value of C has less effect on the total. That's why Parkhead explained to you how .001 and 1m would sound the same as 330k and .001.

The other factor that everyone today seems to forget is cost! Those were the days before the audiophile market was trying to rook you into buying $1000 power cords and $50 coupling caps. Even so, with ordinary coupling caps it's always true that bigger values tend to be more expensive, 'cuz there's more stuff in 'em! Those Fenders and Marshalls had to try to keep their costs down. They were not going to use an unnecessarily more expensive part if they didn't have to do so. 

Leo and Ken would have been well educated in the tube tech's rule of thumb that you should have at least one separate filter cap for every 4 preamp plates you wish to decouple and that it should be at least 10 mfd in size. Going bigger in value would mean more cost. I'm sure that was their first thought! Stiffening up the power supply and thus changing the tone would have been second, if at all. At that time, as long as the amp sounded good who would care? And the mere fact that it was a new electric sound was usually enough in itself. The only comparison was with those older jazz and country tones.

When the Blackface Era began, nobody changed to a solid state rectifier to get a "tighter" and "snappier" tone! They did it because the silicon diodes cost pennies and the tube rectifier cost dollars! PLUS a socket and the space to mount it! It wasn't until years later when we had amps from different eras available to us that people were able to make comparisons and understand the difference between a tube and SS rectifier on the amp's sound.

Hindsight is always 20/20. It's easy for us to talk about all the technical details. Those guys way back then were looking forward, into the murky future. They were blazing the trail and usually had little or no idea what was around the corner. They only knew that if a change they made gave an attractive sound more people would buy their amps! Their knowledge of electronic theory did serve as a guide. If Pete Townsend had an idea of what sound he wanted from his amp Ken's knowledge would give him some idea of where to start making changes but still, it was mostly "trial by terror"!

A good example of this sort of evolution in circuits would be Pete Traynor! He was great for trying different things. Sometimes they gave a benefit and sometimes they didn't but Traynor's assembly line setup was able to be flexible and quickly react to changes. That's why you'll find several different schematics for a Guitar Mate and even then there are Guitar Mates out there that don't exactly fit ANY published schematic!

He had both electronic knowledge AND a good ear! That was essential to rock and roll design. Jazz and country amps were easier 'cuz the amps were just extrapolations of hifi circuits, going for a clean, distortion free sound. Rock and roll was SUPPOSED to be distorted! It's just that there are a lot of different forms of distortion and not all are pleasing to the majority of people. That meant that amp circuits had to be rather different.

WB


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## Rugburn (Jan 14, 2009)

Wild Bill said:


> When the Blackface Era began, nobody changed to a solid state rectifier to get a "tighter" and "snappier" tone! They did it because the silicon diodes cost pennies and the tube rectifier cost dollars! PLUS a socket and the space to mount it! It wasn't until years later when we had amps from different eras available to us that people were able to make comparisons and understand the difference between a tube and SS rectifier on the amp's sound.
> WB


It's interesting that this was done to the larger and more expensive amps in the Blackface line. The smaller amps like the Deluxe Reverb, Princeton Reverb, Super Reverb, Vibroverb, and Vibrolux were all tube rectified. The rectifiers weren't changed in the DR and PR until the Rivera designed II series came out in the early 80's. I wonder what the reasoning was behind keeping tube rectifiers in the "entry" level amps for so long as oposed to the larger professional models. 

Shawn.


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## keeperofthegood (Apr 30, 2008)

What I recall of the early 80's (bearing in mind, in 1980 I was 11 years old) is that THE THING TO HAVE was "Solid State", so much so that manufacturers would proudly label that fact in big letters on their equipment. For the makers, at that time, I have no idea what going from a tube to 4 diodes and cap from 8uf to 40uf cost in terms of price increase. In the 1970's silicon was not inexpensive. I still recall when 1 meg of computer ram cost 75 dollars @[email protected] but sometime in the 90's there was a huge fall in the cost of technology, and prices came down a lot, to where now what you purchase is really just pennies.

Hmm, would need to see a 1979 Radio Shack catalog to know for sure what the costs were like on solid state parts.

However, if you put into your top of the line THE THING TO HAVE, costing you more or less doesn't matter, you have to GIVE THE PEOPLE WHAT THEY WANT to make money


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## GuitarG. (Apr 1, 2010)

Wild Bill said:


> Not likely, GG! Output transformers are one of the most expensive devices in an amp. For years they were taken for granted as far as tone. It's only in the last 10-15 years that some hardworking guitar techs did a lot of experimenting with reverse engineering OTs to prove that they do indeed lend a lot to how the amp sounds!
> WB


Thanks Bill. That's actually what I was getting at in regards too Ken Bran's design of the JTM45. From what I have read, the goal Jim Marshall was initially going for was an exact Bassman circuit or at least sound. The original prototype went through 5 or 6 revisions before going into production. What I was getting at is that originally they may have tried an exact circuit copy of the Bassman which did not quite sound right because of the differences in output transformers between Triad and Radiospares when this was tried. Hence the change to the circuitry to try to get it to sound right or at least what became the Marshall sound.
I agree with you that output transformers have an impact on the overall sound of an amp.


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> What I recall of the early 80's (bearing in mind, in 1980 I was 11 years old) is that THE THING TO HAVE was "Solid State", so much so that manufacturers would proudly label that fact in big letters on their equipment. For the makers, at that time, I have no idea what going from a tube to 4 diodes and cap from 8uf to 40uf cost in terms of price increase. In the 1970's silicon was not inexpensive. I still recall when 1 meg of computer ram cost 75 dollars @[email protected] but sometime in the 90's there was a huge fall in the cost of technology, and prices came down a lot, to where now what you purchase is really just pennies.
> 
> Hmm, would need to see a 1979 Radio Shack catalog to know for sure what the costs were like on solid state parts.
> 
> However, if you put into your top of the line THE THING TO HAVE, costing you more or less doesn't matter, you have to GIVE THE PEOPLE WHAT THEY WANT to make money


Keeps, I WAS there! I know exactly what the parts cost! Manufacturers of electronic equipment would call me up and ask me the price!

In the mid 60's SS diodes were around 25 cents. It was hard to find ones with PIV ratings of 1KV so you would put two 600 PIV diodes in series on each side of the bridge. So you were talking at most a $1 solution. Initially, they didn't bother to change the size of the input filter cap so that wasn't a factor but even when they did, add maybe another 25-35 cents.

A 5U4 was around $5 in production quantities. A socket was another $1. The extra space to put everything is obvious.

By 1979 I was selling 1 KV PIV diodes, 1n4007s, for around $6 per hundred, CDN. Radio Shack would have charged you 25 cents each in onesy-twosy quantities.

WB


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## Wild Bill (May 3, 2006)

GuitarG. said:


> Thanks Bill. That's actually what I was getting at in regards too Ken Bran's design of the JTM45. From what I have read, the goal Jim Marshall was initially going for was an exact Bassman circuit or at least sound. The original prototype went through 5 or 6 revisions before going into production. What I was getting at is that originally they may have tried an exact circuit copy of the Bassman which did not quite sound right because of the differences in output transformers between Triad and Radiospares when this was tried. Hence the change to the circuitry to try to get it to sound right or at least what became the Marshall sound.
> I agree with you that output transformers have an impact on the overall sound of an amp.


The difference between output trannies is less obvious than you think. Usually it's not a "hit you over the head" difference that anybody would notice, especially if the two trannies are of similar quality. Quality shows up in cost. Some brands at the time were REALLY cheap and those show more intense differences. I doubt if the difference in quality between a Radio Spares and a Triad was that much as far as the actual materials used and the construction. An OT depends on the quality of the steel in the laminations and the windings of the copper wires. There's really nothing else to play with. Remember, transformers don't deal with sound. They deal with electrons running around as electricity. It ain't sound until the speaker's cone moves! There are no arguments about rosewood versus maple inside a transformer!

The genesis of the Marshall sound has been documented many times. Any book on the evolution of Marshalls should tell you that the original Bassman clone worked fine as far as Marshall and their customers were concerned. Remember too that Marshall was in Britain! A Fender was an import and imports were expensive! Marshall was looking to compete on price and not that worried about 100 % cloning the sound. Even at that, I'm sure that if we had a Fender and a Marshall unit side by side today few of us could pass a blindfold test as to what was being played.

No, the difference was much simpler and much more dramatic! After a couple of years of sales 6L6 tubes and others in that family became scarce and the price went through the roof! Europeans were frantically looking for an alternative. El34's were considered because they had the same pinout, about the same power rating and all that was really needed was a bias voltage range change, which could be done for the cost of one cheap 1/2 watt resistor!

The splendid surprise was that EL34's were a much more sensitive tube. They needed a lot less drive from the preamp section to achieve full output. This meant that they could be overdriven into power amp distortion far more easily than a 6L6 family tube!

This was the birth of the Marshall "snarl" and instantly their amps were in demand for all the new bands of the time. These bands increased in number dramatically due to the "British Invasion" of North American music charts. Marshall sales went through the roof! They now had their own distinctive sound to compete with in the marketplace. There was no profit in going back to 6L6's to try to compete against Fender with Fender's own sound.

WB


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## keeperofthegood (Apr 30, 2008)

YAY WILD BILL 

Ok, so they essentially saved about 5 dollars (or is that closer to 6, my math is bad this week) by going solid state at that time. With the benefit of being able to do as others did at that time and advertise the Solid State too. Double bonus


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> YAY WILD BILL
> 
> Ok, so they essentially saved about 5 dollars (or is that closer to 6, my math is bad this week) by going solid state at that time. With the benefit of being able to do as others did at that time and advertise the Solid State too. Double bonus


I don't remember specific advertising for the type of rectifier, Keeps. After all, 99% of the customers had no idea what a rectifier was! With everything BUT guitar amps you're absolutely right that SS was pushed as the "latest and greatest". That was what started my education! Nobody fixed a tube radio or tv, they just handed it to a kid like I was to tear apart and blow up while they rushed off to buy something new and solid state! I got tons of free stuff to scrap! All it cost my parents was extra fuses on the grocery list.

There were "transistor" wars for portable radios, where one manufacturer would make the claim "Ours is a 7 transistor radio" and another would say "Ours is a 12 transistor radio". The implication to the ordinary buyer was that the unit with more transistors must be better. Of course, that buyer wouldn't know a transistor if you siipped it into his cornflakes but it did help sales. Techs would laugh when one came in for repair and they saw how several transistors weren't actually hooked up to anything. They were just stuffed onto the board to increase the transistor count!

I worked in a music store in Stoney Creek in the early 70's and tubes still ruled. Manufacturers were often ruled by suits who didn't play guitar themselves. They were always pushing transistor amps but the problem was, they all sucked in tone! I remember the boss bringing in some VT amps. They would go out on trial and come right back again! Then they sat in the store for months and years until finally we would have to slash the price so some sucker would buy them.

Musicians were no different than anybody else in that they responded to the transistor hype but when they actually PLAYED a transistor amp they were very disappointed! It was the same problem then as today. Transistors were ok for clean but did not have a pleasing distortion. As we left the 70's the suits finally got this point through their head and concentrated their efforts on transistor bass amps, where the desired tone was clean and snappy, like Chris Squire. Tubes just wouldn't go away for regular guitar but the manufacturers kept trying. Traynor let their tube line die after Pete left and kept offering solid state guitar amps but the only people who would buy them were students and parents who didn't know any better. No professional guitarist would take one on stage. Yorkville Sound made their money on bass amps and PA gear, until finally they got back into the market with a bang in 2001, with the release of the YCV40 tube amp.

Ah, nostalgia isn't what it used to be!

WB

What DID sell quickly was Marshalls! Lots and lots of JMPs, JTMs and eventually JCM800s.


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## parkhead (Aug 14, 2009)

We tend to give more credit to Leo Fender and Ken Bran for having rock & roll ears than they probably deserve. 

I do think Ken was shooting for a specific tonal objective & it might have even been a rock & roll guitar sound 
and definately it turned out to be exactly that ... but I suspect the design requirements of the product were very simple

Leo was a country music fan. He listened to players & took the time to go to roadhouses and ****** tonks to hear people play. I'm sure the design objective on the early stuff was:

The amp has to Project and get loud in a noisy and rowdy bar 
The amp has to hold together even if the player runs it flat out 
The amp has to sound musical when run flat out because "they do that" even if we tell them not to 
the amp should be easy to repair with tubes and parts available anywhere 
if we limit the low end and push the high end we should get a bright clean sound that does not dammage the amp and speaker if its run hard 

Since Ken borrowed Leo's speaker saving bass amp eq, he was already following the same path. 

Let's not forget the influence of the players: 
country players used to tell Leo they liked his amps because they had "hair on their chest" 

and Pete Townsend has said he wanted his amps loud enough so he wouldn't hear anyone say the band sucked

these days people talk about "tone" but if you remember your first gig with an amp that couldn't fill the school gym 
or the first time you played with a drummer and couldn't hear yourself ... the whole idea is about being louder

Big Pa and monitors did not exisit so good sound required power and when power was maxed harmonics with "presence" 

Being the loudest explains why a 69 marshall superlead, sounds the way it does....
that 5k bright cap on the volume ensures that anyone tired of being out gunned by the drummer 
is ready to throw down their cash to be heard !

p


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## WCGill (Mar 27, 2009)

So true that we give Leo more musical credit than he deserves-design credit too. Basically he just borrowed the Western Electric circuit and adapted it to his amps. These amps didn't hold together when dimed nor were they designed to. Leo never envisioned the volumes needed at the time and the cure for distortion was more power, hence the evolution of the Twin and the Blackface amps. You read so much about how Leo also recognized the superiority of point-to-point and tag-board design. If printed circuit boards had existed, Fender would have used them. If transistors would have been available, he would have used them too. Leo was a radio repairman, a technician, not a musician. Like Parkhead said, he relied mostly on country musicians for their opinions on his products and this was a big influence on his designs. It was a wonderful combination of luck, timing and a man who had the right mix of talents and talent around him at a juncture in American music evolution that happens only once. You couldn't have planned it any better. Awesome amps!


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## parkhead (Aug 14, 2009)

Well I give Leo a little more credit than that ... after all look at his track record 
if it was not all him he certainly had good ears or listened to the right people 


what? 40 + significantly cool amp designs, all of them highly collectable, playable and to a great extent 
still current ...

throw in the Tele, Strat Pbass and Jazz and his productivity was mind boggling

leo didn't start making dogs till the maurauder and the Jazzmaster offset stuff the punks think is cool 
(but isn't really, don't tell them) 

Jim Marshall and Ken Bran really got 6 cool products 
out of Leos design ... and that was enought a build a company 
modern Marshalls while cool are not as universally cool as the 4 input and early MV designs 

4-12 
JTM45 
JTM50 
1959 
super bass 
2203 

p


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## WCGill (Mar 27, 2009)

Hmm, my intent was not to downplay or minimize Leo Fender, perhaps you missed the point of my post. I guess I should have been clearer. Leo was above all else, practical, using whatever was available to him and at reasonable cost, to produce his products. This was his true genius, assembling these resources to produce new products, including adapting the classic Western Electric circuit, which is still hard to beat. His guitar designs were totally ground-breaking and futuristic, changing music forever, inspiring many imitators, as did his amplifiers. But, his products were not produced by craftsmen, instead assembled by blue-collar workers. To design and produce a cheap pine plank that could compare with luthier-built neck-through designs was the ultimate example of Leo's achievement. He was a very rare bird. Sound better?


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## parkhead (Aug 14, 2009)

WCGill said:


> Hmm, my intent was not to downplay or minimize Leo Fender, perhaps you missed the point of my post. I guess I should have been clearer. Leo was above all else, practical, using whatever was available to him and at reasonable cost, to produce his products. This was his true genius, assembling these resources to produce new products, including adapting the classic Western Electric circuit, which is still hard to beat. His guitar designs were totally ground-breaking and futuristic, changing music forever, inspiring many imitators, as did his amplifiers. But, his products were not produced by craftsmen, instead assembled by blue-collar workers. To design and produce a cheap pine plank that could compare with luthier-built neck-through designs was the ultimate example of Leo's achievement. He was a very rare bird. Sound better?



No I didn't take it that way... his achievements were astounding in so many ways... at the same time he was just a guy fascinated with machines, tools hardware and all things practical 
He didn't have the grand vision of the future people attribute to him.

p


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## Rugburn (Jan 14, 2009)

Leo Fender used to loan/give some musicians amps to take on the road. He encouraged the players to push the amps hard and bring them back to him when problems arose. That's a pretty amazing commitment to build quality and a very smart way to assess an amp's design flaws.

Shawn.


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## nonreverb (Sep 19, 2006)

Rugburn said:


> Leo Fender used to loan/give some musicians amps to take on the road. He encouraged the players to push the amps hard and bring them back to him when problems arose. That's a pretty amazing commitment to build quality and a very smart way to assess an amp's design flaws.
> 
> Shawn.


He's my second favourite...right after Don Leslie


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