# Swapping tubes into a simple amp - 12AT7 as a preamp tube?



## Greg Ellis (Oct 1, 2007)

First of all, kudos to The Tube Store, and to Canada Post.

I placed an order yesterday around noon, through The Tube Store website, and it was processed, packed, picked up by Canada Post and delivered to my door (Hamilton to Newmarket) before 9am this morning. Very impressive.

Kudos also for having a very nice selection of tubes to ponder and choose from, in stock and ready to go.

I'm working with a Blackheart Little Giant - that's a 5 watt single-ended class A design, with two valves.

V1 is by default a 12ax7. From the factory, it was a cheap Chinese tube that I've already replaced with an EHX.

V1a takes the input signal and drives it through the tone stack to a volume control and then into V1b, which drives the grid of the EL84.

V2 is an EL84, cathode biased if I'm reading it right. Again a no name Chinese tube by default. I swapped it for a Sovtek about a year ago.

So with my package from the Tube Store waiting for me to get home from work tonight, I've got the following options on the experiment list:

Original no name Chinese EL84
Sovtek EL84
new JJ EL84
new TAD EL84-STR

And on the preamp side, I've got four options to play with:

No name Chinese 12ax7
EHX 12ax7
ancient RCA 5751 from an old Sears amp (60's or maybe 70's)
new JAN-Philips 12AT7WC

The one I'm a bit concerned about is the 12AT7. I realize these are more typically used as a phase inverter, and in part that is due to the operating characteristics of the tube.

(scooped from the internets)

• 12AX7
Gain ~ 100
Current Output ~ 1.2 milliamps
TC ~ 1600

• 5751
Gain ~ 60-70
Current Output ~ 1.0 milliamps
TC ~ 1200

• 12AT7
Gain ~ 60-70
Current Output ~ 10.0 milliamps
TC ~ 5000

I can see that the 5751 ought to swap in rather painlessly; specs are close enough to 12ax7.

But the AT7 passes a ton more current, and transconductance is seriously different also.

Do I need to worry about things blowing up if I try that 12AT7?

I'm talking about swapping tubes without any other changes to circuit values.

There's a schematic here if anyone is interested - BH5_schematic Rev 20070611 | Rowbinet - Rowbi's Guitar & Recording Blog

Any thoughts?


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## keto (May 23, 2006)

Nope, no worries - an AT7 is a common substitute in preamp if lower gain is desired.


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## mrmatt1972 (Apr 3, 2008)

I've done it in that amp. It works if clean is what you're after, but there is significant overall volume drop. Every 12at7 I've bought has gone microphonic on me too, so YMMV. I really prefer JJs in my Blackheart for power tubes, but I'd like to hear about the TAD.


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## Greg Ellis (Oct 1, 2007)

Well, that didn't go quite as well as I was expecting. The JJ worked out fine - it's quite different from the Sovtek. Grindier and louder.

But the TAD blew the amp on power up. Pilot light went out within a second. Hopefully it's just a fuse. It was late and I was too tired to really dig into it last night. Hopefully I can sort it out today.

Edit - yup, the main fuse at the AC intake is blown. I'll have a dig up a handful of those, maybe the Source has them at the mall.


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## washburned (Oct 13, 2006)

Greg Ellis said:


> Well, that didn't go quite as well as I was expecting. The JJ worked out fine - it's quite different from the Sovtek. Grindier and louder.
> 
> But the TAD blew the amp on power up. Pilot light went out within a second. Hopefully it's just a fuse. It was late and I was too tired to really dig into it last night. Hopefully I can sort it out today.
> 
> Edit - yup, the main fuse at the AC intake is blown. I'll have a dig up a handful of those, maybe the Source has them at the mall.


It's rarely 'just" the fuse: find out what made the fuse blow.


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## Greg Ellis (Oct 1, 2007)

I have to wonder if it was me. 

I had the open chassis sitting on top of my speaker cab and that's got a handle on it with metal bits at the ends. Maybe I shorted something out.

I noticed that it blew not only the fuse in the amp, but the circuit breaker on my power bar.

I picked up some replacement fuses, and it's all fine now, back up and running, even with the TAD tube.

All seems good now though. I'll be more careful from here on in, grin.

BTW - the TAD really is quite different from the JJ and Sovtek EL84's. I'm still messing with it, and haven't done any recordings yet, but it seems to be tighter in the bass and have an emphasize at a different part of the upper mids somehow.


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## Wild Bill (May 3, 2006)

Greg Ellis said:


> Well, that didn't go quite as well as I was expecting. The JJ worked out fine - it's quite different from the Sovtek. Grindier and louder.
> 
> But the TAD blew the amp on power up. Pilot light went out within a second. Hopefully it's just a fuse. It was late and I was too tired to really dig into it last night. Hopefully I can sort it out today.
> 
> Edit - yup, the main fuse at the AC intake is blown. I'll have a dig up a handful of those, maybe the Source has them at the mall.


Washburned is giving you good advice. Something made the fuse blow! Best to find out what that was or you may be sorry.

As far as your choice of tubes, the brand of tubes has no difference on tone. There is no sound inside a tube! No little maple or rosewood elements with different tonal qualities. A tube handles electrons! There is no sound until the speaker cone pushes air.

That being said, different gain factors can definitely change the tone, usually with the amount of headroom. However, most preamp circuits have so much extra gain that it tends to swamp gain differences out. With a 12AX7 you have two triode stages in each tube that have gain factors of 100. You have to multiply them together. Count the number of 12AX7s in the average amp and do the math. Mutiply 10,000 times the number of tubes. The amount of gain is such that using a 5751 or a 12AT7 doesn't make an audible difference, except perhaps in the very first position.

Your amp might show more of a difference than another, since it has only one preamp tube! With much less gain involved, there's much less swamping out of any change in tubes. Count the tubes in a Peavey 5150 sometime!

However, there are vast differences between brands as to overall quality! Some last longer than others, some tend to develop noises and microphonics. That's why tubes from the Golden Years seem to last much longer. They were made better! I tend to stick to EH and JJs, as being good quality for the price.

As I said about gain differences, there may not be any real audible difference. Certainly not one that anyone else but a really picky fellow musician might notice. Even then, try him after he's had a few beers! There is one exception - a 12AY7. This dual triode fits the same socket with the same pinout and needs no changing of resistor values around it. It has a voltage gain of only 40 and even ears honed on "cookie monster" metal would notice the change! This was Leo Fender's preamp tube of choice with his first amps and up until the late 50's with the Tweed amps. If you play more rootsy blues, more Delta than Chicago, you may really like it!

Incidently, in the Golden Years the 5751 was never really considered a different tube. It was a 12AX7 that had a twisted heater winding, giving it a bit less hum. That made it a good choice for hifi audiophile amplifiers. Today it is sold as a slightly less gain 12AX7. Nobody ever mentions a lower hum filament. I suspect that what is really going on is that if a production run of 12AX7s turns out to have a bit less gain than they are supposed to the factory labels them 5751s instead of scrapping them!

Or maybe I'm just too suspicious!kkjuw

Wild Bill/Busen Amps


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## Greg Ellis (Oct 1, 2007)

Thanks Bill. I've seen you write that speech before, about tubes not changing the sound.

Perhaps it's just my expectation that makes me hear something different in the way the TAD EL84 tube breaks up when compared to the JJ and the Sovtek, but I hear it.

The Jan Philips 12AT7 seems to be changing frequency response more than anything, when compared to the EHX 12AX7. There's a lot more treble there and less bass.

Perhaps it's all in my head. I'll try a few more combinations and see what's most pleasing to my ears. Hopefully it's not the most expensive option, LOL.

I'm pretty sure I must have shorted something to ground to blow the fuse. I had the chassis sitting wire side down on top of the carry handle for my 2 x 10 cab. Stupid really. While changing tubes I must have brought one of the metal bits on the handle into contact with the PCB or something, so when I powered it up next it went poof. 

It's running great again now with a new fuse and a book underneath keep it clear of any contact with metal.


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## Wild Bill (May 3, 2006)

Greg Ellis said:


> Thanks Bill. I've seen you write that speech before, about tubes not changing the sound.
> 
> Perhaps it's just my expectation that makes me hear something different in the way the TAD EL84 tube breaks up when compared to the JJ and the Sovtek, but I hear it.
> 
> ...


I'm not saying you are crazy, Greg. You are reacting like a human being. We all share a human psychology. We are hard-wired to continually try to make patterns out of our environment. This is a good thing if you have to worry about a lion being behind a bush or tree!

However, it can often fool us and so it does with tone and sound generation. The technical term is "psychoacoustics" and it is a fascinating study in itself. 

Tonal differences with different brands of the same tube have NEVER stood up to true scientific testing! That means blindfolds and no possible means of a subject having any idea what tube is operating or even if a tube was indeed changed! So-called audiophile gurus have either failed such tests miserably or they have conveniently not been available whenever a test has been suggested.

For a guy like me to believe otherwise is to ask me to throw away 50 years of electronic physics and theory and start believing that the Sun goes around the Earth. Or that a politician always keeps his promises!

Being human, we can't help but start to hear things that aren't there if we listen long enough. We are especially minded to hear positive differences if one of the tubes involved cost us a lot more money! Nobody wants to believe they made a mistake and just wasted their cash because of some mojo.

Spin that LP backwards, Greg! Hear those satanic messages! When you first hear the record backwards it sounds like total gibberish. Yet let someone tell you that it is saying "Paul is Dead!" and dammit! It sounds like that plain as day! Your brain has successfully made a pattern that wasn't there, after the suggestion was given.

I've been kicking around the idea for years about setting up a night at a local club here in Hamilton for a GuitarsCanada jam session! It would also be an opportunity to run some blindfold tests on different brands of tubes, or guitar cords, or speaker wire. I think it would be great fun and would be willing to wager a bottle of good scotch with anyone who cares to participate!

I'd have enough drink for life!:woot:

Wild Bill/Busen Amps


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## Greg Ellis (Oct 1, 2007)

Well, I'm rather amazed to say it, but I think he's right guys. 

After swapping tubes back and forth, and recording little snippets and panning them left and right to compare, I honestly can't tell the difference. A couple of the configs were a touch "gainier" than others, and that did make a bit of a difference to how the amp played, but tonally? All the same. Sovtek, JJ, TAD - no detectable difference.

That's just my experience, of course. YMMV.

I guess I've got enough backup tubes to last me several years now, grin.


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## Wild Bill (May 3, 2006)

Greg Ellis said:


> Well, I'm rather amazed to say it, but I think he's right guys.
> 
> After swapping tubes back and forth, and recording little snippets and panning them left and right to compare, I honestly can't tell the difference. A couple of the configs were a touch "gainier" than others, and that did make a bit of a difference to how the amp played, but tonally? All the same. Sovtek, JJ, TAD - no detectable difference.
> 
> ...


Well Greg, I guess now I'm not going to win any scotch from YOU! LOL

I guess I should buy YOU a drink!:food-smiley-004:

Wild Bill/Busen Amps


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## dradlin (Feb 27, 2010)

Wild Bill, while you are talking tubes: in your opinion do they degrade over time or do they simply work or not (like say a lightbulb)? Just retubed my DRRI without noticing any difference.

Also, so the difference between brands is limited to price and build quality (durability)?


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## Wild Bill (May 3, 2006)

dradlin said:


> Wild Bill, while you are talking tubes: in your opinion do they degrade over time or do they simply work or not (like say a lightbulb)? Just retubed my DRRI without noticing any difference.
> 
> Also, so the difference between brands is limited to price and build quality (durability)?


Durability is exactly it, DR! As for tubes degrading, preamp tubes can but not significantly. They can grow weak over the years but this is a very rare thing, more of a failure than a normal occurrence. They are voltage amplifiers and thus have very little strain on the cathode coating, which releases the electrons with which a tube works.

99.9% of the time they simply work until they die, which might be decades. I have pulled 12AX7s that were in service for 50 years and they still tested fine!

Power tubes in output stages are different. They deal with big gobs of current and can indeed slowly get weaker and weaker, till eventually they are no longer useful. This is why I suggest that performing musicians that play their amps a lot take them to a tech every couple of years, just to have the tubes tested. At the same time the tech should check the idle current and if necessary reset the bias.

If the tech doesn't actually own a tube tester then go somewhere else! Why should you pay for new 6L6s if the old ones were still good?

Another mojo myth going around is that the brightness of a tube's heaters is an indication of how strong it is working.

Balls! If it lights up at all the heater is working. It can be cheerfully bright and weak as a politician's promise!

Wild Bill/Busen Amps


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## dradlin (Feb 27, 2010)

Thanks Wild Bill...


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## WCGill (Mar 27, 2009)

What you were hearing when you substituted the 12AT7 into your amp wasn't psychoacoustics. Lowering gain by subbing a tube with higher plate resistance than a 12AX7 will increase bandwidth dramatically. Swapping a 5751 will lower gain by about 25% but won't do much for bandwidth increase but a 12AT7 or a 12AY7 should make a difference. First position will probably be the most dramatic but if circuit limitations won't amplify that extended bandwidth, audibility may be much more subtle. A 12AX7 has a theoretical gain of 100 but is much lower in a circuit with a 100k plate resistor, the most commonly-used value and when loaded with following circuits such as tone stacks, gain drops even further.


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## Greg Ellis (Oct 1, 2007)

WCGill said:


> What you were hearing when you substituted the 12AT7 into your amp wasn't psychoacoustics. Lowering gain by subbing a tube with higher plate resistance than a 12AX7 will increase bandwidth dramatically. Swapping a 5751 will lower gain by about 25% but won't do much for bandwidth increase but a 12AT7 or a 12AY7 should make a difference. First position will probably be the most dramatic but if circuit limitations won't amplify that extended bandwidth, audibility may be much more subtle. A 12AX7 has a theoretical gain of 100 but is much lower in a circuit with a 100k plate resistor, the most commonly-used value and when loaded with following circuits such as tone stacks, gain drops even further.


Absolutely, there was a definite difference after switching preamp tubes.

The "no noticable difference" comment I made was about different brands of EL84 power tube.

I'll get back to messing with the preamp side soon, now that I understand that is where i can make a difference.

It's a neat little amp, very simple, and it seems to have been built with modding in mind. There's even some extra taps on the power transformer to accommodate a tube rectifier, if someone wanted to add one.

I'm more inclined to mess with the tone stack, I think. The controls there don't really seem to traverse the right range of values. There's a sort of "flubbiness" in the bottom end that I haven't been able to dial out, even with the bass and mids controls at zero. The 12AT7 actually helped with that.


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## Wild Bill (May 3, 2006)

Greg Ellis said:


> Absolutely, there was a definite difference after switching preamp tubes.
> 
> The "no noticable difference" comment I made was about different brands of EL84 power tube.
> 
> ...


Greg, if you want to keep messing with switching tubes you can have a lot of fun but if you are not "blind" as to any changes then it all means nothing! 

With a true scientific test even if you had a friend doing the switching you must not see his face or be in contact with him in any way. Maybe if neither of you was in the room at the same time and you just made written notes every time you came in and took a listen it would be better. It's amazing how we can pick up subliminal cues just from body posture that neither of you thought were there!

Good point about the plate resistance/bandwidth, WC! 

Wild Bill/Busen Amps


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## Greg Ellis (Oct 1, 2007)

LOL Bill. I'm just trying to make the best of the amp for my purposes. A double-blind methodology sounds interesting for a get together, but I don't have the time at the moment.

Recording and then playing back, switching tracks, etc, has given me enough to go on for the moment.

I haven't had a huge amount of time so far; mostly I'm tied up rehearsing 30 songs for a jam that's coming up Friday.

But of the 4 tracks I've recorded so far, there was only one that sounded any different:

EHX 12AX7 + Sovtek EL84
EHX 12AX7 + JJ EL84
EHX 12AX7 + TAD EL84
Jan Philips 12AT7 + TAD EL84 << this one doesn't sound quite like the others


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## nonreverb (Sep 19, 2006)

Good point. Also, to further that, a 12AU7A as an example is a superior current source compared to a 12AX7 with about 9X the current capability. The 12AU7 in particular, can be found in phase inverter circuits driving serious output tube arrays. The new Ampeg SVT's use two of them to drive the 6 X 6550 output stage. A 12AX7 wouldn't last long in that application.




WCGill said:


> What you were hearing when you substituted the 12AT7 into your amp wasn't psychoacoustics. Lowering gain by subbing a tube with higher plate resistance than a 12AX7 will increase bandwidth dramatically. Swapping a 5751 will lower gain by about 25% but won't do much for bandwidth increase but a 12AT7 or a 12AY7 should make a difference. First position will probably be the most dramatic but if circuit limitations won't amplify that extended bandwidth, audibility may be much more subtle. A 12AX7 has a theoretical gain of 100 but is much lower in a circuit with a 100k plate resistor, the most commonly-used value and when loaded with following circuits such as tone stacks, gain drops even further.


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## GTmaker (Apr 24, 2006)

damme it guys...you drive me crazy with all this teckie tube talk..
-Yes they make a difference
-No you cant tell any difference
-yes they make a difference but you wont hear it so whats the point
-etc. etc. etc.

I allways looking for CLEAN CLEAN CLEAN
Ive got a wonderfull Fender Super Reverb RI..
If I replace my #1 12AX7 with a 12AT7 what can I expect ?

I ask cause its by far one of the cheapest mods you can do (if you can call it that) and I like to try stuff.

ps.
would it make a difference if I replaced all the 12AX7 with 12AT7 ?


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## WCGill (Mar 27, 2009)

12AT7 is not a direct substitute for a 12AX7 although the pinout is the same and it will work. It likes different operating parameters than a 12AX7 and would be optimum if the socket was wired for them. 5751 or 12AY7 would be better choices.


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## GTmaker (Apr 24, 2006)

WCGill said:


> 12AT7 is not a direct substitute for a 12AX7 although the pinout is the same and it will work. It likes different operating parameters than a 12AX7 and would be optimum if the socket was wired for them. 5751 or 12AY7 would be better choices.


I have waved the white flag and raised my hands...."I GIVE UP !!"

G.


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## WCGill (Mar 27, 2009)

Try a 5751 in V1, about 1/3 less gain, or a 12AY7, about half the gain of a 12AX7.


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## Wild Bill (May 3, 2006)

WCGill said:


> What you were hearing when you substituted the 12AT7 into your amp wasn't psychoacoustics. Lowering gain by subbing a tube with higher plate resistance than a 12AX7 will increase bandwidth dramatically. Swapping a 5751 will lower gain by about 25% but won't do much for bandwidth increase but a 12AT7 or a 12AY7 should make a difference. First position will probably be the most dramatic but if circuit limitations won't amplify that extended bandwidth, audibility may be much more subtle. A 12AX7 has a theoretical gain of 100 but is much lower in a circuit with a 100k plate resistor, the most commonly-used value and when loaded with following circuits such as tone stacks, gain drops even further.


Not meaning to be picky WC but when you talk about increasing bandwidth, is that really a factor? These tubes will amplifiy up to and beyond 50 or 60 MEGAhertz! The entire audio spectrum to such devices is no more than a sniff and a fart!

If bandwidth was truly affected at audio frequencies with a 12AT7 wouldn't we be able to build much simpler bandpass filters or equalizers? It seems to me that while bandwidth changing is technically true in a general sense it is just not really relevant to the typical guitar amp triode amplifier.

Wild Bill/Busen Amps


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## WCGill (Mar 27, 2009)

If someone hears the difference, which the OP did, then I would suggest it's relevant.


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## loudtubeamps (Feb 2, 2012)

GTmaker said:


> I have waved the white flag and raised my hands...."I GIVE UP !!"
> 
> G.


simple answer??
I believe what everyone is saying is that there are certain tubes that will perform better than others in a given circuit.
Bottom line ,all of the tubes mentioned ,12- au,ay at ,ax will work without doing any damage.
Try a 12AU at the phase inverter first, if u find you are loosing too much then put the correct P/I tube back in and try this.....
RE: first stage tube , (your 12 ax) try an AU or AY for "clean, clean clean". 
If u find one that u like and plan to stick with it, I would check the plate voltage on that tube and see if the DC volt reading falls within the design spec. for that tube. I have found that they all seem to sound sweetest (in the pre- amp section)with a plate reading of 220 to 260 VDC. 
The easiest way to bring the voltage into this range is by changing the value of the cathode resistor.
Amp unplugged from the mains::
First, lift one end of the existing cathode resistor out of circuit.Substitute the existing (lifted) cathode resistor with a 10 k pot connected with an outside and center lug clipped to the cathode output and to ground respectively.Measuring the plate volatge while sweeping the pot will give you your target voltage, power down,then simply disconnect the pot, check the dc resistance of your test pot clip leads and replace the read value with the closest resistor.
As well: If U have the test pot mounted ,you can plug in and play an open chord while u adjust the pot and listen for the sweet spot.
Clear as mud?? I can draw u up a quick diagram, if needed.
cheers, d


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## Greg Ellis (Oct 1, 2007)

loudtubeamps said:


> The easiest way to bring the voltage into this range is by changing the value of the cathode resistor.


Thanks LTA 

Per the schematic, there are 1.5K - 1 watt resistors on the cathode pins (3 and 8) of the preamp socket.

Which direction is likely needed here? A bigger resistance value, or smaller (to suit the 12AT7, I mean)?

The amp definitely has a different feel with that tube. Bass flubber is significantly reduced, and that's been a problem.

I wonder if a fixed resistance in series with the test pot would be safer? I'm not sure what happens if there is NO resistance on the cathode. If it's likely that I need a larger value, I could just insert the pot in series with the existing cathode resistor.


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## loudtubeamps (Feb 2, 2012)

Greg Ellis said:


> Thanks LTA
> 
> Per the schematic, there are 1.5K - 1 watt resistors on the cathode pins (3 and 8) of the preamp socket.
> 
> ...


First, I would measure the plate voltage with your AT in and see if U are within the voltages suggested, U may be OK.
If not, you could leave your 1k5 resistor in series with the trim pot and as mentioned measure the plate volatge as u sweep the pot to obtain a target voltage. As you increase the value of resistance, your plate voltage will rise.
Tweaking by ear is the only way to find the spot and if u hear it, stop and see where your plate voltage is at.
You can go as low as u want with the plate voltage but usually anything below 140 VDC starts to sound crappy and u really don't want to go much higher than 300 VDC , you are reaching the max. limits of the tube and it starts sounding stiff and thin.


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## Greg Ellis (Oct 1, 2007)

I'll have to check if my meter can handle 300V.


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## loudtubeamps (Feb 2, 2012)

Greg Ellis said:


> I'll have to check if my meter can handle 300V.


Most meters will, no probs.


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## loudtubeamps (Feb 2, 2012)

Forgot to mention....If u like, you can use the same test pot after u find your target plate voltage to find the sweet spot for your cathode bypass cap(assuming you are using one) Same deal, connect one end of the cathode bypass cap and use the pot to sweep while you strum, you will hear a big difference, more gain as u decrease the resistance. I sometimes end up with 1 to 3k3 in series with the cap for a particular sweet tone, sometimes it sounds best with no resistor required.
Goes without saying, all of the tweaking will change how the amp sounds, the bigger the push at the front ,the bigger the reaction at the end of the signal chain and visey-versey,twiddle dum,twiddle dee.3dgrw


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## Greg Ellis (Oct 1, 2007)

1.5k cathode resistors are bypassed with a 22uF cap on V1a and a 1uF cap on V1b, according to the schematic (I linked to it on the first post).

Not sure I'm following your thought tho - instead of bypassing the cathode resistor with a cap alone, you bypass it with a cap and resistor in series?


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## loudtubeamps (Feb 2, 2012)

"instead of bypassing the cathode resistor with a cap alone, you bypass it with a cap and resistor in series?"

Yup, the cap in series with a resistor/test pot. The 22 mfd will be most pronounced when sweeping the test pot.
This tweak is totally an "ear thing" that U can dial in. Listen for the sweet spot and then measure the value of the pot and stick a resistor of that value in there. Once you've tried it ,you are well on your way down the road to insanity, ask anyone that knows me.










Also.......RE: earlier reply to plate voltage VS cathode resistor value:If u have your 1k5 r in series with the test pot for finding your plate voltage, remember to add your 1k5 with the pot reading to give you your final cathode resistor value.
cheers, d
I'm sure you are doing this correctly but so as not get in any trouble..
Care is required when messing around inside these things. Power down, unplug and stay within your knowledge and comfort zone and stay clear from anything that could bite, that includes your B+ volatge supply lines and of course the big filter caps.
That's what the insulated meter probes are for, they do all the touching, right??!! 
Also be sure that u are measuring on the tube side of the plate resistor and not the B+ supply side or you won't get the readings U are looking for.


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## loudtubeamps (Feb 2, 2012)

just to add to the confusion, my first reply was in response to GT maker re: tube swapping
Greg..The only pre-amp tube u have to play with is the 12ax, obviously , no phase inverter, so any comment I made re: swapping tubes at the phase inverter were directed to GTmaker.
cheers, d


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## Greg Ellis (Oct 1, 2007)

Grin - I'm not confused about that.

12AT7 in the one and only spot I can put a preamp tube seems to make some good headway in the right direction, for me.

12AX7 gives me more volume, but the amp is "flubby" (for lack of a better word) and the bass, mid, treb controls don't seem to cover any sort of useful range. Treble above 11 oclock or so is just way too shrill. Bass above zero creates a very loose bottom end that's not really pleasing at all.

With a 12AT7 in that spot, the volume drops quite a bit, but the EQ becomes a lot more useful (I can get the bass control up to 9 oclock or so and stay tight, even after compensating on the volume control). 

It's still not really where I want it to be, but it's closer.

Making sure the AT7 is happy in that slot is a good idea.


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## loudtubeamps (Feb 2, 2012)

The flubbiness with the 12 AX might be reduced by simply trying the cathode bypass cap and resistor trick. If it's not doin' it for U , then experiment away.
Cheers, d


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## Wild Bill (May 3, 2006)

WCGill said:


> If someone hears the difference, which the OP did, then I would suggest it's relevant.


Well, that's where we have room for discussion. Did he really hear a difference?

That's why I champion blind, scientific testing. We are creatures of flesh and blood with a psychology, not machines. We can be totally honest and sincere yet be dead wrong!

I still chuckle about a blind test concerning the best sounding speaker wire. The techies who set it up invited some gurus with a high Net profile. 

The gurus ended up picking a piece of rusty wire from an electric fence! Yet I have no doubt they sincerely believed what they thought they heard.

"Paul is Dead!"

"I love Satan!"

"Go To The Lobby and Buy Popcorn!"

Wild Bill/Busen Amps


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## Greg Ellis (Oct 1, 2007)

But... but.... rusty wire from an electric fence makes the BEST speaker cable. Everyone knows that!

LOL


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## nonreverb (Sep 19, 2006)

Nope, hate to break the news but it's actually unfurled coat hangers tied together. Five being optimum. Six has too much line loss and 4 the low impedance can fry the output of the amp AND in the case of stereo equipment, they have to be exactly the same length or else the sound will get to one speaker faster than the other....



Greg Ellis said:


> But... but.... rusty wire from an electric fence makes the BEST speaker cable. Everyone knows that!
> 
> LOL


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## greco (Jul 15, 2007)

nonreverb said:


> ...... in the case of stereo equipment, they have to be exactly the same length or else the sound will get to one speaker faster than the other....


WOW, I learn so much about audio electronics through this forum!!

Thanks everyone.

Cheers

Dave


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## GTmaker (Apr 24, 2006)

greco said:


> WOW, I learn so much about audio electronics through this forum!!
> 
> Thanks everyone.
> 
> ...


I read the same stuff and I just get head aches...

saw this and I think its interesting...
someone will have to make sense of it for me.
http://home.comcast.net./~machrone/bjr/bjtubes.htm

G.


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## GTmaker (Apr 24, 2006)

loudtubeamps said:


> simple answer??
> I believe what everyone is saying is that there are certain tubes that will perform better than others in a given circuit.
> Bottom line ,all of the tubes mentioned ,12- au,ay at ,ax will work without doing any damage.
> Try a 12AU at the phase inverter first, if u find you are loosing too much then put the correct P/I tube back in and try this.....
> ...


First ..thanks to loudtubeamps for the detailed reply...

before the easter weekend I got myself a 12AU7A tube from my local L&M . Dammed thing cost me 16 bucks plus tax but that cheap enough for some experimentation.

As directed I first tried the AU7A in the V6 (phase inverter slot) on my Super reverb RI.
This realy sucked the life out of my amp. Volume on 7 and the amp still had NO BALLS at all.
Clean... yes, but the amp was lifeless.
Plan B ...put the AU7A in V1 (first tube stage).
A little better but not by much...I still thought the amp was being sufficated...

final results....I dug up a 5751 that I had hidden away somewhere and put it in the V1 slot.
With my amp volume on 3.5 , I can turn the guitar volume down a bit and play all day long. 
Turn the guitar volume up full and the amp sings as it should.

For now this ends my tube swap quest , untill someone tells me to try another thing...lol

keep on rockin

G.


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## Wild Bill (May 3, 2006)

Good stuff, GT! The article you included didn't contradict anything that has been said. In fact, it only agreed with it! The author confirmed that different brands of the same time sounded the same. He also showed that 12AX7s, 5751s and 12AT7s didn't make a noticeable difference either, since there was always more than enough gain to make the amp sound like what it was.

He pointed out that the 12AU7 had so little gain as to make the amp lame and useless. 

Only the 12AY7 had a useful enough amount of gain but low enough to improve the clean headroom.

That's pretty well what I have been saying all along!

Wild Bill/Busen Amps


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## GTmaker (Apr 24, 2006)

Wild Bill said:


> Good stuff, GT! The article you included didn't contradict anything that has been said. In fact, it only agreed with it! The author confirmed that different brands of the same time sounded the same. He also showed that 12AX7s, 5751s and 12AT7s didn't make a noticeable difference either, since there was always more than enough gain to make the amp sound like what it was.
> 
> He pointed out that the 12AU7 had so little gain as to make the amp lame and useless.
> 
> ...


HEy Wild Bill ...thanks for the clarification. 
please excuse the simplistic question but as I am knee deep in the swapping stuff I have to ask.
As you can tell, my experiment with the 12AU7A swap in V6 and V1 was not that successfull for me.
I ended up using the 5751 for my purpose.
1. Are you sugesting that I also try the 12AY7 in the V6 or V1 position and
2. What difference can I expect from my 5751 that I have in there now?

thanks again for your input

G.


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## Wild Bill (May 3, 2006)

GTmaker said:


> HEy Wild Bill ...thanks for the clarification.
> please excuse the simplistic question but as I am knee deep in the swapping stuff I have to ask.
> As you can tell, my experiment with the 12AU7A swap in V6 and V1 was not that successfull for me.
> I ended up using the 5751 for my purpose.
> ...


GT, as always, there are details when you swap tubes. Not all the tubes will be in the string of signal amplification. Some are needed for tremolo, reverb or whatever. Non-signal amplifiers are best left alone. They will not change your tone, except maybe by not having your reverb work anymore.

When you have a string of signal amplifiers, you have to consider the gain of the entire chain. Every 12AX7 or tube in that family has two separate tubes inside - two separate amplifiers. When you calculate the total gain of the chain not only does each chain count twice but you MULTIPLY each stage, not add them!

Each stage of a 12AX7 has a theoretical voltage gain of 100. In the real world you might see 70. Still, for 3 tubes in a preamp string that's 70x70x70x70x70x70 = nearly 118 BILLION!

The real gain is nowhere near that, of course. You have circuit losses, particularly in the tone stack. Still, you have so much flippin' gain that changing one or two tubes really doesn't matter. Things get swamped out by the sheer total amount of gain.

So again in the real world, we find that 99% of any audible difference will come from the V1 first position. After that any differences just don't jump out at you or really aren't there at all.

I'm suggesting that you play with the first position only and not waste your time after that. You are in the position of the man who has a micrometer to measure with but has to cut with an axe!

You realize that this has all been done before, GT? Bet you a beer that the only useful difference you will find will be swapping a 12AY7 into the first position! :banana:

Wild Bill/Busen Amps


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