# Rectifiers, Tube AND Silicon



## keeperofthegood (Apr 30, 2008)

Hey oh

Ok. Seems a simple two pole switch could switch between the two diodes of a full wave rectifier tube OR a pair of silicon diodes. However, a tube has a MUCH larger voltage drop than a silicon diode.

http://boozhoundlabs.com/howto/pdf/5Y3GT.pdf

Now, that states a drop of 50 volts @ 125ma. Should I series a resistor R=E/I = 50/0.125 = 400 ohms? OR do I need to know the actual voltage drop and current draw of the rest of the amp and then work out an appropriate dropping resistor from there?


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## Guest (Jan 1, 2010)

What would Randall Smith do? Look up his patents at USPTO and there should be one in there for tube/SS rectifier switching that's nearly volume change free.


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## keeperofthegood (Apr 30, 2008)

iaresee said:


> What would Randall Smith do? Look up his patents at USPTO and there should be one in there for tube/SS rectifier switching that's nearly volume change free.



http://www.schematicx.com/view-schematic/mesa-boogie-dual-rectifier-schematic/

Well, google brings up Messa and Messa "dual rectifier" brings up that schematic (hope it opens) and in that was done the "simple" approach of a two pole switch and no voltage drop compensation from what I have so far found. This means users hands are on a switch that has 300+ volts behind it @[email protected] and a voltage increase switching in those SS diodes unless there is a self balancing circuit later in that schematic.

I would not do more than one diode, they are pretty rugged and [email protected] for rectifying [email protected] more than one is over kill isn't it? I would have by passed the diodes with 0.01uF caps, and I would have bypassed the switch with 0.001uF caps too (maybe not, but that's my first gut instinct). The switch I would use a relay for to keep hands and HT separated (Unless someone can tell me that this is needlessly paranoid that no one with beer wet hands would be messing with such a switch). This still needs voltage drop compensation though.

110Uf as the first filter capacitor is @[email protected] large for tubes, even a pair of them. Most sheets I have looked at so far have had max first capacitor values at 20uF or under (one of the rectifier tubes I am buying in a week stipulates nothing more than 8uF first filter capacitor). The 110uF could go anywhere after the first choke, but not off the tube @[email protected] unless all those old data sheets were plain wrong?

:rockon2:

Ok.... any other ideas


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## Emohawk (Feb 3, 2006)

A simpler example might be the Bad Cat Hot Cat schematic. That amp also has tube/SS recto switching with no compensation for volume. Here's the link to Schematic Heaven...

http://schematicheaven.com/newamps/badcat_hotcat_30.pdf

I don't know how much of a volume difference there would be. It may just sound "browner".

BTW, it's not something I'd want to switch on the fly - at least not with this simple approach. I can't imagine that would be healthy, but what do I know!


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Hey oh
> 
> Ok. Seems a simple two pole switch could switch between the two diodes of a full wave rectifier tube OR a pair of silicon diodes. However, a tube has a MUCH larger voltage drop than a silicon diode.
> 
> ...


What is it you're trying to do, Keeps? With Mesas, the idea is to be able to switch between the tight sound of silcon diodes and the bluesy, vintage "sag" of a tube rectifier, which is caused by the voltage drop. That voltage drop increases as you crank the amp into heavier current flow.

Are you talking about compensating for the lag of sag with silicon diodes, then switching between the two rectifiers? There would then be no difference at all!

I'm missing something. I haven't had my coffee yet.

:food-smiley-004:


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## WCGill (Mar 27, 2009)

On my V2 Combo, I use dual switchable tube rectifiers, a 5Y3 and a GZ34. Because it's a fixed bias amp, the bias must also be switched.


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## keeperofthegood (Apr 30, 2008)

Wild Bill said:


> What is it you're trying to do, Keeps? With Mesas, the idea is to be able to switch between the tight sound of silcon diodes and the bluesy, vintage "sag" of a tube rectifier, which is caused by the voltage drop. That voltage drop increases as you crank the amp into heavier current flow.
> 
> Are you talking about compensating for the lag of sag with silicon diodes, then switching between the two rectifiers? There would then be no difference at all!
> 
> ...


:_) Wild Bill. Looking at what happens when you take the tube out and put in a silicon diode in. The tube example I showed first has a forward voltage drop of 50 volts at 125 ma. and that drop is I believe a dynamic value (correct me if I am wrong there) changing as the current draw changes. A silicon diode will have a forward drop of only a volt or two and it is constant over the working current range of the diode (again, correct me if I am wrong).

In essence, when switching in the silicon diodes, there would be a voltage increase due the lack of drop. I am asking about placing a resistance in series with the diode to compensate that voltage increase. That there wont be any sagging with increased current drawn I figured already, I was more thinking at idle the voltage provided out being say 200 volts, jumping by as much as 50 volts when the rectifier means is switched in.

The capacitor bypassing is normally done on diodes, the switch I am not sure on. Spike suppression on the switch is part of that intention.

Also, am I being over cautious with my feeling that switching directly the HT voltages is dangerous and would be better done remotely through a small relay?



WCGill said:


> On my V2 Combo, I use dual switchable tube rectifiers, a 5Y3 and a GZ34. Because it's a fixed bias amp, the bias must also be switched too.


 That is a different one, switchable tube rectifiers. How do the tones compare, does one sag sooner than the other?


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## WCGill (Mar 27, 2009)

Yes, the bottom end is very loose with the 5Y3.


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> :_) Wild Bill. Looking at what happens when you take the tube out and put in a silicon diode in. The tube example I showed first has a forward voltage drop of 50 volts at 125 ma. and that drop is I believe a dynamic value (correct me if I am wrong there) changing as the current draw changes. A silicon diode will have a forward drop of only a volt or two and it is constant over the working current range of the diode (again, correct me if I am wrong).
> 
> In essence, when switching in the silicon diodes, there would be a voltage increase due the lack of drop. I am asking about placing a resistance in series with the diode to compensate that voltage increase. That there wont be any sagging with increased current drawn I figured already, I was more thinking at idle the voltage provided out being say 200 volts, jumping by as much as 50 volts when the rectifier means is switched in.
> 
> ...


OK, first off, do you know the B+ voltage in your amp already? Then you can decide if it will go too high when you switch in silicon diodes. Dual Rectifiers already figured this in.

The worry is that the B+ will be too high for your filter caps.

If all you care about is simply the switch rating and switching "hot", don't worry about it! Just use a REAL switch!:smile: You know, one of those large toggles rated for 10 amps or more at 240 vac. They will handle over 500 volts feeding the plates of a rectifier tube just fine.

Just stay away from those little mini-toggles!

Capacitor bypassing of diodes hasn't been done since about 1966! When silicon diodes first came out they were a bit expensive in higher voltages. In fact, higher voltages than 600 piv were often not available. So the practice was to stack them in series to add up the piv ratings. Theoretically, 2 units of 600 piv would be safe at 1200 piv. However, diodes were not manufactured in those early days to super close tolerances. The voltage drops across the junctions would vary, which meant when you stacked them the voltage would not "share" equally across the diodes. One diode might get more than half and the other less, which meant that one would likely blow and then take out the other one.

Hence the equalizing resistors and the suppression capacitors, to facilitate equal loading across each diode.

Nowadays, diodes are manufactured to very close tolerances. They've also gotten MUCH cheaper! Nobody bothers with the caps and resistors. It's easier and cheaper just to put an extra diode in the string. So if you figure one 1n4007 at 1kv piv is not quite enough, use 2 in series, or even 3. 

Spike suppression on the switch is probably overkill. The currents involved are milliamps and your contacts are rated over 10 amps. Isn't 1000:1 overrating enough?

Same argument against using a relay. Don't worry, be happy!

Besides, you're not the first guy to do this. Lots of other guys have done it, including manufacturers for commercial models. They all get away with it, so you should too!:smile:

:food-smiley-004:


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## keeperofthegood (Apr 30, 2008)

:food-smiley-004: Thank you much Wild Bill!!! Puts my head to ease!!


>.< been a morning of working out switching for things, like switching 12.6 to 6.3 between a 12AX7 and a 6N2P BUT I got that one sorted out  to not accidentally blow the 6N2P w00t!

Ok, 2 diodes at 1000V would probably be enough, it is only 350vac... I think, 700p-p no? Or is a "350" volt transformer actually higher peek valued? Or am I so far out in left field I need a pinch-catcher?

>.< gahhhhh I went to DeVry in Tech Engineering and I cannot remember SO many years they just out the window >.< poof and I so wish my Dr could have said "yea, see you DID have a stroke" >.<


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> :food-smiley-004: Thank you much Wild Bill!!! Puts my head to ease!!
> 
> 
> >.< been a morning of working out switching for things, like switching 12.6 to 6.3 between a 12AX7 and a 6N2P BUT I got that one sorted out  to not accidentally blow the 6N2P w00t!
> ...


My memory was fine until the kids came! Used to be able to do simple math in my head, too!

There's gotta be a connection between having kids and getting Alzheimers...

Anyhow, a full-wave bridge has a piv of 1.41 x the rms voltage applied. Nobody cuts it that tight so we double it. When you're only talking 350 vac we use 1N4007's at 1 kv piv 'cuz they only cost about a dime anyway!

I still keep an ARRL Radio Amateurs Handbook around. All these formulae are in there. Look in the power supply chapter and there's all your rectifier circuits, the voltage multiplier circuits and the PIV rating figures.

So far the kids can't affect my books!:smile:

:food-smiley-004:


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## JMC Amps (Dec 19, 2009)

keeperofthegood said:


> http://www.schematicx.com/view-schematic/mesa-boogie-dual-rectifier-schematic/
> 
> ... unless there is a self balancing circuit later in that schematic.
> 
> ...



Hey Keeps,

Interesting note on the Mesa Schematic:

I would suppose the key word in “Dual Rectifier” is “Dual”.

One would assume the two paralleled 5U4s combine to form one big composite 5U4 rectifier tube with half the plate resistance and twice the current capacity. 

If we knew the current draw in the supply we could then figure out the actual voltage drop of two rectifiers in parallel as opposed to 50v for one. If we assumed that the voltage drop would be cut in half, the a few clicks on the calculator would give us a mere 5% reduction in plate supply, and only about 2mA in bias difference. And since Mesa amps are typically biased cold anyway, a minor variance in plate supply would not affect dissipation (bias) and the difference in actual dB between the two rectifiers would be pretty much inaudible.

Also, if this is the case, the extra current capacity could handle the charging currents of that first big 110uF cap.

I wish I had another 5U4 here to try and prove my curiosity!kqoct

_Wild Bill: I still keep an ARRL Radio Amateurs Handbook around. All these formulae are in there._

Wild Bill, do you have a formula for paralleled tube rectifiers?kkjq

Regards

-JMc


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## keeperofthegood (Apr 30, 2008)

JMC Amps said:


> Hey Keeps,
> 
> Interesting note on the Mesa Schematic:
> 
> ...



Actually, I am rather curious about series/parallel tube rectifiers myself. I have seen dual internals paralleled (tie the plates together) and I have seen the same done with triodes too (grids and plates) but the actual written information is vague to not available at least to Google. I have found nothing in searching on using two Tube diodes in series. I did fine one mention of using a Tube diode as a cathode diode bias (a 6AL5 117 to 150 forward volts, 330V reverse, 10 volt drop 9ma max plate current) but no written information on how to plot that (Lots around using things like zener or LED as bias though).


LOL Wild Bill. I lost three of my more valuable books to my son and his proclivity to tear things apart. I have a few books packed away still and safe. I probably have some PDF's downloaded from Pete Millets files though XD


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