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(2)6V6 Class'A' Champ


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10/1/1997 12:47 PM
Mike Tremante
(2)6V6 Class'A' Champ
What would be required besides another output transformer to take a Champ, add another 6V6 but retain the Class A duty cycle, essentially doubling the power but retaining the character of the amp? Would I parallel the 6V6s except for the new OT primary? A phase inverter would not be necessary as both tubes are working the same phase of the cycle, right? So I could parralell off the driver for the second 6V6, right? Stop me if I'm wrong. Would I burn up th PT because of the current draw? It would seem that the main functional operation that causes the high current draw and operating temperature of Class 'A' is the signal sent to the power tubes from the driver being similar in phase as opposed to Class 'A/B' sending (2) out of phase signals alternatly amplifying opposite ends of the duty cycle. Again, stop me if I'm wrong. So if I hook it up as I stated, what can I expect??  
 
Mike Tremante
 
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10/1/1997 2:16 PM
R.G.

The Tube Amp FAQ addresses this as follows:  
==================================================  
How can I modify my Blender Tweety Bird amp to be as loud as a Marshall Major/ AC30/ Tweed  
Bassman/ SVT/ etc.?  
 
(Alternatively, how can I make my amp twice as loud/more power/ etc.?)  
 
You can't do this in a low power amp, at least not electronically. To put out the power the big amps put out, you need the entire power train to be as beefy as the big amps. This means bigger power transformer, rectifiers, filter capacitors,  
output transformer, more power tubes, bigger chassis, more ventilation to carry off the heat, lots of things. You can't just add a couple of tubes.  
 
An amplifier is properly thought of as primarily a big power supply that has some extra junk tacked onto it to carefully let a little of the power out to the speakers under special, controlled circumstances.  
==================================================  
 
The thing causing high current drain is not the signal going to the outputs in phase as opposed to out of phase as in a push pull amp. It is the fact that the output tubes have to be drawing 1/2 of the maximum instantaneous possible power and 100% of the average total power just sitting there with no signal so the sigal can swing 1/2 of the way up to max instantaneous and 1/2 down to zero to cover the entire signal swing.  
 
Putting in a second power tube means that the power stage would be trying to pull 200% of the max average power with no signal, and this would constitute about a 100% overload on the power supply (I say about because the piddly little preamp tube power use is not duplicated). This would overload the PT, cause twice the AC ripple on the power supply. It would also pull twice the amount of filament power as a single power tube.  
 
If you hook it up as you state, you can expect to cook the power transformer eventually even if not immediately, cook the OPT because the standing current is now twice as much, and potentially overheat the capacitors, overheat the other stuff in the cabinet... AAAARRRGGHHHH!!!  
 
Seriously, it's a bad bet. Put a line out on it, and run it out to a bigger slave amplifier.
 
10/1/1997 9:17 PM
Mike T

I know my question may have sounded a little ridiculous, but I have bigger amps, and I'm just looking for a different sound, like a Champ, but with just a little more power than a Champ, so I could use it at rehearsals. I don't mind changing PTs and OTs and FCs and all that, as long as I have a reasonable idea of what I'm doing. The schematic that Doc mentioned may be just what I'm looking for, as long as the 2 power tubes can be biased into Class 'A', the same as the original 1 without burning them up.  
 
I have a little line out box that I built for the  
Champ and it works fine. But I'd like a self contained unit, you know?  
 
Mike
 
10/1/1997 9:42 PM
R.G.

That makes sense. If you're willing to update power an output transformers, go for it!!
 
10/2/1997 9:11 AM
Steve M.

Mike,  
 
There is a cool schematic on Joe Piazza's webpage that might fit the bill better than Dave Cigna's. It is more in line with what you want. You can get to it by going to the "Resources" Page here on the Ampage. It is under vintage amp schematics on Joe's page and is the Gibson Gibsonette schematic.  
 
The Gibsonette runs two 6V6s in parallel single ended Class A, like the Champ without the paralleled 6V6. It uses a single 12AX7 preamp tube and a 5Y3 rectifier, also like the Champ. You would probably have to change out the OT, using a 2500 ohm impedence primary, vs. the Champ's 5000 ohm primary. You'd also have to change the PT to one with a higher current rating to supply the extra 6V6 with enough current, or you could add an extra filament tranny.  
 
This might be a cool project to build from scratch as you'd have to do extensive rewiring anyway.  
 
Hope this helps,  
 
Steve
 
10/2/1997 6:59 PM
Mike T

This is exactly what I'm looking for. Thanks, buddy. I have a Princeton PT which was made for 2-6V6s, a Mojo champ PT, which is bigger than a stock Fender but I don't know what the primary is, and a Celestion 35 watt 8" speaker in it now. What I'll probably do this weekend is install the other 6V6, and see how it works. If it looks like it will burn up I'll get some trannys somewhere. Thanks again.
 
10/3/1997 3:39 AM
Dave Harris

It is vital to get the impedance ratio right on your OT or you could end up with LESS not more power even with an extra tube and a beefier PT. Is the Mojo transformer the OT ? The Mojo Champ OT in their online cat. is 4 ohm and your Celestion is 8 ohm. That will give less power than the original Champ with 4 ohm speaker. To double the power you will somehow have to find an 10W single ended OT with a 2500 ohm primary and 8 ohm secondary.
 

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