Needing help understanding an odd design...

[otto pärt]

Okay, this starts with the fact that I've got a couple of old 6A6 tubes who are dying to be put into use. 6A6 is equivalent to 6N7 and is designed for use in a class B output stage.

I ran across this schematic of a "class B guitar amp:" http://webpages.charter.net/dawill/t...Guitar_Amp.gif

I would be eternally grateful for any explanation, comments, and/or advice from the gurus here regarding this topology.

[imaradiostar]

Hopefully an old timer or RF guy will chime in but usually class b amps are very similar to class ab amps. It's still a push-pull amp like the majority of tube amps. The difference is that only one of the output tubes works at a time- one tube is off 50% of the time while the other is working. Most guitar amps are class ab which means the tubes share the load for a portion of a given waveform.

This probably won't have a large effect on your amp design. Build it and see how it sounds. You can always bias it a little warmer if you like. It would probably make a decent low wattage amp but it isn't likely to sound like guitar amps you're used to hearing, seeing as most guitar amps use output pentodes instead of triodes.

Power triodes generally require a decent amount of grid drive. This may be why your schematic shows a 12au7 buffer before the triode grids. You're only planning to get low power out of it so why not breadboard a simple circuit with minimal parts so you can see how it sounds? I'd probably start with a pentode or high-gain triode driving a Fender style LTPI and just listen to the tubes and see how they sound. You could safely increase the grid leak resistors to around 100k ohms based on the old RCA datasheet I looked at. You may be able to get away with much more, reducing the amount of grid drive needed but increasing the risk of the tubes running away.

Hope that helps!

jamie

[otto pärt]

Thanks for the response, jamie.

I guess the thing about it that's most puzzling is that the cathodes of the 12au7 driver are negative relative to the grids. That can't be good, can it?

[imaradiostar]

They're direct coupled- it's common in amps that draw grid current to make full power. Each triode is acting as a cathode follower that's directly connected to the output tube grids.

It's also nice because it alleviates grid blocking caused by the grid coupling cap charging up. Those parts could easily be mosfets instead of tubes.

I don't know if the output triodes need that much drive or not- that's why I suggested breadboarding something slightly simpler first to see how it sounds.

jamie

[imaradiostar]

Have you considered making a hi-fi amp with them? Perhaps you could drive a pair of efficient little bookshelf speakers and use a chassis that shows off the tubes. Are they coke bottle shaped?

jamie

[otto pärt]

Yeah, they're coke bottles. They're pretty cool. I bought them by mistake when I was actually after 12A6s to go in a Heathkit monoblock. Totally different tube.