Advice on 6V6 push pull Parallel

[krzanik]

I need some advice on a push pull parallel 6V6GT amplifier that was origionally from a Hammond P-40 Tone Cabinet. The schematic is at:http://www.captain-foldback.com/Hamm...schematics.htm If you scroll down to the bottom of the page to Tone Cabinet and click on the P-40 (sorry it's a zipped file) you can see the schematic. This amp has seperate treble and base output transformers, when I hooked up a cd player and speakers I get: fairly quite music from 4 ohm speakers and very faint music from 8 ohm (95 decible per watt efficiency) speakers. Origionally a preamplifier would provide the input to this amplifier and I imagine the preamplifier had a good amount of gain because my computer can add 12 decible of preamplification and it was still pretty quit on the 8ohm speakers. I have also tried replacing the output transformers with Hammond 1650F output transformers but the amp is still quite. If someone can take a look at the schematic and offer advice for turning this into a usable amplifier I would be very pleased. I had no luck with posting the schematic (too big!) Sorry

[Miles Prower]

The Front End of this is a differential amp, and ideally it should be driven by a preamp with balanced output. One potential problem is applying the signal in such a manner that it becomes a common mode voltage. This really isn't a good diff amp (no CCS and very small resistors in the tail) so that would cause low output, instead of no output (which a good diff amp with a decent CMRR would produce).

Also, did you make sure that the bias is correct? 6V6s usually run with Vgk= -19Vdc. If that back bias resistor has increased in value, you might be getting excessive bias. That, too, would cause low output.

I'd check these first and see what the results are.

[krzanik]

Thanks for the reply and thanks for posting part of the schematic. I'm going to ohm-out the resisters with my volt meter this evening and see if the values have changed (pretty likely as it's 50 years old). I'm thinking about just using the power supply and building a push pull 6V6 per some of the schematics posted on the forum (new resisters and caps). Which brings up a good question...I have not had any luck finding a 6V6GT push pull parallel schematic anywhere. I've been looking for close to a week now. I would really like to use all four 6V6GTs per channel. If any one has a schematic that fits the bill I'd love to take a ganders (look at it).

[Miles Prower]

Originally posted by krzanik
Thanks for the reply and thanks for posting part of the schematic. I'm going to ohm-out the resisters with my volt meter this evening and see if the values have changed (pretty likely as it's 50 years old). I'm thinking about just using the power supply and building a push pull 6V6 per some of the schematics posted on the forum (new resisters and caps). Which brings up a good question...I have not had any luck finding a 6V6GT push pull parallel schematic anywhere. I've been looking for close to a week now. I would really like to use all four 6V6GTs per channel. If any one has a schematic that fits the bill I'd love to take a ganders (look at it).

There's no reason that you couldn't use an existing schematic for a good 6V6 amp already out there and make it a PPP. Paralleling the 6V6's will cut the Rl and Zo in half, double the Ip, Cmiller, and Ci. You may need to add cathode follower drivers to insure that the slew rate stays up there. That's NBD. For stability, add grid and screen stoppers to each of the four 6V6s (never share these).

If you're keeping the original power supply, I'd ditch that back bias scheme in favor of this (see attached). Just make certain that each 6V6 has its own bias pot, so that the Ip's can be balanced.

[krzanik]

Wow, thanks for the info. The power supply diagram is greatly appreciated. I am not sure how much of your information I will be able to utilize imediately. I am a novice and have been trying to stick to schematics of proven disigns that I find. I imagine after reading up some more your suggestions will make more sense to me.