GM70 Self Inverting push pull with SRPP driver

[sampleaccurate]

Hi all.

I'm a newbie to this site, but I've got a couple of amps under my belt (300B SE and an "oddwatt" KT88 PP amp) that my new GM70 project will be based on.

Here's the basic schematic:



The tubes are biased with a CCS using a high voltage transistor to provide adequate voltage to the cathodes.  Current balance is achieved using a 25 ohm potentiometer.  The driver is capable of pushing the GM70s to full power.

B+ will be 1150 volts, and the tubes will be run in class A with an idle bias current of 100mA each for an idle plate dissipation of about 100 watts each.

Comments?  Has anyone attempted to use the GM70 in a self invering push pull output configuration?  What transformer primary impedance would be best for a CLASS A push pull GM70 amp.  10K seems about right for SE operation, but what about class A PP?

Thanks!

[Bas Horneman]

  10K seems about right for SE operation, but what about class A PP?

Usually half of what they use for SE. So I reckon around 5K would be good for PP.

Has anyone attempted to use the GM70 in a self invering push pull output configuration?

Not that I know. I still have to try the self-inverting concept. I'm thinking of implementing that with a 6N3C amp for my son.

[sampleaccurate]

I've been digging through an old RCA tube manual and found the calculations for computing load impedance, power, etc. 

Based on what RCA says, assuming my calculations are correct, 4.8K would be a good load impedance.  Looks like you hit the nail on the head.

I may use a slightly higher impedance for lower distortion at the expense of some power.  The RCA manual guidelines typically result in higher distortion levels than desired at rated power, which is 87 watts in PP with a B+ of 1150 and an idle current of 100mA per tube.  Doubling the impedance should result in a 3dB drop in volume (half power) but with less distortion.

I ordered a 10K tranny for this project.  I may change it to a 7.5K and split the difference.  Or possibly a 6.6K.  My issue is that different books and webpages have different recommendations, and there seems to be a lot of variation in recommendations.  I think I'd rather err on the conservative side and not overload the tubes with too low of an impedance.

Thanks for the response - much appreciated.

[Benny Buis]

Hi Stephen,

over the thumb calculation for the opt is easy, half what se would have been plus 25 %, this always works out, kind of what Bruce would tell you, too.


All the best,


Benny

[rcavictim]

Only 87 watts output from a pair of big GM70's at 1150 Volts?  Something wrong there.  Shouldn't that be 287 watts?  You can get more than 87 watts from a pair of tiny 6L6's at 500 volts.