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kstlfido 9th April 2013 07:54 AM

Just stumbled across this thread searching for PPP 6AS7 circuits...

I have a pair of UTC LS33 20W line to voice coil transformers as well as a pair of Lambda 100-300v 400ma regulated power supplies. they've been stubbing my toes; time to move them all or make something useful. Big and heavy? Oh yes.

Desperateaudio- did you ever finish the amp? Did you find 600 primaries on the OPT a bit to low for your design?


ja2dhc 9th April 2013 08:39 AM


Originally Posted by kstlfido (
Desperateaudio- did you ever finish the amp? Did you find 600 primaries on the OPT a bit to low for your design?

My 6AS7 PP amp is described in the below URL
(sorry, written in Japanese, but you can see schematic and photos).

Data of 6080pp Stereo Amp.

I tested a low imp OPT of 968 Ohm Primary P-P). The below
lists comparison between 2K ohm vs. 968 ohm Primary P-P.
RL (P-P) ....... 2K ohm / 968 ohm
Max Power ....... 9.2W / 8.1W
THD@1KHz/1W ..... 0.89% / 0.98%
Output Waveform is shown in the below URL.

kstlfido 9th April 2013 09:33 AM

Thank you for the links. Hmm, it seems like a 600 ohm primary might work for PPP 6AS7G, but I would get similar results to your 968 ohm load for PP.

Odd, how total power went down with 968 ohm, all other things being equal? I'll study your circuit.


Gusmore 26th June 2019 02:06 AM

6AS7 Push-pull Amp
1 Attachment(s)
Hi, I told you that I am going to build the amplifier that I show. It is a circuit published in the RCA Receiving tube manual RC16.- What opinion do you have? See the attachment

Best regards

kodabmx 26th June 2019 08:21 PM

My opinion is to use triode connect sweep tubes such as 6P36S instead.

Triode power amplifier.

"The results were more than a little disappointing. I could get 12.5 watts out of it but at 6.5 percent distortion. Even when the power was lowered to 5 watts the distortion was still 4 percent. When I connected the feedback I couldn't get the distortion below 1 percent. Oh, well, these tubes were not designed for audio service."

Gusmore 11th July 2019 08:43 PM

Thank`s for aswer, but I have 12 (NOS) Haltron 6AS7 G tubes, then I will try to use them.- I have read a few publications in this forum where artosalo considers that the 6AS7 tube has excellent performance for AF.

Sorry, I write more or less in English, my language is Spanish.-

Best Regards

jhstewart9 12th July 2019 01:02 AM

3 Attachment(s)
Here is my kick at the PPP 6080 amp. I avoided the requirement for a brute force driver by applying boot strapping. That can be done in at least two ways. If a UL OPT is available the drive to the previous stage is easily provided by the UL taps. Otherwise a voltage divider across the OPT primary wdg works. The OPT is a special Hammond made for me.

I did a proof of concept using the UL OPT scheme. 25 watts with a 400V PS provided by a lab supply. That's pushing things a bit, but 125V in the cathode resisters & 250V on the tubes is OK.

The other cct was a simulation only, but shews that the concept is very workable. Cathode bias is a must. Or a bias servo.

All this published in Glass Audio more than 15yrs ago. On on this forum a few times.:D

Both use a two stage diff amp, so -ve 150V is also required.The cathode resisters should be something that can be heat sinked.

Kay Pirinha 12th July 2019 09:44 AM

Why not connect the drivers' plate resistors directly to the other side final's plate instead of a voltage divider or UL tap?
Best regards!

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