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Posted new P-P power amp design
Posted new P-P power amp design
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Old 11th June 2018, 12:53 AM   #1621
smoking-amp is offline smoking-amp  United States
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Quote:
and check what power and setup I can get from my 6JN6 stack.
The 21JV6 tube is pin compatible with the 6JN6, a different heater V obviously (foil cuts for htr). It's rated 18 Watts Pdiss, but has a noticeably bigger plate than the 6JN6. Same plate size as the 24 Watt 6HJ5 used in the monoblock version. It has higher gm than the 6JN6 too, so should be easy to drive to the higher power using boosted B+, possibly without changing the input tubes.
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Last edited by smoking-amp; 11th June 2018 at 01:20 AM.
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Old 11th June 2018, 06:03 PM   #1622
cbueche is offline cbueche  Switzerland
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Thanks for the pointer, interesting approach, but the PCB has to be cut/re-cabled for two supplies voltage, one for the input tubes at 6.3V, on for the output tubes at 21V.
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Old 13th June 2018, 07:58 PM   #1623
cbueche is offline cbueche  Switzerland
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>I think someone would have said something by now if this was a problem.

Hello Jeff,

I came as well to the conclusion that the amplifier just works fine :-) Some more measures and impressions...

This is the engineer amp and I'm an engineer (or so I was told in 1988 before leaving electronics for IT ), so I measured a few more values, and tried to calculate if I'm still in the max dissipation of the tubes. I did some more tests with 1KhZ sinus out of my Tensa generator.

With an input voltage of 4.5V peak-peak, I get 36V peak-peak at the output, which if I'm correct translates to about 13V RMS, into my 8 ohms speakers I'm about at 20W when it starts clipping, as expected. At that power-level, the cathode current is at about 60 mA, B+ down to 380 (from 390 when idle). In a dark room, no sign of red plates.

I had two swap two input tubes to be able to balance the drivers. But I have no reliable way to measure distortion, my Siglent Scope has some FFT but it's not really usable (or I'm too dumb). I shall build something based on an audio card as some other have done. Until then, I used the voltage check on xDP1/2.

I think the tubes are still breaking in (maybe 5 hours playing now), the bias current is slowly growing, right now after one hour playing, it went from 40 to 41.6. As other have written it shall settle down at some point, or I hope so.

I dropped the cheap Denon plastic speakers and connected my Magnat Quantum 508 towers. They are specified at 93db/W/m. At full volume, my two kids came down from their room to ask for some peace... and the only other way to get them downstairs is food . Amazingly enough, the sound quality (source is a Squeezebox 3 playing from a NAS) is excellent even at full-volume... but you suddenly notice you can't talk to each other anymore. Amazing experience.

Next is finishing the box. Waiting on the laser cut of the panels.

Charles

Last edited by cbueche; 13th June 2018 at 08:00 PM.
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Old 14th June 2018, 06:49 PM   #1624
phyciocc is offline phyciocc  United States
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After a few days, today I went back in and redid all the biasing and balancing, and took some more careful measurements. B+ is 364V ( a bit higher because my power transformers is rated at 300-0-300, so as expected). C+ is -62V, and the Screen voltage is spot on at 150V.

I tried to measure the actual power and by injecting a 1 Khz sinusoidal wave, I drove the amp to clipping (speakers were connected and are nominally 8 Ohms). This occurred when the output at the speaker reached 14.8 RMS, thus giving me approx 27W RMS. This is a bit higher than spec. because I have a bit higher B+ and my output transformers are a bit lower in Impedance than the one suggested in the standard BOM. Mine are 6.6K vs. the 8k that Pete specified.

With no imput the amp is QUIET. No hum, no nothing ( my speaker are efficient but not terribly so). All in all, I am delighted.

Do I dare to tackle the 50 W monoblock? I'd need to make two...

Hello Everyone: Now that the amp is running and playing great, the itch to upgrade is now in full force. Currently I have an Edcor XPWR205 (300-0-300, with a 50V tap). I am getting a B+ of 364 V, and the output is about 27W at clipping. My opt are already upgraded (50W CXPP50 at 6.6K), so the only thing should be to increase the B+ another 30V or so. Antek seems the preferred choice for this, so I am thinking of getting an AS-3T325 (AS-3T325 - 300VA 325V Transformer - AnTek Products Corp) plus a 50V or so CT transformer for the Bias supply (Hammond 187D56 187D56 Hammond Manufacturing | Mouser).

What do you think?

Marco
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