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I am making a PP el84 amp out of an organ amplifier. The amp was originally a mono push pull parallel el84 amp with a 5u4 rectifier. I am just going to reuse the chassis sockets and power trans. This original amplifier PT has 2 bias taps one is CT and the other just a single voltage source.
So, I have been going thru various schematics and since the PT has a CT bias I would like to make an amp with fixed bias. The one scheme I like is the Scott 299 with a 6BL8 front end to PP el84 (7189). It seems like the circuit B+ would be about spot on with this power trans. I was just about ready to dive in when I came across a PP 6973 amp in the RCA tube manual. This amp uses fixed bias that is really fixed! I like this design because it is slightly simpler than the Scott 299 with the extra pots and wiring. So, I would use the Scott 299 schematic except with fixed fixed bias utilizing the ct bias supply with a FWBR and maybe a 5K pot so I can adjust the total amount of bias voltage. I also plan to put 10 ohm resistors on the el84 with a test point for each so I can at least determine the idle current for each el84 and select the 4 that are the closest. Anyway, I have not tried something like this before and wanted to know if anyone had suggestions or comments. thanks in advance, Dak
Follow Ups:
For those vintage amps with "fixed," fixed-bias --- good to check the actual idle current draw. Often higher than nominal, prolly due to higher outlet voltages seem to effect the B+ more than the C-.
Adjustable bias supply is the way to go. As not all output tubes (of the same type) draw the same current. Even brand new.
I would never build an amplifier with non-adjustable fixed bias. In fact, I think most push-pull amps don't have enough adjustments. I'd like to be able to not only adjust the bias per pair, but also per tube (for balancing).
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Buy Chinese. Bury freedom.
If you're not in deep Class A, you would also do an AC balance for transconductance differences, to maintain low direct current in the output transformer during loud passages.
Sure I used to feel that way. But, now I would rather worry about what music I will play next rather than if the tubes are balanced to the nth. degree. I mean if cathode bias sounds great and while the output pairs are supposed to be matched, I doubt if in real life that the output pairs are well matched at all. And all those juke boxes and organ amps that operated in fixed fixed bias they sounded just fine. cheers, Dak
"all those juke boxes and organ amps that operated in fixed fixed bias they sounded just fine"
No they didn't. They were mid-fi. That's not what most of us aspire to build.
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Buy Chinese. Bury freedom.
That was a sweet amp. It was my daily driver for 2 years until some crack heads stole it. It was a sad day in stereo land, I must say.
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