6F6G UL Amplifier

[sorenj07]

I have the Z565's, the PA774, and two 80uF 370VAC motor runs I'm thinking of using for the B+. B+2 might well be a couple 10uF Solens I can squeeze under the hood.

I haven't quite figured out the biasing section, but I assume that around 2.2K 1W for the bias PSU resistors should work. With the 6F6G's, around what values for the bias adjustment setup makes sense? Could a 25K pot and 10K resistor work? I assume that 6V6's would be easy to substitute in, so that's always a plus.

Here's a tentative schematic. Sorry, I don't have access to a scanner already, and I'll be sure to clear up any confusion with reading the thing.

tentative schematic, just came up with it. how does it look? if the 6F6G's are close enough to 6V6's, around what values do you think would work for the pot? I could probably use a smaller bias transformer since i'm basically not loading it at all, but that's a minor detail.



I'm also considering wiring in a Noble 100K pot to control volume.

Thanks, and wish me luck!


EDIT: Sorry, the bottom plate resistor on the 6SL7 is meant to be 280K...

[Tom Bavis]

6F6 needs a bit more bias voltage than 6V6, but looks like there's enough...

[sorenj07]

all right, cool. i'm working on getting parts together. the main hurdle will be the chassis, i want something that looks like i could have picked it out of Stereophile magazine, though i apologize for using that name around here

[SY]

What happens on signal peaks to that 1N914?

[sorenj07]

not too much, I don't think. it's just there to protect the cathodes on startup from the high negative tail voltage. unless you mean that a transient could drive the tube into positive bias, which would turn on the diode and flood the LTP with current? i don't think the signal would approach that height, somehow. im biasing at -2V so unless you get a 4V pk-pk wave from a CD player, i think it's safe?

[SY]

Most CD players have 2V RMS output. That's about 2.8V peak, or 5.6V peak to peak. Probably OK at less-than-full volume, but it's on the edge. Why not use a more reasonable negative rail and a CCS to avoid the problem?

[sorenj07]

By more reasonable, do you mean less voltage? I had a spare primary winding so I figured I might as well go with the increased balance the higher tail resistor can permit. Couldn't just changing the plate resistors to 180K and therefore bias to -2.8V take care of the problem a little more easily than needing to figure out a CCS?

That said, it'd probably be pretty easy to tap off 15VDC or so from the bottom 12V winding and feed a CCS. Know any that can put out 1mA of current? Hardly a strenuous task, I imagine

[sorenj07]

Final schematic:


Built it! Pictures and such to follow. It biases fine on dummy loads, but oscillates rather extremely when hooked up to my Cerwin Vega speakers. Sounds like about 1KHz.

I think I wired up everything correctly, and the voltages check out. The B+ is about 20V less than it used to be between today and yesterday - used to be 300V, now it's 280. Makes me suspect that the oscillation happened between today and tomorrow? If that's possible...

Any ideas? I'll go take some pics now.

[SY]

First thing I'd check is the wiring of the primary of the output transformer.

[sorenj07]

All right. Time to try flipping them primaries. Here are the pics. At a louder volume, I can hear music playing on both channels so I hope the situation isn't too bad.





And the wiring:


It was a bit cramped!

EDIT: For future reference, the voltage doubler caps in the bias supply have been changed to 250V. 200V would have been fine but somehow, they were more expensive.