• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6F6G UL Amplifier


2006-05-04 9:15 pm
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...
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?
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 :D


2006-05-04 9:15 pm
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.
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.
Doesn't seem like it, since one tube's wires are blue, while the other are green. Plate is solid, screen tap has white on the jacket as well. I flipped both pairs of wires and I think I'm only getting the noise out of one channel. I'll flip that channel's wires and see how I do. But I have to leave for dinner so I'll do it later tonight.
I changed the 6SN7, no difference. I changed the two front power tubes, and this time the oscillation didn't begin until a moment after the music started, fading in quickly. Switching the other power tubes didn't make any difference. Whatever's happening seems like it's isolated to the circuit around the tubes and/or the OPT of the right channel. I'll go over everything and probably check voltages while it's on my bench hooked up to dummy loads.

This is getting frustrating :(
Just realized that oscillation might happen as a result of the wiring, so I'll be putting those grid stoppers in there pronto. Maybe not tonight though, I'm tired from connecting/disconnecting it from my speakers and lugging it two flights of stairs between my room and my workshop :smash: