help figuring out bias supply

[Jason C]

Hey guys I need help with my bias supply (and probably a bunch of other stuff too). I'm building an Ampeg Mercury (6L6 version) but I used the Gemini II bias supply off the HV. Now I get -94v where the two 270k resistor sum up. How do I get that into the -50v range? The other thing is that when I took out my bias supply and clipped in a 200 Ohm 15w resistor that resistor got HOT and started smoking but my 6L6 plates weren't red plating or anything. Thanks a bunch.
Jason

[Albert Kreuzer]

I'm building an Ampeg Mercury (6L6 version) but I used the Gemini II bias supply off the HV. Now I get -94v where the two 270k resistor sum up. How do I get that into the -50v range?

Try a lower value resistor instead of the 68k (after the diode). Or a smaller cap instead of the .047 off the PT. I'm looking at the Piazza schematics, there may be variations on the values.

The other thing is that when I took out my bias supply and clipped in a 200 Ohm 15w resistor that resistor got HOT and started smoking but my 6L6 plates weren't red plating or anything.

6L6s can take a lot of current before they actually start to redplate. How high is your B+? A shared 250 Ohm resistor is for B+ around 360V, so a 200 Ohm is too low anyway - with a higher B+ the more so. How high is the voltage at the cathodes with the 200 Ohm resistor? Should be around 20V (50mA per tube). I guess it's quite a bit more.

Cheers,
Albert

[Jason C]

My B+ is 375 and the cathode voltage is 27. That's with the 200 Ohm resistor in place.
Jason

[Albert Kreuzer]

Hmm - that's hot, but not too hot for class A. B+ is in the ballpark. Each tube dissipating about 23.6W at idle. Resistor dissipating 3.6W. A 15W resistor should not smoke. Get warm, yes. Smoke, no. You'd need 55V across the resistor to burn 15W. Even more to get it smoking.

How does the amp sound?

[Jason C]

The amp will sound pretty good from what I can tell, there is some motor boating that I'll handle after there's no more smoke. How much lower should I go on either the cap or resistor for the bias supply? I have some .01's and .022's. Also I'm getting pretty different voltages on my PI. On pin 2 of the 6SL7 I get 36 and pin 5 I get 74. Would that cause my motor boat problem? Oh yeah, this last time I clipped in my resistor it didn't smoke but it did get pretty hot. So that's progress right? Thanks.
Jason

[Albert Kreuzer]

How much lower should I go on either the cap or resistor for the bias supply? I have some .01's and .022's.

First try a .022, then measure your bias voltage. If it's still too high, you can either go to .01 or lower the 68k resistor to 56k or 47k. You can also use a 33k with a 25k trim pot in series.

Also I'm getting pretty different voltages on my PI. On pin 2 of the 6SL7 I get 36 and pin 5 I get 74. Would that cause my motor boat problem?

Not sure how the voltages should be, but 36V definitely is too low. Try grounding pins 1 and 4. Disconnect the 100µ/6V cap. No luck? Disconnect the NFB wire. Check the 120k plate resistors. Have you tried a different tube?

Motorboating usually is caused by bad filtering and/or bad lead dress/grounding.

Oh yeah, this last time I clipped in my resistor it didn't smoke but it did get pretty hot. So that's progress right?

Yeah, the smoke should stay inside

Cheers,
Albert

[Jason C]

Well the good news is the PI plate voltage is now 277 on both pins 2 and 5. The bad news is now I have no sound. I grounded pins 1 and 4, removed the 100uf cap and the NFB. The first thing I did was swap out the tube with a known good 6SL7GT and after that I got no sound from either tube so I'm not sure what I did this time. I'll have to go back and check all my connections to see what in the world I did. I guess I shouldn't have let all the smoke out the first time.
Jason

[Albert Kreuzer]

Yes, check all the connections. Especially everything from the 6SL7 cathodes to ground (the 1k and 240R resistors, the 100µ cap).

"If you connect it right, it works."