martin manning wrote:Hi Darin, so that yellow lead in your pic goes to the top of a pot, and the bottom of the pot goes through a 22k to ground, and the bias voltage is from the wiper? If so that is the most negative voltage you can get from that winding, and a bigger resistor to ground won't help.
Martin, I was thinking along the same lines that it's most likely maxed out on that 15V tap. I'm still not clear as to why the HT winding bias scheme didn't work.
TM
Thanks guy's
I have tried a few different values and nothing seems to make a difference
so I believe you guys are right...maxed out the 15V tap.
As far as the HV bias tap...I may take another shot at it, but
if you guys think its OK(safe) to run it with the 15V tap I may call it and move on...I got frustrated trying to figure out the HV tap...it seemed so simple.
Darin
Considering that you have 500V on the plates that might be enough bias, however what may be ok for this set of tubes might not be enough for the next set and it would be nice to have more range. Also, I may would be concerned as the current rating of that 15V winding. I know the 24V bias tap on the 50W MusicMan PT with the bridge rectifier is rated at 120mA. You may very well be fine, but I wouldn't the bias circuit to be my weak link.
The Peavy schematic shows 525V on the high setting, so for 6L6's 30W you need to get the idle current down to about 30 x 0.7 / 525 = 0.040... seems like you are ok.
I've used a similar PV PT before and rigged up the bias like Darin has it.It worked fine PT barley got warm.
I belive peavey has another amp biased like this can't remeber wich one though.
He should be getting -55vdc or so from the bridge I would think that would be enough negative voltage to bias most 6l6's.
I've been wrong before and I don't know near what these guys know about dumbles so you probaly shouldn't listen to me.
I didn't look closely at that part of the Peavy power supply... that "15-volt" winding is +/- 15 volts regulated, +/- 27V (and so 54V rail-to-rail) after the bridge, and sportin' a couple of 1A fuses, so ya, looks like it's up to the job!
I really think you will be fine with using the orange taps you should have plenty of negative voltage on hand .I think you will have plenty of current on tap also.
If you are still having problems I can take apart the amp I built with the PV PT And see exactly what i've got.I was able to get a fairly good range.
You can still get your low voltage for the relays off the same taps.Once you get your bias issue solved.Iwill have to do some thinkin to remeber how to do it.
Considering that you have 500V on the plates that might be enough bias, however what may be ok for this set of tubes might not be enough for the next set and it would be nice to have more range.
Thanks guys
TM...This is what I am concerned about...
here's a pic of the bias pot...yellow from LV bridge white to 220k's
Darin
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