Peavey Donor amp build

[dcribbs1412]

Increase the bias adjust resistor. I generally have to increase to 1k.

Hi Mark
I have a 22k from the bias pot to ground...change that to 1k?

Glad to see you got it going.

Thanks for your help cbass

Darin

[M Fowler]

No you need to adjust the bias resistor which is the resistor infront of the inverted diode.

Depends on the bias circuit some are 470R to 2k2 or so.

[dcribbs1412]

No you need to adjust the bias resistor which is the resistor infront of the inverted diode.

Depends on the bias circuit some are 470R to 2k2 or so.

I understand...the bias circuit( bridge I think) I'm using I do not have a resistor in that spot.I'm using similar to this 100uf/100v and a 15k
I tried a few different values in the 15k spot but none seemed to give me any different adjustment...

Darin

[M Fowler]

Darin,

I was referring to your bias board layout if your not using this then I have missed something in your build. The resistor circled in red.

[dcribbs1412]

Darin,

I was referring to your bias board layout if your not using this then I have missed something in your build. The resistor circled in red.

Hi Mark
I was not able to make the HV tap work for bias.I used a 15VDC tap
off the PT and made "my interpretation" of a bridge. It seems to work
OK, I'm able to bias at 30-35 ma, but that is the lowest the bias pot goes.
here's a pic...yellow lead goes to pot which has a 22k to ground.
http://i1018.photobucket.com/albums/af3 ... ean029.jpg
http://i1018.photobucket.com/albums/af3 ... ean030.jpg

Darin

[M Fowler]

Okay then you have to increase the 22k to be able to increase the mA bias voltage to ground. The next higher common value is 27k.

[dcribbs1412]

Okay then you have to increase the 22k to be able to increase the mA bias voltage to ground. The next higher common value is 27k.

Will do...
Thanks
BTW have a safe Happy New Year

Darin

[M Fowler]

Darin, you have a great weekend and Happy New Year.

[martin manning]

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.

[ToneMerc]

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

[martin manning]

TM, I see no reason the HT tap w/ FWB didn't work either... lots of Marshalls out there with that set-up. Here's another look, Darin. The top of R31 is the bias voltage; right-hand side of C15 goes to one side of the the HT, before the main rectifier bridge.

[dcribbs1412]

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

[M Fowler]

OK, I'm able to bias at 30-35 ma, but that is the lowest the bias pot goes

If you are trying to lower the bias to say 28mA wouldn't you have to lower your resistance to ground the 22k off bias pot to 20k or 18k?

Mark

[ToneMerc]

OK, I'm able to bias at 30-35 ma, but that is the lowest the bias pot goes

If you are trying to lower the bias to say 28mA wouldn't you have to lower your resistance to ground the 22k off bias pot to 20k or 18k?

Mark

True, but I interpreted him as he's wanting to go even more negative verses less, which would be more positive toward zero....

TM

[M Fowler]

I did too but I was reading things over again and wanted to make sure.



Mark