Bias supply query

[fossilshark]

Hi, a few months ago I completed my LA60BL clone. I put in a voltage doubler as i had to use a 28vac line for the bias circuit. The output is controlled by a 250k pot. It was working for about two months until the bias pot burned up when I (naively) biased my amp by ear. When I replaced the pot that one arced instantly. Thoughts?

Side note: has anyone cloned this amplifier before?

[sluckey]

Let's see your schematic.

[fossilshark]

I drew this quick last night and forgot to post it. Also thats a picture of the bias supply (silver cans and 250k pot)

[2deaf]

It looks to me as though the pot is wired incorrectly.  The wiper should be going to the 220K grid leaks but it appears to be going to a bias cap.  If this is the case, you can probably short-out the bias circuit with the pot rotated to one extreme.

[sluckey]

There's nothing on your schematic that would burn up a pot. I agree with 2deaf, I don't think the pot is actually wired IAW the schematic.

A couple things about your circuit bother me... Being able to adjust the bias voltage to zero is dangerous. I would put a resistor between the pot and ground, rather than connecting the pot directly to ground. 2000µF is excessive for the filter caps. Not because of over filtering, but because those big caps take a long time to charge up, much longer than it takes the B+ filters to charge. I'd drop the value to about 100µF.

[fossilshark]

You guys were right, i made those changes and it works. Well theres another issue with the gain pots but thats another story