So I biased my Ampeg SVT-3 Pro mosfets the other day

audiomitch · #1 12-17-2010, 04:52 PM

They were pretty far off, and setting the bias to the correct value changed a lot.

If I replace the 12AU7 and 12AX7 on the power side, should I turn the bias pot down before powering up again and re-bias afterwards? I'm paranoid about frying my Mosfets. I wonder if replacing those tubes could send too much current? Or is it enough to just replace the tubes and do a quick check of the bias voltage after? Is it even an issue at all?

Sorry for all the questions, I hope they made sense and/or aren't dumb.

Jerrold Tiers · #2 12-17-2010, 08:54 PM

Quote:

Originally Posted by audiomitch

They were pretty far off, and setting the bias to the correct value changed a lot.

If I replace the 12AU7 and 12AX7 on the power side, should I turn the bias pot down before powering up again and re-bias afterwards? I'm paranoid about frying my Mosfets. I wonder if replacing those tubes could send too much current? Or is it enough to just replace the tubes and do a quick check of the bias voltage after? Is it even an issue at all?

Sorry for all the questions, I hope they made sense and/or aren't dumb.

Don't worry.

The tubes have zero effect on mosfet bias.......

It's not a stupid question, by any means...... in some amps the drivers do affect output bias, as in an SVT. But the 3PRO has a blocking capacitor so there is no interaction.

audiomitch · #3 12-17-2010, 09:36 PM

Excellent! Thanks for the great information!

One more thing - some people are talking about swapping out the bias pot for one that won't go out of adjustment as easily. This is a 10k bias pot, right? Could somebody simply swap that pot out for a Fender style 10k bias pot? This is kind of hypothetical, I don't think I'll be yanking out the board and desoldering the pot anytime soon, but you never know.

It will be interesting to see if, or how soon the bias drifts out of spec again.

Jerrold Tiers · #4 12-18-2010, 08:10 AM

The pot really isn't that bad... the real issue is that the amp is super-critical about bias, due to its design...... We used the same pot on a lot of products, and the 3PRO is really the only one with a consistent problem.
I would also have probably (if I had been doing the design) done the bias a little bit differently. But the existing style is effective and workable.

You often hear about the bias being LOW, but rarely about it suddenly going HIGHER and blowing the amp......which it could. So I suspect many were simply adjusted to the low side, and temperature changes etc bring out the issue.