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cowpoke 25th July 2011 09:09 PM

Help Ampeg SVT-3 Pro
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It seems as though I blew up an Ampeg SVT-3 Pro while adjusting the bias.

I shorted the gate of R34 to ground. It blew all of the Mosfets except 1, also blowing R25 and D4, D5. Consequently R32-R38 were gone too.

Now I will be replacing the IRFP FET's as well as other parts. I would appreciate any advice or comments as I usually do not work on SS amps. My plan is to replace all silicon parts in the power amp section, acually all of the parts in red boxes. See attahced PDF.

I will then use the light buld in series trick, with the bias bot all the way down. I will hopefully bring it back to life. Any tips or tricks would be very helpful.

nigelwright7557 25th July 2011 10:04 PM

Easy done, I dropped a metal screwdriver while adjusting the bias, there were lots of sparks before i could turn it off.
So i bought a plastic tweaking tool.

I just wiped out the output MOSFETs and everything else was OK.
However sometimes the output MOSFETs going can take out the drivers and/or the VAS too.
So I guess what your doing is probably best, to replace all transistors etc.

cowpoke 25th July 2011 10:57 PM

Nigel, did you use matched mosfets? If so, how did you match them? I was talking to a local tech and he said that he never matches them, they just pull them from stock and have had good results. I am fixing this amp to keep for myself, so I want to do it correctly. Did I miss any parts that you think should be replaced? See the pdf with all items to be replaced in RED boxes.

Zero Cool 25th July 2011 11:06 PM

I have this note from ampeg:

If the mosfets are not matched for their Vgs(on), within 0.3V, they will never share well, and may fail at high currents. That was the factory spec, and the mosfets were sourced pre-matched and color dotted.

Before that procedure was followed there was a too-high failure rate. Once we introduced matched mosfets, there were a lot less failures.

I match them using methods similar to what Nelson Pass and Rod Elliot have posted on their websites. The amp will work just using mosfets out of the tube WILL eventually blow up again especially if you(your customer) plays at high volumes!

I also change the gate resistor from 1K to 47 ohms as suggested by an ampeg tech and they seem to live a lot longer as well. some models have 1K and others seem to have 150, 100, 47 whatever parts they seemed to have laying around that day i guess in them.

ALSO.... I have found on some Ampeg models...that if the BIAS pot is turned too far one way or another they will oscillate and draw power on start up, even blow up if you are not looking for it. be careful. if you see the amp break into oscillation before you get to full line voltage. turn the bias pot a bit and it should go away. you can then power up all the way and set bias.

from Ampeg:

Adjust the trimpot closest to the mosfets for 25mv across the 0.47 ohm resistors in the mosfet source circuit. Average, so check each bank to be sure they are matching pretty well. No signal no load, obviously.

Be sure to have the pot at the LOW end of the bias voltage range to start with, or it will immediately draw a lot of current...........

Best of luck


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