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Author Topic: Ampeg SVT 350  (Read 1596 times)

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  • Chipper
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Ampeg SVT 350
« on: June 27, 2016, 08:11:15 PM »
Hello Forum! Just let me say first off that my knowledge of solid state amps is very minimal. I usually try to stick with tube amps and have a pretty good record with repairing and modding them. Solid state, not so much. I've fixed a few with minor issues, but generally steer clear of them because of my lack of knowledge of solid state circuits.
A friend dropped off his tube amp for retube and bias and with it asked if I could take a look at his other friends SVT 350. Finished up the tube job and figured I'd look at the Ampeg. The fan wasn't working and I thought it might be something minor. That was my mistake. I opened the amp an was checking voltages when I had a probe slip at Q14. Spark and popped the 10amp fuse. I put a new fuse in and said a prayer, but it wasn't my day. Fuse stayed good, but smoke from R20, R21, R45 and they died. I removed Q14 from the circuit and replaced the resistors listed above. I hooked up my current limiter and plugged it back in and got an initial surge but then the light dimmed. The resistors still got pretty warm but I didn't leave it plugged in too long so I wouldn't do to much more damage. Where do I go from here? Schematic attached. Thanks in advance.  :'(

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  • Chipper
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Re: Ampeg SVT 350
« Reply #1 on: June 28, 2016, 01:19:48 PM »
Any ideas? Anyone?

Enzo

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Re: Ampeg SVT 350
« Reply #2 on: June 28, 2016, 10:17:34 PM »
Well first, Q14 is not part of the fan circuit.

OK, resistors never burn up on their own, something always draws excess current through them to do it.  You are up on tubes?  So imagine a screen resistor on a power tube.  The resistor only burns up if the tube runs excess current through the screen, like when the tube shorts internally.

Just a suggestion, when citing resistors, maybe tell us where they are.  I found R20,21 by Q4, took me longer to find R45.

R20,21 burning tells me instantly that Q4 is now suspect.  If you sparked Q14, good chance it is not bad too, pull it and see.  R45 burnt becuase it was the best path for current for something.  R47 next to it, is that now open?  And Q3, is it now shorted B-C?

Since Q14 is suspect, check the gate resistor R44.

DO NOT put a speaker load on this amp until it is stable and not making DC.

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  • Chipper
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Re: Ampeg SVT 350
« Reply #3 on: July 04, 2016, 07:53:03 PM »
Thank you so much Enzo. As soon as I have a chance to work on this again, I will you know how I made out.