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Ampeg SVT-3 repair advice

K

kaygee

Jan 21, 2008
11
1
UK
#1
I have an 11 year old Ampeg SVT-3 PRO that has stopped working in the middle of a gig and started making a continuous high pitched noise. Opening up the amp one of the resistors has burnt out. On the power amp schematic diagram it’s R8 (2.21K 1/2W 1%) between V1, V2, Q1 and Test Point 7.

I usually use the amp with an Ampeg 8 x 10 but when it stopped working I was using a Peavey 2 x 15 instead, both cabs are 4ohm and I was not driving the gain, eq or master volume hard so I don’t think the cab has caused the fault.

Please could any of the knowledgeable members advise if by replacing the resistor the amp should start working again or if another fault may have caused the resistor to burn out?

Thanks in advance
 
#2
I believe the only "V's" are in the preamp... Maybe you had a tube short or a cap go leaky causing a tube to short.

I would suggest testing the preamp tubes and repairing the resistor. Most mom and pop stores will do this free of charge but it will take some time.

Good luck!
 
#4
There is a significant risk of electric shock when poking around inside an amp, even if it isn't plugged in, because of the high electric charge stored inside the capacitors. This charge is sometimes sufficient to kill people. There is a proper method to safely discharging the capacitors that amp techs are familiar with. If you don't know how to do this, then you are putting yourself in jeopardy of being electrocuted.

I assume you know the resistor has burned out because it looks cooked. If so, replacing the resistor may very well fix the amp. Or it may not. It depends on why the resistor burned up and if anything else burned up at the same time. Also, if there is an underlying problem that isn't resolved, you may then just burn up the replacement resistor and you're right back in the same spot.

Troubleshooting skill is both an art and a science and with so many things that can and do go wrong with amplifiers, it's almost impossible to diagnose this sort of problem on a discussion board like this.

It's worth a shot if you can safely replace the bad component and it fixes the problem. The above advice about checking the preamp tubes is good. There are five tubes in this amp. 4x12ax7's and 1x12au7 in the preamp section. There are no power tubes.
 
#6
Bassmodder is wrong. The power amp has tubes also....... the mosfets are basically a "replacement" for the outputs/transformer, impedance matching to the speaker, the tubes provide all the gain.

That resistor is the "plate limiting resistor" for the POWER AMP tube driver section...... if it burned up, the plate of V2 would appear to have been shorted to ground somehow.

V2, C4, C6, C7 are suspects, R7 is a secondary suspect.

If C6 or C7 failed, there would be DC on the output, the relay should have opened up and you would hear no sound, so they may be "out" of the picture.

Yes please be careful poking around. let the amp sit for 15 min or so after unplugging, and it is best to check with a meter for voltage.
 
#8
Interesting design Jerrold. Never seen the schem for it.

Been seeing SO many weird things happening with new tubes in my amps lately- heater to cathode shorts, shorted rectifiers, and new tubes that have low emission and drifting bias that I start suspecting tubes right off the bat.

Sad but my old Fender stuffed with NOS tubes and a full rebuild (by me) seems to just keep on trucking. Spensive but I keep telling myself that price was for reliability.:(
 
#9
Thanks for all the advice, it was a fantastic help.
I took the amp to a tech who looked at the amp for 10 seconds and said he needed to replace all the Mosfets and it would cost £180 ($290). I then showed him what Jerrold had written but he was still sure the Mosfets were the problem so I walked away.
I decided to try and fix it and replaced the burnt out resistor, but the amp still it did not work, then I replaced valve V2 and it came back to life, looking forward to my next rehearsal to hear it turned up.
 

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