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Unread 09-29-2011, 11:58 AM   #1
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Amp Techs unite...Ampeg Vt-60

So, a guy I know picked up a late 80s Ampeg vt-60 (Tri-AC). Its supposedly a clone of a Mesa. He has given it to me to look at because it has an issue. He picked it up at an auction for $15. Thats a deal!

It hummed and had sketchy pots and such. He also told me that after a little bit of use, the volume would drop. The first thing that I did was re-tube it with extra stuff I had laying around, and clean up the pots. Right off the bat it was WAAY better.

I played it for about 2 minutes and it was great...until... after about 2 minutes, the volume cut drastically...almost like there was a small tweeter hidden in the cabinet and the sound was coming out of that.

If I put it in standby for a few seconds and switched it back on, the full volume would come back for a couple seconds, then drop again.

This amp is notorious for cold solders, so I took the chassis out and got into the guts of the amp (3 separate PCB's all crammed really close together...ugh!), and saw that on 4 of the big caps, the solder joints were burned.

I haven't tested the caps yet with a multimeter (I think you can do this). I've read that these particular amps have a complicated layout and that if you can find the official schematics, they are different than the actual amp (missing components and such).

My question is, are burnt solder joints indicative of the actual caps being bad, or something before them being bad? Does the volume drop issue sound like something that any of you have run into in the past?

Thanks in advance for all your copious amounts of knowledge you are about to bestow upon me

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Unread 09-29-2011, 01:03 PM   #2
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Low Power or Loss of Volume

Intermittent Problems

From what you describe, I'd guess something with the capacitors is bad. It seems like they're only holding/delivering power for a couple minutes then shutting down.
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Unread 09-29-2011, 02:06 PM   #3
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+1 on Steve. One or more filter caps giving up and pulling B+ (the high-voltage line) down.

Rip, are you sure the solder joints for those caps are actually burned (overheated in some way?) ... not acid leakage? An amp from the late 80s is borderline-ready for a cap job anyway.

There are ways of checking capacitance but they don't simullate the operating conditions ( ie with hundreds of volts applied). IMO you don't need to check capacitances so much as monitor the B+. If the voltage plummets at the same time as volume loss.... change the filter caps. Clip a voltmeter across one of the big fat filter caps and set it to a high DC voltage range. SAFETY RULES APPLY... this is the lethal part of the amp we're talking about. I hope you get it working.
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Unread 10-10-2011, 07:40 PM   #4
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I replaced the caps...the problem is still there. At least the amp powers on...but still, after a minute or two, it pops and goes really quiet
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Unread 10-11-2011, 04:06 PM   #5
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Even though it seems to be a heat up problem, I would try this just to make sure its not a solder joint/bad connection issue that only manifests itself after something gets warm enough. When the volume drops off, I would bang the amp (firmly, but not too hard...) and see if it comes back, at least briefly. If the banging does affect it, I would suspect a broken solder joint, or possibly bad connection - like the tube sockets or any ribbon type multi conductor cables. I would disconnect and re-connect any connecting cables like that in there, and also look at all the tube sockets to make sure the pins have enough "springiness" to firmly grip the tube pins.

You changed every tube ? I'm trying to think of components in a tube amp that would have a heat-up problem other than tubes. Resistors and caps in my experience typically don't have heat up issues, except maybe wirewound resistors, since they usually handle a lot of current and get warm / hot..

Tube sockets - the pins and the solder joints - the tubes get pretty hot, pretty quick, and would heat the sockets up relatively quickly.

Transformers - maybe. You could check the output voltages of the power transformer after it heats up.

Solid state components - heat up issues are common with any of these, it can take a few seconds or even an hour to occur. A lot of amps have IC's, transistors, and other SS devices in them for various things like reverb, channel switching, effects loop, etc. Could it be switching to the other channel, or other functiionality that is turned way down? Did you try using a cable to connect the FX out to the FX in? You can get "freeze spray", commonly used for troubleshooting solid state circuit heat up problems. After the thing is heated up and malfunctioning, you simply spray it on one component at a time; when you spray one and it the problem disappears or gets better, that is the culprit. I would not spray it on high voltage stuff like tube sockets, though. It can leave some condensation behind that could cause arcing; injecting high voltage into low voltage circuits.
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Unread 11-26-2011, 01:53 PM   #6
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Diving in...

OK.... I just received a VT60 I purchased from a Minneapolis Craigslist seller. I am looking forward to playing the amp out but first I will have to solve the same problem with loss of power after warmup (starts after 5 minutes) I can momentarily get the amp back to full volume by switching the triode pentode switch but that only works for a minute or 2. I tried banging the amp (thanks Major Tom) and the volume again came back momentarily. So I just wanted to thank you guys for forging the trail on this amp. The VT 60 is the little brother to the 4 power tube VT 120. You can see the spot where the 2 additional tubes would go. Atomium Amplification has posted the details of a thorough rebuild of a VT 120 google ampeg-vt-120-epic-overhaul
Hope this helps it's what I will attempt when I crack the amp open.
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Unread 06-08-2012, 11:52 PM   #7
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hello i have a ampeg vt60 that has been gutted, but i still have the power and output transformers and the combo box. does anyone know what the transformer wire codes are for them? i would like to reuse them for another amp build soon.
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Unread 03-24-2016, 10:21 PM   #8
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does anyone know what the transformer wire codes are for them?
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