[sammyr]

I've got an ampeg VT-120 Triax head that's giving me grey hair. It's developed loud popping that starts as the amp warms up and turns into a deafening storm of static and pops.

To give a little history. I've replaced the power tubes and they're biased properly. I've cleaned and retensioned all the tube sockets, jacks, pots, switches and replaced the power resistors and plate resistors and some suspicious coupling caps all with no result. The filter caps look like they've been replaced recently.

With v3 (12au7) removed there is no popping. I've replaced the coupling caps and plate resistors in that section. With v3 in and v4 removed there is just the odd quiet static pop. Jiggling the tubes or sockets does not seem to have any effect. I have subbed in many different tubes without any luck. Chopsticking also yields no results.

The amp itself is a real pain in the ass to work on so I'm curious if anyone has any ideas or suggestions while I steel my resolve and prepare for more toil.

Schematic http://www.thetubestore.com/lib/thetubestore/schematics/Ampeg/Ampeg-VT120-Amp-Schematic.pdf

 

[nonreverb]

When just v3 removed, what happens?

 

[sammyr]

with v3 removed the problem stops. I've swapped a couple of 12au7s in there to no effect.

 

[sammyr]

I thought perhaps the reverb transformer might have a soft short but checking the resistance with my multimeter doesn't show any blips or anomalies.

 

[nonreverb]

Static measurements rarely reveal intermittent shorts in transformers. Only with B+ applied and a load connected would it reveal the fault.
I does appear, based on your description, to be in the reverb circuit somewhere.

 

[sammyr]

The main reason I suspect the reverb transformer is that it beyond replacing the socket itself for v3 is that i've swapped all the coupling caps and plate resistors in for v3 and v4. As well I figure it and v3 have higher voltage than the rest of the preamp. I have some suspicions about v3's socket but without a tube in it it doesn't make any noise when chopsticking so it doesn't seem to be the culprit.

I replaced the PI plate resistors measures for good measure while i had the board out to access the reverb transformer. I'm going to see if I have a neon lamp and to do rg keens transformer test The Super-Secret Transformer Tester

this amp is really stumping me. the popping is quite loud which makes me think its arcing or a loose tube socket. its diffrent from the frying bacon sound of crackling resistors. process of elimination leaves tube socket, resistors other than plates in v3/4, or reverb transformer. i may shotgun the rest of the resistors in v3/4 while the boards out just in case but I don't see that fixing the problem. I'll have to get a board mount preamp socket and if that doesnt cure it ill have to track down a replacement transformer. fortunately i found a spec sheet but loud purchased st louis music which used to distribute the replacement transformer for ampeg and crate but they don't sell parts anymore.

 

[nonreverb]

I don't suppose you have another reverb transformer lying around. That would be the definitive test.

 

[sammyr]

I wish. I don't even think I can buy an original replacement part might have to improvise unfortunately. at least after much digging I found the specs

 

[jb welder]

Until you can test/replace the TX, try injecting a signal to the cable coming off the tank 'out'. At least that way you will know if the return path (and other half of V3) is ok.

 

[sammyr]

thanks JB I'll give that a shot.

 

[nonreverb]

Here's something else to try. Just unplug the tank completely and tell us what happens.

 

[sammyr]

there doesn't seem to be a difference in noise with the tank plugged in or unplugged.

 

[jb welder]

That seems to imply the problem is in the return circuit and the reverb TX is ok. I suppose there could still be crosstalk through the 2 sides of V3 if the transformer was arcing, so you could disconnect the TX primary just to be sure.

 

[jb welder]

And I guess the reverb control or switch does not get rid of the noise?

 

[sammyr]

adjusting the reverb control adds some hiss as you turn it up but has no effect on the loud pops. I can barely let the amp warm up before the popping and crackling gets so loud I have to turn it off

 

[dtsaudio]

with v3 removed the problem stops. I've swapped a couple of 12au7s in there to no effect.

At this point i would suggest disconnecting the reverb transformer at the 12au7. It is the plate load and takes all the current that is required by the tube.

 

[sammyr]

I had a chance to do some more testing on the reverb transformer. It seems to be ok or at least it passed R.G. Keens transformer test. I connected a 120v neon indicator lamp and limiting resistor across the secondary winding and then briefly connected the primary to a 6 volt lantern battery and the lamp flashed. I guess that at least narrows it down to a bad tube socket or a bad component in the reverb circuit.

update: pulled out the tube socket and and it looked okay visually but i noticed that even with a tube inserted a couple of the socket pins are still pretty wiggly. hopefully I can track down a pcb mount tube socket locally tomorrow

 

[nonreverb]

There is a condition that that test might not reveal that would also explain the noise. If the transformer primary winding has breached insulation between windings that arcs under operating load, that would cause all kinds of problems. It's not a common problem but one never the less.

 

[sammyr]

I hadn't thought of that. I'm going to try a new tube socket while I'm up because finding a replacement for that reverb tx is going to be difficult.

 

[sammyr]

seems it was the socket or the joints at the socket because replacing it with a new socket today seems to have cured the loud pops. now just have to attend to some minor crackles background noise and we're off to the races.