VHT Special 6 Ultra Very Quiet

[extordi]

Hey guys,
I've lurked on this forum for a while. When I ended up with this issue, I figured there would be no better place to ask about it than here.

I've got a VHT Special 6 Ultra. Nice little 6 watt tube amp. I just took it upon myself to do the fairly popular negative feedback mod. I have done little work with tube amps, but am quite experienced in electronics, soldering, reading schematics, etc. So I thought I could do this pretty easily, especially considering the steps outlined here (you can also see the schematic for this amp if you scroll to the top of that page). However, upon completion I've found that the amp is super quiet. It "works" per say, but you have to turn everything up real loud to hear it. If I plug into the effects return, the amp acts as it should. I've tried jumping the FX loop, and the tubes also seem to be fine. At one point I found that by plugging my guitar into the jack halfway and jiggling the connector into just the right spot, the amp would work exactly as normal. However, this doesn't even seem to be the case any more. Seeing as plugging into the FX return works, it seems like something got messed up with the input stages. I'm just not really sure where to proceed to figure out this problem.

I'm really stumped by this one! If anybody has anything you think I should try, it would be greatly appreciated. I just want my amp to work again!

[Enzo]

OK power amp works.

I've tried jumping the FX loop, and the tubes also seem to be fine.

What does this mean? How does that check tubes? Or do you mean jumping the loop makes the sound come out?

If that is the case, it sounds like the cutout contact on the FX RETURN jack is not making contact. You can plug into it and it works, but that doesn't go through the cutout contact.

Clean that cutout contact, or look to see if it is bent. Check the contacts with an ohm meter to se if they measure very low resistance when closed.

[extordi]

What does this mean? How does that check tubes? Or do you mean jumping the loop makes the sound come out?

Sorry if that was a little unclear. I just meant that I tried connecting the FX SEND and FX RETURN jacks with a patch cord to test if it was a bad contact. I also just checked that cutout contact, and it seems to be totally fine. Also, to do a really basic (and not completely conclusive) test of the preamp tubes I tried swapping them out, since only one of them is in play after the FX loop. I know it doesn't mean that the tubes are definitely fine, but since the post-FX loop preamp section works fine with either tube, plus a few minutes ago I was able to get the amp to work properly with some clever positioning of the input connector, I figured I could (at least temporarily) look past bad tubes and see if there is something else that will work.

[extordi]

OK power amp works.



What does this mean? How does that check tubes? Or do you mean jumping the loop makes the sound come out?

If that is the case, it sounds like the cutout contact on the FX RETURN jack is not making contact. You can plug into it and it works, but that doesn't go through the cutout contact.

Clean that cutout contact, or look to see if it is bent. Check the contacts with an ohm meter to se if they measure very low resistance when closed.

Sorry if I was a bit unclear. By jumping the FX loop I meant connecting a cable between the send and return to eliminate the cutout contact. I also just measured it with a meter, and it's working fine. And about the tubes - I know it's not conclusive but I figured I would try swapping the two preamp tubes around (i.e. pull each out and put it in the other socket) and see if it affected anything. The remainder of the preamp post-FX loop is only in one tube, so I figured that if everything behaved the same with both arrangements of tubes, plus the fact that just a few minutes ago I had the thing working perfectly normally by cleverly jiggling the input jack, I could probably rule out the tubes, at least for tonight (since I don't have any 12AX7's at hand).

I'm also noticing there is a relatively noticable hum when you turn the inputs up. It's maybe slightly more noticable than when the amp is operating normally, and goes away when you turn the volume down. Perhaps the issue could be a bad input jack?

[Randall]

"What does this mean? How does that check tubes? Or do you mean jumping the loop makes the sound come out?"

I took that to mean that jumpering the loop makes no difference, so it appears that neither that nor the tubes are the issue.

extordi, this amp has two inputs? Same issue with both? When plugged into Ultra input, signal goes thru V1A gain stage, Ultra vol pot and passes thru Clean input switching jack on it's way to Clean V1B gain stage. If the issue is only with the Ultra input, then look at the switching jack on the Clean input. If it is on both inputs, then the issue is after the Clean input (providing all connections are verified as good) and before the Send jack.

[Enzo]

OK, so jumping the FX loop did NOT restore sound.

Input jack? Sure could be.

Is the chassis out right now or is it in the head cab? With signal running through it, either whack the top of the head with your fist, or bang on the end of the chassis with a rubber mallet... or your fist... Does sending that sharp vibration through it affect th problem? If you can hear hum that the volume control changes, then teh problem is likely before that.

I'd apply a signal to the input, and use a scope or signal tracer to follow along the signal path looking for where the signal disappears.

[extordi]

"What does this mean? How does that check tubes? Or do you mean jumping the loop makes the sound come out?"

I took that to mean that jumpering the loop makes no difference, so it appears that neither that nor the tubes are the issue.

extordi, this amp has two inputs? Same issue with both? When plugged into Ultra input, signal goes thru V1A gain stage, Ultra vol pot and passes thru Clean input switching jack on it's way to Clean V1B gain stage. If the issue is only with the Ultra input, then look at the switching jack on the Clean input. If it is on both inputs, then the issue is after the Clean input (providing all connections are verified as good) and before the Send jack.

The problem happens with both inputs, but I'm not completely convinced it's entirely after the clean input. If you plug into the Ultra input and turn both that and the Clean input up, you can get a bit more volume out (it's basically acting like I have my guitar volume turned waaaaaaaaaay down, though that's not the case nor is it a bad cable) but still quiet. I had my oscilloscope on it and I was checking voltages after each of the gain stages. After both ultra and clean the signal was smaller than that coming from my guitar, but all of a sudden it started working normally again. At this point, the signal after each gain stage became significantly larger than my guitar's input level, as it should. It's stopped working agian, but this little blip of functionality seems to imply that the problem is affecting both gain stages directly, not just reducing the signal level after.

[extordi]

Ok first off, sorry for the delay in reply. For some reason every other time I have tried posting, the message did not go through.

I have been poking away at this for a while, following the signal through with my oscilloscope and whatnot. I think the problem may be to do with the clean input jack. If I plug in there, I have the issue of a quiet output. If I plug into the Ultra input, I can get more volume, but still quiet. However, if I turn the "ultra" knob all the way up and play for a little bit, the amp will suddenly "pop back" to working normally. If I then plug into the Clean jack, though, the issue comes back. Perhaps this is due to a bad connection somewhere? I will get a hold of some contact cleaner and see if that helps at all.