Repairing a hissy, crackly VHT Special 6 Ultra

  • Thread starter Thread starter shmaller
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shmaller

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I have a VHT Special 6 Ultra that hisses and crackles, and not in a good way.

Video of the behavior: https://photos.app.goo.gl/Mf9Eag9UJ8Fg1ywh9

It doesn't always happen at first, it can sometimes take up to 2 minutes for the crackling to start. Or, maybe it won't hiss until I make a sound with the guitar, and then it starts. Once it starts hissing, it does not stop until turned off.

The behavior happens on all switch and dial settings, whether a guitar is plugged in or not, and can be heard on the Line Out. I've tried different guitars, cables, power supplies, and outlets.

So I brought it back to the shop I bought it from. I only know enough about amps to know NOT to go messing around in there myself with no idea what I'm doing.

Just documenting the issue and my troubleshooting, will report back in a few days when I know what the problem is!

s
 
My very old Sony receiver was doing something very similar, and I set it aside for about 15 years. A couple of years ago, I replaced the electrolytics and it was like new again.

It could be a bad tube as well. You only have a few tubes in that thing. You should be able to pull the preamp tubes, and if it still crackles, you know its not in the preamp section. If it quiets down, then you can look at the front end.
 
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Because that noise was under the control of the master VC it was likely in the first stage and yes, likely the first valve. You can't always diagnose noise problems by "pulling tubes" because you can for instance have a noisy anode load or associated bad joint. With no current through the resistor there will likely be no noise so you often need to fit a good valve.

Dave.
 
Update from the shop: they hit the whole thing with contact cleaner and tried several different power tubes (it takes a 6V6GT) and the problem is still happening. Seems to maybe be set off by twiddling with any of the controls of the amp?

Reaching out to VHT at this point.
 
Did they look at the front end? I looked at the schematic, and there is as 12AX7 in the front end. If that's leaky, that will surely show up in the output. I'm no tube genius, but for a few buck I would look there. @ecc83 is the tube guru. I would check with him.
Sounds silly, but if you had a clip of the noise that would really be helpful.
 
Old Music Guy said:
Sounds silly, but if you had a clip of the noise that would really be helpful.
Click to expand...
Video of the problem is in the original post!

Other than fiddling with the power tube while I was there in the shop, I have no idea what's going on. I think they'll probably have to call in a tech
 
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The fact that the noise changes when you roll down the tone control and roll the volume means that the problem is before the volume and tone stack, which rules out the power tube. I suspect that V101 (12AX7) or possibly one of the capacitors or resistors is bad. They need to check the section in red. Volume and tone are in the yellow box. So the problem has to be before that.

VHT6.webp
 
Totally agree with @TalismanRich. The 12AX7 also does double duty (V1-A and V1-B).
They're not a lot of $$$. I'm surprised your tech didn't go the the front end first.
Schaltung klein.webp

EDIT: In all my years in electronics, the chances of the problem being a passive component (R, C, L) are miniscule if there is an active component in the circuit. Not nil, buy close.
 
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