VHT 12/20RT - Watts VVR non-functional - Fix or return?

[Gurduloo]

I just bought a "Open Box" VHT 12/20RT on eBay at a great price ($360). After 2 weeks I've noticed that turning the Watts knob does nothing. Since the amp was open box, I doubt it is covered by warranty, but I could probably ship it back for a refund. The problem is ... I doubt I could get another one at the same price that I paid, plus I'd be out whatever it costs to ship it. And I really like this amp. I can do basic wiring, but I've never worked on an amp before. So I'm trying weight between returning the amp or fixing it. Does anyone have any advice on how to diagnose my problem and what kind of potential costs I could be looking at to repair it?

VHT 12/20RT Schematic

[Jazz P Bass]

Welcome to the Forum.

You may be expecting something else out of the "Watts" control.

It is the internal high voltage that is applied to the output tubes.
It is utilized only when switching tube types.

When using 6L6/ EL34 output tubes, the switch is set to 20 watts.

For 6V6 tubes, the switch is set to 12 watts.

https://images.thomann.de/pics/prod/vht_1220_manual.pdf

[Leo_Gnardo]

What Jazz P Bass said ^^^. And if you got an amp you're really happy with, for only $360, why not hang onto it?

Also, "when all else fails, read the directions (owners manual.)" A cheap lesson in life's realities.

[Gurduloo]

No, we're talking about two different controls here. There is a switch behind the rear panel that toggles between 12/20 watts. I have not touched that one - my understanding is that it's only for when I put in a different set of tubes.

There is also a knob on the top of the unit that varies the wattage from less than 1 watt to 12 or 20 watts, depending on the aforementioned toggle - see page 4 on the manual you linked. That's the one that isn't working. I'm in contact with the seller to see what they can do, but I also want to try and get a feel for how feasible it might be to repair it.

[g1]

Do you have it cranked real loud when you are checking?
What kind of tools and experience do you have for working with amp innards?

[52 Bill]

Boy, that's some power control, variable B+ 306vdc to 29vdc.

The control VR8A & B, changes not only the B+ voltage, but also the bias to the power stage. At first I thought that the FET could be shorted thus allowing for the full 300 volts to pass, but if the control was turned down, it would still reduce the bias voltage and would probably cause the output tubes to redplate.

The circuit isn't all that complex, should be fixable if you have the correct set of skills.

[Gurduloo]

Do you have it cranked real loud when you are checking?
What kind of tools and experience do you have for working with amp innards?

I've tried it both with the volume turned low and up. Either way, turning the watts knob has no audible effect. I have no experience at all with amp innards, just guitars and tiny robots. I have basic soldering experience and not much else. One of the reasons I chose this amp is that I was hoping to get some experience modding it, as it is designed with modders in mind. If I start out with repair experience, I guess that's good too.

[g1]

Boy, that's some power control, variable B+ 306vdc to 29vdc.

The control VR8A & B, changes not only the B+ voltage, but also the bias to the power stage. At first I thought that the FET could be shorted thus allowing for the full 300 volts to pass, but if the control was turned down, it would still reduce the bias voltage and would probably cause the output tubes to redplate.

Just to note, the owners manual in post #2 contains a schematic that is not for this reverb/trem version.

Bill, I think that other half of VR8 just limits the trem excursion?
So I believe the shorting of Q1 is a likely source of power level being stuck on full.

[Gurduloo]

So it could be as simple as replacing that one MOSFET? That sounds encouraging and something I could handle. From what I understand, I can't test the MOSFET while it's connected, is that correct?

Could there instead be a problem with VR8? If VR8 also has some effect on the tremolo - should I hear some change in the tremolo when I turn VR8? I'm at work now and can't test that, but maybe that could backup your theory?

[g1]

Yes, VR8 could also be responsible for your problem.
I mentioned the FET because I have seen it cause the same problem in other brands (Suhr) and also when used as an electronic standby switch (Traynor), so it is not uncommon.
Even if the trem side of VR8 is working, the other side that controls the high voltage level could still be faulty. It is a dual pot and either side can independently be bad.
But see if the trem side is working anyway. Set trem depth to full and see if the power level control has any audible effect on the depth.
If you do end up changing the Fet, they are cheap: http://ca.mouser.com/ProductDetail/F...nbAy5CeA%3D%3D
Be careful not to damage any insulators if they are to be re-used and also heatsink paste.
Also check the related zener diode ZD1 and make sure it is not shorted.

[Gurduloo]

Ok, I played around with VR8 with the trem at full depth and there's a definite effect there. That's a big relief because I'd only tried the trem with the watt knob on low, but since the overall power level was stuck at full, it sounded awful. Now it sounds awesome. Loud, but awesome.

I'm going to take it apart tomorrow to see if I can even find the FET and zener diode before I go any further...

[52 Bill]

Bill, I think that other half of VR8 just limits the trem excursion?
So I believe the shorting of Q1 is a likely source of power level being stuck on full.

Right!

I just looked at the schematic again and see that it is a cathode bias power amp with that depth switch.