VHT Special 6 replacement parts

[0xDEED]

Could someone suggest compatible replacement parts for VHT Special 6?

In particular:
- transformers
- switches (ON/standby/ON)
- volume pot
- tone pot

I am troubleshooting my VHT Special 6 and it seems I may need to replace one of the above.

Thanks!
Chris

[The Dude]

Since they don't release schematics, here's gut shot that may help.



I'll add:
1) Transformers rarely go bad. Have you tested them?
2) Those switches look pretty standard. There are any number of toggle switches that would work there. Check continuity first to see if they are bad.
3) I can't see the pots in the photo, but they don't appear to be PC mount, so it shouldn't be a problem finding something to replace them. Try cleaning them first if they are noisy.

[DrGonz78]

I think this the right schematic.

[Jon Snell]

I wonder what 'compatible' parts you require and most importantly, what is wrong with it?
A resistor is a resistor, a capacitor is a capacitor etc.

[Mick Bailey]

Hi Chris, and welcome to the forum. If you could provide more information on the fault we can help you to narrow down and diagnose what the problem is. In the main, with repairs don't need model-specific parts and very often what appears to be an involved fault can be resolved inexpensively once you've identified where the problem lies. It gets expensive if you throw random parts at it - basically, you can end up replacing perfectly good parts in the hope that one or more replacements fixes it.

[0xDEED]

I wonder what 'compatible' parts you require and most importantly, what is wrong with it?
A resistor is a resistor, a capacitor is a capacitor etc.

I specifically asked about transformers, pots and switches. Resistors and capacitors are always easy to identify.

[0xDEED]

Hi Chris, and welcome to the forum. If you could provide more information on the fault we can help you to narrow down and diagnose what the problem is. In the main, with repairs don't need model-specific parts and very often what appears to be an involved fault can be resolved inexpensively once you've identified where the problem lies. It gets expensive if you throw random parts at it - basically, you can end up replacing perfectly good parts in the hope that one or more replacements fixes it.

Mick,
Thanks a lot for your reply. I am not going to replace random parts. I plan to test each component first to hopefully identify what the problem is. If all parts look ok, then it means the problem is more complex. But I have a feeling the problem could be pot or switch, that's why I asked proactively, so that I am ready to order when needed. Also, VHT support claimed that "I can't find these pots and switches in my country(!)" which sounds to me like "we don't want to tell you what we use" (weird).

I will explain what the problem is and hopefully some of you can give me feedback: (perhaps it's better to open a new thread?)

The amp gets extremely loud suddenly even when the volume is at 0. By switching from HI to STANDBY and back to HI (or from LOW to STANDBY and back to LOW) the problem could resolve, sometimes. But then after a while the same thing happens, while I play it gets loud suddenly and stays there. Lately, it looks like the problem is practically permanent. As soon as I turn the amp on the audio is really high (even with volume at 0) and it takes a lot of switching between STANDBY and ON to resolve (or it does not resolve at all). While the audio is very loud the volume pot makes small difference in the actual volume (it goes from very loud to a bit more loud - small difference)
The volume pot is not scratchy.
Also, it should not be related to the tubes, as the problem persists with whichever tubes I use.

Thoughts? Should I open a separate post to ask about this problem?

Thank you all so much for your help!

[0xDEED]

Since they don't release schematics, here's gut shot that may help.



I'll add:
1) Transformers rarely go bad. Have you tested them?
2) Those switches look pretty standard. There are any number of toggle switches that would work there. Check continuity first to see if they are bad.
3) I can't see the pots in the photo, but they don't appear to be PC mount, so it shouldn't be a problem finding something to replace them. Try cleaning them first if they are noisy.

Thank you very much for your help. It's a good tip that transformers rarely go bad. I will focus on other parts first during my troubleshooting.

[Helmholtz]

Your volume pot might have a bad/broken ground connection.

[Mick Bailey]

Agreed. With the amp switched off and discharged, check with your DMM from the pot wiper to chassis ground to see if it goes to near-zero ohms when fully counter clockwise.

[Enzo]

And it may not be a bad part at all, especially as described, that could easily be a broken solder connection between parts.

Troubleshooting is not testing parts to see if we find a bad one, it is systematically isolating the problem. This is not a complicated circuit. SO when it does this, does your B+ voltage jump too or does it stay steady? If you apply a steady signal to the input, does the signal at the plate of either triode jump? There are test points on the schematic, are they stable?

You have a pull boost switch on the volume control. ANy chance that switch is failing?

[0xDEED]

Thank you all for your helpful comments!

Let's give names to the states:
OK = the amp works as expected
NOT_OK = the amp has very high audio level

Enzo, you said it could be a broken solder connection between parts. Given that the problem manifests only sometimes and there are no interim states (it's either fully OK or fully NOT_OK) would this mean that the "broken solder" sometime makes full contact (as it should be) and sometimes not? Which means that tiny movements of that solder make it loose connection? Is that a common behaviour? (I don't have much experience with electronics, I am trying to learn)
I am thinking that the switch movement ON==>STANDBY itself could be causing a vibration that is strong enough to move that bad solder? (It's quite a "heavy" switch)

Also, the pull boost switch seems to be irrelevant to the problem, i.e. at OK both PULL and PUSH positions work as expected, when at NOT_OK both PULL and PUSH positions have high volume (albeit different level, i.e. one of them is boosted)

[Enzo]

OK, so we find the pull boosts works, even in failure mode the switch still boosts the sound, so it is not at fault. We are not getting stuck in boost.

Can solder be intermittent? You bet it can. Vibration happens, but also thermal expansion of the metal wires happens. It doesn't take much. The standby switch does not appear to be soldered to anything, so that is safe.

And it doesn't really matter if something is common. Whatever it is is happening to YOUR amp right now. Whether a hundred others all have the same problem or if it never happened to anyone else ever, really doesn't matter. We are fixing what we have before us. Or another way, statistics do not apply to individuals.

[0xDEED]

Enzo thanks so much for your feedback!


Folks,

I am thinking of this approach first:

1. unplug amp
2. open amp carefully (take all safety precautions)
3. discharge capacitors (take all safety precautions again)
4. visually check for any obvious loose connections (especially the ground of the pot, as Helmholtz and Mick Bailey mentioned)
5. electrically check connections and elements using my DMM

For the moment I would prefer to avoid/postpone checks while the amp is live. I hope the above will not be a waste of time.

Do you think my approach is correct?

(Note: I have never tried to troubleshoot a tube circuit before, I have only done some low power low voltage circuits before, so I am very cautious with high voltages etc)

[Mick Bailey]

That's what I'm referring to in post #10. Always disconnect and discharge the amp when carrying out this type of check - apart from safety, you don't want any voltages present that may skew resistance readings. I like to take a measurement before anything else so I can be sure of what the fault is before tracing it. Sometimes even moving a wire can 'fix' a fault before you get chance to determine what it is (or was), especially with an intermittent problem if it's the result of a crack somewhere. Apart from visible problems, pots can be intermittent where the terminal connects to the track - either by crimp or rivet, or the wafer that carries the resistive track can crack. Of course, it could be something else other than the pot ground, but that's a likely cause at this stage. Different problems can produce the same symptoms so it's worthwhile keeping an open mind when troubleshooting.