VHT Special 6 replacement parts

[0xDEED]

Just opened the amp, haven't done any proper checks yet, but here's a photo of the push/pull pot next to the tone pot. If anyone can recognise what kind of push/pull pot this is please let me know!

It's probably something like this: https://uk.farnell.com/bourns/pdb183...ain/dp/1867163

... but 1M (based on the schematic) instead of 500K?

(I will start checks with my DMM in the next few days)

[Mick Bailey]

That's most likely a 16mm Taiwan Alpha pot. You need to remove the knob to see what the shaft type and length is. The bushing looks to be 7mm. The Bourns pot has a 3/8" bush so the mounting hole would need to be opened up, but Bourns is a good brand if the shaft is correct and would otherwise fit and work. The Bourns is 500K, which is OK instead of the 1M pot specified. Be aware that there are many pots advertised as 'Alpha', but they are junk. Taiwan Alpha pots are far superior to the Chinese and Korean tat that floods the market.

[Jazz P Bass]

Quote of the year: "We are fixing what we have before us. Or another way, statistics do not apply to individuals."
Thank you, Enzo!

[0xDEED]

Ok I think I am getting somewhere...

The volume pot has a total resistance of 66.7KOhm (not ~1000K as it should). The one of the pot's terminals is connected directly the ground as it should. Which could maybe mean that:

1. the pot is broken (internally)
OR
2. the pot short circuits via a different resistance to the ground (the R22 is 68K, which is connected to the push pull switch and to the ground and is suspiciously close to my measurement above).

I will continue the investigation tomorrow. I am not sure yet if the problem is out of the pot (the connections) or inside (faulty pot)...

[The Dude]

You are reading across the resistor in the circuit. To properly measure the pot resistance, you'd have to unhook one end of the pot. However, that's not really relevant here. What's important is that the pot wiper is at near ground potential when the pot is fully counterclockwise as Mick suggested earlier. Have you tested that, yet? Also, you said earlier that the pot was "scratchy". That is a clue. Have you tried cleaning the pot?

[0xDEED]

You are reading across the resistor in the circuit. To properly measure the pot resistance, you'd have to unhook one end of the pot. However, that's not really relevant here. What's important is that the pot wiper is at near ground potential when the pot is fully counterclockwise as Mick suggested earlier. Have you tested that, yet? Also, you said earlier that the pot was "scratchy". That is a clue. Have you tried cleaning the pot?

Thanks for your advice! Looking at the schematic I understand now exactly what you mean. I will check the wiper at counterclockwise to see if it's near ground (having first unhooked the ungrounded end of the pot that connects to C11, R10, R12).

The pot is not scratchy but it is very easy to turn compared to other pots. That's what makes me think that it could be broken. Which product is good for cleaning pots?

[Mick Bailey]

Don't unhook anything at this stage - all you need to do is connect your DMM to the wiper (centre lug) and chassis ground. Check the reading with the pot at zero and report back. Deoxit or Kontakt PR is what I use for cleaning pots.

[0xDEED]

Don't unhook anything at this stage - all you need to do is connect your DMM to the wiper (centre lug) and chassis ground. Check the reading with the pot at zero and report back. Deoxit or Kontakt PR is what I use for cleaning pots.

Done. The wiper shows about 930K at 0 position and about 67K at 10 position (and roughly logarithmically in between) so I guess it works well.

I have no idea where to go next. I need to think harder

[The Dude]

The wiper should go to ground or near ground in the fully counterclockwise position if one end is actually connected to ground. Either ground is not connected, a measurement error/misunderstanding, or the pot is bad. Are you measuring wiper to ground?

[Mick Bailey]

With the pot at 0 position you should read 0 Ohms (or just a few ohms due to meter lead resistance etc). At 10 you should read the full pot value - close to 1M Ohms.

[0xDEED]

First of all, let me tell you that I am learning so much from this process and I feel grateful to all of you!

So, I measured the wiper again (the middle ear of the pot) against the chassis.

It does show about 930K at 0 position and about 67K at 10 position. Looking at the schematic again I believe that measurement makes sense if the circuit is broken at the X mark I am showing below:




When VOLUME is 10:
The path to the ground is R12 + R13 + R15 (and +R22 when the pot is at PULL position) So the 66.7K measurement makes sense (which becomes about 135K in the PULL position)

When VOLUME is 0:
Only the resistance of the pot is added to the above mentioned path, and that's why it increases to about 930K (the pot is rated 1M)

Next I checked if the left ear of the pot is indeed connected to the ground. It is. Which makes me think that the connection is broken INSIDE the pot (the RED X ABOVE is inside the pot)

Is my thinking correct?

If it is correct, I need to get a new pot. If I do need to get a new pot, would it matter too much if I get one that is rated 500K (i/o 1M)? I can't really find 1M in the market that matches all the characteristics I want.

Thank you all!

[Helmholtz]

Yes, that's what I meant in post #9. Perfectly explains your symptoms.
Regarding symptoms it makes no difference if the broken connection is inside or outside the pot.

A 500k audio pot should work.

[0xDEED]

What I realise now is that it's impossible to find genuine Alpha Taiwan pots in the UK. There are plenty with the same geometry (shaft 6mm, thread 7mm etc) but they are clearly not genuine (probably made in China). I think my only good quality alternative is Bourns, but I may need to make the panel hole a bit bigger to accommodate the larger diameter of the thread.

If you have any recommendations for good quality push pull pots (that are easy to solder like the one I already have), please let me know!

[Greg Robinson]

This style of pot is more commonly used in guitars than amps, so I searched "guitar parts uk", and found this. The bushing may be longer than what's in the amp now, but you should be able to use a second nut on the inside to adjust for the correct length if so. Available in 250k, 500k and 1Meg, so you can stick with the stock value.
Axesrus UK

[Mick Bailey]

I had a run-in with this company and submitted photos to Alpha for verification, as the parts supplied were poor quality. Alpha said they were not made by them. Alpha pots have the company name stamped on the body.