Klark Teknik KT-2A DC on Gain pot with any 12AX7 other than Stock

[bluesprocket]

Hi All,

I've got me a bit of a brain twister.  I have a pair of Klark Teknik KT-2As that belong to a friend.  He asked me to take a shot at "upgrading" them with some NOS tubes and some cap upgrades.  First stop, cause its easy is popping a few different tubes in there to see what the difference is. 

So, using tubes that I've tested and they test "fine" on an EICO tester, and seems to play nice in a little 15W guitar amp I've got sitting next to the bench...all of them cause the Gain pot to make the "sound of the ocean" when turned.  Odd...

So, I start poking around.  With the stock 12AX7 in the signal path, there is no measurable DC on the wiper or high side of that pot.  And turning the pot is quiet.  If I put ANY of the other tubes I've got lying around in that same position (NOS GE and RCA, new JJ and EH...I've tried more than a couple) all of them end up with around 60-100mV of DC on the pot, and the delightful ocean sound returns.

It looks like the signal path and sidechain are more or less the same as the schematics I have for an original 2A, and I'll start chasing this down bit by bit.  But wanted to see if anyone might have an "oh check this thing out, because its the almost always the culprit."

I tried pulling the T4 out to see if anything was coming from there...and nothing changed.

I appreciate your time and any thoughts you might have.

Thanks!

-Chris

 

[Khron]

Posting a drawing of that piece of circuit might ease the flow of ideas from us lot?

Or if it's wired just as a volume pot, would adding a DC-blocking cap upstream of it be an option?

 

[analogguru]

Put a capacitor >100nF between the wiper and the grid and a 1M resistor from grid to ground - this should cure the problem.

 

[bluesprocket]

Posting a drawing of that piece of circuit might ease the flow of ideas from us lot?

Or if it's wired just as a volume pot, would adding a DC-blocking cap upstream of it be an option?

Here's a snippet of the schematic for the original unit.  Everything is PCB in the KT one, including the pots, but I might be able to work something out.