Tweed Twin scratchy volume pot on guitar

[Randall]

I have a 5F8A clone here that came in for a harsh spiking noise on low loud notes. One tube was red plating, and another was very microphonic. He brought a spare set of vintage tubes that he thought are matched, but they are not. I swapped out the set, and now the bad noise seems to be fixed.

But now, and this was the case with the original set as well, when I turn the volume pot of my guitar it is very scratchy in the amp, both normal and bright channels. Pretty sure it's not the guitar's fault. I am at a bit of a loss to understand why this is. Anyone have any ideas?

I measure 0.25 vdc on both .02 coupling caps on the volume pot sides.

http://www.prowessamplifiers.com/sch...5f8a_schem.pdf

[Helmholtz]

Though very uncommon, it sounds like there is DCV on the input grid. Could be caused by a leaky/gassy input tube. Try to measure or replace the tube. For a test you can use a 12AX7. Also cathode and plate voltages could give some clue.

[Randall]

Replaced 12AY7 input tube with a bench 12AX7, no change. Cathode voltage is 1.9vdc. Plates are 225vdc and 246vdc.

[Helmholtz]

Plate voltages look too high (schematic says 145V). What is the supply voltage? Measure plate and cathode resistors.

Some problem with all inputs (to different degrees)?

Is the chassis safety grounded?

Does the guitar have active electronics?

I measure 0.25 vdc on both .02 coupling caps on the volume pot sides.

Looks like somewhat leaky coupling caps. Is the DCV at the vol pot side positive or negative wrt ground?
But I don't think that's the reason for the scratchiness.

[nosaj]

If you unplug your guitar what voltage is on the instrument cable plugged into the amp?

nosaj

[Randall]

B+ is 426vdc. Cable measures 0.9vdc. Chassis is well grounded. Same issue in both channels. Guitar is not active, it is a cheap Squier strat.

[pdf64]

...Cable measures 0.9vdc. Chassis is well grounded. Same issue in both channels. Guitar is not active, it is a cheap Squier strat.

Is that still the case if the amp vol controls are turned down to minimum?
To eliminate oscillation as a cause.

[Helmholtz]

B+ is 426vdc

I did not ask for power amp supply but for input tube supply. Should be around 300V.

Cable measures 0.9vdc

That's definitely not normal and could be the problem. Is the voltage positive or negative wrt ground?
Does the voltage disappear when you pull the input tube?

[Randall]

Yes, still the case,0.88v with volumes down.

[Enzo]

Well that is what is making your guitar knobs scratchy. ANy chance contaminated eyelet board?

[The Dude]

It's a clone so maybe not, but aren't the connections from jacks to input tube grids off the eyelet board- straight from jack to socket?

Are jacks wired correctly and well grounded?

[pdf64]

It's a clone so maybe not, but aren't the connections from jacks to input tube grids off the eyelet board- straight from jack to socket?

Are jacks wired correctly and well grounded?

The 68k input mixers look to be board mounted; so there’s opportunity for a crappy board to leak some voltage onto them.
https://el34world.com/charts/Schemat..._twin_5f8a.pdf
Hence the query of post #8.

[Leo_Gnardo]

pdf64, Enzo & The Dude point to a good possibility. Try a "home run" wire straight from input jack. What I do in cases like this, also in new builds, mount 68K stopper right on the tube socket. Skip the board entirely. See where that gets you. A quick cheap experiment that may pay off. Those leaky boards!

[Chuck H]

Since the input grid (that connects to the instrument cable) is virtually decoupled from the circuit by the grid/cathode and grid/plate spaces in the tube the only possible causes would be a bad tube or a semi conductive board. Since you tried a different (and I assume known to be good) tube that only leaves a semi conductive board. There's a possibility of contaminant on the board surface causing the problem. Dirty flux residue that should not be conductive, but as with all things that might have been cheapified for lower production cost made in unregulated overseas markets, and we cannot know this isn't the case. You could try cleaning the board surface in the areas between the input tube plate resistors and the grid stop resistors. That said, I've been seeing more reports of new boards being semi conductive in just this last year. Another example of the unregulated overseas manufacturing I think since that problem didn't exist with newer black boards until recently.

If cleaning the board surface doesn't help I think Leo has the best solution. Remove the grid input circuit from the board.

RANT: Why did anyone ever think tinting a circuit board with carbon (all black ink is made with "lamp black" which is carbon) was a good idea. Especially back in the day when most resistors (a semiconductor) were made of carbon!?! And somehow the problem persists because now unknowing people, techs, players and other mojo hunters want to duplicate the vintage circuit. Which is equally obtuse since the problem is well known now. Today whenever one of these black fiber boards is used it's because someone knowingly did the wrong thing for $$$. That's technically criminal isn't it? But as criminals have been known to say, it's only a law if you're caught breaking it.

[Leo_Gnardo]

Today whenever one of these black fiber boards is used it's because someone knowingly did the wrong thing for $$$.

This ^^^. One of the perpetual offenders is Mojo. There are others besides. My advice on kit builds: avoid these boards completely totally 100% or suffer the consequences. Randall was put through the mill in a major way when he used one of these carbon-black boards in a Deluxe Reverb build a couple years ago. Rebuilt on a turret board made of proper non-conductive material, no further problems. What a long fezzle...

I'm looking forward to Randall trying out the home-run on this Twin and reporting "bingo! that was it!" Sure hope so.

Sure hope the red-plate problem you reported Randall in the original post wasn't also due to leaky board material. That stuff can easily upset bias in any of the preamp tubes as well. It's circuit poison. Cleaning the board, heat treating with hair dryer or heat gun, all well & good but as far as I'm concerned those are temporary solutions. Very iffy especially in amps used for serious live & studio work. You don't want those to fail and every means should be used to ensure they do not.

And thanks for your support Chuck!