Odd behavior of OD Drive pot - need help "solved"?

[mlp-mx6]

250KA pot in OD drive position. HRM (switchable, but tuning for HRM tone), if that matters.

Here is what happens with no guitar plugged in and the master turned up: Up to about 10 o'clock it functions normally, the hiss/hash gets louder. Above there it actually suddenly diminishes. If I plug in and strum, I can still play it turned up, but as I hold a note it sputters a bit then goes WAY down in volume. It is acting kinda like grid blocking when I turn up the pot. If I turn it back down it resumes normal behavior.

I replaced the pot in case the pot was bad. No change. I replaced both wires going to & from the pot (both wires are shielded). No change.

Any ideas?

[novosibir]

Any ideas?

Yes!

Leaking coupling cap!

Is your pot scratchy by tuning? What proofes this!

Larry

[mlp-mx6]

1. I wish it were. I replaced the coupling cap, and the same behavior is there.

2. The pot is a little scratchy because the pot actually is the grid leak resistor (in series w/ the 180K grid stopper) at that point in the circuit. It has a small negative voltage on it.

[novosibir]

Try another tube in the OD section!

I've had this problem with some NOS Tesla and also with JJ's - enhanced grid current.

Much success!

Larry

[jelle]

Did you replace the tube?

Edit, Larry beat me to it.....

[mlp-mx6]

Yes. First thing. Multiple tubes.

It has me baffled.

[jelle]

Parasitic oscillation/ crosstalk?! Can you post a pic?

Maybe I'm first this time, Larry's should be asleep already!

Jelle

[novosibir]

... Larry's should be asleep already!

No! Still about 5 hours My sleeping time usually is 4:00 or 5:00 a.m. German time

Back on topic:

- Creeping path in the preamp tube socket?
- Bad filter cap?

Larry

[mlp-mx6]

OK - new information. It appears there is an oscillation that is killing me here. If I watch the power tube current it jumps up by 20ma just by turning that pot, with no guitar plugged in.

So - good news, I know what it is. Bad news, I know what it is...

[mlp-mx6]

Did I mention that I really dislike chasing down these things?

Grrrrr.

Pointers, anyone?

[Bob-I]

Did I mention that I really dislike chasing down these things?

Grrrrr.

Pointers, anyone?

Start by posting some pictures, then use the chopstick wiggle wires method of locating the area. Use a scope and watch for changes as you move wires.

If you used a tried and proven layout it should be easy to find. If you made up your own layout it may be more difficult.

[mlp-mx6]

Yeah, this is my own layout as it is a "TripleDrive" type - dedicated clean, plus clean/OD (with switchable HRM), plus Dumbleator built in. The biggest challenge is the length of the runs from the OD tube to the OD controls. I recently rewired this amp to have a BM approach to the HRM, and changed the pots to be 250K/1M instead of 100K/100K. (HRM pots, rather than non-HRM pots)

I have isolated the oscillation location to be OD1. Using a tip from Gil Ayan in another thread ( https://tubeamparchive.com/viewtopic.php?t=3320 ) I put a .001 cap across each plate resistor sequentially and found that around OD2 the oscillation went away, but also around OD1 it did. Therefore the oscillation is around the OD1 triode.

I will post pictures as soon as I can.

[Bob-I]

Yeah, this is my own layout as it is a "TripleDrive" type - dedicated clean, plus clean/OD (with switchable HRM), plus Dumbleator built in. \\

I've done a 3 channel too so I know what you mean. I'm in the process of reworking it. After so many mods I've decided to put in a new main board.

Do you have the leads to the OD tube shielded? I found it was absolutley necessary on mine.

[mlp-mx6]

Bob, it's so cool that you did one of these too.

I've "solved" it for now by turning down the OD trimmer. Max the volume, max the OD drive, and turned down the trimmer just below where the oscillation started.

And, yes, the wires are shielded.

I'll turn it up later this weekend to see if it still sounds like I want it to. (almost 11pm here, so I don't want to have angry neighbors)

I have thought about a new mainboard also. I did the board for this in 2 pieces - PI and Dumbleator on one board, all the preamp stuff on the other. Still thinking... but for now it is not oscillating. It is so nice to have a scope to see this stuff with.

[Fischerman]

The amp I just fired up for the first time has HRM pots on the front panel and I also have some pretty long runs from the board to the pots. I also used shielded for all those runs. Could the wires at the controls be a possibility? Like the wire from the HRM treble wiper to the OD Master? I have a 100k resistor between the HRM treble wiper and the OD Master...installed right on the OD Master.

Just thinking in terms of phase...the output of the entire OD circuit is in phase with the input to the OD circuit so maybe look for ways that the OD OUT could possibly be 'coupling' with the OD IN?