TRex 18watt, gating/farting/blocking?

[soma_hero]

Been posting on this on the 18watt forum but may need more opinions:

Built an Trex, which is an 18w lite channel and a Trainwreck style channel into an 18w power section. http://mhuss.com/18watt/schematics/18wTRex-S.pdf

I'm getting close to done, but nagging issue here. On the Trainwreck side there is a gating effect like a misbiased or misreferenced tube. When I roll the guitar vol down or let the note ring out and die, it cuts out in a farty manner.

If I roll the treble down and mid up the problem goes away. Any ideas?

This is a sound sample. Treble is turned down and mid turned up at about 48 seconds.

[Blindog]

It sounds like a high frequency oscillation to me. I had a Plexi build that would oscillate when the presence and treble were turned up, especially with a Strat. Check your lead dress and chopstick around.

Also, do you have a shield for the bottom of the chassis? Sometimes that helps HF oscillations. My 2 cents

Hope you get it fixed. Sounds like it could be cool amp!

MArk

[John_P_WI]

Sounds like you are missing a grid leak to ground somewhere.

[dartanion]

Post some picts please. It'll be much easier to help you diagnose the issue.

[soma_hero]

Okay pics. Don't judge me, my other builds are much neater I promise! I tried to shoehorn this into a small organ chassis and my own layout. So things are a bit cramped. And V5 is rotated 90 degrees to use existing mounting holes.

I reduced the stage one cathode cap to 2.2uF and increased the grid stopper between stage two and three from 150k to 470k. As long as the gain control isn't maxed out, I think the problem is okay.

Any recommendations as far as cleaning things up if needed?

[Structo]

Well you have more room in your organ chassis than I did in my Hammond.

Looks pretty cool.

You say it does this only at certain tone settings.

So you should look there. What size slope resistor do you have?

Or it could be a high freq parasitic oscillation that is causing the blocking.

Do you have a scope?

[soma_hero]

Yeah it did this when the Treble was down and the Mid was up, with Vol up. With the changes I made it seems to be fine now. And no I don't have a scope. Wish I did though...

Slope resistor is 100K.

[flood]

hey,

i'm having similar problems with my 2061x build.... see https://tubeamparchive.com/viewtopic.php?t=8666

this thread is a great help though. will drop the cathode bypass cap on the lead channel to about 680n or 1u and see how that helps. also, the grid stopper in the switch path will be increased a bit. any other tips? thanks in advance.