Variable resistor for 6sn7 grids??

I have a type of push pull limiter I picked up and the low frequency was creating some strange behavior from motorboating and chirping when the threshold was turned. . It's a modified Hammond pre amp/limiter....
Anyhow, the quick fix apart from getting some frequency limited transformers was to put some 100k resistors in parallel to the 1meg resistors that are on all 4 of the 6sn7 grids going to ground. This cut the low end pretty good but I was wondering and tried to put some  pots in instead of the resistors so I can bring back some of the low end and maybe be able to balance things out a bit. I figured if I could put 4 pots total (1 for each grid) I'd be able to have more control over the frequency this unit is having to work with and play with the balance at the grids a little...... If that even sounds right....

Ok.... I'm dumb.....  :-[

Can you just put 1 meg pots (pin 1 and 2) on the grids with a 100k in series on last pin of pot to ground?????



Do I need to worry about wattage ratings????? Other stuff??? I'll experiment....

unmodified schematic is attached....the 6sn7s are the same....

Let me know if you need more info....

Thanks!!!

Messing with the 6SN7 grid to ground resistors is not going to help you.

Take some pictures of the unit top (or a view of the tubes) front and innards. List the tube types.  This will at least give people an idea of what kind of circuit it is.
If its motor boating when your trying to compress that means its probably a varimu type limiter and the variable mu tubes (most likely not the 6SN7's) are mismatched. One is stronger or weaker than the other. Usually there is a cathode balance pot for varimu limiters that, in simple terms, wipes the ground between the two cathodes of the vari mu tubes so you can balance out the current draw between tubes. Some times the tubes are just too mismatched for the pot to work and you need a new set of tubes.

Again pictures would help. There might be a screwdriver adjust pot somewhere on the back or inside that says "balance" you would want to turn this while playing music and compressing, if the tubes are still good you should be able to twist out the thumping.

You don't want to find frequency limited transformers. People want to avoid those! The unit probably just needs some tube love and possibly some new filter caps.

Get us some pictures.

Bwhahahahah. Scott that's a real hum dinger Lol. I've been working on tube stuff my whole life and Its hard for me to work out whats going on with that circuit. Maybe Doug or PRR can give a good description of that original circuit but it definitely was not a compressor as you said, but I can't figure out a circuit even close to that that would produce good compression.

You really would have to trace out what was going on in there for anyone to help with the current circuit.

I have a better idea...

I would gut the whole thing and build a Federal Am-864 compressor. It pretty much has the exact right amount of tube sockets, a 6SN7 output amp just like the Hammond. Its really not that complicated of a build. On the same level as a guitar amp.

I think you could do it with everyone's help.

Here's the schemo

What's with the vibrator connection? Was this some sort of preamp for a car radio? Doesn't look like a limiter to me, either.