Missing Treble Control - Fixed!

[Chris-in-LA]

So treble is being reduced at full max or minimum?
If so that’s weird, have you got a scope, checking for oscillation may be a good idea?

No scope. The reduction is very slight but it’s something I’ve seen on a few of my tweed builds. We’ll see what happens when I get the new pot.

 

[Pete Farrington]

Tweeds (which includes Marshalls!) typically don’t have grid stoppers at their output valve socket terminals.
Or even at their input valve socket terminals.
Vintage practice may have just about worked with vintage valves etc, but maybe won’t with current production stuff?
Grid stoppers mounted at grid terminals at the socket please, on the input and output stages at least - you know it makes sense

But really, if you’re building / fixing amps, stop working blindfolded, bite the bullet and get a scope!
A Velleman / Picoscope etc USB dongle takes up next to no space, will work with your PC, and can be incredibly helpful in getting your head around this stuff.

 

[anitoli]

Scopes are CHEAP on ebay.

 

[Chris-in-LA]

Tweeds (which includes Marshalls!) typically don’t have grid stoppers at their output valve socket terminals.
Or even at their input valve socket terminals.
Vintage practice may have just about worked with vintage valves etc, but maybe won’t with current production stuff?
Grid stoppers mounted at grid terminals at the socket please, on the input and output stages at least - you know it makes sense

But really, if you’re building / fixing amps, stop working blindfolded, bite the bullet and get a scope!
A Velleman / Picoscope etc USB dongle takes up next to no space, will work with your PC, and can be incredibly helpful in getting your head around this stuff.

You’re right. I don’t really know that much and I struggle to wrap my head around some of the concepts. In the 2204 kits, they add a 5.6k resistor to pin 5 as the grid stopper, right? I did build this Bassman as a 2204.

I did some research and it looks like 1.5k is what Fender uses for grid stoppers with 6L6 tubes.

 

[mickeydg5]

Treble is always truncated at minimum and maximum. It is the nature of the beast.
The Bassman uses 6L6/5881 types and 1.5k on the control grid suits it due to operational characteristics. I actually use 2.2k.
EL34 do better with higher resistance due to their characteristics in general.

 

[Chris-in-LA]

Again, @Pete Farrington is a genius. The Bassman needed the grid stoppers to work as a 2204 circuit. The tone controls are more responsive, the treble finally actually works. Even the bass pots seems to work better. But since the treble pot is audio taper, it really only works between 7 and 11. I’m waiting to see if Mojotone responds to my request for a 250KL pot to replace the one they sent. They are usually pretty responsive. They sent me a new PT for this amp when the first one developed a short, no questions asked.

 

[Chris-in-LA]

The 250KL pot that I ordered from Valvestorm arrived today. No word from Mojotone about my complaint about having the wrong pot in the kit.

 

[Chris-in-LA]

Thanks again to @Pete Farrington I just installed the linear pot and the tone stack works as it should.