How stable can you get an Express?

[RockinRocket]

Yes it looks like you followed Kens layout minus the cap cans. Your gunna need to get your hands dirty if you want to fix it..

[Matthews Guitars]

Checking and resoldering all tone circuit connections has caused no change.

Hmm.....why would M and B act as if not in the circuit at all? When T acts as a volume control?

[sluckey]

Hmm.....why would M and B act as if not in the circuit at all? When T acts as a volume control?

If the bottom lug (left in your photo) of the treble pot were shorted to ground, the tone stack would behave exactly as you describe. Check with your meter.

[Matthews Guitars]

Found it. I'd installed some shielded cabling to reduce noise and found that I had a shiner shorting that cable to ground, thus grounding part of the tone stack.

Fixed now.

[Matthews Guitars]

Replacing those two short wires that WERE shielded cable with unshielded wire made enough of a difference in noise level that I'll be going back to shielded cable, but installed VERY carefully.

[Matthews Guitars]

Shielded cables reinstalled, more in use now than before. This has helped the noise floor even more.

I'm far from an expert on this kind of amp but I recommend using as much shielded cable in the signal path as possible,
and I don't hate my choice to use two multi-section canister caps rather than the six individual axial leaded 20 and 40 uF caps.

If I build another one I want to experiment with putting the OT on one end of the chassis and the PT on the other. Makes for a better
mechanical balance and should reduce transformer crosstalk. Can't say if that'll take any magic out of the tone but I'd try it.

Even understanding what's going on in the circuit, I'm still amazed at the level of drive this amp gets when one half of one preamp tube
isn't even in the circuit!

It really has a thoroughbred racehorse kind of feel to it. It WANTS to be run hard. I absolutely love it! When circumstances allow, the only
way to play this amp is turned way up.

It's one of my two top favorite amps, the other is my 1973 vintage Marshall 20 watt Lead & Bass, model 2061. Which actually does a similar thing.

[rdavy]

A grid stopper in front of the first gainstage adds hiss. I prefer a 100pf from the anode to the cathode instead. Remember these amps were designed for high capacitance cables that lowers the treble peak of the pickups into the mids

I had a 22K grid stopper on the first triode and had quite a bit of instability (oscillation with volume full up). I added the 100pf snubber cap from plate to cathode and reduced the grid stopper to 2K2. This made a huge difference. Thanks for the tip! I may try it without a grid stopper, per your previous suggestion, to see if it improves further.