How stable can you get an Express?

[Matthews Guitars]

I think I've done a pretty good job of making my Express build pretty stable, but I know there's room for improvement.

As things stand now, I can dime the amp, all controls, with the head isolated mechanically from the speaker cabinet, and not get any oscillations. Provided that the V1 preamp tube isn't TOO sensitive. A really hot preamp tube like a Chinese square getter type will cause oscillation at less than maxed out settings.

How good should it be possible to get it? And what are the best proven stability enhancing strategies for an Express?

[xtian]

As things stand now, I can dime the amp, all controls, with the head isolated mechanically from the speaker cabinet, and not get any oscillations.

You did it! Achievement unlocked!

[Roe]

One of mine is quite stable, not just due to lead dress but a few grid resistors:
gainstage 1: no grid resistor
gainstage 2: 820R
gainstage 3: 68k
PI: 68k (on each triode)
el34s: 47k (on each grid)
The result is less blocking distortion and tighter bass as well

[dblgun]

I suppose it would depend on the reason for it becoming unstable. As mentioned, lead dress and a grid resistor can help. The second stage (V1 p1/2) can sometimes benefit from a small amount of capacitance between the plate and grid. Marshall did the "hot shield" trick tying the braided shield of the coax to the plate. I'm not real big on putting plate voltage on the shield for safety sake. Try putting a 6-8pf 1000v volt cap between p1 and p2, it often does the trick and could allow you to use the other tubes.

[jrdamien]

Swapping out the preamp tube for something more stable—like a Sovtek or JJ—could really help with keeping things smooth. Those Chinese square getters you mentioned can be a bit tricky, especially in a hot circuit.

Also, have you thought about adding some grid stoppers? A small resistor at the grid of V1 can really help with taming any unwanted feedback

[dblgun]

These circuits do prefer some tubes to others for sure.

[Roe]

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

[Stevem]

Amp build neatness and function are often at odds with each other.

A very very important rule of thumb is grid wires should be short .
Extending a plate wire so that the down stream coupling cap can be soldered right to the tube socket can cure a lot of problems.

[Matthews Guitars]

The way I built this, I built it to keep leads carrying signals as short as possible. I specifically built the preamp board so that there would be no reason EVER to have to lift the board to
access underneath it. All possible service can be done from the top side. This allows me to use very short wires to the board.

[Matthews Guitars]

The amp does have a brighter than needed top end. Much more brilliance is available than I'll ever need. A little careful taming of the top end frequencies should be useful
as a means of improving the amp's stability without negatively affecting tone.

[Matthews Guitars]

Got to figure this out. All controls worked when I built it, now the treble control works but bass and mid literally do nothing. Presence is OK.

If I turn down treble, mid, and bass all the way, the amp goes silent. If I then turn bass and mid back up, the amp remains silent. Treble is both treble AND a volume control.

I'm thinking tone caps but I'm not measuring any leakage in them.

All caps on the board are MKT1813 630 volt Vishay caps. Good quality that should be totally good long lasting parts.

[RockinRocket]

My mind was a bit blown when you said amp was stable with all controls maxed on first try.
Lets see some guts? Maybe you got a cold joint somewhere.

[Matthews Guitars]

I was pretty happy with the stability at first sound, too. But that was with some pretty low gain preamp tubes.
When using higher gain tubes I had to start playing with wire dress and ended up adding some shielded cables,
so now it's stable up to a very respectable gain point. I can make it oscillate but not at settings I would actually be playing it at .




https://marshallforum.com/media/img_0147-jpg.14948/full

[RockinRocket]

That sounds about right stable where the amp sounds best.

Id say Vol pot ground looks wonky at the wiper and you over did the solder at the buss bar at that point. Ken bent the wiper lug over and soldered directly to the pot.
Id reflow all the connection in the tone stack and pots. At fist glance it looks wired correctly.

[Matthews Guitars]

The ONLY wires under the board are from the supply turrets (four in a row at the end of the board) to their respective nodes. So there's nothing under the board that could be in play.

I used canister filter caps (triple 20 and triple 40 values) rather than the "traditional" six pack of axial caps.