Express New Development.

[passfan]

I just noticed a weird oscillation in my express. As I turn the volume up at 9:00 I get a dull thump and in between 11:00 and 1:00 I get this wine that sounds like and old radio being tuned. It changes in pitch as I go up and disappears around 1:00. You'd swear a radio station was dialing in. At 3:00 it starts again at an even pitch (around 14k) and continues all the way up. I've swapped out tubes all through the preamp. I've added a 22k on v1 grid and added a 15k on v2 grid, nothing stops it. The amp still sounds great when I play it just when I stop I can hear the whine.

[passfan]

Okay I pulled v1 and it all went away , hiss, oscillations, everything.

With v1 in, the treble and mid and presence controls have an effect on it. If I turn either treble or mid down it goes away. The bass doesn't effect it. The extreme bright setting makes it go away.

[ampgeek]

Sounds like something in the bright switch area.

Can you make the offending character change by prodding around the switch and caps?

Interesting/timely topic as I was going back through my build and Ron's build guide (cleaning up wiring etc..) I observed a trebley oscillation at various knob settings. Sure enough...I had failed to "goop" the two bright caps together as directed. I squirted some silicone in there and mashed them into the goo to cure for the night. And...bingo...this morning there are no signs of that condition.

Cheers,
Dave O.

[passfan]

I just discovered something odd. I've been messing around with this chassis out of the cabinet. I stood the chassis up vertically, with the transformers on the bottom and the condition disappeared. Freaked me out so I sat it down again and it came back. Stood it back up and it's gone. Now when it does it you have to be close to hear it as it's not loud but at 14k all I need is a hint and my ears hurt.

I went a little further and discovered that if I take the chassis off of the 2x12 cabinet I just built it stops doing it. I must have gotten the preamp section far enough away from my top speaker when I sat the chassis up vertically. What would cause that?

[geetarpicker]

You need to have a ground plate in place on the bottom of the amp if you crank up the gain much at all. You definately can get oscillation issues from a Liverpool/Express if it's on top of a speaker cab without any shield plate. The originals had a metal plate screwed directly onto the chassis to form a sealed box, however most builders use an open chassis and attach a ground plate to the inside of the cabinet.

[passfan]

You need to have a ground plate in place on the bottom of the amp if you crank up the gain much at all. You definately can get oscillation issues from a Liverpool/Express if it's on top of a speaker cab without any shield plate. The originals had a metal plate screwed directly onto the chassis to form a sealed box, however most builders use an open chassis and attach a ground plate to the inside of the cabinet.

That must be what it is , I didn't have this problem when I had it installed in my combo but then the bottom of the chassis was mounted onto a rear panel which faced it away from the speaker compartment , thanks Glenn.

[JamesHealey]

I built my Express using a Marshall style layout without a bottom shield and there isn't a hint of oscillation or noise (minus usual TW hiss).. I'm wondering if Ken's layout was as well thought out as it could have been.

I mean I might have just been lucky but I've not had a single problem at all and it sounds fantastic, no band aids to the circuit or shielded wire other than input jack.

[passfan]

I built my Express using a Marshall style layout without a bottom shield and there isn't a hint of oscillation or noise (minus usual TW hiss).. I'm wondering if Ken's layout was as well thought out as it could have been.

I mean I might have just been lucky but I've not had a single problem at all and it sounds fantastic, no band aids to the circuit or shielded wire other than input jack.

Just one look at the astroid layout and you can see what must be the natural evolution of layout and grounding for these amps. I too haven't had the problems that most have (minus the cheap pots) and this one snuck up on me as it's barely discernable until you get up around 3:00 , which I just don't do. The one common factor is the placement of the chassis in relation to the speaker cabinet. I will add bottom plates now to my rocket , which is still on the bench and my express. I recently decided to put them in "head" cabinets and do away with the 2x12 combo because of the weight (too old to deal with that anymore). I recently finished 2 2x12 cabinets which I made stackable (and much easier to handle individually). Now I can go 2x12 for each head or drive both with one head (much more versatility). The downside, if at all , is that I have to put bottom plates on my amps.