New Express build with pictures!

[Cliff Schecht]

So as I mentioned, I finished an Express last night and fired it up today. I guess I wasn't too surprised to find that it oscillates at a relatively high frequency (still audible) although I took care to do a "proper" job on the lead dress, etc.. Today I tried different tubes (Sylvanias, Telefunken, Bugle Boy, etc..), tube shields and a 15k grid stopper on V2a, none of which tamed the amp much if at all. I'm pretty damn sure the problem is around V2a and the tonestack, chopsticking these wires would sort of tame these oscillations but I could never walk away from the amp and actually play it. There is audio coming through and the "right" sound is there, but of course it will never sound good with parasitic oscillations taking up all of the headroom.

So where should I be looking? I'm trying to kill the oscillations without having to resort to band-aids like shielded wire to V2a and the like. I haven't tried reflowing any of the solder joints yet, they look good and dandy, but maybe this would help? What should I be looking for/avoiding as far as lead dress goes around V2a and the tonestack? I'm thinking about pulling all of the tubes except for the first position to take a peek at this on the scope, but just throwing scope leads on there can also induce oscillations a lot of times, especially in such a high gain/unstable amp design. Anywho, any help is much appreciated!

Also, big thanks to Ron, David, RJ and Andy for helping me source parts and tubes!

[dartanion]

Does it squeal all the time or is it only with certain controls in certain positions? If it's all the time, swap OT primaries. If it's only certain settings, what are those? Also, have you tried shielding the chassis? I have built a few that are unstable on the bench without any shielding.

Post a few pics too.

[Cliff Schecht]

So yeah, I disconnected the NFB wire and it worked fine.. That means I flipped the damned leads, doesn't it... Grr, now I have to add some extra wire to the lead that I shortened too much. I guess I should mention that I did this build with an A3801 which I would usually be reluctant to even cut the leads on (I left them LONG in my last one) but this amp isn't coming apart so I said fuck it. Looks like I shouldn't have said fuck it quite so quickly..

[geetarpicker]

One day my original Express started to oscillate. The fix was to push the OT primary wires down tight against the chassis with a small amount of foam rubber right as they go up under the PS board. The foam has been there ever since.

I recently had one of my two clones start to oscillate after doing some rewiring though the lead dress was exactly the same as before when it had worked fine. The fix was to tighten the middle pot though it already seemed snug enough, somehow with the heat of soldering things weren't quite making as good a connection. Also when you use a wood front panel you even have less chance of a firm ground connection, plus you can't really tighten it up too far or you will crack the wood. I noticed that even though the pots and buss bar all tested as fully grounded with a meter, just a little looseness in the pot mountings still caused oscillation problems. I've also had this issue a couple times with my original but at the input jack. Even though it appears you could ground things more firmly in the design, my gut feeling is leaving some of the signal paths forced through the chassis may be part of the tone. Also of note I don't think you need shielded wire going to V2 if everything is done correctly.

Oh, I almost forgot I also had a intermittent oscillation issue with one of my clones 2 years ago that turned out to be bad solder joint on one terminal on the pots. It looked perfect but was broken. Check that area VERY closely as there is alot of gain going on there!

[Cliff Schecht]

Flipping the feedback wires made the amp stable at low volumes so good call there. I reconnected the NFB wire as well. The amp works at low volumes and can get some raunchy distortion up to a point but when I turn up the treble too high, it starts to cut into oscillation. The thing is the transformer wires are color coded like the Francesca layout and so I assumed that this was the correct way to do the leads (blue to V5, brown to V4) but turns out that was wrong.. Interesting.

But yes, definitely some progress. Now it's teetering on the edge of oscillation, this is especially evident when I tap the chassis with a chopstick while adjusting the treble knob. Up until a certain point, there is a resonance that dampens off at a rate dependent on where the treble pot is. But as I turn the treble up past about 4 or 5, the resonance builds slowly instead of dying off until the amp starts feeding back badly. Hrmm, still seems like something hairy in the early stage of the preamp. The bright switch affects the oscillations as well, i.e. if it's on the 500pF cap then it tends to oscillate rather easily. I also noticed that with my Strat on the single coil pickup I get some AM radio station coming through as well, but this goes away when I switch to the bridge.

I'll try to get some pictures posted tonight. I did a good job on the lead dress as far as keeping everything neat and tidy but there could be some little booger that I'm not seeing/considering..

[Cliff Schecht]

Good call Glen. I'll check the joints on the pots tonight. They are tightened firmly with star washers between the chassis and ground lugs (small ones that come on the AB Type J pots). Hope those little bastards aren't the cause, they would be a bitch to remove .

[Cliff Schecht]

Here are some pics. I think the build came out clean (as far as Express builds go) and so I'm hesitant to start pulling it apart blindly to find the problem. Lemme know if you guys see any trouble spots or the like.. Click the images to make the bigger.

[IMG2136]http://imgur.com/ht3PM.jpg[/img]
[IMG1922]http://imgur.com/zGBYl.jpg[/img]
[IMG1709]http://imgur.com/wmmmZ.jpg[/img]

[hairyandy]

Oh, I almost forgot I also had a intermittent oscillation issue with one of my clones 2 years ago that turned out to be bad solder joint on one terminal on the pots. It looked perfect but was broken. Check that area VERY closely as there is alot of gain going on there!

Yes, poke around the pot connections and the bright switch and see if anything is microphonic. I've had issues like that in the past as well.

Andy

[passfan]

On your input jack , I can't see your 1 meg resistor to ground from the positive. It possibly looks like your shield is connected at the shunt contact instead of the sleeve , correct me if I'm wrong. It does all you say with no cable plugged in correct ?

[redshark]

I would check the wiring on the input jack, where is the 1 Meg resistor and how its wired? . The leads of the caps that go to the bright switch are a potential anthena so check those wires too.
I would also check the way your pots are grounded. Eevrything else looks normal, why did you use a grid resistor on V1?

[Cliff Schecht]

Was trying to kill oscillation on V1 so I used a 15k grid resistor like KF did on the undocumented Express.

On the input jack, I'm using the shield to connect the shunt contact to ground. The resistor is going to ground as well.

I'll dig in again later and see if the tonestack is still acting up.. Thanks for the help so far guys.

[KellyBass]

I see you have the 15k grid resistor at v1b...do you have one at v1a as well? My eyes aren't what they used to be...

You might try a slightly longer run of wire coming off of the 500pf cap.
Run it straight down to the deck, over to the treble pot and straight up. I've had some success with keeping these wires down on the chassis.

I like your build! Are those vitamin q's? Nice...

[Cliff Schecht]

Yeah I got the Vitamin Q's and C-D 0.1uF caps from David, I like them a lot myself. They look nice placed on a board too .

I'll try that trick later Kelly, this amp is CLOSE to being stable and I really think it's just one little troublemaker now. The grid resistor IS on V1b, not V2a like I kept saying. Your eyes aren't that bad!

I think the reason that Ken did the strips across the open space in the chassis by the heater wire is to help dampen some of the resonance of the chassis itself. As I tap the chassis, I can hear the amplifier start to slowly build up a resonance that goes away when I push on the chassis in that open area with a chopstick or my finger. Interesting..

[KellyBass]

The resonant chassis thing is interesting!

I'm trying some different things to cancel it out. If i can't get it to settle down, I'm gonna put a PZM microphone in that open space and run a stereo rig!

[funkmeblue]

are you 100% positive your not dealing with a microphonic tube? I've had the same problems as you. These things are very particular about there v1 tube. Tubes that work fine in my boogies v1 freak out in my build. Went through a couple of tubes that I ordered for low microphonics and could not be used. Try wrapping v1 in foam or get some tube damping rings and see if that helps.