The edge of instability......

[M Fowler]

I run new stock JJs or GT Gold pin and do not find problems using those tubes.

I'm not sold on using NOS and prefer to get tone with what is currently available.

Same with the EL34's those are GT 34M or 34L.

Mark

[redshark]

Still that mighty high pitch squeal!!! a little bit less intense and definitely at the end of the volume pot only with treble on 10, presence on 10 and volume 8.5-10, before those levels is fine, only with guitar unplugged....

Let's try some luck with amperex on V1 and V2....

[M Fowler]

The brite switch wiring is a big problem I swithced over to the DPDT switching which really helped in my Express combo. Also the presence wiring can be a bugger causing oscilliation. If you do not have a phase problem then I would check those areas closely.

Other than that I wonder if a component value is wrong. Most of us do that, I recently found my resistors mixed up in my drawer and went back over an amp to find they indeed had the wrong resistors. Should measure each one as I build.

Mark

[redshark]

Mark, thanks for the suggestions....I had been through the bright switch, basicaly chopstick all the wires between V1b and V1a. Relocated them to the less noisy place. I did this with the amp cranked up with the oscillations at their peak level (of course wearing earplugs).
I rewired the pots, also been through the presence wire, moved that one, tried the foam under power board suggested by Glen to move the OT secondary to stay close to the chassis, actually glued them to the chassis and used the foam....what else? I've tried many things.
I had measured the components in circuit (mostly resistors) and they test normal, I didn't misplaced any and they all seem to be in spec. I haven't measured the power caps because I would need to unsolder the leads but I might.
Maybe I should not worry about this issue anymore. I will never play the amp with both presence and treble dimed and all volume dimed, and even if I did with a guitar plugged in I don't get the squeal, only when unplugged.

[M Fowler]

1. You have tried reversing the OT primary wires.
2. You have worked on the NFB presence circuit.
3. Replaced the filter caps. Possible leaky filter cap.
4. Is the input jack grounded?
5. Is the input jack switching lug working? Since it oscillates when unplugged.
6. Brite switch wires are short as possible and the caps up against the chassis?
7. Did you need to use shielded input wire to V1?
8. Was shielded wire from V1 to volume pot needed?
9. Tighten all jacks and pots; all ground points/lugs. Are speaker jacks grounded?
10. Did you use a grid stopper on V1 15k to 33k?

Mark

[redshark]

1. No, if that was the problem I would have oscillations present all the time.
2. Yes I did.
3. No, I haven't.
4. My input jack looks like the layout, the shield goes to the ground of the input jack so yes.
5. yes it is,
6. Lead dress is fine and caps are syliconed to the switch
7. Yes I did.
8. Yes I did, that helped the amp have a better transition from sustain to feedback not sounding congested.
9. Yes
10. 15 k.

[M Fowler]

Well crap your done with that amp then.

Pull the wires away from the pots especially if you are using thin jacket wire.

Most of the time larger grid stoppers can add some noise but in your case perhaps its time to pull the big guns like 68k and work your way down to 33k.

[Cliff Schecht]

1. No, if that was the problem I would have oscillations present all the time.

You'd be surprised. I was 100% sure I had my leads in the color correct order on an Express build but the damn thing wouldn't sound right when cranked. When I turned down the volume, the oscillations went away. The amp still sounded alright up to a certain point but it would always oscillate. I flipped the primary leads and the oscillations went away..

[redshark]

Ok guys please help me....I think I'm getting close.....

with presence and treble dimed (no cable plugged) I started turning the volume up and when I'm getting close to 10 and the mighty squeal starts I noticed V3 looks a little bit inclined so I go move it and it wiggles in the socket, with my right hand I kept turning volume up and with the left moved the tube to hold it in position then suddenly I touched the tube and the squeal goes, take my hand off and the squeal comes again, reposition again and it goes .....WTF?? is it the natural capacitance of my hand? or is a loose socket or a cold solder joint in the pins? \\

SUGGESTIONS? I tried a tube shield and that didn't work...what should I do?

[redshark]

Damn my ears hurt!!! I reflowed all the pins in V3, also tightened the screws and now I cannot stabilize it like I did a few minutes ago. is the high treble interacting with volume, tubes and my hand on top of all preamp tubes that I can make it squeal or not. This is strange and I'm clueless.....no one has any idea??

[PlinytheWelder]

FWIW. I just tried diming everything on mine and it squeals ,too. If I back off on either the gain or the treble, it stops. In no way am I going to play the amp like that, so I'm not going to worry about it.

With SO much gain on tap with these it seems inevitable that if you put the controls in the worst case possible to get some oscillation. In any case, the amp sounds better with the treble down from max, so why worry?


Edit: When I tried this, it was just with the guitar un-plugged. Yeah, it was getting noisy, too....

[redshark]

Well, here the information is confusing. As I wrote before a friend of mine did something similar to a real trainwreck and he made it oscillate but here Glen and other guys say the amp shouldn't do that. So I've been trying to make it stop and it's been like an insane war to me.
I've been trying diferent tubes, a second shielded wire, grid resistors in pin 7, pin 2, ceramic cap across the plate resistors, etc. and for a second it was my hand what cut that oscillation. Then when tried to fix it I couldn't stop it again...
By the way I just made an experiment according to the Trudy schematic, soldered a ceramic 0.002 cap in parallel to C8 and the amps gets darker. I'm not sure if I will leave it like that but I just wanted to make a comment that it is a valid tweak for those guys that think that is amp is bright.

[PlinytheWelder]

Just trying to be the voice of reason...


Patient: "Doctor, it hurts when I do THIS".

Doctor: "Well, just stop doing that".

[Cliff Schecht]

Perhaps the confusion is with some people leaving the input floating when diming the knobs. I'll bet that nearly every Express starts to oscillate (even if you can't hear it) at maxed out settings, there's a SHIT TON of gain! Combine this with the layout, he uses coupling between the wires as another form of controlled feedback, and it's not that hard to imagine why these things go unstable. My Express oscillates when I leave the input floating and have the knobs maxed out.

As I have it now though, the input jack shorts to ground when a guitar cable is unplugged (I think Ken did this in some Express builds as well). Keeps the amp nice and quiet (all normal hiss aside) even when all of the knobs are maxed. But who runs their knobs all the way up? That would probably cut your head off.

[redshark]

It will be interesting that Glen give us his opinion on all this thread. He says his amp even unplugged and with all the knobs maxed out still doesn't oscillate.
I came back to experiment with the oscillations and it seems that when I place my hand barely touching the preamp tubes I don't get the high pitch squeal....all this thing is driving me crazy specialy when a friend told me Suzanne does the same thing....