Fender custom shop 68 Twin reissue Nasty HF Oscillation

[Chuck H]

Helmholtz asked earlier what happens when you unplug the reverb send to the tank. I'm just repeating the request so it doesn't get missed. You could also unplug the reverb drive tube.

EDIT: Ah... Helmholtz was reposting at the same time.

[BiBi]

thanks for the replies...

yes the oscillation certainly occurs before the power amp.
Pulling the reverb driver ECC81 doesn't help neither does pulling the opposite ECC83 from the other channel.

included are some pics of the oscillations at different frequencies before it goes completely haywire.

[Chuck H]

Gut pics?

[Chuck H]

If this model has the big capacitor on the board near the preamp, did you replace that one too? Or just the caps in the doghouse?

[BiBi]

so here are the guts...

obviously this is after some changes I made like separating common cathodes, redressing the wiring from the power transformer, etc.
nothing has hurt so far but nothing has helped either

[BiBi]

all the filter caps have been replaced.

[Helmholtz]

neither does pulling the opposite ECC83 from the other channel.

Which tube is this in the schematic?

At which circuit point were the scope pics taken? What was the time base setting (horizontal deflection coefficient)?

[Chuck H]

Helm is considering the same thing I am. Divide and conquer. Trace the signal path to find the first place in the circuit where the oscillation shows itself. Oh, and the scope settings.

[Helmholtz]

so here are the guts...

obviously this is after some changes I made like separating common cathodes, redressing the wiring from the power transformer, etc.
nothing has hurt so far but nothing has helped either

If you actually replaced C34 you obviously used the same Illinois cap crap as before (sorry Illinois people, love your area and tradition).

[Chuck H]

If you actually replaced C34 you obviously used the same Illinois cap crap as before (sorry Illinois people, love your area and tradition).

It's alright. I don't think they're made there

[Helmholtz]

I don't think they're made there

Doesn't matter, headquarters are responsible for product quality from all production sites. I know that you can get identical quality out of anywhere if you care, invest and check. Quality management is the key - and installing qualified/experienced quality managers with direct reporting. Also quality improves if the factory is charged with after sales costs (closed feedback loop).

[Chuck H]

Doesn't matter, headquarters are responsible for product quality from all production sites. I know that you can get identical quality out of anywhere if you care, invest and check. Quality management is the key - and installing qualified/experienced quality managers with direct reporting. Also quality improves if the factory is charged with after sales costs (closed feedback loop).

Absolutely. I think the humor I intended must have been missed. And yes, I've had enough experience with Illinois caps that I avoid them. Maybe they make some really good ones, but those aren't going into any guitar amplifiers I'm aware of.

[Helmholtz]

I think the humor I intended must have been missed.

I think I got it. But it's such a shame that a US (Illinois) based company with a long tradition got that bad a reputation.

[Chuck H]

I think I got it. But it's such a shame that a US (Illinois) based company with a long tradition got that bad a reputation.

Hopefully that reputation is just with us guitar amp guys. By that I mean that it's the amp manufacturers that choose what capacitor to pay for. If a long standing, reputable company gives you the lowest bid that doesn't automatically mean your getting their the product that earned them that reputation. I can go to the local deli and buy a grilled Ruben and it'll be delicious. Or, at the same deli I can save some money and just get the bologna sandwich.

[BiBi]

Thanks for the heads up on the Illinois caps-- I had one 22/500 from them still for some reason, but I didn't realize they had such a bad reputation.
I replaced it with another and it didn't help however.

By the other channel ECC83, I meant I can pull V1 while using the "vibrato" channel or V2 while using the "normal" channel and there is no change in the oscillation.

Scope setting was at ,1ms for the photos as I started at around 40Hz on the generator. The scope was connected directly to the dummy load (directly from the amp output jack is the same)
Like I wrote earlier, I am quite certain the problem is situated around the grid of V4b. The previous amp's problem was corrected here while and I can remove the following tubes and still see
oscillation on V4b's anode. Grounding V4a's grid and separating the cathodes here have not shown any improvement.