Need held diag. 68' Super Reverb.

[fusionbear]

A client brought me a 68 SR that needed a cap job bad. Ghosting like crazy. this a pretty routine repair, or so I thought. Anyway, replaced the filter caps and just the cathode caps for Normal And Vibrato Channels, the rest looked good. But here is the problem. Normal channel works fine, no issues at all, but the vibrato channel will begin to track a note exactly an octave below the notes you play on the low E string. It tracks so perfectly, it sounds like the old MXR Blue Boxes. I have never encountered such a problem. Any ideas? My signal generator is broken, so I really don't know how I am going to trace it down...

I replaced all the cathodes caps in the amp and the plate resistors on the vibrato channel to no avail... Normal Channel works perfectly still...

[pdf64]

How about a sig gen app for smart phone or pc?
Try it with V3 reverb driver removed.
Check all resistors for drift.
Trying swapping in different tubes.
Are you sure that it's not just the trem channel driving the power amp a bit harder than the normal?

[Stevem]

The amp is osscillating either due to a tube ( preamp),poor connection in a tube socket( high resistance) or the wire / parts layout!

Pull out the first 12at7 that drives the pan and swap-it in the vibrato 12ax7 tube locations starting with the V2 slot, does the issue stop or get reduced?

Are the small Ciramic caps on the output tube sockets still in place, if not that would be why the amp oscillates as they where put in place to make up for the poor wiring layout they used on these latter amps to speed up production!

Also is the output transformer original?
The vibrato channel has one extra gain stage over the normal channel ( the 4th preamp tube) and it takes at least 3 stages of gain to make a amp oscillate from wiring or layout issues and in light of this that means from the first gain stage to the reverb mixer stage in the vibrato channel, or from the reverb mixer stage to the output stage!
You may be able to track down the gain stage where the oscillation is taking place by removing the cathode bypass caps one at a time, which is a easy as cake thing to do and takes little time!

[fusionbear]

I will try these tips this evening. This is a drip edge 68 with BF layout thankfully. Not the rats nests that came slightly later...

[pdf64]

it takes at least 3 stages of gain to make a amp oscillate from wiring or layout issues

If the 2 triode sections of a 12AX7 are cascaded then oscillation can be a hazard.
With 12AX7 in V1, even a 5E3 preamp can do it.
All it needs is 2 polarity flips and enough gain.

[Stevem]

Make sure someone did not unhook the feedback lead from the speaker Jack or lower the value of the feedback resistor!
Sometimes folks disconnect the tremolo circuit at the intensity pot for less signal drain and more gain, so you might take a peek at that also!

[fusionbear]

I want to thank you all for your awesome advice. It turns out the the .022 from the plate of the V2B (first gain stage of the bright channel) was the culprit. You could vibrate it by flicking it with a chopstick and it would give you the octave down. I replaced it, but I also replaced some plate resistors that were out of spec and 4 cathode resistors that were out of spec. The amp is back in all its glory again. Woo hoo!

[Stevem]

If you still have the amp you should do the owner a tonal favor and replace that crappy sounding PI input disc cap with non disc type as Fenders use of this defies logic in many ways!

[pdf64]

Wow, that's a weird one!
Well done tracking it down.

[fusionbear]

If you still have the amp you should do the owner a tonal favor and replace that crappy sounding PI input disc cap with non disc type as Fenders use of this defies logic in many ways!

Yes, I agree... that little .001 ceramic is a goner.....

[premiumplus]

Love me some chopsticks...best diagnostic tool ever.