67 Super Reverb ugly distortion

[vintagekiki]

Greg_L

What makes this hard is that the distortion is upon a hard transient, like a heavy strum of a low note on a guitar.

Without hearing it I am having a hard time trying to figure out how to replicate something like that with a sine wave signal...I don't know how to set up so that it would be similar.

I guess I'm gonna have to experiment with that more...maybe input 330hZ (low E) with the signal generator, with the amp connected to a dummy load, and keep cranking it up until it goes weird...
so when it goes bad, I would use the scope to trace back thru the output section until it doesn't look wrong??

Just thinking out loud here, looking for some ideas.

Get the story back in the technical flow.
How many voltage gives a clear signal on dummy load of 2 Ohm

Unanswered question

Are you able to compare your FSR with the same or similar (from your colleague)

Maybe your guitar gives a heavy strum of a low note.

[vintagekiki]

Two weeks the thread is inactive.
Does this mean that the FSR has been repaired, and run around the stage.

[earache]

Vintagekiki

No, the amp isn't working yet. I'm getting together a more credible signal generator and will continue testing once I have it working.

I've also printed off the entire thread and am going to review every thing that's been suggested to make sure I haven't missed anything.

Thanks for your continued interest! You and all of the other posters on this thread have taught me a lot!

[vintagekiki]

Easy, easy pace.
Just so you know the forum has not forgotten you.

[Sweetfinger]

I'm going to relate an anecdote from years ago.
I had a silverface Bassman head that worked fine, then seemingly started cutting out momentarily while playing. A bunch of techs took a stab at it, including Gerald Weber, twice. Nobody could fix it.
Turns out that what was happening, is that I had started to play louder in the band I was in. I was resting my palm on the strings just in front of the bridge. With the pickups I was using, as I picked, I would slightly compress the lower strings sending very low frequency signals to the amp.
The amp was trying it's best to amplify those 1 to 10 hertz, or whatever, pulses, but just didn't have capability. It would spend all the available power trying to amplify those low transients and the audible signal would cut out. I could even make a tremolo sound by hitting a note or chord, and push my palm into the low strings at intervals.
Of course, once the problem was identified, it could be properly addressed. The amp worked as originally intended, but I was definitely not using it as Fender intended.
Some small circuit tweaks and a bass cut cap on the input cured it right up.

The moral? This problem might be outside "the box". Is there a small strand of steel wool stuck on the reverb tank recovery that only shorts on high volume transients? ... or something like that?

I just had a guy bring in a Les Paul that had been to a Gibson warranty tech but still had "ground noise issues".
The "issue" was that he was using coated strings, which had partially worn through, so as he played, he was alternately grounding and ungrounding and getting that little static pop that you hear, a gajillion times as he made and broke, and made contact with every wind along the strings.

EDIT: I forgot to add the time a Traynor head would make a godawful static on one note in particular. After some sleuthing, and figuring out the amp HAD to be on the speaker cab, so the problem was vibrational, but poking everything inside failed to make any noise, I saw that on the most problematic note, at volume, one ceramic disc cap with slightly long leads, would resonate and physically oscillate quite a bit further than I would have guessed it could, and was shorting on whatever lead was nearby. BzzzzzzTttt!
A little bend and some goop, good to go.

Did a voice coil rub get ruled out? Does this amp make the same ugly distortion plugged into a completely different cabinet?

[Chuck H]

I just had a guy bring in a Les Paul that had been to a Gibson warranty tech but still had "ground noise issues".
The "issue" was that he was using coated strings, which had partially worn through, so as he played, he was alternately grounding and ungrounding and getting that little static pop that you hear, a gajillion times as he made and broke, and made contact with every wind along the strings.

Great catch. And could only be diagnosed by an outside observer who knew all the things to consider. Very interesting.

[Helmholtz]

Turns out that what was happening, is that I had started to play louder in the band I was in. I was resting my palm on the strings just in front of the bridge. With the pickups I was using, as I picked, I would slightly compress the lower strings sending very low frequency signals to the amp.

Wondering where in the amp's signal chain the cutting out actually happened. Surely the additional LF (quasi-DC) would shift the bias of the preamp tubes and cause some amplitude modulation like a bias shift tremolo circuit. But as the effect mainly showed at high output I strongly suspect OT core saturation. Transformers just can't handle (near) DC.

[earache]

Hello to All of You Very Helpful People

I was able to fix the amp by replacing the output transformer. Not exactly what I wanted to do to a vintage amp. I realize that I had been resisting that probable outcome, and trying every other possible fix first.

In getting to the final diagnosis I had to enlist the help of a technically more experienced friend, Russell. I will quote his e-mail that he sent me where he correctly surmised that the transformer was bad. I had asked how he came to the conclusion and he said:

" Well, a couple of things. It's a 2 ohm secondary, so ohms measurement was of no use. I noticed that, as you said, it was ok at low levels, but when pushed, it began to distort as though something was 'shorting' showing an increase in current draw, yet a reduction in output level.

Somewhere along the line, I noticed that there's a feedback line from the secondary to the driver tube. Eventually, I was able to get it to exhibit the same symptom with NO output. Current draw with no signal to the speaker output. It would come and go for a while and finally gave up. There's signal going to the primary side and nothing coming out. It still may start out ok, but eventually quits... thus; shorted output transformer.

Signal in at roughly 'guitar' level, load resistor was 8 ohm at first, then 4. The symptom got more pronounced as the load was lowered towards the design limit of 2. It just all pointed towards a bad transformer in the end I guess. All done with signal generator and scope, DVM for voltage checks, which seemed to confirm a short somewhere, yet "none could be found".


Hope this is helpful to someone in the future. Thanks to all of the people who read and followed my lengthy posts and offered help. I really learned a lot with this one.

[g1]

Back around post #63 (pg.2) a simple neon bulb tester was recommended and the schematic was also posted.
I'd think it may have detected the OT problem, but it would also have been very good to know if it could not.
From reading through the rest, I get the impression the neon bulb test was not done?

Glad to hear you got it fixed though! Thanks for posting the update.

[earache]

G1 -

You’re correct - I didn’t make the neon bulb test rig.

I was admittedly impatient to get the Amp fixed. I also wasn’t sure of what type of neon bulb or where to get it..the closing of Radio Shack makes it a little harder to go out and grab small items like that...

[vintagekiki]

Welcome dear friend.
It's nice to see you after a long time.