67 super reverb has distorted sound when note decays

[pm_EE]

Hi,
My 67 super reverb has had this strange "after sound" for forever. It is a a distorted multi phased sort of sound that occurs when a single note decays to less than half the attack volume. It is more pronounced on some notes than others. Notably c# 3rd string 6th fret. It also seems to be very intermittant - I thought I got rid of it several times, and it's always back

I have done a complete cap job to the amp, changed tubes, tested with speakers in another cabinet. Both preamp channels behave the same. My suspicion is that there is some sort of oscillation due to parasitic feedback going on. I think I have seen posts about this before but can't find them now.

While I am an engineer and have a complete lab at my disposal, this is a really hard problem to find since I must wait for a decaying note to produce the effect. If anyone knows the answer to this or has some hints they'd be much appreciated.


Thanks,
pm_EE

[Buddy Boy]

Check the bias circuit.

[davemccarthy707]

change the 470 ohm bias resistor and the diode. Also check the 820 ohm feedback resistor to see if it is cracked.

[ScottR]

could it be the speaker rubbing?

[scott1568]

Like Scott said, you could have bad voice coils on one or more speakers. This happened to me recently when I bought a used pair of Webers. Just sent them to him this week for repair.

Here is an easy way to eliminate the possible cause(s) of your problem before getting inside the circuit.

Remove the speaker grill, and GENTLY use your fingertips to push in on the cones a little, relieve some pressure and let them release back out. If you feel like something is rubbing or grinding in there, the voice coil is rubbing on the cone. I would also try this on a known "good" speaker to see how they compare feel-wise.

Go to Weber's site. I think in the FAQ section there is a step-by-step process of fixing this if you want to try.

Or you can send it to be repaired by someone qualified. Weber is only taking speaker repair work for his products right now from what I understand.

[phsyconoodler]

OP quote: "I have done a complete cap job to the amp, changed tubes, tested with speakers in another cabinet."

He has tried other speakers.
There is something amiss in the circuit.Could be any number of things.A tech could localize it better than random guesswork on a forum.

[donnyjaguar]

Put your signal generator on 554Hz and adjust the amplitude until you hear the distortion. Disconnect the NFB and trace the signal backwards.

There are some curious cost-saving measures in this amplifier's design that could cause this symptom if a component is out of tolerance. Checking voltages is a good idea here.

[pm_EE]

Thanks for all the quick replies so far.

1) As it may have been overlooked in my original post, I have the chassis hooked to en entirely different cabinet with a set of 68 cts's newly rebuilt bt orange county (the original cabinet has the original 67 cts's in it, so that was suspect).

2) I already had the thing over to a tech who's supposed to be the best in Austin, twice, and he couldn't solve it. But as I have always found, if you want something done right you have to do it yourself. I just relegated it then out of laziness an not particularly liking to work on something I didn't design myself. But I've been out of work for several months now due the company I worked for cutting about 40% and no one's hiring. So, I have the time and I've been studying the circuitry of this and many other tube amps for several years now, so from a circuit and theory point of view I know more about it than a tech. What a tech has that I cant get from theory is the experience of what they've seen from a long time with certain resistors that crack, etc. So that's why I have put the question out to the forum.

3) I have been going to set up my bench to put a signal generator on it, thanks for the idea of disconnecting the feedback. I'm not sure if a static signal generator will produce problem. I have a programmable digital one, but it's a fair amount of work to set it up to produce a loop simulating attack and decay. So I have been looking at some ADSR generators, but I'll go ahead and see what I get get out of what I've got. I'll also try the suggestions about the
resistors mentioned below. A cracked one in right place could certainly be the culprit. And these are really hard to find because the crack may not be easily visible. A could solder joint could do it too. Anything that causes the resistance of an element or node to change with a change in voltage or current.

4) I'll let you all know what the outcome of tests are when I get to them in the next day or so. Thanks again for the tips so far.
-pm

[phsyconoodler]

The best way to find bad resistors is to move them with a plastic probe.Cold spray works very well too.
And as far as digital signal generators go,is your guitar a digital AC generator?
Go over your cap job,paying atention to the under-board wiring.

[RussB]

2) I already had the thing over to a tech who's supposed to be the best in Austin, twice, and he couldn't solve it.

Jon?

if it wasn't him, you didn't have "the Best Tech in Austin"

[RussB]

Tell me about your bypass caps. Were these replaced? Could one be leaking?