Twin Reverb RI buzzy undertones

[sonicmojo]

Aloha,

I have a twin reverb in for repair as it just does not sound right. It has what seems to be a slight "coil" rub type sound where there is some slight fizzy buzzing in the background on notes no matter what cabinet used. This after the prior repairman and I have both gone through it pretty good. It has all new filter caps, I've tried other tubes in all positions, checked the choke, etc. I even broke out my oscilloscope but the nature of the PCB makes it very hard to grab the signal at various points as the caps are right down on the board.

There is one symptom that maybe offers a clue. When I strike the "E" note on the low E string at the 12th fret, I get a tremolo type oscillation. It doesn't happen on other notes, even E in other places. Very strange.

Does anyone have any ideas on what else I can check? I'm about to give up on it.

Thanks,

Bryan

[martin manning]

Can you get any of this to happen driving the input with a signal generator?

[Stevem]

So are both channels doing this ?
If so then you issue is in the power supply section , or from the input to the PI on out to the speaker.

If it’s only seems to be on the vibrato channel then you can confirm this by pumping a test tone into the channel and then scoping the output from the reverb send jack on the rear of the amp.

If the amp did not do this before the filter swap out then that’s another clue!

Sometimes replacing shot tubes with new one with good gain will bring out inherit issue that where not there before.

Long grid runs be they wire or circuit board traces can make for oscillation with certain notes when there played.

If you have any big value coupling caps ( like .2 ) with a 600 volt rating you can try jumping out the power supply filters and see if it helps.

I also don’t think that the grounding layout on those RI amps where all that they could be, but the two darn circuit boards makes that hard to change!

[pdf64]

Instability can manifest similarly to voicecoil rub.
I suggest to scope the output across an inductive load, whilst running a variety of test signal levels and frequencies. Look out for any weird stuff, eg

[sonicmojo]

It appears on both channels. It is subtle so is very hard to tell if it is sympathetic vibrations but I'm convinced there is something going on. Unfortunately my scope has a mostly blown LCD so I am limited on what I can see and do). I need to get a new one.

I just ran a signal generator through it and swept the frequencies. Sounds normal. Whatever noise is happening is hard to hear within the signal. I could not get that tremolo oscillation to happen at any frequency with the signal generator.

How do you scope the output with an inductive load? I was going into my simple resistor load box but I could not get a good display. Or maybe it is my scope itself. I don't trust it right now (or myself with it yet).

[Stevem]

If this is not one of your amps, but amp fora customer and this issue was not part of his reason for getting the amp worked on, then let it go as it is if it's so hard to hear this problem.

Otherwise you may find yourself married to this amp for a long expensive time!

If you feel you must play with it still then also try moving the wires around ( up off themselves) on the PI tube.

Now that I think about it since I have worked on a ton of reissue superreverb amps that do not have this issue, I think your issue in this amp is related to the longer PI output wires runs to the added two output tubes .

Both the reissue SR and the TR use the same main board yet all the SRs that I have worked on do not have such a issue present.

I think it would be worth while to rerun one of the PI outputs with shielded cable , trying this mod on one PI output at a time .

These things to try, first assume that you have jumped in new and tested for leakage power supply filters since if the PI section is not uncoupling as needed from the output section then this could be the sourse of the whole problem.

[dgrainger]

Does it happen with any guitar? How close are the pickups to the 6th string? Hot pickups right up next to the string will cause a warbling/oscillating sound sometimes.

[Stevem]

That's certainly true so you need to confirm the issue by injection a test tone that you turn on and off.

[Stevem]

If the pickup deal for the issue is not the problem then in keeping with what I posted about keeping grid wire runs as short as possible try moving the two PI output caps to the grid of the output tubes.

You should have the most shielded side of the cap on the down stream side of the audio flow ( output tube side) , you can look up how to test coupling caps for this on line.

You can also try as a last resort increasing the value of the 1.5k grid stoppers on each output tube socket to 5.6k .
If this clears it up then back down in resistance until the issue comes back again, and only go back up to what is needed to stop the oscillation,

[Gaz]

May be helpful if it was the same noise I was hearing.

https://music-electronics-forum.com/for ... distortion

Also try removing the reverb tube just to rule out the reverb circuit causing it. If you search you’ll find it’s another common reason for weirdness overtones when the amp distorts. That is usually corrected by reverb driver lead dress