Early 70's Vibrolux "snare drum" distortion

[H Chandler]

Hi,
I'm going crazy trying to find the source of some crappy high end distortion in a early 70's vibrolux reverb. It sound likes snare drum snare rattles on low notes when played at moderate to loud volume. It has what appears to be a replacement output transformer with 3 impedance taps. The numbers stamped in it are
017450
EIA606-045.
I'm guessing this is a generic Schumacher replacement transformer but I have not been able to turn up any information on it. Could a bad transformer cause this sort of distortion. Where else should I look?

The power tube section has been rewired to BF spec. The suppressor caps have been removed. Could it be oscillations?

Any suggestions will be most appreciated.

Thanks

[drewl]

FWIW I had a similiar distortion in my Pro Reverb that turned out to be the speaker, have you tried another cab?

[H Chandler]

I've had that happen with speakers too but I tried it on another speaker that sounds fine with another amp.

[pdf64]

If not a bad speaker then tube rattle is a likely culprit, can be pre or power.
Does any of the controls alter the snare?
If in normal channel, does removal if V2, V3, and V4 get rid of it?
Does swapping the 12AX7s around alter it?
Does different pair of 6L6 tubes get rid of it. Peter.

[H Chandler]

Did all tube swaps including pwr and rectifier. Tried yet another speaker cab.

No Soap.

[tubeswell]

Maybe try isolating the gain stages with a 630V cap to ground from each plate (one at a time) and putting a signal onto the grid of each following stage - work backwards from the output stage to the input stage and when it the problem starts you know you will know which stage it's in. Could be DC from a leaky cap, or something from a haddit plate resistor. (I had a similar problem once - a nasty treble smack with the tone controls maxed out - turned out to be a bad plate resistor in V1)

[mac1amps]

Ah yes, those suppressor caps! If the amp has been doing that ever since you removed the .002 µF ceramic discs at the power tube control grids [to ground], then it sounds like parasitic oscillations to me (SF's are notorious for that). The easy solution is just to re-install the .002's (you probably wouldn't like the "not-so-easy" solution).

[H Chandler]

Probably not, but what is it anyway?

[pdf64]

I guess it would be to re-wire the amp (or at least the signal path) with a BF type lead dress, and to fit the correct OT.
If it is parasetics, here are a few lead dress ideas which seem to keep them at bay for me.
1/ Use any mixer resistors as grid stoppers, by mounting them right on the tube socket legs, ie the 68k input mixer resistors, and the reverb mixer resistors, move them off the board and right on to the socket, insulating the free end with shrink wrap etc. Not as pretty as stock method, but complies with general tube circuit best practice.
2/ When routing wires to the tube sockets, form a twist or 2 between the grid and plate of each tube stage where ever possible, ie as with heater wiring. This will increase the normal miller effect capacitence to roll off the high frequencies a little earlier.
Also twist together the wires which are carrying push pull signals, ie to the power tube grids, OT primary leads
3/ Route any grid wiring so as to be right up against the chassis for as much of the run as possible. Obviously doesn't make any difference if using screened cable, but with these methods I've found that screened cable isn't needed.

Always bear in mind when wiring that parasitics etc can generally only occur when there's coupling between a grid circuit and the plate circuit of the next stage, assuming normal common cathode inverting stages.
Peter.

[Bruce / Mission Amps]

I don't find these amps to be notorious for oscillations. Sure, you could have one.
Check to see if every ground wire, from the eyelet board to the brass plate, is actually soldered and not cracked.
Check out the ground wire from the NFB voltage divider near the phase inverter... I have seen these break loose more then the others.
The other place to look is at the actual eyelets of the coupling caps, resistors and ecaps.
I've seen lots of cold or oxidized solder joints in these amps.