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Hello everyone and thanks for having a look here,
As the title says, I have a 68 Twin reissue Custom Shop on my bench, actually the 2nd one in two weeks with a similar problem of HF oscillation. This circuit is essentially the same as the 65 Twin reissue, except that they have paralleled the two input sections so the "custom" channel also has access to reverb and tremolo. Oscillation occurs first at around four on the volume control and appears as a heavy squiggle on a sine wave on the oscilloscope. Past this, there comes a point where the amp simply explodes into full oscillation and becomes unusable. The owner of the amp first noticed the problem as a sort of background noise to certain notes but actually didn't ever the play the amp very loud and may not have noticed the oscillation beforehand.
The amps have grid stoppers on nearly every tube grid, preamp and output. I believe they must have noticed an issue in the factory and made some adjustments (This amp also has a reworking of the traditional tremolo section as in Europe LDRs are outlawed. It would have been nice if this was the source of some trouble, but as I removed the tube and the power to the LDR replacement circuit it isn't the case.)
The source of the trouble appears to be at the tube used for mixing the reverb return and dry signals. Grounding the grid here removes most, but not all, of the oscillation. Also, when I removed the phase inverter and output tubes, oscillation still occurs at the anode of this tube. Leads from the PCB to the socket aren't particularly long, and there is a grid stopper of 10k on the grid of the mix as well as the reverb return! I have tried to reposition these wires (it worked on the first amp) but here there is no difference. As well, a different tube makes no difference. I also tried to snubber the grid with a capacitor from grid to ground, but this only helps with such large values that the amp would end up sounding completely flat.
Obviously there is a gremlin in this amp, but I can't seem to find it! If anyone has some ideas, I would be most appreciative...
As the title says, I have a 68 Twin reissue Custom Shop on my bench, actually the 2nd one in two weeks with a similar problem of HF oscillation. This circuit is essentially the same as the 65 Twin reissue, except that they have paralleled the two input sections so the "custom" channel also has access to reverb and tremolo. Oscillation occurs first at around four on the volume control and appears as a heavy squiggle on a sine wave on the oscilloscope. Past this, there comes a point where the amp simply explodes into full oscillation and becomes unusable. The owner of the amp first noticed the problem as a sort of background noise to certain notes but actually didn't ever the play the amp very loud and may not have noticed the oscillation beforehand.
The amps have grid stoppers on nearly every tube grid, preamp and output. I believe they must have noticed an issue in the factory and made some adjustments (This amp also has a reworking of the traditional tremolo section as in Europe LDRs are outlawed. It would have been nice if this was the source of some trouble, but as I removed the tube and the power to the LDR replacement circuit it isn't the case.)
The source of the trouble appears to be at the tube used for mixing the reverb return and dry signals. Grounding the grid here removes most, but not all, of the oscillation. Also, when I removed the phase inverter and output tubes, oscillation still occurs at the anode of this tube. Leads from the PCB to the socket aren't particularly long, and there is a grid stopper of 10k on the grid of the mix as well as the reverb return! I have tried to reposition these wires (it worked on the first amp) but here there is no difference. As well, a different tube makes no difference. I also tried to snubber the grid with a capacitor from grid to ground, but this only helps with such large values that the amp would end up sounding completely flat.
Obviously there is a gremlin in this amp, but I can't seem to find it! If anyone has some ideas, I would be most appreciative...
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