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Pentode Triode mod but get crazy hummmm... change c6?


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If you could post a picture, the problem may be easy to spot. From your other post mentioning your chopstick test, I suspect it could be more of a lead dress issue. The OT primary is a very strong source of oscillations, but not the steady 60Hz hum type (like heaters and HT secondaries, and AC wall power). OT's can setup a rather loud and totally rude, but steady 120Hz motorboat buzz if it's out of phase while using NFB.

 

Generally speaking though, improperly routed OT primaries will cause a squeal that'll vary in freqs as the wires are moved away from the more sensitive signal wires picking it up and injecting it into your tone. So adding a wire to it (at pin 7) that runs to a triode switch and back (to pin 9) is simply making that noise source an even bigger problem. Instead of just shoving the wires around, why not get that triode switch away from the tone stack and the wires to the control grid. Keep those wires twisted, well away from everything, and as close to the chassis metal for the length of the wire run as much as possible to reduce the field strength. Worst case, you might try shielding those OT primary and triode switch wires. Even coiling a wire (grounded at both ends) around a noisy bundle of primaries can be enough to help if you can't keep it close to the chassis. Even the big players like Fender use that trick. That's where I learned about it.

 

As to the cap question: The typical voltage at B+ is about 370 - 380vDC. A 400v cap may work, but probably won't live long without at least that extra 50v safety margin. A cap rated for 500v would be a better choice for C6, but a 400v cap would be fine fo C9 or the rest of the B-rail. However, better filtering and shielding bandaids probably won't solve a lead dress problem. If R5 is connected directly to the grid, no shielding should be necessary anyway. For example, I've got an 18watt with no hum, no noise, and not a single millimeter of shielded wire anywhere in it. It's all in the lead dress.

 

Gil...

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