Single Ended Master Volume Hum

[Bruce / Mission Amps]

The schem looks wrong! Your maximum load for the power tube grid can't ever be what it should, because of the tone stack loading down the volume control, at low settings the power tube grid might even be seing a comparative short.

Ground a 220K to 470K resistor from pin 5. Now add an additonal coupling cap between that resistor and the volume pot wiper.

If you mean the tone stack (held up from ground by that 50k pot) in parallel with the MV at the power tube grid, it won't really matter if the wiper is grounded because the power tube bias is developed at the cathode... he could short the grid of the power tube to ground with a screwdriver and it won't really effect the bias level... of course no sound will happen. ha ha

Oh and you can not unload the gird or pot with a cap from ground because then the power tube's grid
will have no DC reference to ground, resulting in zero bias and the tube will turn on full blast.. possibly blowing the tube.

[Hugo]

Oh and you can not unload the gird or pot with a cap from ground because then the power tube's grid
will have no DC reference to ground, resulting in zero bias and the tube will turn on full blast.. possibly blowing the tube.

Is that the reason the power tube's cathode bypass cap blew up when I installed the coupling cap MWJB suggested?

Hugo

[MWJB]

"I first added a 470k resistor from pin 5 of the power tube to ground. This had absolutely zero effect on anything, hum was exactly the same." It would be, all you have done at this point is add a larger value resistor, in parallel to ground with your tone stack & vol control...therefore the value of the new resistor is negated (tone stack & vol will only ever be a lower resistance).

"Then I added the coupling cap you suggested. At that point the hum got louder and louder and after about 15 seconds the output tube catode bypass cap violently exploded like a fire cracker." Replace the cathode resistor with 10W version, replace the bypass cap with a 100v rated item, keep some air between the bypass cap & cathode resistor, the resistor gets very hot & can cook the cap...cap shorts & takes out the power tube. Post a pic of the power tube wiring. Be sure that the shielded cable has the loose shielded end insulated, to avoid shorts.

"If you mean the tone stack (held up from ground by that 50k pot) in parallel with the MV at the power tube grid, it won't really matter if the wiper is grounded because the power tube bias is developed at the cathode... he could short the grid of the power tube to ground with a screwdriver and it won't really effect the bias level... of course no sound will happen. ha ha" no sound apart from hum?

[MWJB]

It shouldn't do, because I didn't suggest putting a cap between the grid & ground...the cap goes from the grid of the power tube to the wiper of the Master vol. The power tube grid is always referenced to ground via the 470K you just added from pin 5 to ground.

[Hugo]

It shouldn't do, because I didn't suggest putting a cap between the grid & ground...the cap goes from the grid of the power tube to the wiper of the Master vol. The power tube grid is always referenced to ground via the 470K you just added from pin 5 to ground.

That's exactly the way I did it. 470k to ground first, no change in hum, no other effects. Then added the coupling cap between grid and MV, cathode cap blew. I'm just trying to understand why that happened.


Hugo

[MWJB]

The bypass cap probably blew, irrespective of the mods to the grid load & coupling cap.

[tubeswell]

That's exactly the way I did it. 470k to ground first, no change in hum, no other effects. Then added the coupling cap between grid and MV, cathode cap blew. I'm just trying to understand why that happened.
Hugo

Hi Hugo, what was the voltage rating of the cathode bypass cap? and what voltage is being dropped across that cap?