Grainy/Trashy noise on tube amp

[JoeM]

You stated the noise didnt happen until you added the voltage doubler. Is the noise coming from the doubler circuit itself? Maybe try returning the bias circuit to stock and see if the noise disappears.

[Ampzone]

You stated the noise didnt happen until you added the voltage doubler. Is the noise coming from the doubler circuit itself? Maybe try returning the bias circuit to stock and see if the noise disappears.

Thanks for your input Joe...that's a thought but the bias was too enemic stock and was putting the 34's around 39-40ma.

[JoeM]

If the noise disappears, than at least you'll know it's coming from the doubler. Maybe someone can suggest a different doubler circuit that may work well for the amp.

[Ampzone]

BTW what is the screen voltage on this amp?

Also when you're biasing, you wanna get it up to 50mA, recheck the plate voltage, then redo the 70% calculation. As you bias hotter plate voltage will drop (plate voltage and plate current are out of phase).

Good points. Last I checked It was appx 498 on the plates with the 34's, the screens are on average 3-4v less. I have to run out door but when I get back I am going to recalculate the bias for 88's and see what gives....stay tuned.

[Rhodesplyr]

If the bias adjustment doesn't fix it, I'd second Alex's suggesting of looking for oscillation. I had this problem in a Dean Markley amp in the wiring to one of the 12AX7 sockets. It sounded a bit like one of those rain-stick percussion instruments and was very dependent on the wire position. It's possible that during your bias mod, you moved a wire and inadvertently caused this to happen. The permanent fix in my case was to replace the higher impedance wire of the two with shielded cable. The oscillation was 20V peak to peak, but it was above the range of human hearing. The only way to observe it directly was with a scope.

[Ampzone]

If the bias adjustment doesn't fix it, I'd second Alex's suggesting of looking for oscillation. I had this problem in a Dean Markley amp in the wiring to one of the 12AX7 sockets. It sounded a bit like one of those rain-stick percussion instruments and was very dependent on the wire position. It's possible that during your bias mod, you moved a wire and inadvertently caused this to happen. The permanent fix in my case was to replace the higher impedance wire of the two with shielded cable. The oscillation was 20V peak to peak, but it was above the range of human hearing. The only way to observe it directly was with a scope.

Some good suggestions. I was able to build the bias on the existing strip for the most part but I will double check the wires. My gut says the doubler may be flawed. I don't know.

[Ampzone]

BTW what is the screen voltage on this amp?

Also when you're biasing, you wanna get it up to 50mA, recheck the plate voltage, then redo the 70% calculation. As you bias hotter plate voltage will drop (plate voltage and plate current are out of phase).

Wilder - with the bias at 48ma on the highest reading 88 the plate voltage is appx 489. According to my calculations I can go up to appx 50ma. This particular set of 88's are not super matched i.e one is sitting at 44ma while the other is at 48ma etc. I will go with the highest tube reading and set it for 50ma now.

Oh the screens are now sitting at appx 487 pin 4 and 488 pin 6.

stay tuned....

[Ampzone]

ok...after running the bias up to 51ma on 88's the sound was still fizzing. So I removed the 88's and grabbed another set of 34's I had stashed in a box. I set the highest reading tube to 36.5 the others were sitting around 31-32ma. I realize that 36.5ma is really hot at 500 plate volts but I wanted to try it. At the same time I moved some of the voltage doubler wires around.

Result: the fizzy sound is gone but the tone is coldish still as the other three 34's are sitting around 31-32ma. I can't raise them without raising the highest one sitting now at 36.5ma. I wish I could lower the B+.

Maybe I can find another spare tube that's more matched or another set. Hmmmm...

[olddawg]

Result: I wish I could lower the B+.

You could always put some 10v 5 watt zener diodes in the B+ line. They're cheap and easily removable.

[Ampzone]

You could always put some 10v 5 watt zener diodes in the B+ line. They're cheap and easily removable.

Interesting...this will not present any issues? Never tried this before. Thanks for the input here...gotta figure out where I could tap in without tearing to much up on the turret strip...