Additional bias supply filtering

[deiseldave]

Hello. Is it worth it to go from (1) RC filter to (2) stages on a fixed bias supply? I mean, is there a decent amount of hum decrease? I used the Fender style bias circuit (attached) on my Plexi6v6, and ended up with a 10K resistor from the 10K pot pin 1 to ground, to get voltage right.
If I were to go with a 2nd stage, I would like to keep the 10K pot (since it is already purchased and mounted). Would these values get me in the ball park?: Swapping the first 470 ohm for a 100 ohm, adding 47uf/1K RC, and swapping the 10K to ground from pot pin 1 with a 5k.
If I'm calculating voltage drops right, that seems like it should put me in the ball park. Thanks.

[vibratoking]

I didn't calculate your proposed changes, but the existing circuit that you posted is a low pass filter with a corner frequency of 7.2Hz. This value should have plenty of filtering if working correctly. The circuit you showed has been used on a plethora of amps with no hum problems. What are you looking to gain with this change?

[deiseldave]

What are you looking to gain with this change?

A little bit more hum reduction. I only have a little hum, when cranked. I may be geeking out too much, here.
I've elevated heaters (which helped), chop sticked, and got super meticulous on the grounding. All those helped with buzz and hum.
It seems pretty quiet. But, not quite as quiet as some other amps (Mesa, Soldano, etc) that I've played. I just noticed that those amps have 2 stages of filtering, and starting to geek a little about it.
Maybe it's just a little noisier because the chassis is out of the box (while I'm tweaking), and exposed to fluorescent lights, computer, the powers of yaga-bamba, etc.
I should probably play more, and geek less. Thanks.

[Leo_Gnardo]

Any hum in the bias supply has no effect on hum from the preamp. You wouldn't hear hum from the bias supply unless gains were at low settings and you listened carefully. If hum from the bias supply was there, it would be there even if you removed the output drive tube thereby stopping any noises from the preamp.

In case there is hum from the bias department, my quick n dirty solution is simply add a second cap across the grounding resistor on the back of the bias pot, 27K in this case. Tack or clip on a 4.7 or 10 or even 22 uF and see if that doesn't drop the hum to nearly nothing. Takes seconds and nothing lost if it doesn't work. If it helps, then solder that cap on permanently.

[jjman]

Most any hum there is essentially cancelled in the OPT if it's PP.

[deiseldave]

Thanks for the feedback, guys.