5f11 bias and hum help...

[Phil_Tre]

Hi !

-last december, I built a 5f11 Vibrolux, everything went smooth and I'm pretty pleased with the result...
Except for one thing, there is a slight hum (60Hz, not 120Hz)...

-I can completely kill the hum by grounding the tremolo (foot switch) and turning the intensity/depth
pot pass 5 or 6... I also found that if I bump the bias capacitor from 25uF to 50uF, the hum almost disappear.

-Is't not a big noise, I can live with it but as I'm a bit of a perfectionist, I would like to fix it the right way.

-Now the question is, what is the drawback of a larger value bias capacitor ? the schematic called
for a 25uF, what about 50uF or maybe 100uF ? The power trans have a 50VAC bias tap so I think
that there is no charge/time problem ??? Tone wise, I don't want something different like a cold
or stiff amp...

-The circuit is exactly like the Fender 5f11 schematic except for a bias pot and a 1Meg resistor @ jack 1.
I have try 3 matched set of power tubes, different rectifiers, PI and preamp tubes...

-Bias is adjustable (Sluckey mod).
-plate: 376
-bias: 23mA
-A very cold bias reduce the hum as well (say 10mA)

Any input is welcome,
Thanks for your help !!

Phil.

Photos of the amp + circuit: https://www.flickr.com/photos/96104445@N05/sets/72157644520362199/

[Phil_Tre]

answer to myself or for those interested:


changing the bias capacitor for a larger one does makes a difference !! or maybe it's in my head 


-First, the tremolo depth and speed is reduced by 10~15 %. At first, I thought it was the bias that
has shift but no, same thing...


-Second, the tone seem rounder/fuller, more bassy, but in a good way... again, I'm tired and it could
be my imagination but this time, I really like the tone, not just good but great, because I always feel
that this amp was lacking bass...


Now feel free to comment as I'm still trying to understand what happened, I cannot see how going from
25uF to 50uF could makes a difference...

As for the hum, it is reduced by 7dB... I did try an other 25uF in case the first cap was defective but no,
same hum, the 50uF did it...


Ph.

[PRR]

25u to 50u would indeed be 6dB. You say 7dB, whatever.

[Phil_Tre]

25u to 50u would indeed be 6dB. You say 7dB, whatever.

I "measured" it with an iPhone app, it's more precise than I thought