NFB on 5f1 champ HUM

[freakshow__]

Hey all

I just finished a 5F1 champ build and it works great except for one thing.....

With the NFB NOT engaged it sounds great, only a slight low level hum, which I am seeing is fairly common?

However....when I engage the NFB switch adding the 22K NFB resistor to the 8 ohm tap of the OT...it gets a really loud hum and the actual guitar volume gets quieter.

Any ideas in why it's doing this?

My OT is a Hammond 125CSE model for reference.

I have already tried reversing the OT primary leads and it doesn't make a difference.
I have gone through my component values and wiring 2x as well and I can't seem to see anything out of place.
My plate voltage is 349 and idle current is 42mA. So bias seems pretty common...
I did not use a tube rectifier...I have the common (4) 1n4007 diodes instead.

Any help would be greatly appreciated!

[sluckey]

Sounds like you don't have the OT secondary connected to chassis ground.

[freakshow__]

the layouts do not show it connected to ground. One end to the first filter cap and the other end to pin 3 of the 6V6 tube.


EDIT
my bad...I just read that too fast...no...the secondary is not chassis grounded.....hmmmm

[freakshow__]

That was it Sluckey. Appears to work correctly now....

NFB OFF...louder and less compressed sounding to me.

NFB ON...quieter and more compressed sounding.

is 22K the happy place for the resistor...or are lower/higher values "better" on a 5F1?

[sluckey]

Leo thought 22K was the cat's meow. Seems that nowadays every champ owner likes a different value. Experiment at will.

[Stevem]

If you play around enough the bottom line is that pickup wise, Humbuckers and single coils will want different values, especially depending on how loud you commonly play the amp!

You might even find that between neck pickups and bridge pickups on some guitars that you can optimize the average needed value of that resistor better.