gain vs hum (AC heater)

[kambo]

while i was messing with AC heater on slowblow mic pre,
i noticed peaking hum at 120hz was canceling (null) when gain was ~50%
(60Hz and upper harmonics too)

dead quiet, no noise, great sound ???  but only at ~50% gain

any idea why ?



thanks
kambo

 

[ruffrecords]

Possibly heater hum in anti-phase with residual hum on HT cancels out at one gain setting.

Cheers

Ian

 

[kambo]

thanks Ian,
i will mess with AC heaters more when i get time...

 

[PRR]

This happens a lot with AC heat tubes on both sides of a gain pot.

The tubes invert. It often happens that V1 hum is out-of-phase with V2 hum. At some magic gain setting, the two hums cancel.

The FIX is to cure BOTH hums.

 

[kambo]

thanks PRR,  makes more sense now.
kambo