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hum balance adj. on Fender amp

9/17/1997 10:16 PM
hum balance adj. on Fender amp
I just bought a master volume Super Reverb and on the chassis is a slotted adj. screw labeled"hum balance" . My question is what does it do and how do you use it? I didn't want to mess with it without some info. Thanks  
9/18/1997 12:48 AM

You get massive amounts of hum induced if your filament supply is floating. The lowest hum seems to occur when the filament supply is exactly centered on ground. Some amps use a centertapped winding and connect the centertap to ground, some use two resistors across the filament winding and tie the middle of the two resistors to ground. A variation on the "two resistors" method us to use a low value (usually 100-400 ohms) pot with the wiper tied to ground. This is what you have. By adjusting the pot, you can balance the filaments around ground and minimize AC hum from the filaments. Just turn it one way or the other, listening for minimum hum. If the best position is not somewhere near the center, something is wrong.
9/18/1997 9:48 AM
John Greene

Actually, on the later Fender models this adjustment allows you to tilt the bias between the two halves of the output power amp. The thought being when the output tubes are biased exactly the same, any AC ripple on the center tap of the output transformer will be cancelled because it will be common mode to the two output halves.  
The adjustment instructions tell you to adjust this pot for minimum hum. The heater supply is already center tap grounded or has two 100 ohm resistors from each side to ground, depending on the model and year.  
9/18/1997 11:16 AM
Mike Schulze
Are you sure you are not thinking of the "output balance" pot? My twin reverb II has both hum balance AND output balance. Output balance - as you said - varies the amount of bias between the two pairs of output tubes. My amp was so quiet I had to measure idle current with transformer shunt to adjust this...
9/18/1997 11:39 AM
John Greene

You're right! I read one thing but my brain heard something else.  
I read the instructions for adjusting the output balance and totally overlooked the fact there was a control there called 'Hum Balance'.  
I just went back over the schematics and tried to retrace what made me think that to begin with and I can't. Jeese, musta needed another cup of coffee.  
9/18/1997 12:38 PM
it's also important to remember that if you can't get the hum to almost disappear, or if it gets really loud and "crackles" as you adjust the pot (sounds like a dirty audio pot), the pot may be blown (open). this usually happens if one of the power tubes shorts the filament within, putting 400vdc or higher on the filament circuit -- thereby using the 100 ohm pot as a fuse (thru the pot wiper to ground). typically, you'll notice a power tube problem, change the tube(s), the amp will sound fine again -- except for the damn hum. unsolder the leads to the pot, put on an ohmeter, and you'll find it OPEN. time for a new pot.
9/19/1997 9:52 AM

That happened to one of mine. I was turning the hum balance pot and saw arcing in there - then BANG! And little streams of smoke came shooting out all the 1/4" jacks! It looked really cool but boy did it stink! Had to replace the pot and a few resistors.
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