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How to use the hum balance pot?

2/29/2004 9:58 PM
Carlo Pipitone
How to use the hum balance pot?
I'm going to do a general overhaul and setup of a Bassman 135 amp (please reply also to my other post in the Mods & teaks forum ;) ): cap job, component check, etc.  
This is my first time with a late-70's MV model, and it has the hum balance pot.  
How do I use correctly such a pot?  
P.S.: it is the export model with a 3-prong power cord.
2/29/2004 10:22 PM
A Colman

The hum pot is to reduce hum when you are using slightly missmatched tubes or compensate as tubes age ,just turn it one way or the other and leave it where it hums the least.  
3/1/2004 8:01 AM
Le Basseur
"The hum pot is to reduce hum when you are using slightly missmatched tubes or compensate as tubes age "
The hum pot has nothing to do with the age or mismatched tubes.It's only a "symetrical device" ;)between filament's ends and ground.  
The hum pot found in Bassmans can be replaced with two 47 Ohm/0,5W resistors,if you don't want to bother too much by rotating the pot each time you "feel" your amp has a little too much hum and in order to provide a higher degree of safety when a tubes fails (the small resistors will act as fuses,but the rest of the circuit will stay unharmed).  
3/1/2004 1:39 AM
Jack Hoff
Is it anything like a bias control?
3/1/2004 2:19 AM
A Colman

3/2/2004 2:32 AM

There are two kinds of hum balance pot and some amps have both.  
One is the ground reference for the heater supply and that does indeed have no relation to tube wear or mismatching. Twiddle it for laest hum. It mainly affects the preamp.  
The other is a balance for the output tubes and very much depends on the tubes themselves. When you see Fenders with the bias pot wired as a balance rather than a straight voltage adjust, that is what it is for. Twiddle for least hum. A push pull output stage will cancel its own hum if the sides are balanced.
3/2/2004 9:07 AM
Carlo Pipitone
Is hum affected by the main AC line?
Thank you guys. :)  
This amp (a Bassman 135) has got two pots on it's back, labelled as follows:  
This seems to fit perfectly the two cases listed by Enzo (hello!)  
Now I'm gonna twiddle with both for leat hum, keeping an eye on tubes current (four 6L6's) with the bias meter.  
One more question:  
is hum affected by the main AC line?  
I mean, shall I adjust the hum balance put according to the place?  
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