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Re: Tube amps

11/6/1999 3:46 AM
Re: Tube amps
thanks Ben, I had forgotten about all the good stuff @ the machine gun site. I had already converted the bias to BF style, just thought it would be nifty to be able to balance it too. The power tubes in this ampa re old fender brands one has no getter left at all and is weak, so maybe I'll talk the owner in to a new pair...  
11/6/1999 4:54 PM

It's stupid simple.  
You just remove the one reistor that is grounded.  
That is the one that sets the overall bias voltage, and usually solder fromt he bias balance pot right to it's shell.  
Replace it with a little 22K-25K trim pot...(wired as a variable resistor...use only two of the lugs)... and put a 6K8 resistor in series with that trim pot right to ground.  
Now you can vary the amount of negative voltage that is shunted to ground and also balance how much is left over to send each side of the OT's power tube(s).  
You should be able to swing somewhere close to -40v down to about -60v.  
If the little trim pot goes open, for some reason, the negative voltage goes up and the tubes will not be hurt.  
If the trim pot goes bad and shorts out some how (this never has happened to me in many many years), then the negative bias voltage will rise (more positive) and the tubes are in danger of being underbiased.  
I wouldn't worry about this myself.  
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11/6/1999 7:21 PM

Hi Bruce:  
I knew it was simple, but I musta done it wrong cause it didn't work. This works with the tapped balance pot , right?  
I used a 25K trim pot and a 15K resistor to replace the 33K resistor that came from the center lug and was soldered to the backside. the outer lugs send the bias to the tubes and the tap feeds the bias to the pot.  
would there be any effect, or any point, in changing the 47K & 68K resistors to the grids?  
Did I mention this is the 77 SF 70W srper I've been noodling around with?  
Thanks for your help!!!  
11/5/1999 10:35 PM

OK I'll bite.  
Have you got a diagram of this set up? A description would be OK too. I've got one of those tapped balance pots that I'd like to do that to, or I'll just set up two separate bias pots. I haven't decided, yet  
Bruce C. has a cool way of coverting it to get the  
best of both worlds.......bias adjust and hum  
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