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Vintage threads from the first ten years

Re: Fender hum balance...help me

:

8/18/1999 5:15 AM

Re: Fender hum balance...help me

Bruce:

    If Jason is going to add a second pot to adjust the bias, why not scrap the bias balance control and just wire up 2 bias adj pots to the 2 output tubes? I believe that you just wire them up in parallel from the same negative bias supply- right? Since each pot would be sending the bias voltage to only 1 tube, you might need to change the value of the bias set resistor- right?

    With independent bias pots for each of the output tubes you could just dial in the current you want for each tube, rather than first setting the overall voltage and then adjusting the balance and then readjusting the voltage and then readjusting the balance...

    Actually a single bias adjustment pot is a big improvement so Jason might just want to rewire it per typical BF schematics (he can look at the layout drawing for my Hot Rod Tweed mod to see how I rewired the stock '72 SF bias control (ignoring the 4th terminal on the pot):

http://www.techaccessinc.com/blueguitar/hrt_bm_l.pdf

Steve Ahola

8/19/1999 7:42 AM

Another decent way to do it is just like VOX did in the old days in the AC50 with fixed bias and with a seperate bias pot for each power tube.

It works pretty good for what it is.

Changing the "12K" to ground changes the overall bias voltage dump off pretty well too.

Up or down from that resistance value nets a very useable adjustable bias supply.

I guess a couple saftey resistors around the pots would make sure there was negative bias voltage if the pot went bad.

I just finished refurbing an antique AC50 today and had a chance to mess with it again....pretty easy to work with.

Check it out again and look what the circuit is doing.

http://www.ampage.org/schems/voxAC50-4pwr.gif

Bruce

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