can't get the bias negative enough

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gammelnalle
Junior tube assistant
Posts: 20
(2/24/04 3:56 pm)
 can't get the bias negative enough
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 Hi again !!! Working on my homebrew bassman-vox following a schematic from Kevin O connors TUT 3 (or actually 2 schematics) I can't get the bias "under" -44V. I wanna be sure to reduce (?) it even lower.
The minus is taken via 2 diodes direkt from the PT 2x 285V to 2 resistor 82K (100K in original) 10uF cap 15K resistor and 3 33v zener hooked up in serie and a 10uF cap. With NO load on this I get 99V but when loaded with the 2 trimpots + 6,8K it drops to max 44v.... (if I ohm-measure the pots in oaralell I get aprox 12K....)
Can I increse the 6,8K to 15 k ????

thanks a lot
 
ganzonimx
Hey get your own solder
Posts: 403
(2/24/04 5:21 pm)
 Re: can't get the bias negative enough
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 I think you complicate things unnecessarily (zener's!?). Look at the bias circuit of a Marshall like in this schematic.

The circuit is a simple voltage divider taped off of a HV secondary. To change the bias range, you can lower o higher the 150K resistor. Try to run about 1 - 2 mA trough the divider.

Cris
 
ernie jr
Junior tube assistant
Posts: 68
(2/25/04 3:41 pm)
 Re: can't get the bias negative enough
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 you could get a separate transformer, a small one with 12.6 volt sec. hook the 12.6 to the 6.3 heater tap, this will give you about 60 volts ac before the rectifier.
i have used this before, works well
ernie
 
Lucid Alice
Senior tube assistant
Posts: 165
(2/25/04 8:18 pm)
 Re: can't get the bias negative enough
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 The bias power circuit is basically a voltage divider generating a negative voltage. The first resistor coming from the transformer is the one that sets the maximum voltage. It should be a fairly large value, 75K to 200K?... something like that. Making this value smaller should make the bias voltage higher. Making this value larger should make the bias voltage lower. Try different values until you get the voltage where you want it.


 
6G6
Junior tube assistant
Posts: 94
(2/25/04 10:19 pm)
 Re: can't get the bias negative enough
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 Somehow I kinda like Ernie's idea.
It just feels like it would leave the B+ a touch higher.
I suppose that in reality, whichever way you go, there isn't enough current involved to make a noticable difference.
I would think that the bias would be more stable with the transforer off the heater circuit.
 
 
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