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previous: GrungeMan Thanks for the answer to my confusi... -- 11/24/2000 10:19 PM View Thread

Re: Bias Winding Supply Specs?

11/25/2000 6:51 AM
Bruce
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Re: Bias Winding Supply Specs?
Hmmm....I'm not positive here so please cut me a little slack.......  
 
I'm not sure all the Bedrock amps used fixed bias... (cathode biased)... but if they were fixed bias, there's still a good chance you were looking at 18v-0-18v AC to FW rectified DC switching circuit supply leads.  
 
In anycase, the chatter about bias supply current is rather moot because it is extremely low, much much less then the 20ma you think sounds low.  
 
I'd guess the actual current rating is based more about the bias supply DC resistance to ground,... the part of the grounded voltage divider.  
 
With that in mind I'd still figure to build the bias supply with about 2ma - 5ma in mind or at most under 10ma.  
 
Keep in mind the bias voltage is more of a static thing who's voltage is regulated by the divider mentioned above, so, I think that's where the current is really flowing.  
 
There is no DC ground at the grids themselves and the bias supply filter caps are probably the lowest impedance to ground for the AC signal developed across the grid load resistors.  
 
Here's an example of what I'm saying....  
 
say the bias supply in the Marshall amp mentioned in the other replies, has one leg of it's high voltage winding connected to a diode through a 100K resistor.  
 
Well, this rather sets the limit of the current flow, if that HI-vac stays constant.  
 
If the high voltage was say, 300vac RMS, and the 100K was dead grounded, Ohm's law would show that there is 3ma flowing to/from grounded center tap of the PT... 300v/100,000ohms = 3ma.  
 
We know there is a little less current then that because the 100K resistor is not quite dead shorted to ground.  
 
The voltage goes through a divider network and a pot to adjust that amount of useable divided voltage before the left over voltage is grounded.  
 
 
 
Bruce

 
Replies:
Steve A. Bruce: -- 11/25/2000 9:30 AM