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Re: vibrochamp bias

12/4/1997 12:49 AM
Dave S.
Re: vibrochamp bias
Doc, you had wrote:  
What I'm wondering is for all the interest in ways to select reduced power  
modes, why not vary the screen grid voltage (within certain setpoints) on  
the outout tubes with an adjustable voltage divider?  
If I tried this method of output power control... what limits would I have to stay inside? Did you just mean that one should'nt get the voltage to high (exceeding G2 max dissapation) or to low (starving the tube)?  
Do I have this right?  
Dave S.
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12/4/1997 9:42 AM
J Fletcher
Well I was thinking along those lines and I lowered the screen voltages by a 100 volts,using zeners in series with the screens,in a homemade 40 watt amp.What I found was that once I adjusted the bias to get the plate current back to it's previous value,and changed the reflected primary impedance of the output transformer by selecting a 8ohm load as oppossed to a 4,power output remained the same.So I was back where I started,although perhaps my tubes were happier with lowered screen dissipation.In the end all I'd seemed to have done was increase the impedance of the tube.
12/2/1997 5:57 PM
great help. I was measuring V to ground. and have gone back and replaced the cap too. I was thinkig in terms of ma not watts so the 44ma( and the slight glowing in the plate) was making me real uneasy. subtracting out the catode to ground V makes the dissipation right at 14 watts which is how this guy likes his amps...  
By the way I set the resistance with a reostat and a 1 ohm resistor in place of the fixed resistor in the cathode and measured the V across the resistor.

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