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Strange pot in the Hybrid-A schem.


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9/12/2004 7:57 PM
Tommy Dale
Strange pot in the Hybrid-A schem.
In the Hybrid-A schematic, the "middle" pot in the tone stack, looks like it's a trim pot, but it's supposed to be a panel pot. Is it a panel mounted trim pot, or is it a regular pot, with three lugs, that is just drawn like a trim pot?  
 
Tommy
 
9/12/2004 8:09 PM
Daver

It's a regular pot. That tone stack is one of the unique features of the Dumble amp.  
Daver
 
9/13/2004 5:46 AM
Tommy Dale

Ok, so since it's drawn like a resistor with a arrow across it, I suppose it's connected between the middle lug, and one of the side lugs, while the third lug is not connected at all?  
 
Also: The power transformer has a secondary for bias. How many volts is that secondary usually (nice to know when I shop for transformers). I have never used a adjustable bias before.  
 
Tommy
 
9/13/2004 9:38 PM
Daver

When it's drawn with an arrow through it, it's wired as a rheostat. Looking at the back of the pot, the left lug would go to ground, the center and right lugs would be wired together and that connects to the cap from the tone stack. You could just use the center lug to the cap, but since that's the wiper, if the pot gets dirty you may lose your connection there.  
 
Typically the bias tap will be about 50 VAC. After rectification it will be around -60VDC.  
 
Daver
 
9/14/2004 6:09 AM
Tommy Dale

What cap are we talking about..the 0,047F ?  
 
You say -60VDC. Is that 60V less than the plate voltage, or is it 60V "less than zero". ?  
 
Tommy
 
9/14/2004 8:01 PM
Gary (aka Glasman)

You say -60VDC. Is that 60V less than the plate voltage, or is it 60V "less than zero". ?  
 
 
The bias voltage is negative with respect to ground or zero volts. Both B+ and bias share the same common return path of the transformer secondary.  
 
Gary
 
9/15/2004 6:37 AM
Tommy Dale

As a newcomer to amp making, I have some problems with understanding how the voltage can be negative with respect to ground... What is done differently on the bias secondary compared to the other secondaries, that produce positive voltages....? Maybe a stupid question...  
 
Tommy
 

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