Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dscottguitars on April 21, 2013, 12:05:49 pm

Title: bias supply filtering
Post by: dscottguitars on April 21, 2013, 12:05:49 pm

Hi,  I would like to know about the values used for the filter cap on a bias circuit.

I see schematics with quite different uF values.  Ex:  Bassman 5e6 uses a 100uF/25v and the Bassman AA864 uses a 25uF/50v.  The Bassman 5f6-A uses two 8uF/150v, one before the dropping resistor and another one after.

I would like to use the following setup on a bassman clone: (the 20K load resistor is an estimate, it may change due to the actual amount of negative voltage)

Title: Re: bias supply filtering
Post by: sluckey on April 21, 2013, 12:20:08 pm

I like the better filtering offered by a 2 pole filter rather than one single cap. For that dual bias arrangement I would use two additional 10µF caps, one on the wiper of each bias pot.

Title: Re: bias supply filtering
Post by: dscottguitars on April 21, 2013, 12:30:06 pm

Thanks, I can do that quite easily.  Wouldn't that be the same as using one 20uF cap?  What does the 2 pole filter do compared to a single, larger value?

Title: Re: bias supply filtering
Post by: HotBluePlates on April 21, 2013, 12:55:26 pm

The added caps are separated from the original cap by a resistor, say 15kΩ or thereabouts.

It's the added resistor along with the new cap that increases the filtering.