Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on September 22, 2018, 10:24:53 am

Title: Capacitors in the bias circuit?
Post by: plumcrazyfx on September 22, 2018, 10:24:53 am

I'm wondering about the wide variety of different uses of caps in the bias circuit.  I've seen everything from single 50uf or 100uf caps (Fender) to multiples (10uf) with a dropping resistor in-between.  What are the pros and cons for these different methods.  Does this differ whether you are getting your bias voltage from a tap or from the B+ line?  I started to think about this with a Bassmate that has a 100uf/100v then a dropping resistor followed by a 10uf.  I was going to replace the second with a 33uf but began wondering about what is too much filtering (if there us such a thing) in the bias supply.  Is there too much?

Title: Re: Capacitors in the bias circuit?
Post by: shooter on September 22, 2018, 10:58:57 am

Quote

too much filtering

No such thing  :icon_biggrin:
there is very little if any current, but I would think you want the DC rock solid, so the less AC variations (ripple) the better, that said, the supply is typically 1 diode, simple, so the smart engineers didn't seem to be bothered

Title: Re: Capacitors in the bias circuit?
Post by: sluckey on September 22, 2018, 11:07:30 am

You really don't want the caps so big that they are still trying to charge up long after tubes are warmed up and the B+ voltages are up.

Here's a short read that should make you a bias expert...

     http://www.valvewizard.co.uk/bias.html