Tantalum Capacitors in the bias circuit
Moderators: pompeiisneaks, Colossal
Tantalum Capacitors in the bias circuit
I recently started building a Fender Pro Reverb clone and decided to include a dual bias circuit. Each 6L6GC now has a separate bias adjustment. I am tapping the bias voltage into two circuits consisting of a 50kL pot, a 22k resistor in parallel with a 22uf 100v tantalum capacitor + to ground. The wiper of the 50kL pot will now be supplying the bias voltage to the output tubes. I've read much about Tantalum's not being reliable. Does anyone think I'll have a problem using the Tantalum's in this circuit and do you think it will affect the tone of the amp since the Tantalum is part of the 6L6GC bias to grid circuit?
Re: Tantalum Capacitors in the bias circuit
Tants don't like over-voltages - some even explode. I don't use them in amps, elcos are fine in a bias circuit, they are more reliable and cheaper.
BTW, the bias circuit you describe would lead to thermal runaway if the pot fails - no wiper connection, no bias, max current thru the tube.
You could consider a fail safe bias circuit, with the pot acting as a variable R to ground and the bias signal taken at the junction of the dropping resistor and the pot. If the latter opens, max bias would be applied.
BTW, the bias circuit you describe would lead to thermal runaway if the pot fails - no wiper connection, no bias, max current thru the tube.
You could consider a fail safe bias circuit, with the pot acting as a variable R to ground and the bias signal taken at the junction of the dropping resistor and the pot. If the latter opens, max bias would be applied.
Last edited by FYL on Mon Jan 17, 2011 2:07 pm, edited 1 time in total.
-
Cliff Schecht
- Posts: 2629
- Joined: Wed Dec 30, 2009 7:32 am
- Location: Austin
- Contact:
Re: Tantalum Capacitors in the bias circuit
Yeah listen to FYL and go for the failsafe option, there is no reason not to. I have no qualms about using tantalum caps in amps if I know they are going to be within their safe operating region.
They are quite dangerous when they do explode, they shoot molten tantalum gel everywhere and it can burn you badly!
They are quite dangerous when they do explode, they shoot molten tantalum gel everywhere and it can burn you badly!
Cliff Schecht - Circuit P.I.
Re: Tantalum Capacitors in the bias circuit
Too many amp designs with adjustable bias pull the voltage off the wiper.FYL wrote:Tants don't like over-voltages - some even explode. I don't use them in amps, elcos are fine in a bias circuit, they are more reliable and cheaper.
BTW, the bias circuit you describe would lead to thermal runaway if the pot fails - no wiper connection, no bias, max current thru the tube.
You could consider a fail safe bias circuit, with the pot acting as a variable R to ground and the bias signal taken at the junction of the dropping resistor and the pot. If the latter opens, max bias would be applied.
It's not the way I would do it. It's just so easy to design as FYL says fail safe.
It's also a trivial matter to design for the failsafe mode to still apply a bias that's at the cold side of normal operation and able to continue usable operation.
Concerning the Tantalums - in theory they look superior to other types.
My experience is that they fail to a short.
OK for low voltage preamp design, not as attractive when there's the potential to take out other more expensive components.
rd