Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on February 24, 2011, 08:05:37 pm

Title: 1/4 Watt Bias Pot?
Post by: jeff on February 24, 2011, 08:05:37 pm

 Is it ok to use a regular 1/4 watt pot for bias adjustment?

I got a Bais pot and I don't know what it's rated for. It's only a 10K and I want to replace it with a 25K for a wider range of adjustment but all I have is standard 1/4 watt pots.

Also does 39mA for 5881 AB at 410V sound right?

 I have a "matched set". One's pulling 39mA the other 33mA. When I swap tube sockets the current follows the tube not the socket. Is this close enough? Will it hurt anything if I run it like this until I can get a better matched set?

Title: Re: 1/4 Watt Bias Pot?
Post by: jjasilli on February 24, 2011, 08:29:23 pm

1/4 watt pot is OK.  Watts = volts X amps.  The bias circuit draws about -0- cuurent.  So no watts are dissipated.

25K is not necessarily more usable range.  Usually the bias adj pot is wired as a variable resistor, sittng atop a fixed resisitor to ground.  The fixed resistor insures adequate bias voltage in case the adj pot is turned to -0-.  Note that a few +/- K Ohms makes a big difference.  The range to shoot for is a min bias voltage for hot bias, but not hot enough to damage the tubes -to- hi bias voltage for cold bias; plus some extra leeway for tube drift and different tubes.  Any further range is too hot or too cold, so not useful.  You can probably get full useable range from a 10K pot.

Note that a 10K pot is easier to fine tune than a larger pot, unless you use a multi-turn pot.  For more info see:  http://www.harpamps.com/micKfender/Princeton-Adjustable-Bias.html

Title: Re: 1/4 Watt Bias Pot?
Post by: jeff on February 24, 2011, 08:51:07 pm

 Cool thanks.
 
How 'bout the matching?

Does 39mA sound right? 5881 @ 410V

Title: Re: 1/4 Watt Bias Pot?
Post by: 67polara on February 24, 2011, 11:14:18 pm

I figure your idle is at about 30 watt for both tubes.  You have about 60 watt available for you to use, if you want.  But right now your tubes should last a really long time.  30 watts can be really loud anyways, you may not need more power than that.  If it sounds good don't mess with it LOL.

Tony 

Title: Re: 1/4 Watt Bias Pot?
Post by: HotBluePlates on February 25, 2011, 03:30:30 am

Your tube matching is fine. At least, it sure won't hurt anything.

In any event, idle current matching is only a starting point, especially in class AB amps and in non-triode tubes. But we don't have a way to readily verify any other form of matching, now do we? Which kinda makes the whole matching business a little silly.

In any event, all the ancient tube books talk about matching in push-pull amps being important to achieve maximum clean output power. And we often want output tube distortion, don't we?

Title: Re: 1/4 Watt Bias Pot?
Post by: jeff on February 25, 2011, 06:55:56 am

Good point. And if it won't hurt anything I'll just use these for now.