Fixed bias question....

[dehughes]

Okay, this is a totally newbie question, but I'm tired and having my doubts....

On a fixed bias amp, with two octal tubes (EL34s), that have a 1 ohm resistor going from the cathode pin on the socket to ground, I can adjust the bias by connecting my DMM across the resistor and setting it to measure in millivolts. This will give me the bias reading in milliamps, correct?

My question is this: do I set my DMM to read DC or AC millivolts? My instinct says DC, but my tired intellect is causing me to doubt myself. I've done this before, but it was a LONG time ago that I owned a fixed bias amp (all my other ones in the past many years have been cathode biased).

Thanks much!

[oldhousescott]

Trust your feelings Luke... DC millivolts

[dehughes]

Trust your feelings Luke... DC millivolts

Yeah, see, that's what happens when you're awake for 22 hours and THEN you try to bias your amp.

Thanks much...

[Funkalicousgroove]

In that case, ALWAYS keep one hand in your back pocket. It only takes 60 mA to stop your heart if the path to ground is through your other hand sitting on the bench or touching the chassis!!

[dehughes]

In that case, ALWAYS keep one hand in your back pocket. It only takes 60 mA to stop your heart if the path to ground is through your other hand sitting on the bench or touching the chassis!!

Oh yeah. I've been shocked before, thanks to a filter cap and my own incompetence. That one hurt.....but I didn't have the excuse of lack of sleep, just my own carelessness.


david

[v00d00blues79]

whats an ideal pin 5 voltage for 6L6's?

[Moose]

whats an ideal pin 5 voltage for 6L6's?

That's like asking "What's the perfect dress size for a girl?" Every girl is different. Even those who are ostensibly very similar are different. What is she shaped like? Does she like her clothes tight, short, loose, long? It's really best (though dangerous if you aren't careful) to make some measurements first, then adjust from there depending on personal tastes.

You need to do the math. Calculate your plate dissipation, figure what max dissipation is based on the voltage on your plates, then adjust your bias voltage until you're in the 70% of max range. You can fuss with it from there by ear, knowing that hotter will wear tubes out and colder risks crossover distortion..

So, start by reading this:
http://aikenamps.com/Why70percent.html

Figure max dissipation by dividing max rated wattage by the plate voltage. So, if I had a theoretical amp and measured 450V on tubes that are rated for 25 Watts, I'd get

25 / 450 = .0555 A

*1000 = 55.5 mA

So, max dissipation would be 55.5 mA and 70% of that would be 39mA or so.


So, go measure your plate voltage, do a little math (Look at the max dissipation from your 6L6 data sheets. it's usually 30W but it's a good habit to check for yourself) and then tell US what the best voltage turned out to be.

[v00d00blues79]

Moose,

Thanks for the info. I've been fuzzy on the subject of fixed bias outputs for a while. Still trying to understand the intricacies of it.


Thanks,

Andy

[Moose]

It's actually a lot easier than you think -- once you find a way of conceptializing the whole deal. I started appying a metaphor of a valve with the grid voltage being a measure of how far open the valve was and one day it clicked for me.

Just do the math on your amps every time you change tubes and you'll eventually be able to do it without thinking about the formula or having to look up max dissipation anymore. You'll know them by heart.