ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Ideas on biasing

:

8/17/2000 5:05 PM

Ideas on biasing

Hi,

I've thought about this idea for a while and was wondering if anyone has ever tried it. I'd like to have some kind of constant current source for the output tube at a fixed value, set at say 30mA, regardless of what tube I use(same tube type of course). I'm using fixed bias right now, so I can't think of a way to do this, if its even possible. So when I pop a tube in it should draw 30mA. I was thinking to lower the negative voltage to the grids and have the tube draw just a bit more current than I want, then using a MOSFET or transistor or some type of regulator in either the cathode or plate conection of the output tube to make a constant current draw of 30mA. Am I dreaming this idea up or can it be done?

Any comments??

Thanks!
Ben

8/17/2000 5:21 PM

ben, it can certainly be done.

something to keep in mind is that the CCS has a minimum voltage which must be maintained across it in order to function. thus you might need to impliment a negative supply rail.

is this for a SE or PP application?

i have seen schematics for using a mosfet in the (shared) cathode circuit of a PP amp, but the outputs were run in class A.

the advantage of the mosfet is that they draw no grid current, so that you can bias them VERY quietly with a simple 9V battery. it will last years in that kind of service.

the other advantage in a PP amp is that it raises the CMRR, so that you will get even LESS noise injected by the phase splitter and output stage.

there was some talk on here a while back about using 3 terminal voltage regulators as CCS's with the intent of biasing output stages. the dropout voltage is higher with that kind of approach, but it has simplicity going for it. iirc, someone actually TRIED it out.

kg

8/17/2000 5:29 PM

Thanks for the reply Ken.

This is for a PP hi-fi amp actually, using 6AR6 tubes for the outputs. I like to check my bias and keep the tubes matched fairly close, but who wants to keep on tweaking 4 pots every now and then? Can you point me in the direction of a schematic or website, if you know of any, because I haven't done any extensive building with MOSFETS? Well, I'm glad to know this can be done.

Thanks again!
Ben

8/17/2000 5:26 PM

Bias current is IDLE current (when no signal is present). What's going to happen when an audio signal is sent to the output tubes? In class AB at full clean power, an output tube can easily draw well over twice the idle current during the top (or bottom, depending on which side of the push-pull circuit we're looking at) of the signal waveform. So, wouldn't a current regulator interfere with the normal operation of the output section?

8/17/2000 5:34 PM
Ben	Hmmm, thats a good point Jim. Is there a way around it? Thanks! Ben

8/17/2000 10:01 PM
Brian	Well, one way would be to not use a CCS seriously, I am curious about jim's question. BT

8/18/2000 3:28 PM

There was an article a few years back in Audio Amateur that featured an op-amp and a small valuie resistor in the output tube cathode. It formed a bias servo that worked well in the audiophile amps I used it in. I just don't think bias is that critical anymore. Once set (and with separate pots per tube), and if checked periodically, it shouldn't go too far unless your tubes fail or are of poor quality.

Page 1 of 3