Bias settings on ampifiers...

[Gino Iorfida]

The grid current is generally in the 5mA range, give or take

The 1% on a 1 ohm resistor is nto that big of a deal, but you also have the tolerance of the meter, and since we are dealing with such small voltages, the readings have more chance for error. WIth a 10 ohm resistor, and say you read you are getting 390mV across the resistor, you know you are drawing 39ma etc.. basically, you drop the last digit, however your error is greatly decreased. I just like to know the EXACT value when I do this etc... but then again, to be honest, I always use the transformer shunt method, quicker for me, however, I had one amp here that I used as a prototype/testbed amp, that I had 10 ohm resistors installed, simply because I was ALWAYS tweaking SOMETHING in the amp etc, and I had flying leads coming out into a separate box, so I could try different tubes on the fly/play with different bias settings etc.
<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Gino Iorfida on 30 June 2004 at 07:26 PM.]</p></FONT>

[C Dixon]

I assume you are talking about the "screen grid"? Since there should never be any control grid current with no signal. And even with signal, there should be no control grid current unless the tube is being driven beyond clipping.

I still would like to see an official set of procedures to adjust the bias on a Fender amp. Again, I believe this to be a serious blunder on their and any other amplifier manufacturers, who does not provide this info right on the schematic drawing.

I base this on the fact that it is general "consensus" how important setting the bias is. Thus, 3 different technicians adjusting it using 3 different goals* means at least TWO of them are almost assuredly leaving it wrong.

This to me leaves the customer with an amp repair less than optimum in upwards of 66% of the cases; or more!

carl

*Note: I use 3 since the following 3 methods are the most common procedures I know of:

1. Adjusting for minimum hum (many of us feel this is indeed incorrect)

2. Adjusting for an arbitray amount of cathode current. (WHAT is this current, and HOW was it arrived at?)

4. Adjusting "til it "sounds good". (the problem here is "who" is the judge? IE, way too subjective to be a technician's procedure, IMO)

[Paul Graupp]

Carl: The worst shock I ever got, gaging by my reflex actions, wasn't electrical in nature. When I was a BMET, I was on call a lot an wore a pager all the time. When I left that job, I swore I'd never wear one again; it felt like a growth on my hip.

But a new wife insisted on getting me one for a birthday present and you know I had to wear it. To make it inconspicuous, I put it in my pocket on the vibrate mode. That moring I was working on an airborne radar transmitter; in the maggie section with a huge HV probe and just as I was about to make contact, that *&&%^ pager went off.

It would be hard to describe my antics but the other techs just knew I had killed myself as I threw probes and wires and test sets, chairs and stools etc every which way trying to get away from it. I was absolutely unhurt except for my knee jerk responce and feeling very stupid. To this day it still doesn't seem funny to me but the other guys got a real laugh out of it.

I never wore the pager again...

Regards, Paul