Jcm 900 4502 bias

[radcochran]

I have a Jcm 900 combo and have never biased an amp before. I think I can do it with a little advice. I have a bias probe, and a multimeter. it is a fifty watt 5881 amp. where on the amp is the bias adjustment, and where should I set it. I am going to retube with JJ 6L6. Thanks. Any other info someone would be willing to offer a newbie would be greatly appreciated.

[Mars Amp Repair]

hello,
It really depends on your level of experience with a meter & working around LETHAL voltages. Honestly, you can be killed if you get yourself across 450DC & ground thru your body. It's not that hard to be careless around those voltages...dangling watches..rings touching wrong things...etc...really.

Also need to know if you have a decent Mili-amp range on your meter. That's pretty much a must for the output transformer shunt measurement method.
If your meter is of the LCD digital variety, then you're already ahead of the game. Your probes should be very sharp on the end or you'll likely slip off the point you're measuring & possibly do much damage...at the very least you'll need a new pair of shorts.

You need to calculate what the current should be for a given idle wattage of the output tubes. For 6l6's you probably want to be around 18-22watts.

If your plate voltage is around 475vdc, then your idle current would be around 40ma per tube. Sorry I don't know what the plate supply is for this amp. but I'd guess setting the idle current for around 35-40ma will do the trick.
I think I'd stop at this point & ask you how you're doing with all this so far...

glen

[radcochran]

bias 900

Isn't there a simpler and safer method using a bias probe that fits into one of the tube sockets, and then the tube goes in that.

[Mars Amp Repair]

Sure it would be much easier with the adaptor if you happen to have the little gadget around.
I think the cheapest I've seen are still around $50.
They ususally insert an 1ohm resistor in series with the cathode & then you're measuring the voltage drop across the resistor which translates directly to milliamps (ohms law dictates).
I actually have the Ruby Bias tool with the LCD readout (procured it with a bunch of other stuff). I only use it if I am not needing to take the amp apart & if the bias adjustment is accessible from the outside.
Also note that you will be measuring the plate current as well as the screen grid current with the cathode method, so your readings will tend to lean on the higher side. glen

[Mars Amp Repair]

oops,
I just noticed that radcochran did mention he had a bias tester...I'm basically answered the question in there...if the tester has an internal 1ohm resistor then you will need to use the dc volts range & set the bias pot for between 35 & 40mv...which translates by ohms law across the internal 1ohm resistor in the tester to 35-40ma.
If the tester just allows you to insert a milliameter between the cathode & pin 8 with no internal resistor, then you'll use the milliamp range & set for 35-40ma. Note with this type of setup, you can get weird reactions if your leads are too long.

Sorry for the initial confusion......glen

[Enzo]

Yes, if your bias probe is like most, you plug it into the amp and then the tube into the probe. So the probe is between tube and amp. Then the probe connects to your meter by a cable coming out of it. The meter should be set to read low DC voltages, since in most cases the reading will be about 35 to 40 millivolts. That is the same as 0.035 to 0.040 volts. Millivolt means 1/1000 of a volt. If your probe is fancier with meters on it, then it might read out in milliamps directly. READ THE INSTRUCTIONS for it.

Since the probe has a 1 ohm resistor in it to take the reading across, the milliVOLT reading on your meter translates into milliAMPs running through the tube.

MAKE SURE there is a speaker connected while working on this.


The voltages inside this amp can KILL YOU. Please be careful. You touch that 500 volts while leaning on the chassis, and it can stop your heart instantly.

With the amp running and the standby switch on too - so the amp makes sound out the speaker if you play through it - and warmed up for at least ten minutes, and your probe installed under a tube, it is ready to adjust. Adjustment is made while NOT playing though.

There will be a small trim pot on the circuit board inside - the kind you adjust with a small screwdriver. Unless you are really confident, wrap the screwdriver blade with masking tape except the very end. This is to prevent touching something else with the tool shaft. WHile monitoring the tube current with the meter and probe, adjust the bias voltage with the trim pot to get the target tube current.

Bias is not nearly as critical as people seem to think it is, so if it is a couple milliamps off, no big deal. If the two tubes are "matched" then they should measure similar current for a given setting of bias. If one tube is off from the other, adjust so the target current is centered between the two values.

You will have to make readings on each tube to find out how close they are - or are not. Without adding a whole discussion of the math, 40ma is a reasonable setting for that amp. 40ma per tube.

[radcochran]

formula?

how do I measure the plate current of the amp? Could you show me the formula of how that figures into the equation? In other words, how do you arrive at the 40ma number?

[Enzo]

The whole purpose of the bias probe is to measure current through the tube - plate current.

DO you have the instructions for the bias probe? If not, what brand is it? COntact the maker for replacement instruction. READ THEM.

Plate voltage - actually voltage from plate to cathode, but in this case they are the same thing - times current gives dissipation in watts. We are measuring at idle.

The 6L6 is a 30 watt tube, and the populat target is 70% of that - 21 watts. You will have to measure your plate volts, no one knows what you have there. But I will use 480 volts as an example. If I am off 10-20 volts, oh well, not a great deal.

480v times X = 21 watts. Solving, we get: X = 21/480 = 0.044 = 44ma

So if your plate has 480v and there are 44ma flowing through, then your tube is dissipating 21 watts. 40ma is a nice round figure and works the tube a watt less. As you make adjustments, be aware that the more current through the tubes, the lower the plate voltage will sag. SO if you have 30ma and the plate is at 490v, turning it up to 45ma might drop the plate to 465v. I made up those numbers to illustrate the point, you have to see how your amp reacts.

Set the current once based on the original plate voltage. Then recheck that voltage, recalculate a taerget, then tweak the setting. It ain't rocket science, just get close.