Have not biased before...

[playwell]

I posted this in wrong category.

I have never biased an amp before - need some help in doing so.

I have a basic DVM and nothing else. What do I need, and how do I do it? It's a JCM900 SL-X with EL34. Thank you.

[Flames1950]

Are the JCM900 output tubes mounted in sockets with leads going to the board, or are the sockets mounted to the PCB's themselves? If your sockets are PCB mounted you may not be able to try this.
The usual quickie method is to get some resistors that measure EXACTLY one ohm (get a bunch so you can match them exactly.) Get something in the 1 to 2 watt range. EL34's have pins 1 and 8 connected together and then going to ground. Remove the ground connections and substitute the one ohm resistors in their place.
Now you can measure your cathode current at idle by Ohm's Law. Since Ohm's Law states that current = voltage / resistance, and your resistance is exactly one ohm, you get current = voltage. Set your DVM to DC millivolts (or whatever the smallest range may be) and measure, by the formula above the millivolts of voltage will also equal the milliamps of bias current at idle.
Again, if the 900's have power tube sockets mounted to the PCB this is not a good way to start. In that case hit eBay for a bias probe-style tool that will let you hook up your DVM to the tool and measure the millivolts. The one I got has sockets for two tubes at a time and ran about $60, money well spent if you bias often enough (that company was called Amp-Head IIRC.)

[playwell]

The tubes are mounted in sockets on their own - they are not attached to a PCB whatsoever.

I just checked Radio Shack online - they do not carry any 1 ohm resistors. =( (typical).

When you say remove the ground connection - is there an easy way to do that, or do I have to unsolder a lead from the tube socket?

Also - since there is only one bias trim pot, I assume I only need to do this to ONE tube? The bias pot controls both tube sockets, right?

This is my DVM - is it capable of doing this? I do not know where to set it.

http://www.partsexpress.com/pe/showdetl ... er=390-550

[Flames1950]

There should be a bare piece of buss wire (typically) between pins 1 and 8 and ground. You could just cut it in half and solder the resistors to those halves. George probably has the resistors in his store.
You will want to do both sockets, so that you can ensure that the tubes are matched properly.

From the specs it looks like you'll want that meter set to the 200m DC voltage range.

[playwell]

Flames.. first off, never underestimate how much I appreciate your help here. Some take it for granted.

Moving on... I'm looking at my tube sockets. Pin 1 has ganged red wires. Pin 8 has a black wire which is "jumped" to pin 7 with either bus wire, or more likely, the same wire just extended...

Both sockets are the same.

So you are saying to cut the link between pin 8 and pin 7 and use a 1 ohm resistor there instead of straight wire?

If I am correct on this, where does my meter go?

[Flames1950]

I think you're counting backwards -- pins 2 and 7 should have the "ganged" wires as you put it, those are your 6.3VAC filament connections and they run from one socket to the next -- typically black and red wires in Marshalls but I can't be certain on the 900's. (Usually the sockets have tiny numbers on the bottom side -- IF you can read them!!)
The pins joined together with black wire should be 1 and 8, and there should be a wire going to a chassis ground near the tube socket. Cut into the middle of the chassis ground wire and place your 1 ohm resistors-- solder one end of the resistor to each of the cut wire's ends. The link between the pins stays put.
Then you place the black (common) lead of your meter to a good chassis grounding point (like the mounting bolt holes on the sides) and VERY carefully place your red lead to either pin 1 or pin 8 (doesn't matter since they're shorted) and read your mV, which as we now know is the same as your current in mA.