Metropoulos Forum

Okay... so I've built my first amp, and everything went well, and I'm comfortable poking around in there...

Now I am going to start trying to do some easy stuff on some other amps...

I have three 80s Marshalls here that I need to retube and rebias - a 50w horizontal input JCM800, a 100w JCM900, and a 100w horizontal input JCM800. The 100w 800 has other issues that I will tackle last...

I've found some articles that describe different ways to determine the correct bias values for your amp, and I've found diagrams of the pinout on an EL34... but the instructions on determining the values assume you are quite familiar with the pinout, and the pinout diagrams assume that you know how to read the diagram without any actual labeling...

From what I understand I need to figure out the plate current, then plug that into the handy weber bias calculator, then adjust the bias to the suggested value and see how things sound...

I read more about the cathode current method of determining the plate current...

What I haven't been able to figure out is a list that goes something like:

'Measure from x -> y with your multimeter set to z, then do, etc. until you have figured out the plate voltage.

Now measure from pin n to check the bias'

Instructions of that sort would be really helpful... or at least some clarification on the pinout and how to measure some things...

Thanks for any help!

Here's how I do it:

1) If not using a bias probe, you need to have a 1 ohm resistor on each power tube socket (one end of resistor to ground; the other end tied to pins 1 & 8 ) Use a 1% tolerance resistor.

2) Turn power on and let tubes warm up. Switch standby to "on".

3) *Be very careful on this step (high voltages)*
Set your meter to the highest DC volts (600 or higher) and put the common (ground) lead in one of the chassis mounting nuts or alligator clip it. With one hand behind your back for safety, touch the positive lead to pin 3 (plate) of one of the power tubes. This is your plate or "B+" voltage. Write it down.

4) EL34's are 25 watts max and you want to set the bias to be between 60% and 70% maximum dissipation watts. (60% = 15 watts; 70% = 17.5 watts) Divide your desired wattage by your plate voltage from step 3. (example: 15/450 = .033 or 33mV) This is your desired bias voltage (probably between 30 and 40 mV).

5) Set your meter to lowest DCV setting (200mV or less) and touch the ground lead to the ground side of the 1 ohm resistor and the positive lead to the pin 1 side. Do this on all power tubes and adjust the tube with the highest value to your desired bias voltage from step 4.

Plate voltage will change a little when adjusting the bias so you may need to take another reading from pin 3 and adjust your equation accordingly and repeat steps 4 & 5.

Thank you very much! That seems quite clear to me. Appreciated.

I have one question...

The JTM45 instructions call for a 1ohm 'bias' resistor that is attached to pin 1, pin 8, and then to ground...

http://forum.metroamp.com/download.php?id=867

I only ask because that's very similar to what you describe, but slightly different from what I understood in your instructions...

I understood yours as:

Ground ---> resistor -->pin 1 --> pin 8

while the JTM45 instructions call for:

Ground --> pin 8 --> resistor --> pin 1

Which of those is correct for the EL34?

Thanks!

pin 1 and pin 8 are tied together and the 1 ohm resistor runs from pin 1 to ground.

Okay, so :

Ground --> resistor --> pin 1 --> pin 8

I'm just trying to make sure I'm doing this all correctly! I appreciate the kid gloves.

Oh, also... I don't remove the 1 -> 8 tie and 1ohm resistor until I've gone all the way through the steps (toner mentioned maybe needing to repeat steps 4 and 5)?

remove ? once installed they stay on each output tube.

rockstah wrote:remove ? once installed they stay on each output tube.

Oh, really? Forever? Ok.

yes the 1 ohm resistors stay in place on the tube sockets. forever.

What wattage and voltage resistor? 5? I'm seeing 5% resistors, but not 1% resistors...

This is the one on my JTM45... is this what I want?
http://store.metroamp.com/product_info. ... cts_id=279

Bob wrote:What wattage and voltage resistor? 5? I'm seeing 5% resistors, but not 1% resistors...

This is the one on my JTM45... is this what I want?
http://store.metroamp.com/product_info. ... cts_id=279

Yes. That's what the Metro kits use. 1 or 2 watts is fine. I should have said "low tolerance". Tubes and power can fluctuate more than 5% anyway so that's fine.

Regarding the resistor wiring, George's instructions do show:
ground > pin 1 > resistor > pin 8

However, I read somewhere that this layout is more accurate so that's how I do it:
ground > resistor > pin1 > pin 8

More info and diagram: http://forum.metroamp.com/viewtopic.php?t=10518

If you use the transformer shunt method OR if you buy a bias probe, you don't need to install the 1 ohm resistors. I used the transformer shunt method for a long time, then bought one of these and I'm happy with it:

Compu-Bias.com

Okay... I've got this 800 open and there is a piece of bus wire connecting 7 - 8 - ground on each of the power tubes... what's up with that? When I install this resistor I'll have 7 - 8, and 8 - 1... does that pose any problems?

Also, it seems the resistors we have are the smaller metal oxide resistors... those okay? From what I understand these are 1 watt, and the slightly longer ones are 2 watt?

Are you sure the buss wire isn't on pin 8 > pin 1 > ground?

Pin 7 should have a heater wire connected to it.