Need help biasing JCM900

[RJF]

I have been reading on "how to bias" amps that don't have bias points that are super easy. 3 pages of paragraphs on cathode, pentode, frog tode, blah blah.

I need to know 2 things to bias my Marshall, as far as I know.

1. Plate voltage

2. Where to actually read the bias.

I believe I get #1 by touching pin #3 with my multi meter with the red lead and black lead to ground on the chassis. That is supposed to give me a reading of plate voltage. (likely between 460-480).

What I am confused about is where I can test at to read bias with my multi meter. I don't have a fancy bias probe adapter like some people use. Can't I get the bias by touching the right number pin?

 

[Dale B]

Taken from the following website:

http://www.duncanamps.com/technical/lvbias.html

WHAT YOU WILL DO:

A) Replace the ground wire on each power tube
socket with a 1-ohm resistor.
B) Read the voltage drop across this resistor
(in millivolts) with your DMM.
C) Read the plate voltage.
D) Use the above readings to calculate the
static dissipation wattage.
E) Adjust the bias to obtain the best tone,
while keeping the tubes within specifications.

 

[Loudness250]

Good job Dale B. That link tells you what you need to know if you can read. It's not easy but not rocket science. Just keep that dmm grounded always!!

 

[RJF]

I read all that already. I'm not interested in soldering in resisters or any of that boloney. I thought it was easier than that, you can't just read the dang bias somewhere? It's not like an amp with test points has some magic, where are those test points connected to?

Looks like I'll just buy one of those bias probe things that you plug into your DMM.

 

[bananaladonpcp]

You can get a bias read where the power tube fuses are. Put your DMM on DCma. Find the wire that goes into the PCB board that the fuse holders are on and connects to the fuses. Put the pointy end of your DMM probe in where the wire meets the board and you will get a reading for a pair of tubes like on your Herbert. Be safe man!

-Alex