#1
I'm getting some new tubes for me amp, tis Marshall JCM900 in my sig which has been converted to take EL34's. At the mo I'm using Ruby tubes in power section and the pre-amp is some marshall ecc-83 (got this amp 3rd hand and know the pre ain't been changed since original owner over 10 years ago, probs longer).

Anywho I'm thinking of getting some tungsol's for both pre and power (12AX7 is the same as ECC-83 aint it?) and I think I will need to adjust the bias on the amp, or at least I want to check what it's running at.
Now I know i'll need to get a multimetre but my question is, once I have it what the hell do I attach it to in the amp.

Incase you can't tell this is my first time changing tubes on my own as I can't afford £200 odd to get the only tech in town to do it like I could last time so any further help on this matter would be a great help.

Cheers
Gear:
Epiphone SG Standard - Natural Wood Finish + SD Alnico Pro 2 Bridge Pickup
Epiphone Les Paul Standard - Limited Edition Green
Ibanez S470
Blackstar HT-100 Head
Harley Benton 2x12 Vintage 30's
Vox AC4TV
Vox VT15
#2
Yes 12AX7 and ECC83 are same thing. I recommend JJ's or Tungsol.

To be honest it would probably be a lot less trouble for you to install the tubes yourself, then pay a tech for a few minutes of time to bias the amp for you. Bit if you really want to do it yourself, here ya go -

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.
Last edited by Matt420740 at Aug 4, 2010,
#3
Nice one mate, yeah I was thinking I might ring the tech and ask if he can just do the biasing and I watch him do it for future like.

I just found this video aswell http://www.eurotubes.com/euro-Generic-Bias.htm doesn't look all too difficult from that.
Gear:
Epiphone SG Standard - Natural Wood Finish + SD Alnico Pro 2 Bridge Pickup
Epiphone Les Paul Standard - Limited Edition Green
Ibanez S470
Blackstar HT-100 Head
Harley Benton 2x12 Vintage 30's
Vox AC4TV
Vox VT15
#4
To bias a JCM900 with EL34s (which is what they normally have outside of the US) you can put your multimeter probe at the junction of R26 and R27 and adjust the bias trimpot until you read -39V. That will get you close enough to get you rolling. Don't go below -40V or above -38V.
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