1983 USA Marshall JCM800 2204

[shampoch]

I have a 1983 USA Marshall that has 6550's in it. The tubes look very old and could even be original lol. Not sure, but I wanted to run EL34's in it. The Bias Pot is rated at 22k. Can i just pop EL34 and bias it (using my Bias Rite) ?

[soundguruman]

No, you will need to change a resistor to get the bias voltage dialed in, probably.
But go ahead and measure to see, with the bias buddy.
I think your current will be too low, but check first.

[Gtr_tech]

USA production 6550s can last a *very* long time....'specially when just loafing along in a 2204. The circuit wasn't designed for them, so operating voltages are pretty low and the amp just won't do them justice. It'll sound better with 34s, but yea, you need to have the bias circuit changed to get the correct range to keep 34s happy. Save the 6550s for an amp that can take advantage of their capability.

[Jazz P Bass]

You could also check the bias voltage range.
Remove the output tubes & monitor pin #5 as you rotate the bias pot.
If the reading goes to -45 something volts dc, you may be o/k.
Depends on the plate voltage too.

[shampoch]

Thanks guys for your feedback. By the way, the cobwebs have since been cleaned off the amp - these are pics from when I first bought it. The amp sound fantastic as it sits and it's good to know the Philips 6550's can last years becasue they just look old lol. I will not invest in converting to EL34 being I have another JCM800 EL34 already. I just have the tubes laying around to try and didn't want to damage the amp doing so. Is there any risk trying to bias up EL34s as it sits ?

[big_teee]

Here's the link to the Mark Huss Schematic, that shows Values for both Output tubes.
His is a 1983 2204 Also.
http://mhuss.com/MyJCM/JCM800_2204.pdf
Good Luck,
Terry

[Jazz P Bass]

Nice post.
Thanks.