JCM800, tubes, and you! (Tube/Bias help)

[pinto79]

Hey guys.

I recently bought a 2203 100 Watt JCM800 head.

It desperately needs new tubes, and I can't get a straight answer locally about whether the bias on the amp is adjustable.

I realize that if it wasn't origianlly it could have been changed at one time.

So, who here can tell me if it would have been adjustable originally, and what to look for inside if it has been modified to be adjustable?

And part 2 of the question: I want to put a set of GT No. 7 E34LS Tubes in. What should I do? Can I just stick them in if the bias isn't adjustable? Any potential problems? Should I consider having a bias adjustment mod done?

Thanks in advance!

 

[jerryjg]

Re: JCM800, tubes, and you! (Tube/Bias help)

imo you should take it to a reputable tech for a good going over.
Let him install and check the bias on the tubes.

how do you know about those tubes? Have you spec'd then out. They could still be well into the good range..are they old stock?

Its not going to make a huge difference if you just go ahead and slam in some new tubes and play away. Thats waht mpost people did anyhow b4 the frenzy of all this biasing ...but refer to my first sentence in my post.

 

[Robert S.]

Re: JCM800, tubes, and you! (Tube/Bias help)

All 2203s have a bias adjustment.

My 1982 2203 came with 6550s from the factory. If your amp does not have el34s in it now you need to have some minor changes made for it to accept EL34s. I had mine converted over to EL34s and it was cheap and easy.

It is now out getting a tube loop installed.

 

[metlking]

Re: JCM800, tubes, and you! (Tube/Bias help)

It's not hard to bias an amp.....but it can be deadly if you're not carefull! A JCM800's bias is adjustable......watch and learn grasshoppa'

http://www.eurotubes.com/euro-Generic-Bias.htm

 

[pinto79]

Re: JCM800, tubes, and you! (Tube/Bias help)

I know about the tubes because I've had them installed in a 50 watt JCM800 many years ago and loved the bottom end they provided, so I'm trying to replicate that again. I like the later break-up compared to lower numbered tubes I've tried, it lets me push the amp harder from the front.

I need to find some 7025s for the preamp... then I'll truly be in tone heaven...

 

[pinto79]

Re: JCM800, tubes, and you! (Tube/Bias help)

All 2203s have a bias adjustment.

My 1982 2203 came with 6550s from the factory. If your amp does not have el34s in it now you need to have some minor changes made for it to accept EL34s. I had mine converted over to EL34s and it was cheap and easy.

It is now out getting a tube loop installed.

This one is already equipped with EL34s. I think all the Canadian issue ones were and the US ones got 6550s. I could be wrong, but that's been my observation.

 

[jerryjg]

Re: JCM800, tubes, and you! (Tube/Bias help)

This one is already equipped with EL34s. I think all the Canadian issue ones were and the US ones got 6550s. I could be wrong, but that's been my observation.

Oh yes, I had forgotten the Heads came originally with 6550's( like my JMP 100 watt thats also been converted to EL34's) and that would have been very bad to stickk in el34's without rebiasing. Just goes to show you to be carefull with advice .
That said, Id have no problems putting in different EL34's and [playing away.
As long as your tubes dont stress out and glow bright red or look too cool and dont seem to have that nice lovely blue glow your in the clear AFAIK>

 

[Rich_S]

Re: JCM800, tubes, and you! (Tube/Bias help)

Yes, it's adjustable, but not enough to switch between 6550's and EL34's without a minor part swap. Since the amps came both ways, and you have no way of knowing whether previous owners swapped tubes properly, have a tech check to make sure it can be biased for the EL34's you put in. Just because it has EL34's in it now doesn't mean it's set up right.

 

[pinto79]

Re: JCM800, tubes, and you! (Tube/Bias help)

I found this:

http://www.marshallforum.com/marshall-amps/615-canadian-jcm-800-a.html

Mine is a Canadian model, so the EL34 arrangement is correct.

 

[Lake Placid Blues]

Re: JCM800, tubes, and you! (Tube/Bias help)

You should always check (or have checked) the bias whenever you change tubes, even if the tubes your putting in are supposed to be the same as the ones your taking out. It's a good idea to keep tabs on the bias regardless.

 

[Fresh_Start]

Re: JCM800, tubes, and you! (Tube/Bias help)

You should always check (or have checked) the bias whenever you change tubes, even if the tubes your putting in are supposed to be the same as the ones your taking out. It's a good idea to keep tabs on the bias regardless.

+1 on all of this, but only for power tubes and usually only in fixed bias amps. LPB knows this, but somebody else might not.

Preamp tubes in virtually all modern amps are "self-biasing" (cathode biased).

While each power tube may reach a slightly different balance point in cathode bias, the only way to change the bias for a cathode biased tube involves replacing (or jumpering) a resistor. IOW adjustable cathode bias is extremely rare.

Chip

 

[Lake Placid Blues]

Re: JCM800, tubes, and you! (Tube/Bias help)

I'm glad you clarified that. I sometimes take for granted that people would know certain things.