How can I tell if this 1989 JCM800 2204 uses 6550s, EL34s or 5881s???

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Most 6550's are shaped like a KT88.

Tungsol%206550.jpg


The only other 6550 that I've seen that wasn't shaped like a KT88 (i.e. EL34s) are the GE 6550s.

GE6550AMP.jpg
 
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SgtThump":0ebf0 said:
My buddy has a 1989 Marshall JCM800 2204 that he bought years ago with 6550 power tubes. This is a 6550, correct?
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It looks like one to me.
 
Like you said, I'm not sure if the later models come stock with 6550's. I would at least think that it has been set up for 6550's if it has sounded OK. Otherwise, it would probably sound awful, if it would work at all.

I don't think going from el34's to 6550's is that crazy of a mod......but couldn't tell you what the hell it is. :? I've drawn a complete blank. I've had it explained to me but can't remember.





Wouldn't the plate voltage have to be quite a bit higher for the 6550's?
 
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SgtThump":f72f4 said:
My buddy has a 1989 Marshall JCM800 2204 that he bought years ago with 6550 power tubes. This is a 6550, correct?

6550.jpg


He's been running it that way for years with no problems and now it's time for a retube. The guys at GC gave him EL34s, which got me wondering if this amp should be fitted with 6550s? I know Marshall used 6550s in the late '70s and early '80s, but did they use them in the 2204 in the late '80s? I don't think so, but I don't know for sure.

How can I tell if these are the right tubes for this amp? Maybe these have been wrong all along? Can I tell by measuring the plate voltage with my Weber Bias Rite or something?
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It is more then possible that his amp had a mod done in order to accept 6550 tubes. If his amp was in fact using the 6550's he cannot use el-34's unless a tech or someone handy swaps out a few resistors.
 
oh yeah, i forgot... the picture you posted is a 6550 tube.
 
The Marshall book says they only used 6550's from '74 through '86.

If it was changed to 6550's they probably just did it like this;

http://www.voodooamps.com/tubeconversions.html
 
Usually, when they've been modded for EL34/6550 Swap, you'll have to either add a resistor, or change one out by the bias pot. Here's one that's been converted from 6550 to EL34.

SLGuts011-1.jpg


If the one you're looking at was an EL34 amp, and changed to 6550's, they would have taken out that bias resistor and put in a different value.
 
That tube is a 6550. A 1989 2204 should have come with El34's. It's hard to tell if those resitors were changed, there should be a 5.6K resistor there. Those red resistors are added, probably 1 ohm resistors to monitor bias. You need to look at the resistors near the bias pot.
Jerry
 
It looks like someone changed a lot of the resistors in that amp. That carbon comp resistor near the bias pot isn't original. Many of the other resistors look like they've been replaced, they aren't the original manufacturer. The bias supply caps are Chinese, change them. I'd get out a multimeter and start measuring voltages and resistors to see what's been done there.
A lot of guys install 1 ohm resistors on the cathode of power tube socket so you can measure voltage drop across the resistor and measure the bias. It works like a bias probe.
Jerry
 
I've got an 86 2204 and it had Telefunken EL34's in it when I bought it in 89. I replaced them with KT-77's
 
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JerryP":ef903 said:
It ...Many of the other resistors look like they've been replaced, they aren't the original manufacturer...
Jerry
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Jerry, Do you mean the yellow resistors on the board? Marshall was using ones that looked like that by '89. I had an '88 2210 and an '89 2204 that had those type of resistors from the factory.
 
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Marshall Freak":87c14 said:
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Jerry, Do you mean the yellow resistors on the board? Marshall was using ones that looked like that by '89. I had an '88 2210 and an '89 2204 that had those type of resistors from the factory.
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2210001.jpg
 
The Silver Mica isn't stock.

Here's a schem with both: circuity for 6550 and EL34, Chris.
 

Attachments

Yup. Here`s a 6550 chart for ya . .

6550.gif


Plate voltage should be in the 440V area . . . check it with a DMM at PIN 3 vs. ground, DMM set to 1000V DC.
 
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SgtThump":36f66 said:
Thanks alot guys! I really appreciate it. Is it pretty safe to say that this amp was modded to accept 6550s judging from those pics? Sounds like it to me.

The tubes are on order and when I get to replacing them, I assume it will be more obvious to me whether or not the amp was modded to accept them. If I can get them into the correct bias range, I suppose I'm good to go. RIGHT? :)
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If it hasn't been modded, you won't be able to get it biased for 6550's. So yes, if you get them into range, you should be good to go.

That Brown resistor next to the bias pot was changed to get it into bias range. If he ever wants to mod it for EL34's he'll need to add a resistor to it like in the picture I posted, or change the resistor back to the stock value.
 
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