Marshall amp gurus I have a question....

[Mike Aimer]

concerning the Mashall JCM 800 model 2205, are there two versions of this amp circuit, one domestic (England) set up for EL34s and one for export (US) set up for 6550s?

OR

Are they same amp circuit and output tube socket just fitted with and biased for one or the other tube types?

A friend recently purchased one off CL and it is running on KT88s. Mine has EL34s. I'm not able to put them side by side and check out the guts in person any time soon. I'm just wondering if it should have EL34s in it and has been modded in some form to use the KT88s, or if it would have to be modded to use EL34s.

I have the Marshall book. No help there. I can't seem to get a straight answer from "the web" either.

Anyone have the truth?

[bigsmitty]

I believe their the same just different bias resistor. KT88s are similar to 6550s. Good place for marshall info is metroamp.com forum.

[Mike Aimer]

Thanks for the reply. I've posted at metroamp and am waiting.....

[solderstain]

I believe their the same just different bias resistor.

Make that resistors. Yes - same circuit. Change three resistors and you pop out the 6550s and in with the EL34s, rebias and rock.

I had an '83 JCM800 I did that very operation to (along with a few others)... Sold the amp in a weak moment and miss it.

I also did the same bias change to a mid-'70s Super Lead. I've never tried any Marshall with KT88s, but in my ears, "The Marshall Sound" rides on the glow of EL34s.

[bigsmitty]

I have a 74 super lead that had 6550s and to change to el34s just had to put 47k in parallel with the other 47k to achieve proper bias as described in aspen pittmans book. You also need to change NFB to 8ohm to keep proper gain with this mod.

[paulster]

The EL34/6550 changes are the two bias splitter resistors, the bias range resistor and the tap that the negative feedback is taken from.

[martin manning]

This schem seems to have the changes:

http://www.drtube.com/schematics/marshall/4210.gif

Note 4210 combo and 2205 head are same diagram. It shows 1K screen resistors added for US only, and no change to the OT tap used for the NFB. Later versions, i.e.

http://www.drtube.com/schematics/marshall/2205pwrm.gif

show EL34 only, switchable mains, and include screen resistors.

MPM

[bigsmitty]

I stand corrected.

[selloutrr]

correct change the bias section the rest is of the amp shares the same circuit.
The reason for the output tube difference between US and Europe was due to tube failure during shipping, the US got a stronger built tube that was easier to obtain in the states most where installed with 6550's after arriving at port.