Power Tubes for JCM 800

[eino aho]

A friend of mine just bought a 1983 JCM800, and he wants to put KT-88's in the power section, so we opened the chassis and discovered two el_34's. The question is: Has the circuit been changed to accomodate el34's as opposed to 6550's? Or is it just a bias issue? Also, are there any simple mod's he can do when putting in the kt-88's?
Any replies would be appreciated greatly.

Eino

 

[John Phillips]

Where was the amp sold (new) originally?

All non-US-market Marshalls (including Canada I think, but someone correct me if I'm wrong) were fitted with EL34s from the factory. These are the 'correct' tubes that the circuit was designed for, the 6550s were over-spec substitutes requested by the US distributor to cut down on warranty returns. They sound quite different too - some people like them.

To convert one to the other, you need to replace three resistors and move the feedback supply wire on the impedance selector.

The bias range for KT88s should be roughly the same as for 6550s, you should be able to use the same values and adjust it with the trimmer.

 

[eino aho]

John, not sure where it was made but will find out on sunday as he is bringing to the studio. Thanks for the input. Is there any one to call and speak with about these mods you refer to? He nor I are techs by any stretch but alas he likes to noodle with this stuff against his better judgement

Eino

 

[jaimz]

Ok, so I am the friend he was talking about. I have an 83 jcm800 and it is a US version (6550). I guess the person who owned it before me swapped the 6550s for el34s and I am trying to figure out what he did exactly and how to get it back.

I went over everything on the circuit board and from what I can see, the two bias resistors are intact and still 150k (I believe they are two of the three you were saying needed to be changed, and from what I read they are to be changed to 220k to accomodate el34s). I compared everything else in the head to the schematic and it looks like everything is stock, with the exception of a 47k resitor (which is the third resistor you were talking about, and I read it is supposed to be a 56k for el34s) that was changed to a 27k for some reason, and I am changing it back to 47k.

So can you tell me more about the feedback supply wire? I havent read anything about it and I would like to be able to figure out where it is in my head and whether or not is has been altered.

Also by throwing El34s into this head without making the correct modification to support them, could anything have become damaged in the amp that I should have looked at?

Thanks for the help!!

 

[John Phillips]

No, nothing could have been damaged. EL34s require a far lower bias supply voltage than 6550s, so the worst that could happen is that it would run very cold and sound horrible.

The resistors you would need to change are the 150K bias feeds, and the first bias supply resistor (next to the bias diode), which is 150K in a 50W 6550 model, 220K in a 50W EL34 model, 15K in a 100W 6550 model, and 22 or 27K in a 100W EL34 model.

So it sounds like you need to just change the 27K resistor to 15K (I'm assuming this is a 100W amp) - not 47K, which will take the bias far too low. That is, assuming it is that resistor, but that's the main one you need to change for the conversion (you can't do it without) so it seems likely that it is.

The feedback wire is connected to the 4-ohm tap on the impedance selector switch for 6550s and the 8-ohm for EL34s, but there's a good chance it wasn't moved either, if the bias feed resistors weren't done.

Good luck! It will probably sound great with KT88s...

 

[jaimz]

Thank you for helping me figure this all out. I will check out the feedback wire tonight.

My head is a 50W. I am pretty sure the printing on the circuitboard underneath the resistor says 47k, I will have to double check. Let me know if that doesnt sound correct, and what size the resistor should be for the 50w.

I cant wait to hear the way this head sounds when it is set up properly.

Thanks again!!

J -