JCM900 modified ?

Psygoat

I have a JCM900 model 4100.with EL34's
The screen grid dropper resistors are usually 2k2's or 470R for the 6L6.
This is on the schematic and in my notes.
The one I have has 1k 5W resistors, it's probably been like that for a long time.
Has anyone seen this before or can they figure out a reason why anyone would change the value of these?

ICBM

Some of them did come like that, I can’t remember which revision. It should be fine with either valve type - most EL34 amps use 1K anyway.

Psygoat

Ah thanks for that.
It was working before and the work looked original, that's why I didn't just dive in and start changing it.
I think the reason it blew in the first place was dry joints on the rear pcb, could have been any of a number, but probably the fuse holder.

I sometimes come across ones where people have used EL34's in the 6L6 model, but never the other way round for some reason.

I appreciate your help, I've had a lot of questions lately, so i have to apologise for doing 'hit and run' posts.
I've got a lot of amps in, but my main thing is synths, so on other forums I have a lot to say; on the subject of amps, not so much.
It makes a nice change, especially working with valves again.

ICBM

Psygoat said:

Ah thanks for that.
It was working before and the work looked original, that's why I didn't just dive in and start changing it.
I think the reason it blew in the first place was dry joints on the rear pcb, could have been any of a number, but probably the fuse holder.

The usual ones are the fuse holders, the high/low power switch (which can also arc between the pins sometimes), and the 100-ohm 'choke' resistor. The other common JCM900 problems are the FX loop jacks, the pots and occasionally the switching ICs in the preamp (Dual Reverb models in particular which have more of them).

Psygoat said:

I sometimes come across ones where people have used EL34's in the 6L6 model, but never the other way round for some reason.

Because no-one would put 6L6s in one on purpose . It's a very common mod to put EL34s in them, it gives the proper 'Marshall' sound. Some people forget to change the screen resistors if they come with 470s...

Marshall only changed to using 5881s in the early 90s because at the time, getting a supply of reliable EL34s was proving difficult - Western production had ceased, but JJ, Svetlana etc hadn't started making them. The only ones available in the quantities Marshall wanted were the first-generation Chinese ones, which were total crap and they had a lot of problems with warranty failures.

At the time it looked as if audio valve production worldwide would be cut to only three types - 12AX7, EL84 and 6L6/5881 - so Marshall decided to change the valve type. This is also why Fender dropped the valve rectifier on their otherwise quite accurate early-90s Vibroverb reissues. It was only in the later 90s when better EL34s and some of the other types became available again that Marshall switched back.

Psygoat said:

I appreciate your help, I've had a lot of questions lately, so i have to apologise for doing 'hit and run' posts.
I've got a lot of amps in, but my main thing is synths, so on other forums I have a lot to say; on the subject of amps, not so much.
It makes a nice change, especially working with valves again.

No problem at all, it's good to pass on the experience. I'll know who to ask if I have to fix anything to do with a synth now too .

DJH83004

The 2.2K screen grid resistors are unusual, for EL34s, but it is associated with the triode / pentode (low/high output) switching and creating sufficient differential. You would be amazed how much control the screen grid has on an output pentode (just look at the output on one that has gone O/C for example!).
Don't ever worry about asking questions on the forum we all want to help, I still ask IC for the benefit of his experience and first tinkered with them as a 14 year old with my dad, 46 years ago   

Psygoat

Yeah, I'm always up for giving advice about synths.
When I first started which would be mid 90's, I suppose, I think Marshalls stock valves were the Sovtek EH's which I always thought were a terrible valve, especially the 12ax7eh.
Maybe the reason you gave for them needing large quantities explains it.
That's really interesting.