JCM 900 - conversion from 5881 to EL34

[Platypus]

Hi all. I'm new here, so I'd like to say hi to everyone.

I own a 50 watt JCM 900 that has allegedly converted from 5881 to EL34 power tubes. I'm curious if anyone knows what is involved in this conversion. I've checked the schematics for both versions, and to start with, there are several resistors that have different values, but in mine, they still have the appropriate resistors for 5881 tubes. Now I'm starting to think that maybe the mod wasn't done properly (or at all). I recently had a tube blow and after replacing the tubes, and rebiasing, the amp does not seem to be right. There is very little gain, and the amp sounds very compressed, and filtered, almost as if I had a wah pedal in my signal chain.

Any insight would be greatly appreciated.

[Structo]

Howdy,

In a tube amp the first suspects are always the tubes unless there is something glaringly wrong that is apparent.

So I would try swapping known good 12ax7's starting at the Phase Inverter V3.

That drives the output tubes so if that is bad the whole amp suffers.
Some recommend replacing that every time you replace the output tubes.

Now on the 5881/ EL34 thing.

Are pins 1 & 8 connected together?

The 5881 doesn't have anything connected to pin1 but a EL34 requires those two pins connected. (1 &

I see the 5881 draws .9 amps where the El34 draws 1.5

So make sure the PT is up to the task since the EL34's are going to draw 1.2 more heater current.

Also there will be a difference in the bias so make sure you have that right and enough range to allow the adjustment.

Looks like the screen resistors on a 900 are 2K2 so you're fine there.

So really the two considerations are current draw and pins 1 & 8 joined together.

I'm sure some others will chime in that know a lot more about Marshall amps than I do.

[jjman]

I had a 100w (model 4100 ?) head.

http://www.schematicheaven.com/marshall ... w_4100.pdf

The screen resistors were 470 so I changed them to 1k. I believe the 1 & 8 pins were already tied together and I know I did not have to move any resistors off the socket. I added 1ohm resistors so I could bias and learned that the factory tubes were not even somewhat matched.

I did not notice any change in tone after the switch to el34s. I had enough range in the bias control to bias the el34s properly. I rarely played it with the amp cranked which may explain the lack of a difference to me.