jcm900 4101 4x5881 pc0112 1kohm screen resistor.... doh!!

[-Elepro-]

hello i have this 4101 jcm900 100w with 4x5881 but it has 1kohm screen resistors...

also the power amp board is labeled pc0112.... and the bias trimmer is PR1 22kohm (i found this value in 50w amps!!)

can you help me about exact schematic?

[Jazz P Bass]

1K screens are what 5881 tubes require.
I don't know anything about board numbers or bias pot numbers.

Here is all of the JCM900 schematics that I have. (some are real hard to read)

JCM 900.zip

[DRH1958]

There are some 100 watt amps with the bias trimmer as 22k but the resistor in series with it was changed from 47k to 56k. In Feb. of 1994, Marshall issued a service bulletin for changing EL34s to 5881s and one of the changes was to change R28 from 47k to 56k to increase the bias voltage. The normal EL34 bias voltage for Marshalls is -42 to -46 volts but they spec. -50 volts for the 5881s. There was a worldwide shortage of EL34s then, so they changed over to 5881s and I would assume yours was one of the transitional amps and they didn't get the screen grid resistors changed to the 470R value for the 5881s or issue an official schematic since they were scrambling to deal with the EL34 shortage.
EDIT: JazzP, apparently Marshall thought 470R was the correct value as that's what they have for the value in the service bulletin.
5881_EL34conversion.pdf

[g1]

Does it have the 5881 label on the chassis?
As mentioned above, it could be a conversion, or maybe even an EL34 version that used 1K's ?

[DRH1958]

Does it have the 5881 label on the chassis?
As mentioned above, it could be a conversion, or maybe even an EL34 version that used 1K's ?

Good point, g1. The official changeover to 5881s on Marshall's assembly line took place on Aug. 12,1993 and they did indeed place a "Marshall 5881 Power Valve Equipped" sticker on the front panel. I suppose the service bulletin was issued later for amps that needed service(power tubes) and no EL34s were available, so mods were required.
As a side note, Michael Doyle and Nick Bowcott's revised edition of the "History of Marshall the First Fifty Years" is a great read. Copyright, 2013.

[-Elepro-]

Amp is not mine. I have to repair it....
someone changed r35 to 10ohm
r28 is 47k...
This morning i tried to rebias and i did not get value lower than 50mA per tube.....
so i thought someone swap to 5881 without mods

....thanks at all

[g1]

The official changeover to 5881s on Marshall's assembly line took place on Aug. 12,1993 and they did indeed place a "Marshall 5881 Power Valve Equipped" sticker on the front panel. I suppose the service bulletin was issued later for amps that needed service(power tubes) and no EL34s were available, so mods were required.

Yeah, I never saw that bulletin til many years after the changeover, maybe more like around the time of the "change back", where they switched back and some owners wanted to change their 5881 versions to EL34.
And my understanding was not that they couldn't get any EL34's, just that they didn't feel they were reliable enough to warranty. I don't remember who their last (reliable) EL34 supplier was (that petered out) just prior to the switch? Tesla? And I believe that it was the Svetlana's that enabled them to "change back".

[g1]

This morning i tried to rebias and i did not get value lower than 50mA per tube.....
so i thought someone swap to 5881 without mods

I think you are correct, sounds like this is an EL34 version of the amp.

[DRH1958]

Yeah, I never saw that bulletin til many years after the changeover, maybe more like around the time of the "change back", where they switched back and some owners wanted to change their 5881 versions to EL34.
And my understanding was not that they couldn't get any EL34's, just that they didn't feel they were reliable enough to warranty. I don't remember who their last (reliable) EL34 supplier was (that petered out) just prior to the switch? Tesla? And I believe that it was the Svetlana's that enabled them to "change back".

In the History of Marshall book I referred to, on page 175 there is a section called "The Great EL34 Famine: August 1993-1996/97". It says "due to a global shortage of EL34s, Marshall was forced to go 'back to the future' and repeat history". As a result, they were forced to use 5881s. They go on to say they worked with Svetlana for 2 years until they got tubes that they feel were good enough. Steve Grindrod stated that this is the reason the power tube failure LEDs were removed when they released the JCM 2000s after they got these tubes because of the reliability of them. He stated that "these new Svetlanas were the closest you could get to an original Mullard". So yes, there was a severe shortage because there was a backlash from Marshall loyalists about the 5881s and yes you are correct about the Svetlanas bringing them back to reliable EL34 supply.

I agree, Elepro just needs to replace the 5881s with EL34s. This should straighten out the bias situation.

[g1]

I guess it was a matter of scale. In '93 when they told us this, we were scratching our heads because we had no problems getting EL34's for repair , but when you need them by the thousands or tens of thousands I guess that's a different story.
When the Svetlana's came out, they were such great tubes. For a while we saw some heater failures (intermittent) due to the solder in the pins, but otherwise they were fantastic.

[Mike_88]

hello i have this 4101 jcm900 100w with 4x5881 but it has 1kohm screen resistors...

also the power amp board is labeled pc0112.... and the bias trimmer is PR1 22kohm (i found this value in 50w amps!!)

can you help me about exact schematic?

[ATTACH=CONFIG]37101[/ATTACH]

-Elepro- Hello, could you help me? Take a picture of the JMP52C board from the side of the tracks? I would be very grateful to you)