strato56 7/27/2018 9:29 AM
JCM900 4501 grid resistor
Hi,

I have a 50W JCM 900 DR 4501 on the bench.
One output tube EL34 has an extra 2k2 grid resistor in series with the 1k5 resistor (R27) on the PCB.
Anyone familiar with this modification ?

greetings,

Strato56
 
strato56 7/27/2018 10:08 AM
Posting the schematic didn't go as planned I guess

https://drtube.com/schematics/marshall/cd0112-iss7.pdf
 
Pedro Vecino 7/27/2018 10:31 AM
Itīs not a modification. It comes from the original construction.
In the JCM900 there are similar variables in relation to the grid resistors that do not appear in the schematics.
 
strato56 7/27/2018 10:48 AM
Thanks !
It just seemed odd to me that only one output tube has the extra resistor.
 
Jazz P Bass 7/27/2018 1:50 PM
Odd it is, two 2.2K's in series.

I have not seen that one!

For the record, I attach one separate 2.2K resistor to each tube grid right at the socket tab.
 
Enzo 7/27/2018 1:59 PM
Maybe someone went to add 2.2k resistors, and forgot to do both. MAybe the phone rang.
 
Leo_Gnardo 7/27/2018 3:41 PM
Quote Originally Posted by Enzo View Post
Maybe someone went to add 2.2k resistors, and forgot to do both. MAybe the phone rang.
At the Marshall factory? On every one? Just had a '900 in last week, it had the same strange setup. Maybe the design engineer's phone rang . . . . more likely it was beer thirty for him, time to head for the pub for a pint or three.
 
Enzo 7/27/2018 4:18 PM
Well if they are all that way no, but we only had the one unit in evidence when I wrote.
 
g1 7/27/2018 6:32 PM
In that pic from the first post, it looks like it's just daring the chassis to touch it.
 
The Dude 7/27/2018 6:50 PM
Yep. I might put a heat shrink sleeve over that one.