Marshall JCM900 Valve change and bias

[Enzo]

They may be stock. Parts change all the time, that is why I mentioned the schematic you posted is the 14th version of that circuit.

[J M Fahey]

I think I need a Time Out. This amp is the JCM 900 100W Hi Gain Dual Reverb. I check a couple web sites for the schematic. But the PC board that is in the amp (PC0112) does not have the same value R28 and VR2 as shown on the schematic. R28 has a 68K and VR2 measures out to be 17K. I am searching other Marshall schematics but do not see anything else out there.

Then maybe somebody modded it.
Too large R28 means negative voltage will be larger than normal and too small VR2 (did you measure it in circuit? not a standard value; normalized ones are 22/25k or 47/50k ) woukd mean smaller adjustment range.
47k and 47k as specified shoud give the proper range.
And basically follow Factory suggestions; biasing by % dissipation is Internet nonsense, sadly spread like Gospel.
Of course, Internet is run 99.98% by amateur non Engineers; Factory ones rarely have the time or inclination.
Follow schematic values and all should be fine:

[g1]

The 22K bias pot and series 68K resistor are in the issue 7 circuit.
See page 10 of attached pdf

[Enzo]

There you go.

I still think the message to take is just put in the thing whatever values you NEED to make it work to your liking. Ignore the steady march of schematics across the galaxy.

[TomCarlos]

Thanks again everyone... and thanks Greg for the EL34 conversion schematic. The amp is up and running... neighbor is very happy!

[Mick Bailey]

Just to add that the later Marshall 5881 is a Shuguang 5881/6L6WGC.