JCM900 4100 IC supply problem

[fuzzman72]

Hello!
Playing my new acquired JCM900 I've realized that when I strike a chord on the guitar, using the high gain channel, the red light on the switch dims.
I've opened the amp and monitored the dual voltage supply for the IC, to find out that when I keep the channel B gain control at 12 and play the guitar, I have a voltage sags from +-15 to +-11V.

I've replaced C22 and C21 and D4 and D3 with two 15V/1W Zener diodes, but the voltage is still sagging.
Is this normal in your opinion?

[JMFahey]

Weird.
What´s the voltage at the other end of the power resistors feeding those Zeners?
Does that voltage sag a lot?
Do you have a clean readable schematic to upload? Mine is a murky mess, unreadable values.