RD50 mod

Hi,
I own a 112RD50 and a 110RD50. The 112RD looks original (as far as I can tell). The 110 has a replacement Fender output transformer.
The 112 has a very fine clean sound. The 110 breaks up very early and has a kind of a cruchy edge, also when played on low volume.
I noticed a difference when looking at the inside of the two amps. On the 110RD diode D9 (a 1N4151A, between the +30 VDC that goes to pin 5 of the power tubes, and groud) is replaced by two types of that diode in series.
What could be the use of such a mod, and can that cause the early breaking up?
greetings
Leon

Looking at the schematic, it appears to be a zener diode. Perhaps having two in series changes the voltage to the control grid (pin 5) of the power tubes? Perhaps checking the voltage at the negative side of the zener that goes to R48, and perhaps shorting out one of the zeners, and re-checking to see if there is a difference in voltage, and sound? Please check the schematic, and make sure you know what you are doing - I am only thinking out loud. Good luck.
Peter Maziar

I recently changed D9 on my 110-RD but I had 18 and 12 volt zeners. (The original was bad and chared the resistor). I put them in series to make a 30V.
They are cooler because the power is split between the two so I left it that way.