[Boatanchors] JOHNSON 500 speech clipper mod?

[Gary Franklin]

I have a Johnson 500 which has developed a problem which I believe to be 
in the audio chain up to the 6B4G driver.  The factory tube line up is:  
12AX7 ( first and second audio amplifier),  6AL5 (audio peak clipper), 
6AU6  (third audio amplifier),  6B4G (audio driver).   My transmitter 
has been modified in the clipper stage. Instead of a 6AL5 it has a 6C4 
....  Anyone know of this modification?  could it be a WA1HLR mod??    I 
also have a small board mounted to the bottom of the chassis which has a 
SK 3021  NPN power transitor. Handles 300 vceo,  500 vcbo, 35 watt, 
2amps in a TO-66 case. On the board is also (3) diodes and a 10K ohm 2 
watt resistor.  What mod might this be ???  may or may not be related to 
the 6C4.

Gary

K8BKB