Strange bias red plating

guitarxtc

guitarxtc

Member
I have 50w JCM800 build;
on two el34 Plate voltage is 450V
Screen 448V
if i set Bias -40V and 3mV on pin 1and8
if a try bias -30V current on pin 1and8 is only 10mV and red plating... can't figure out where is the problem

and amp is unplayable from loud buzz
 
You’re measuring across a 1 ohm resistor?

What are the screen grid resistor values? Have you checked ground connections for the bias supply? What are the values of the bias dropping resistors after the CRC filtering in the bias supply? 220k? 150k?

What tubes are you using and are they known working tubes?
 
yes across 1ohm, i have 4.7k on grid and 220k grid leak,1k on Screens

bias is from separate secondary 0-70ac,after bridge rectifier i got -100V DC
220uF-20k- 220uF-15K-10uF-50K bias pot with 15K to ground and then to two 220K grig leak resistors
 
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