![]() In this amp, R157 looks like it's also there to protect the amp. In many amps, this resistor doesn't exist and the connection is a short. What I did do, was stand the replacements further off the PCB. ![]() If the same thing happens again, there's a chance of doing some serious damage. There's a temptation to uprate the resistors to say 2W or even 5W but this isn't a good idea. ![]() It's actually a safety feature to protect more expensive parts of the amp like the output transformer. This has the effect of heating up the screen grid resistors, eventually exceeding their power rating and so they blow. As this condition occurs, more current is pulled from the screen grid supply. The reason that R139 and R140 failed so dramatically is because of a valve going short-circuit. The schematic marks resistors R139, R140 and R157 as 'fp' meaning 'flame proof' which is rather ironic as they were burnt out! EVH 5153 Mk III Schematic with crucial screen grid components highlighted in red and cathode to ground and V8 plate components shown in blue. On the other side of this small PCB is a pair of 6L6 valves. The image below however, will give you an idea of where the power amp circuit board is (top right). Unfortunately, I didn't take any pictures of this prior to changing components. ![]() The power valves were fried but it didn't look like 'normal' wear was responsible so I decided to open up the amp. This amp came to me because it was blowing fuses. seemingly, not so important! The fact that I had to do this EVH 5153 Mk3 screen grid circuit rebuild kinda says it all. I'm sorry to say this but I'm not exactly a big fan of modern guitar amps! □ So sorry but I think there's far too much stuff going on for something that should in essence, be quite simple with the important stuff being well. ![]()
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