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I’m just wondering if that means actually “dangerous” or just not as per factory spec

Yeah I thought of doing that. But space does not really allow. You mentioned that you didn’t think I was “safe” to run a mark iii biased that high. In what sense?

Ok, so before your last reply I managed to install a new trim pot (100k, not 50k) and now the bias sits the lowest I have managed to get it, 43mA, which with a plate voltage of 469v now, is about 68-69% so I am ok with that… for now. But it’s right at the end of its travel. Next time I have the...

Thanks for the input. I had a 50k pot in there but I have now removed it and put the resistor back in. I have since got a 100k pot to try. The fact that all tubes aren’t created equal is the reason why I wanted a trim pot installed. I want the freedom to get whatever tubes I like (or that I...

Thanks for the reply. I didn’t actually measure anything before I did the service and installed the trim pot. My error there. But once I removed the trim pot and put the resistor back in (68k, not 6.8k. Measured) and also the mesa tubes that were previously in it (415’s) the red plating...

Anyone?

This really is a strange one… I just removed the trim pot I installed and put the factory 68k resistor back in. The bias is now reading higher (49mA) yet the tubes are no longer red plating! What the..?

I think another part of the issue may be the resistor that I replaced. My searching didn’t give me a clear indication whether I should replace R117 or R102, due to some amps having one resistor or the other, some having both. I replaced R102 with a 50k cermet pot. Did I replace the wrong one?

Thanks for that. I’m not sure I understand why a tube with a known 30w rating would need to be biased colder in a particular amp. 70% is the accepted bias point for everything as far as I’m aware.

Isn’t it the tubes that determine plate % dissipation?

Thanks for the reply. The 454v was measured pin 3 to pin 8

Hi everyone. Been a member on there for a bit but this is my first post. I have a 60w blue stripe mark iii that I have just done a cap job on as well as installed a bias trimpot. When I fired it up afterwards the tubes appear to be redplating, but the strange thing is that I was able to bias...