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In Reply to: RE: Thoughts on red-plating a tube? posted by boeingpilot on May 21, 2015 at 10:15:16
Ok, may have answered my own questions..... (or may have generated more questions!)
Looked closely at the PCB, found the land had pretty much lifted and possibly started to melt attaching to the coupling cap. (see pics, sorry for bad picture). So, either this is the problem, or I have multiple problems. You can see that the board has seen better days. It's sort of coated with waxy coating that I suspect is the board going bad. And there are several other lands that have started to lift.
So, fortunately I have a spare ST-70 to use, but now have to decide what to do with this one. The spare has new PC-3A (7199) driver board. Do I just get another (I do obviously have good 7199's), give up and get a PC-3B (6GH8's) or something else. Joe Curcio's? VTA's?
And I was just starting to like this amp (again!).
Back to the red plating, opinions, did I find the issue, or are there more?
Follow Ups:
Check the warranty for the new tubes. Some places give up to 6 months warranty for a bad tube. Also, you should add a 470 Ohm, 1 w resistor in series with each screen grid. That may prevent future red plating caused by screen shorts. I did it and haven't had a tube go red since a few years ago.
I replaced the four power tube and rectifier tube with Japanese ceramic octal sockets in case loose pins are the issue.
The wax has been there since the board was made. The problem is, as you noticed, the pads and traces are not reliable. If they have lifted or have been damaged in different places, then you need to either make repairs or replace the PCB. There are a number of good choices. IMO, the original 7199 driver still makes a fine-sounding amplifier.
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