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Mystery rectifier: what is it?
- Thread starter Sam Cogley
- Start date
I didn’t say they were “burnt out,” I said that one of the getters is black above a line through it near the upper mica, and that the other one is a weird light grey that looks almost white under bright light, while two have nice, shiny silver getters. I haven’t plugged them into my Hickok yet.
I’m not sure which socket each tube was in. They were caked in grime when I pulled them before loading the cabinet, I couldn’t see much of anything about the condition of the getters before I washed the tubes off. I rarely wash tubes in the sink, but these were an exception. I haven’t touched the Telefunken 12A_7s from the amp yet, as I don’t want to obliterate their markings in a bath. (The four Telefunkens in the preamp were beautifully clean inside their annoying spring-wrap shields.)This circuit is relatively simple and conventional.
This amplifier worked well despite the high voltages and simplistic screens supply until it failed. The solution was to install a 5U4G to reduce the sudden abnormal high voltage that caused the destruction of 2 EL84 (just guessing).
If all components supplying voltage and bias current to the now-roasted output tubes meet their specifications, it may be worth checking whether the output transformer of that channel is still working; output transformers react poorly to short circuits or to the absence of a load.
Other members may have better suggestions.
I’ll test some components and report back. I’m not at all opposed to tweaking circuits for better behavior.
Photo of the amp and the tubes in question.
Yes, the one on the left looks white, like a tube that’s lost vacuum. It doesn’t look like that in person. The one with the burned-looking getter is second from the right.
For comparison, a depressing photo of a WE mini pentode gone to air:
Yes, the one on the left looks white, like a tube that’s lost vacuum. It doesn’t look like that in person. The one with the burned-looking getter is second from the right.
For comparison, a depressing photo of a WE mini pentode gone to air:
that very light getter still makes me suspect air or at least gas.
not sure the dark one means anything specific, usually when they are badly overheated they get a brown sort of look to them. They all look like GE tubes though, normally the color within a brand is fairly consistent. In the pic the two on the right look about the same to me.
fair guess leaky coupling caps and other usual issues exist here too.
not sure the dark one means anything specific, usually when they are badly overheated they get a brown sort of look to them. They all look like GE tubes though, normally the color within a brand is fairly consistent. In the pic the two on the right look about the same to me.
fair guess leaky coupling caps and other usual issues exist here too.
Picture's worth a 1000 words. Yeah the one on the left looks ready for the bin. The others I'd test.
My only point was the getter isn't always an indicator of health. We've all likely seen adds for used tubes and comments attesting to a tube's level of usage based on getter color as though it was a wear indicator.
The Sylvania / Baldwin on the left tests strong / new on my TV7. The poor KT88 is dead as a doornail.
My only point was the getter isn't always an indicator of health. We've all likely seen adds for used tubes and comments attesting to a tube's level of usage based on getter color as though it was a wear indicator.
The Sylvania / Baldwin on the left tests strong / new on my TV7. The poor KT88 is dead as a doornail.
I suspect it’s gassy, but I haven’t checked yet. It’s just not that chalk white “gone to air” color.
They are GE, the etching on the side was all that survived the gunk other than the 188-4 EIA code and a tiny bit of the date code (might have been 57-59?) on one tube.
