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It's been a while....thought I come back and check to see what's been happening around here.
I've been a long time user of signal tubes like the 6DJ8 and 12AU7 varieties. I also have a headphone amp that uses EL84. But that was the extent of my experience with tubes. I even bought a TV7-D/U and had it calibrated a few times through the years by Dan Nelson for testing all these signal tubes that I've dug out of old electronic stores and flea markets for the past 15 years.
My preamp had been the ARC LS-25 Mk I with GNSC Ref level mod that uses 4 6DJ8/6922 tubes for a long time. In the fall last year, a Ref 2 Mk I with the same level mod came up on Sleezygon and the seller was local. I couldn't pass up on it. So I bought it! Although it had the "same" reference level mod, it was much more complex in the Ref 2. Life was great until I decided to try some NOS rectifier tubes, of the 5AR4 and 6L6GC variety. I had purchased some on eBay and they tested good in my TV7-D/U. No shorts, no gas leakage, and measured well above the minimum.
On the last day of Thanksgiving holidays, I decided to take out the current production rectifier tubes in the pre and try out some NOS tubes. Well, when I inserted them in and turn it on, I smelled burning vinyl odor. It took a while for my turkey-rotted brain to register before I shut it off.
Well, that "adventure" cost me an arm and a leg to fix. I took it to Nick Gowan of True Sounds here in the San Francisco Bay Area. He does a lot of work for ARC. Unfortunately, since this was a heavily modded unit by Steve Huntley, the work was even more complex.
What had happen was the bad tube toasted the power supply as well as an expensive black gate capacitors. The preamp just got fixed and returned to playing position. What I found out was that out of the stash of tubes I bought, I decided to select a bad tube to start. It arc'ed, killed the power supply, and sent AC current through and killed one of the capacitors. Normally, a bad tube would just kill the fuse, but I guess I was double unlucky.
I asked Nick how do I know whether the tube is good or bad? He pointed out some signs on the bad tube:
1/ that reflective coating on the glass near the base. (It should normally only be on top or side of the glass);
2/ burn smoke inside the glass. The glass should remain clear if the tube is good.
Well, I am still trying to get him to spell the beans on what he uses to test these tubes. Or, what can I use in the future to test these rectifier tubes. If any one knows, please do share.
FrankC
Follow Ups:
I'm not really sure if I understand what you said, BUT, in the "how to tell if a tube is bad" part of your post:
/1 "reflective coating" being near the bottom of the tube.....
IF you are talking about the getter flash location, that has NOTHING to do with whether the tube is good or bad. The reflective coating or getter flash doesn't move to the bottom of the tube as the tube goes bad.
The location of the reflective coating (I'm still guessing you mean getter flash) is entirely a result of the design engineer's decision about where to put the getter. It has nothing whatsoever to do with whether a tube is good or bad.
Perhaps I am misunderstand you, or maybe you misunderstood what Nick said/meant. Either way, sorry to hear of your problems and hopefully you'll get your gear all back up and running soon (and with a minimum of expense).
Keep up posted.
cheers,
roN
"Life was great until I decided to try some NOS rectifier tubes, of the 5AR4 and 6L6GC variety"
6L6GC is not a rectifier tube. If you plugged a 6L6GC into a rectifier socket,that would account for problems/smoke.
The ARC Ref 2 uses (1) 5AR4, (1) 6L6GC, and (1) 6922 tubes in the power supply section. Rectifier might be the wrong word.
It turned out 1/2 of the 6L6GC I had purchased were bad. The 5AR4 tubes were "unstable" during the warm up stage, but then they are fine after the initial warm up. Nick suspect that there might be mini arcing when they were warming up. Well, I guess I won't be plugging them into the preamp anymore.
FrankC
Forgot to mention that at Nick's shop, there was a headphone amp that uses a pair of power tubes. The guy had purchased a pair of power tubes from some online tube dealer. The boxes had some measurement on them. But Nick said he tested them, and they both failed. Anyways, the tubes somehow damaged the amp so it is in there for repair.
I would love to go into that discussion with him on what it really means to "match" a pair of tubes and what does measuring mutual conductance mean in audio terms, but I didn't want to take up too much of his time. Maybe someday he will decide to write about it and share the knowledge.
FrankC
Mutual conductance is basically a measurement of the tube under dynamic or working conditions as we say and you are essentially applying an AC signal on the input grid of the tube along with its DC voltages and then measuring current at the output which would be at the plate.The ratio of the current variation at the output to the voltage variation at the input would be what we call transconductance which is measured in Micromhos.
Break down the word transconductance to figure its meaning..The word trans means across and of course that would come out as conductance across the tube
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
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