[rfeenstra]

You're probably right! It says Telefunken on it.  Who cares what's inside.  I've purchased stuff off from PayBay. Sometimes it's a BOHICA moment.              (Bend Over, Here it Comes Again)

[rfeenstra]

Wow!  I was just on the TubeDepot website and saw the RCA NOS 12AX7a at $180 each.  I'm pulling the RCA that I just put in the tuner out, re boxing it and selling it with the other 2 I have.  That's crazy!

[morzh]

Telefunken 12AX7 are very expensive today, especially NOS.
Any 12AX7 or ECC83 that is original production from any major old tube house comands a hefty TTNP.
Today's productive from Russia is considered very good. Still the original Telefunkens are the most desirable type.
I just bought a bunch of Sovteks from various lines (Genalex, Sovtek, ECC803) plus a couple of original Mullards from Jim McShane. Prices are good but still $35-45 apiece is no cheap.
I intend to write about my experience rolling the tubes in my McIntosh C22, I published it on AK forum.

[rfeenstra]

Some of this tube stuff is interesting.  I'd gladly sell my RCA's for $100 each!  I bought them maybe 25 or 30 years ago to service electronic organs.  I didn't even realize I had them until I pawed through some boxed up stuff looking for something else.  I found quite a few audio related tubes.  I need to organize and sell on _____bay (you fill in the blank!)

[KCMike]

He doesn't mention brand but the prices seem good for new ones. Unless I'm missing something.
https://docs.google.com/spreadsheets/d/1...0foD#gid=3

[Phlogiston]

You're probably right! It says Telefunken on it.  Who cares what's inside.  I've purchased stuff off from PayBay. Sometimes it's a BOHICA moment.              (Bend Over, Here it Comes Again)

That's funny - in a totally off the subject - -

One of the techs at the Verizon switch built a "toy cell" used to test parts and software but not for com. service. He named it Bohica Hill. It remained that way for at least a year before a supervisor saw the name on a report (shows how often he looked at the report). He proceeded to have a *** fit.

No, it wasn't me.

[TV MAN]

I find it quite interesting about solving the MPX problem by blowing a tube and 2 diodes. Wonder if it was the tube, or the diodes? A HH Scott was my first tube Hi-Fi unit. I passed it to my parents and bought my Fisher 500c. I always thought the Scott sounded good. It always had FM issues in the tuner and was finally boxed it away and bought a new tuner. I inherited it back when my dad passed. It has been in the family since the mid 1980's. It got played a lot until mom passed in 2000. I don't think it even got turned on after that.

[rfeenstra]

I'm sure it was the tube, even though it tested very good when I put it in the tester. The diodes were part of the automatic switching circuit and that was working correctly. I have since pulled out the RCA I put in there and replaced it with an older NOS Solvtek that I found. Works just fine.

[rfeenstra]

Well Crap!!!  I was watching Buzz's video on a Philco 45 when a buzzzzz started in my audio.  I thought it was a Buzz problem till till I looked over at my Scott amp and saw one of the 7591's quite red inside.  So my buzz was not Buzz but my buzzing Scott output.  Quickly turned it off.  More work to do.  I've left this thing on overnight many a time (I am over 65 so I forget to turn it off).  I'm glad it happened while I was watching Buzz. I'll blame Buzz for my buzz.  Don't know if it's a tube or the bias supply.  I think now is the time to instal the "IBAM" bias upgrade that you Fisher guys are using!

[rfeenstra]

I cleaned the tube sockets and turned it back on with a close watch on the outputs. Buzz is not buzzing any more! Still , I think I'll put it on the bench and check the bias supply and the screen supply. In the meantime, I'll be back to solid state.

[rfeenstra]

OK. It's been about 3/4 hour. Not redplating. Buzz is still not buzzing. I suspect a bad connection on the screen grid. I think I will still work on the biasing system and will work on the tube sockets..

[Phlogiston]

If yours has the 2 6u8As or a variation, better check them. They don't last as long as often expected and can become noisy. I just changed out both of mine.

I would also throw away the tube that red plated. It probably deposited metal on the micas.

A small fan blowing across the OPs will make them live longer - slow speed, muffin fan, switched.

[rfeenstra]

It's been playing perfectly now for several hours under my watchful eye. I am concerned about the condition of the red plated tube. The problem was audible and visible for only a few seconds but one never knows the damage caused. this was a matched set of 7591's. Probably not matched any more!

I sent you a PM.

[rfeenstra]

Russ, the 6U8's are capacitor isolated from the 7591's so I doubt they would cause the red plate condition. Only one of the pair did it so I don't think it was bias. Probably screen. I put the original, very weak, Scott 7195's back in. Sounds good even though they are tired. Heat generated is relatively low. I have them biased so they are not working as hard as designed. Still, I need to install the 1/4 watt resistors I bought for the cathodes, and then re-bias the outputs.

[Phlogiston]

Yes. I suggested checking the 6U8s because you had it on the bench.

Those tubes can self bias through conductive traces on the micas. The more often the tube overheats, the more likely this is to happen. I have seen it in both 7591s and 7868s.