Odd trouble with new Tung Sol 12AX7's

[hasserl]

I recently had an amp (Pignose G40V) in here for some work. The owner had recently installed some new preamp tubes, the reissue Tung Sol 12AX7's, a week or two prior to the problem developing. The symptoms he had was a progressive loss of power / volume. He only uses the amp on Sunday's at church, so the new tubes didn't have a lot of time on them, just a couple of hours at most. I found very low output on the cathode follower out to the tone stack, the signal was very weak. I swapped that tube out and the amp was back up to full output. I called him to let him know what I found. He had bought the tubes somewhere with no warranty. So he ordered another Tung Sol 12AX7 from a different supplier and had it sent to me. I installed it, checked it out, everything was tits. So I let him know it was ready to go and I buttoned it up and set it aside. When he came to pick it up we plugged it in and gave it a try, and damn but the same exact problem was there again. This time the amp had less than a couple of hours of use before the tube failed.

Besides a very weak AC voltage at the cathode follower (with a 1k sine wave at the input) the DC voltage was very low. The schematic calls for 162vdc at the CF, we had something around 60 IIRC. I grabbed a spare tube out of a drawer and stuck in place of the Tung Sol and the DC voltage came right up to about 166 and the output was right back up to full. He had also installed a Tung Sol in the V1 position, I swapped it from the V1 position into the V2 CF position and stuck the spare tube into V1, and the voltage and output dropped right back down again with this tube. I pulled another spare tube out of the drawer and swapped out the Tung Sol in V2 and everything was right back to normal again. We left the spare tubes in V1 & V2 and he tossed the Tung Sol's into my waste basket before he left.

I have a hard time believing three different tubes from two different suppliers, would have the same condition. But there is no denying that the Tong Sol's had very low output at the cathode follower. They seemed to work fine to begin with but quickly the performance went away.

I know the cathode to filament voltage at a CF can approach the limit for a 12AX7, but they seem to work fine in plenty of amps with a CF in the preamp. Any ideas what's going on with these Tung Sol's? I did pull them out of the waste basket after he was gone, and I'd like to do some other testing on them to verify they are sub par. Any ideas?

[Enzo]

I'll have more ideas after you do that testing. Chances are they all came from the same batch, regardless of who sold them. ANy chance the pins wer small and they made poor contact in the socket? Or they were all oxidized? Did we check what was happening at the grid of those tubes under failure condition?

You say they worked OK in other sockets in his amp though?

[hasserl]

Well I'm not sure if they worked fine in other positions, they seemed to, or there were no symptoms of poor performance except with them in the CF position.

I'll measure the pins and compare to others.

[MWJB]

One thought...the Tubestore makes a comment about the filaments being "encased by the plate structures", like on the spiral filament EH 12AX7LPS (aren't they both from the Saratov factory?). I have heard that the LPS is not recommended for CF stages due to the high voltage (300v+) on the plate connected to the B+ rail.

Whether related to the LPS situation or not, it would seem that this is nature's way of telling you to avoid the TS in this position.

[hasserl]

I've only had the opportunity to measure the pins on the tubes. All the Tung Sol pins measured .040", for comparison I pulled several old RCA, GE, Westinghouse tubes and measured them, some measured slightly less at .039", some measured the same as the Tung Sol's at .040"

The pins all look to be in good condition, as you'd expect for new tubes; not a lot of corrosion.

[loudthud]

I remember hearing about cathode to heater problems on some Russian tubes but I don't remember the specifics. You might be able to measure some leakage right on the pins of the tube.

[Mark Black]

I realize tube testers don't give you the whole story, but I find it interesting that every new Tung-Sol 12AX7 I have tested with my B&K 747 showed bad and took forever to warm up to even that. Gotta be something crummy about the coatings or vacuum or whatever on those babies. They are the only brand that tests that way...

[hasserl]

Finally had a chance today to put these tubes into a different amp (Carvin Vintage 16) and try them. Ths was just a typical input gain stage and second gain stage, both just common cathode amplifier stages. All three tubes, all Tung Sols with very low hours had close to zero output. On all three the plate voltage on the B side (input) was close to the rail voltage, for comparison a GT labeled Sovtek shows 195 plate voltage. The B side is the cathode follower side in the Pignose amp these came out of.

Note that one of these tubes was working just fine in the V1 position in the Pignose amp before I moved it over to the V2 slot to see how if it would work there. It was only in V2t long enough to test it and find the voltage low, only a minute or two, and that was enough to cause it to fail completely.

[Bruce / Mission Amps]

Do you have a decent tube tester with a way to increase the filament voltage to something like 18v-20v?
I have found that some Russian preamp tubes are just horrible and suck dead donkey butt until I heat the living crap out of them for a minute or two with 6-8 v higher filament voltage.
After that they seem to work OK, like any other cheap 12AX7.
Careful, too much and they burn out.

[hasserl]

Nope, I sure don't. My tube tester has always been just an amp.

[GCA]

I realize tube testers don't give you the whole story, but I find it interesting that every new Tung-Sol 12AX7 I have tested with my B&K 747 showed bad and took forever to warm up to even that. Gotta be something crummy about the coatings or vacuum or whatever on those babies. They are the only brand that tests that way...

The heaters on the EH 12AX7's seem to blow goats. I rigged up a JJ ECC83S, a Sovtek 12AX7WA, the EH, and a Fender-branded tube (looked like the Sovtek on the inside) and ran all 4 off of a 6.3V transformer secondary in parallel. All 3 of the others' filaments glowed cherry; the EH could only manage a dim ember. I switched out the EH with another tube to make sure my socket wasn't crap, but no. The EH heaters suck. The other tube glowed just as brightly as before.

I've also heard that this tube doesn't work in a CF... I suppose you found that out the hard way. (two years ago, this is a really old post I'm replying to.) I havent' tried it, however.

[Enzo]

Keep in mind the only part of the heater that matters is inside where it cannot be seen. That some heaters extend out and are brighter is beside how they function.