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strange behavior of Belvu and S&H 5751 tubes in my EICO ST84 preamp

Posted by DAK on July 3, 2017 at 14:36:40:
I was trying some 5751 tubes in place of the 12ax7 in the preamp and 2 of the ones i tried behaved oddly. Well, to begin with i tried some Tele ECC83 in the line stage and output tube of the EICO st84 with no problems. These 3 tubes, 2 for the preamp line stage tone control and 1 12ax7 is for the anode follower or output of the preamp. BTW, the preamp is completely rebuilt with all new parts and power supply except in the phono stage which i am not using. As i mentioned with the Teles i have no problems and the sound is good top to bottom at the neutral settings of the treble and bass controls. And the tone controls is quite useful with the Altec 19s which can have a treble that is too in your face. But, back to the problem.
So, i next tried a silver plate Belvu aka Mazda aka Adzam 5751 in the V5 position which is the anode follower tube and out of one channel is a loud hum. It is quite loud. I replaced that tube with a Raytheon windmill getter 5751 and no problem with hum or other noises. Pretty good sound maybe a tad better than the Tele. I took out the Raytheon and then I tried a Phillips 5751 SQ which is actually made by Siemens and is triple mica version. In the same V5 socket this tube has no sound in one channel. I tried wriggling the tube and could not get the missing channel to come on. I put the Raytheon back in and no problems. I guess i should mention that i ran all the tubes through a pin straightener before using. Then i tried all Raytheon 5751 in the preamp and no problems. It sounded very good and i could have more adjustment of the volume control which was too sensitive with just the ecc83.
Next, I tested both of these "bad" tubes in my EICO 667 tester and they both tested very good. I also tried both of these tubes in the V3 and V4 sockets and the Belvu still hummed and the Phillips was ok.
Pretty weird, no? Any suggestions and input is appreciated. regards, Dak