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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
Edits: 07/03/17Follow Ups:
The tube that hums, when you tested, did you test for shorts? Filament to cathode or grid?
5751 gain is considerably less than 12ax7, but I encountered a no sound situation with 5751's where 12ax7's sounded fine and it was a power supply problem causing the tubes to be biased off the chart for either tube. Suggest you measure DC voltage on cathode, grid and anode on both sections (pins 3,2,1 and 8,7,6) and take a look at the curves for both 5751 and 12ax7.
I will check it and see how the measurements compare to the 12ax7 tubes and the good 5751. thanks for the suggestion.
The voltages shouldn't vary much between tubes, so measuring once is fine. We want to see how the tubes are biased.
I think you have socket issues in addition to a bad tube or two. If a dual triode has a channel out in one socket, but both channels work in another, the socket is almost certainly the culprit.
Buy Chinese. Bury freedom.
after putting different tubes in the socket and they worked ok. Maybe i will just change out the socket anyway. I checked all the connections to the socket, looked really close for carbon or solder traces that might be causing problems. Everything looks ok.
Why do you suppose the Belvu tube hums? but tests ok and also works without humming as a driver tube? I have not run across such an issue with a tube.
Guess the humming tubes are bad.
As another test, i put the Belvu 5751 as the driver tube in a SE amp and it was ok in that amp. I am really puzzled as to the behavior of this tube. Anyone experience a tube with such oddness?
This is off topic but I think you should stick with US made 5751s. I think they are much more likely to be authentic. The Raytheon is a good to great tube and Sylvania is widely admired, then of course RCA and the multitudinous GE offerings. Did Mazda really produce 5751s? Not that this answers your question....
The silver plated ones are especially desirable. The rest of my mazda ones work fine in other equipment but i have not gotten them out to test in this preamp to see if it could be related to the brand.
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