Re: Valuable 12AX7 and all this audio foo foo.

Stan or Patricia Griffiths <w7ni@...> wrote:

Has anyone ever wondered why Tek used Telefunken, Amperex, and Mullard tubes in their scopes and not RCA, GE, Westinghouse, or Sylvania, at least in the small signal positions? I would guess that they knew these were higher quality, more reliable, quieter, more linear, and more consistent. Those same parameters make a difference at 10kHz as they do at 5MHz.

Chris wrote:
I'm not really sure what you mean by "small signal positions" . . . do you mean the first stage of a sensitive amplifier? For letter series tube amplifiers, the first stage was often a cathode follower, probably a 6AK5 (5654), Tung-Sol or RCA, or maybe a stage with gain containing a 6AU6 or 12AU6, Sylvania, RCA, GE, or Westinghouse. In some cases, it was a 7586 nuvistor, RCA or Hitachi. Virtually none of these tubes were Amperex, Telefunken, or Mullard.

After a couple of years working in the test department at Tek in 1960-62, I can say from personal experience that Tek had the very best luck with Amperex 6DJ8's over any other brand used in 530/540 scopes. They tried GE, Sylvania, and some CBS if I remember right. The differences were not subtle. Tek bought untested tubes because they were cheaper and every brand they tried had ten times as many bad new tubes right out of the carton than Amperex. It simply
took too much technician time to weed out the bad tubes that were new right out of the box, so Tek tended to stick with Amperex. I am talking about failures like open filaments, cracked glass, dead shorts between elements, and stuff like that. I don't recall much performance difference between brands of 6DJ8's once you got past the gross infant failures. No new stuff I worked on used many 12AX7's . . . I think there was one in each scope power supply and I
don't recall what brands were used or any significant failure rate of those, any brand. Same for 12AT7's and 12AU7's.


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