[Phlogiston]

I was working on a Scott 800-B. It uses 3 6SK7 tubes in the receiver. The radio had a mix of metal and GT style tubes (wrong for this radio).

I was rounding up a batch of metal RCAs  from my stash. I noticed different test values. I know that I have seen this before and probably just assumed an error or deference in the testers.

Well, today I looked it up in the Hickok testing chart only a 6SK7 is listed at 2000 (mu) - but on my 533 roll chart it is 1900. So I went to the RCA manual (1966). There are TWO sets of parameters, one for the metal tube and one for the GT (glass) tube.

The biggest difference is the plate voltage. A metal tube is speced at 100V. A gt tube is speced at 250V.

The transconductance is 2350 for the metal tube and 2000 for the gt.

There are other differences - mostly ALL of them. These are different tubes! I have observed the metal tubes as testing better than the gt - - well duh. So, it was really never good to sub a gt for the metal even if a shield is added. IT WILL WORK, but, since these are often in the front-end the transformer (trans-coupled) will probably need to be retuned - always.

With the difference in plate voltage, subbing a metal tube for a gt could be really bad. You know how some tube sockets will list both the gt OR metal version? I can't recall ever seeing one that says 6SK7/6SK7GT - have you?

So if you all know this, ignore, and, why did you not tell me?