[Laudanum]

Hi GRD

Different topic this time.  I think we were discussing 7058's a little while back and you mentioned that you had some that worked in the Cornet.
I aquired several a week ago and got around to plugging them in yesterday.  They also worked.  Now, heres the rub.  By all rights, from what I can gather, they shouldn't work in the cornet with it's 6.3 volts filament circuit.   After some comparisons and some research, I'm 99% sure that my 7058's are mislabeled and are actually just 12AX7's.  
On the bright side ... these are  Westinghouse labeled, NOS 12AX7's which are actually RCA's.  They are RCA grey plates, round getter, not the earlier black plate sqaure getters but still a very nice sounding tube and one of my fav 12AX7's.  I got 5 of them them for $9.99 so all is good.
Anyway, you may want to check out your "7058's" further .... they quite possibly aren't true 7058's but if you didn't pay much, you may have some diamonds in the rough, so to speak.

[GRD]

So you confirmed they work!  I agree - they shouldn't work with 6.3v.  I picked mine up (an RCA labeled 7058) in a bunch of used 12AX7 pulls.  I plugged it in, it worked, and then I read up on it and pondered if it was running at 1/2 voltage (not that it seemed to operate any differently than the 12AX7's).  I put it back on the shelf and never went any further.

Quit possibly some 7058's were re-labeled or modified 12AX7's.

[Laudanum]

The 7058's I have are late production RCA 12AX7's.    My understanding is that when tube production was waning, the most common tubes were the last to have production stopped.  So, it makes sense that relabeling was common to meet demand on tubes that were no longer being made.  So it makes sense that a tube like the 12AX7 that would work in a 7058 application or a 6681 application for example, would have been relabeled as such to meet demand.  I've heard that plenty of this type of relabeling went on occassionally anyway but really wasn't uncommonn at all nearing the end of the tube era.

I read that a true 7058 is internally the same as a 6922 ... different specs but same construction.  That would make pin 9 an internal shield connection.  My 7058's pin 9 is connected to both pin 4 and pin 5 heaters (heater mid-tap)  just like a 12AX7 .... they are relabelled 12AX7's ... again, 99% sure because there isn't much info on this tube and, it works in the cornet and definitely shouldn't.
They sound great by the way, just like an RCA 12AX7  

Take care

[GRD]

I went back and checked my 7058 for further ID.  It's an RCA, black plate, square getter, date code of 59-?? (1959).