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Hope someone can help. I have 5 Westinghouse labeled 7058's that appear to be RCA's. I've had these for many years and I believe they are actually 12AX7's labeled as 7058's. My understanding is that pin 9 on a true 7058 is connected to an internal shield. On a 12AX7 it's the heater center tap. I know that 7058's can only run on a 12V supply, not 6V.Comparing the "7058" with a 12AX7, they appear to be the same internally. But since I can't see very well and I'm not positive what I would be looking for anyway, I'm thinking that I can verify what these are with a DMM. Please correct me if I'm wrong. On a 12AX7 I would have continuity between any of pins 4, 5 and 9. On a true 7058, pin 4 to 5 would show continuity just like a 12AX7. But pin 9 to either pin 4 or 5 (or both) would be open. So if I have continuity between pin 9 and pin 4 and/or 5, it's a 12AX7 incorrectly labeled as a 7058. Correct, or no?
Thanks kindly.
Edits: 08/23/15 08/23/15Follow Ups:
You are correct.You should know that RCA made a bunch of 12AX7s that were labeled "7058". I had over 100 here at one time and while most were labeled RCA some were relabeled. You have 12AX7s labeled as 7058s.
Edited to fix typo...
Edits: 08/23/15
Thanks Jim. I'm not the brightest bulb in the shed on this stuff so I wasn't sure but I thought it made sense that I should be able to test it as I mentioned. I appreciate the information and confirmation.
According to the rca tube manual, the 7058 is for mobile use, with 13 v heaters. That would mean 12.6 would work fine. Also pin 9 is shown as IC or internally connected which may mean it could be connected to a shield. So, to use it instead of the ax7 would mean you need a 12.6v heater connected to pins 4 and 5 and pin 9 unused. I am not certain if the other parts of your circuit would need "fine tuning" but it would work if you did the pinout changes. regards, Dak
Yes, 13V +/- 1.5V. But I don't think they are actually 7058's.
Info appreciated anyway. Thanks.
,
Understood. But can you tell by using the meter as I described?
My guess is u r right. If it was a center tap for each 12.6V fil, then overall, fils in series would be 24V; not likely. It could be a center tap for two 6.3V fils in series; which would make it a mobile ready (ruggedized and higher than usual filament V capable), capable of using 13.5 to 14V DC filament 12AX7.
Get a 12AX7 setup ready for 6.3V AC filaments, using pin 9 as CT; but be prepared to somehow raise the filament voltage. Testing it like a 12AX7 in circuit, how bright is the filament with 6.3V ? If very dull orange, I would not be afraid to step up the fil voltage to say 7-7.5V. If it is bright orange at 6.3V, we have our answer that you are in possession of a ruggedized 12AX7 mislabelled, or simply capable to run as a filament series connected, higher than usual filament voltage 7058 with mobile ready ability.
My old sportscar was very capable of a 1.0 G force with its flat riding(no tilting while turning; only limited drift) hard suspension. The thought of using 7058s to drive 1625s(12.6 to 14V capable 807 type), using the car battery to light the filaments did cross my mind. However, when you have a buzz bomb, you usually want to hear the engine buzzing; which it did very well...
I don't know what I would have used to vary the voltage but I appreciate the suggestions. Fortunately, Jim chimed in and confirmed my multimeter test as being viable. I have 12AX7's.
Thanks
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