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Ash Small

Mon May 30 2011, 11:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve, the ECC82 is the european equivalent.

Apparently it was used in virtually every valve TV ever made.

Loads on Ebay cheap. for example Â£6 buy it now, Â£3 postage for a Phillips one. others are cheaper

eg 10 for Â£1.20, ends tomorrow

Proud Mary

Mon May 30 2011, 12:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You can download this very comprehensive volume

Rider, J. Vacuum Tube Voltmeters, John F Rider Publishing Inc, New York, 1951, 430 pages, as a free pdf here:

Nah

Mon May 30 2011, 02:07PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

12AT7 tubes are almost identical and can be found in any AA5 radio (you being in europe.......)

The 6SN7 is a great tube and is still cheap. I think the octal tubes are better than the 9 pins

Steve Conner

Mon May 30 2011, 09:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No they aren't, the 12AT7 has considerably higher mu, about halfway between the U and the X. I guess it would work in a valve voltmeter though.

I always forgot which was which until I realised they go in alphabetical order, the T is the ECC81, the U is the ECC82, the X is the ECC83.

The All American Deathtrap is a story for another day. They were imported to the UK to electrocute the unwary. To convert to 240V, a resistive line cord was added to drop the excess voltage, this also sets the carpet on fire and makes the death look like an accident

Proud Mary

Mon May 30 2011, 11:19PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

In 1936-38, Osram produced a dedicated valve voltmeter triode, the A577.

The recommended circuit diagram is a splendid lesson in minimalist design:

Ash Small

Mon May 30 2011, 11:35PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The All American Five
Cautions, Precautions and Troubleshooting.

Wikipedia-The All American Five

Nah

Tue May 31 2011, 12:02AM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

The AA5 is the flagship of Darwin's army.

anyhow, yea, I forgot that they were different, I just remembered that I found a 12at7 in a tube tester socket for a 12ax7........

Um....... What does this have to do with Ash's plan..............?

magnet18

Tue May 31 2011, 12:26AM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

Proud Mary wrote ...

In 1936-38, Osram produced a dedicated valve voltmeter triode, the A577.

The recommended circuit diagram is a splendid lesson in minimalist design:

OOH, I shall keep an eye out for one of those, tubes with an everyday function are hard for me to come by.

I find the approach interesting, by varying the grid voltage they change the current on the ammeter, which can be calibrated to a voltage scale, if I read the circuit correctly.

Was the idea to make sure that there would be no current drawn by the act of measuring since the grid is operated purely on potential?

Steve Conner

Tue May 31 2011, 06:17AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, that's exactly the idea of the VTVM in general, not just that one circuit. The grid theoretically draws no current, so you can get a perfect measurement of voltage. Of course in practice it draws a tiny bit, but in a tube designed for electrometer use it can be very small indeed. Bob Pease wrote about this, he estimated about 1nA for the 12AX7s used in the Philbrick tube op amps. He also mentions that the LMC660 can beat it easily.

Nah, if Ash needs some Alnico for his magnet, he can smelt it over the flames from an AA5. No, sorry, it is completely off topic.

Ash Small

Tue May 31 2011, 08:57AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve McConner wrote ...

Nah, if Ash needs some Alnico for his magnet, he can smelt it over the flames from an AA5. No, sorry, it is completely off topic.

It's interesting nontheless, and the discussion on suitability and availability of valves for a VTVM is very relevant.

I've never bothered learning more than the basic principle of valve technology before, but this thread (along with the 'magic eye' LCR mater I recently aquired, and another item I've not yet posted about) is motivating me to learn more about the subject.

While FET's could be used to amplify the signal for a meter, a valve is not going to fail short-circuit, so as long as the circuit is designed with the valve(s) as the 'weak link' it will be virtually indestructible (I think!). (the AA5, especially the ones with the resistive cord for 240V, is an example where this wasn't the case)

As for the subject of magnets, There was one 1" cube N38 neodymium magnet on Ebay recently that went for nearly Â£10. I'm after at least three, probably five, and preferably N52 (or at least N45 or N48), and preferably unplated (as I'm going to 'attempt' to shape them), if anyone can help.

And thanks to everyone who has contributed so far.

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