Testing the STC 4212D Triode - UK Vintage Radio Repair and Restoration Discussion Forum

David Simpson · 7th Oct 2015, 7:17 pm

Copied from this thread: https://www.vintage-radio.net/forum/...d.php?t=119783

Have bit the bullet & built a test rig.
With only 500V(on load) approx. maximum HT available, was only able to compare with STC's lowest Gm graph.(See copy). Slight kink :- due to HT Voltage range change switching from my DC PSU(ex AVO VCM Mk1's Transformers & switchgear), as Ia starts to rise significantly.
Anyway, good results. See graph sheet & picture.

Regards, David

David Simpson · 8th Oct 2015, 11:03 am

This unused valve was found amongst a box of stuff at a S/K sale some time ago by my retired engineer pal, Martin. Seemingly, the deceased collector was a cinema sound engineer who used these valves back in the pre-war heyday of cinemas. Having successfully tested Martin's STR18 Tx's PA valves a while back, we thought - why not this beast.
Martin built the temp. c/t 14V transformer supply, which he slowly powered up with my Regulac. Vf & If was carefully monitored, along with the filament connections. All tickety-boo. After several minutes, I then connected HT & -ve Vg, and slowly reduced Vg from -50V whilst monitoring Ia & Vg with AVO8's. Martin recorded the tabulations. Va was also monitored to keep as close to 500V as possible, as Ia began to rise.
Having built the valve holder assembly out of clear perspex & paxolin, it was easy to constantly view all valve pin & lead connections. All remained sound. Obviously, anyone considering building a permanent test rig or amplifier would acquire proper valve holders.(Also jolly expensive).
As well as heat dissipation considerations, these valves are seriously cumbersome. Hence they require a sturdy upright mounting. (Just like a waaf I used to know).

Regards, David

David Simpson · 8th Oct 2015, 11:14 pm

Many thanks to Bill for continuing the 4212D thread in this sub-forum. Even if I do flog it, its been a very interesting delve into the world of obscure huge vintage valves. Bit outside my comfort zone.
But, I would like to see a circuit diagram or two of amplifiers which used them. Also, if anyone else has one of these valves, and the equipment to power the filament & supply a KV or more of HT - then go on - have a shot. You'd probably be able to do better than I at getting more accurately nearer to STC's family of Gm graphs.

Regards, David

turretslug · 11th Oct 2015, 8:57 pm

Interesting set of photos of valve and test base- it must have been just a bit nerve-testing merely handling this large, tall, fragile and venerable valve considering its possible value! Thanks for posting.

Even relatively hum-drum middling power valves such as 6L6, 807, EL34 etc. are a bit difficult to test meaningfully where it really counts- high voltage and high emission and when it comes to more serious devices like the 211, 845, 813 types a "proper" test jig is going to be large, heavy and eminently lethal if handled carelessly. By time we get to a monster like this, even a very flat one would probably look usable on a mere AVO tester where it really isn't being at all stretched- and those with a fat wallet and the inclination to buy will be quite exacting about their rare and pricey purchase as the favoured single-ended usage means working at the edges of voltage, current and dissipation capability for meaningful output. And I do hate to read about lovely old AVO testers being used for prolonged "burn-in" of power valves.

7th Oct 2015, 7:17 pm	#1
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,851	Testing the STC 4212D Triode Copied from this thread: https://www.vintage-radio.net/forum/...d.php?t=119783 Have bit the bullet & built a test rig. With only 500V(on load) approx. maximum HT available, was only able to compare with STC's lowest Gm graph.(See copy). Slight kink :- due to HT Voltage range change switching from my DC PSU(ex AVO VCM Mk1's Transformers & switchgear), as Ia starts to rise significantly. Anyway, good results. See graph sheet & picture. Regards, David Attached Thumbnails

8th Oct 2015, 11:03 am	#2
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,851	Re: Testing the STC 4212D Triode This unused valve was found amongst a box of stuff at a S/K sale some time ago by my retired engineer pal, Martin. Seemingly, the deceased collector was a cinema sound engineer who used these valves back in the pre-war heyday of cinemas. Having successfully tested Martin's STR18 Tx's PA valves a while back, we thought - why not this beast. Martin built the temp. c/t 14V transformer supply, which he slowly powered up with my Regulac. Vf & If was carefully monitored, along with the filament connections. All tickety-boo. After several minutes, I then connected HT & -ve Vg, and slowly reduced Vg from -50V whilst monitoring Ia & Vg with AVO8's. Martin recorded the tabulations. Va was also monitored to keep as close to 500V as possible, as Ia began to rise. Having built the valve holder assembly out of clear perspex & paxolin, it was easy to constantly view all valve pin & lead connections. All remained sound. Obviously, anyone considering building a permanent test rig or amplifier would acquire proper valve holders.(Also jolly expensive). As well as heat dissipation considerations, these valves are seriously cumbersome. Hence they require a sturdy upright mounting. (Just like a waaf I used to know). Regards, David Attached Thumbnails

8th Oct 2015, 11:14 pm	#3
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,851	Re: Testing the STC 4212D Triode Many thanks to Bill for continuing the 4212D thread in this sub-forum. Even if I do flog it, its been a very interesting delve into the world of obscure huge vintage valves. Bit outside my comfort zone. But, I would like to see a circuit diagram or two of amplifiers which used them. Also, if anyone else has one of these valves, and the equipment to power the filament & supply a KV or more of HT - then go on - have a shot. You'd probably be able to do better than I at getting more accurately nearer to STC's family of Gm graphs. Regards, David

11th Oct 2015, 8:57 pm	#4
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,393	Re: Testing the STC 4212D Triode Interesting set of photos of valve and test base- it must have been just a bit nerve-testing merely handling this large, tall, fragile and venerable valve considering its possible value! Thanks for posting. Even relatively hum-drum middling power valves such as 6L6, 807, EL34 etc. are a bit difficult to test meaningfully where it really counts- high voltage and high emission and when it comes to more serious devices like the 211, 845, 813 types a "proper" test jig is going to be large, heavy and eminently lethal if handled carelessly. By time we get to a monster like this, even a very flat one would probably look usable on a mere AVO tester where it really isn't being at all stretched- and those with a fat wallet and the inclination to buy will be quite exacting about their rare and pricey purchase as the favoured single-ended usage means working at the edges of voltage, current and dissipation capability for meaningful output. And I do hate to read about lovely old AVO testers being used for prolonged "burn-in" of power valves.