Testing Transmitter Valves in AVO VCM's - UK Vintage Radio Repair and Restoration Discussion Forum

David Simpson · 26th Oct 2014, 1:27 pm

I see that a 4X150A is listed in the Avo Valve Data Manual. But, my oh my, what a heater current it draws ! 2.6A is a lot to pull out of a Mk3, or any other old VCM for any length of time.
A pal has aquired an STR18's Tx, and wants me to test the valves, but I'm wary of those thumping great PA's in a 60 year old VCM, or even my new LeyMarcAvo. Has anyone ever tested one ?
TT22's, EL37's, 807's, QQV06-40's, PL500's, all tested on my testers, no problem, but I'm just wary of 4X150A's.

Regards, David

trh01uk · 30th Oct 2014, 9:22 am

David,

I have faced the same problem - not with 4X150As - but with some big German PA valves that expected some 1200V on the anodes, quite apart from excessive heater current: both clearly well beyond any ordinary valve tester. Not to mention that the unusual valve based wasn't featured in my Mk.IV VCM.

What I did was to rig up a separate heater supply from an isolated DC supply, which was then hooked into the cathode connection on the VCM - it was a directly heated valve, btw. For the anode voltage I just used the 400V I had and made some intelligent extrapolation from the datasheet I had. Not brilliant, but I was able to spot a low gm valve (I was testing several) and one that had gone soft.

The only question I had was whether testing at a low anode voltage holds good when you push the voltage up to 1200V or 2000V (or whatever the valve normally uses)? I see no reason why they should not apply - unless it suffers some sort of flashover at the higher voltages - and probably the only way to determine that is to put it in the original equipment and see what happens.....

Richard

Radio Wrangler · 30th Oct 2014, 9:40 am

With a DC tester, you can get a result which should be aperfectly valid point on the characteristic curves. If you can find published curves then you can compare the actual valve to what it should do.

With an AVO there is the added complication of their corrections for half-wave operation to worry about but the result should be comparable.

DAvid

trh01uk · 30th Oct 2014, 10:43 am

Yes - but often the problem is a lack of characteristic curves in any published data - some educated guesswork is then necessary. I usually find valves fail catastrophically though - with blown heaters or some electrode shorts.

Richard

David Simpson · 30th Oct 2014, 11:40 am

Thanks Guys for the comments.
I'd failed to mention before that these squat fearsome beasties need to be forced air cooled even when in heated s/by mode. Hence AVO test procedures need to be quick. Another aspect of these valves is the wide but thin Anode Cap. Thankfully, I had already made up a 1000V insulated lead with a big crock clip from a spare Megger set.
My pal still hasn't received his Tx yet, but we did a trial with a 26.5V Heater version of the valve(ex PTR175). All went well.
Obviously, AVO's DM's anode & screen settings run the valve pretty near the bottom of the Ia/Vg curve. Even so, it still indicated 45mA on the Mk3, and a good green zone reading.
The required speediness of testing rules out acquiring tabulated readings for a full graph on either the Mk3 or my DC L/M/A. I suppose one could always make up a homebrew ducted fan unit. But I doubt I'll ever see more than two or three of these valves in my remaining years.

Regards, David

Radio Wrangler · 30th Oct 2014, 11:54 am

You want to try checking 4CX250Bs or 3-500Z!

And some 8877 devices have grids which bend away/towards the cathode when really working. These are fun to find at 3kV.

4X150 were very common in all makes of airborne HF, and EIMAC published full curves.

If you're playing with these beasties you need to read "The care and feeding of power grid tubes"

THere's a lot of good stuff on high power valves and how not to wreck them on the web pages of Ian White GM3SEK

http://www.ifwtech.co.uk/g3sek/

David

Sean Williams · 31st Oct 2014, 10:55 pm

The 4x150 will need forced air for any application of heater volts.

The risk of cracking the base seal is very high without.

You can substitute a different heater transformer to allow limited testing in a vcm, but the biggest problem with these type of valves would be that of internal flashover, a very real problem on any with long storage.

There are several articles on the www regarding the use of surplus Russian power tubes, the advice contained is worth knowing

Hth
Sean

26th Oct 2014, 1:27 pm	#1
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,859	Testing Transmitter Valves in AVO VCM's I see that a 4X150A is listed in the Avo Valve Data Manual. But, my oh my, what a heater current it draws ! 2.6A is a lot to pull out of a Mk3, or any other old VCM for any length of time. A pal has aquired an STR18's Tx, and wants me to test the valves, but I'm wary of those thumping great PA's in a 60 year old VCM, or even my new LeyMarcAvo. Has anyone ever tested one ? TT22's, EL37's, 807's, QQV06-40's, PL500's, all tested on my testers, no problem, but I'm just wary of 4X150A's. Regards, David

30th Oct 2014, 9:22 am	#2
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,654	Re: Testing Transmitter Valves in AVO VCM's David, I have faced the same problem - not with 4X150As - but with some big German PA valves that expected some 1200V on the anodes, quite apart from excessive heater current: both clearly well beyond any ordinary valve tester. Not to mention that the unusual valve based wasn't featured in my Mk.IV VCM. What I did was to rig up a separate heater supply from an isolated DC supply, which was then hooked into the cathode connection on the VCM - it was a directly heated valve, btw. For the anode voltage I just used the 400V I had and made some intelligent extrapolation from the datasheet I had. Not brilliant, but I was able to spot a low gm valve (I was testing several) and one that had gone soft. The only question I had was whether testing at a low anode voltage holds good when you push the voltage up to 1200V or 2000V (or whatever the valve normally uses)? I see no reason why they should not apply - unless it suffers some sort of flashover at the higher voltages - and probably the only way to determine that is to put it in the original equipment and see what happens..... Richard

30th Oct 2014, 9:40 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,903	Re: Testing Transmitter Valves in AVO VCM's With a DC tester, you can get a result which should be aperfectly valid point on the characteristic curves. If you can find published curves then you can compare the actual valve to what it should do. With an AVO there is the added complication of their corrections for half-wave operation to worry about but the result should be comparable. DAvid __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

30th Oct 2014, 10:43 am	#4
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,654	Re: Testing Transmitter Valves in AVO VCM's Yes - but often the problem is a lack of characteristic curves in any published data - some educated guesswork is then necessary. I usually find valves fail catastrophically though - with blown heaters or some electrode shorts. Richard

30th Oct 2014, 11:40 am	#5
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,859	Re: Testing Transmitter Valves in AVO VCM's Thanks Guys for the comments. I'd failed to mention before that these squat fearsome beasties need to be forced air cooled even when in heated s/by mode. Hence AVO test procedures need to be quick. Another aspect of these valves is the wide but thin Anode Cap. Thankfully, I had already made up a 1000V insulated lead with a big crock clip from a spare Megger set. My pal still hasn't received his Tx yet, but we did a trial with a 26.5V Heater version of the valve(ex PTR175). All went well. Obviously, AVO's DM's anode & screen settings run the valve pretty near the bottom of the Ia/Vg curve. Even so, it still indicated 45mA on the Mk3, and a good green zone reading. The required speediness of testing rules out acquiring tabulated readings for a full graph on either the Mk3 or my DC L/M/A. I suppose one could always make up a homebrew ducted fan unit. But I doubt I'll ever see more than two or three of these valves in my remaining years. Regards, David

30th Oct 2014, 11:54 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,903	Re: Testing Transmitter Valves in AVO VCM's You want to try checking 4CX250Bs or 3-500Z! And some 8877 devices have grids which bend away/towards the cathode when really working. These are fun to find at 3kV. 4X150 were very common in all makes of airborne HF, and EIMAC published full curves. If you're playing with these beasties you need to read "The care and feeding of power grid tubes" THere's a lot of good stuff on high power valves and how not to wreck them on the web pages of Ian White GM3SEK http://www.ifwtech.co.uk/g3sek/ David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

31st Oct 2014, 10:55 pm	#7
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Testing Transmitter Valves in AVO VCM's The 4x150 will need forced air for any application of heater volts. The risk of cracking the base seal is very high without. You can substitute a different heater transformer to allow limited testing in a vcm, but the biggest problem with these type of valves would be that of internal flashover, a very real problem on any with long storage. There are several articles on the www regarding the use of surplus Russian power tubes, the advice contained is worth knowing Hth Sean __________________ Engineers make things work and have spare bits when finished