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Old 12th Sep 2018, 6:12 pm   #21
David Simpson
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Default Re: Vintage BBC Valve Testers

Lawrence, you've done a great job in delving back into the 1930's for valve testing info. This latest info involved the BBC working with the NPL. So the resultant standardisation circuitry(Fig 2) must be at the top of the tree, nationally, back then. I'm surprised that AVO in the 50's just plumbed for CV455's & 491's in a simple manual recommended Ia/Vg circuit.
Like Martin, I reckon a shot at building a replica is a great idea. Maybe using the old B5 based valves, or later Octal or B7G's.
bollox, you've upset my plans for outstanding winter projects. Just now I'm knee deep in fruit picking & pruning. This hot summer has induced exceptional crops of fruit & veg on our wee smallholding. Then the effin shooting season starts next month. Where will this 73 year old f*** find the time(or energy) ?

Regards, David
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Old 13th Sep 2018, 8:15 am   #22
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Default Re: Vintage BBC Valve Testers

A quick search on Google Patent and some other patent sites didn’t show any related patent.

I also spotted an error in the first schematic in Fig.1, the cathode of the second AC2/HL isn’t connected to anything. I wonder if it should be connected to HT- or to the common cathode of the other two valves - I guess it depends on if it is part of the circuit that should draw current through R or not, if it is just working as an amplifier or as an integral path of the feedback current/voltage.

/Martin
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Old 13th Sep 2018, 10:28 am   #23
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Default Re: Vintage BBC Valve Testers

The description says -ve feedback from the third valve to the first valve so I would say that the second valve's cathode should be connected to HT -ve via its own cathode bias resistor.

Lawrence.

Last edited by ms660; 13th Sep 2018 at 10:33 am.
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Old 13th Sep 2018, 10:39 am   #24
David Simpson
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Default Re: Vintage BBC Valve Testers

Aye Martin, but I wouldn't worry about it. It was Figure 2 that was eventually produced seemingly, in collusion with the NPL. I too am thinking about building a replica, but reckon I'll try & keep it in a vintage format.

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Old 13th Sep 2018, 6:33 pm   #25
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Default Re: Vintage BBC Valve Testers

Even if the second design is what they used back when the report was written I am interested in the first design as it might work better with modern valves. I'll do some experiments with LTSpice and see if I can get it working and what needs to be adjusted.

The second design also use a 4.5V battery for the grid of the first valve, that either needs an extra power supply (or of course a battery) or it can be delivered by an isolation transformer and voltage regulator from the heater supply.

I'll probably also check how this "mutual conductance standard" can be solved with modern semiconductors as I am well along on my "semiconductor valve" which I designed to work as an ordinary valve when testing AC-based AVO VCMs as well as DC-based valve testers like my Roetest.
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Old 14th Sep 2018, 8:34 pm   #26
David Simpson
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Default Re: Vintage BBC Valve Testers

Martin, talking of semiconductor valves, some have been around for a few years now. In the UK we've been calling them "Fetrodes". I was given one a while back, but haven't got round to see how it works in a valve tester yet.
Its a SPL802 - a solid state version of the B7G PL802.

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Old 14th Sep 2018, 10:23 pm   #27
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Default Re: Vintage BBC Valve Testers

David, here's an equivalent circuit of the PL802 if you didn't have that already: Solid State PL802 schematic

I've got a bunch of Fetrons, manufactured by Teledyne, and they all work as designed but being early solid state tubes they die if you do gas tests on them (they warn about that in the datasheet) and also if you put the voltages backwards and also with positive grid voltages in some cases. They work very well in a well behaved/designed circuit but if you put them in an AVO VCM they will give up their smoke very easily (or you'll burn the internal fuse in them).

There are also Tubesters with semiconductors, however these are almost always made for a specific replacements in specific tube positions in Drake transmitters/receivers and are not really "universal" replacements.

There are also the modern AMT Warmstone solid state tubes manufactured in Russia if I remember correctly.

I have designed a few different circuits, the first designs working like a high voltage JFET where most of the voltage drop is performed by simple high voltage silicon transistors as pass elements in front of the low voltage JFET (High voltage JFETs are quite expensive today and not manufactured by many companies nowadays, the 2N6449 is one that costs some USD $15 or more a piece and is quite easily destroyed). I've also designed a few different circuits with depletion mode mosfets which handle the high voltage themselves. The tricky part is to get these designs to work in an AVO VCM where there is half wave rectified AC voltages or full wave AC voltages present and also to get them to work properly in DC-based valve testers like the uTracer or the Roetest. The operating point for semiconductors like JFETs and depletion mode mosfets vary a lot between each component and you are also affected by the heating with high voltages and high currents, I even found out that the extreme heat we had this summer affected the circuits when the sun shone through my window on the device I ran tests on. Valves are also affected in this way but my goal is to design something that is in some way compensating itself for these effects, but it seems like I have a long way to go to get that to work as well as I want to. Protecting the design from being destroyed by gas tests or wrong polarity worked like a charm with just a few diodes and zeners. I am simply not good enough at electronics to get this deep into designing stuff, but I try and I try hard not to give up!

All of my designs came from Googling and reading a lot about different designs used over the years and they aren't really anything new except for that they work well enough for my needs on AVO VCM's.

The biggest problem is to get time enough for all the real life tests and simulations in LTSpice that I want to do!

So getting this "Mutual Conductance Standard" to work would be very nice!
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Old 3rd Oct 2018, 8:52 pm   #28
chriswood1900
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Default Re: Vintage BBC Valve Testers

For those wondering what a VT5 looks like in the flesh one has shown up on eBay, I hope it is OK to reference it here.
https://www.ebay.co.uk/itm/VALVE-TES...53.m1438.l2649
I'm not making any recommendation just a chance to see a picture.

Chris
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