Marconi Valve Voltmeter TF1041C - repair thread

[Alistair D]

How about running the meter for a few hours? Perhaps the grid current will improve with use. It could also be worth running the valves outside of the meter at say something like 10-20mA anode current to re-age them.

Al

Edit: Try running the meter with the 6BS7s removed. What voltage do you get on the probes then?

[Cathode Ray]

That sounds a reasonable idea Al.
No harm to try it anyway.

I'm not in a position to run them outside of the chassis, but I can certainly run them for several hours in place.

[Alistair D]

Ray, I added an edit to my post. You may have missed it. Something else to try.

Al

[buggies]

The section on the electrometer in this datasheet says that not all of their valves are suitable. Pages 5 and 6 also say stabilised 5 volt heater supply.
http://www.novacon.com.br/audiooutpre29b4.htm

I would not bother buying a random replacement valve.

[Cathode Ray]

Thanks George, that's confirmed I would need to get lucky.

And yes Al, I missed your update. I can try that tomorrow.
If the offset is still present them it's not the valves.

[Cathode Ray]

Hi Al,
With both 6BS7's removed there is no measurable offset.

And an overnight run of the unit didn't change anything.

So I guess I can only order a couple of 6BS7's and see how it goes.
Marconi used Brimar so I will see if I can get some NOS on eBAY.

Just a thought, I wonder if any sellers would measure grid current for me?
A matching pair with <5 uA would be good.
Langrex perhaps?

[Alistair D]

Are you happy that the cathode voltages on the 6BS7s are OK? I can measure mine if you need.

Before ordering new valves I would try a wanted ad on here. A good number of 1041s were scrapped because their meter movements went faulty, perhaps someone has still has the valves in their collection.

Al

[Cathode Ray]

The cathode voltages (tied together on the two 6BS7's) reads 2.55v with the offset and 2.45v when I ground the probe tip.
At the same time I can read the offset arising at the probe tip to be about 90 to 100mV
So there is some correlation there.

The earlier 1041C cct diagram it shows 4v at the cathode wheres mine (later redesigned version) shows an asterisk symbol and 1.2 (*1.2) , I'm not sure of this is supposed to be a voltage comment or something else?

The wanted add sounds like a good idea Al, no harm to try, thanks.

[buggies]

Originally Posted by Cathode Ray

Just a thought, I wonder if any sellers would measure grid current for me?
A matching pair with <5 uA would be good.

I very much doubt it!
Just re-read some of the stuff and note that the 5pA (not uA) grid current mentioned in the electrometer circuit would give 500mV with 100M grid resistance - if I have managed Ohms law with all those zeroes...

[Cathode Ray]

Cheers George,
Your right, it's pA ... That's a different story, I'd say that's difficult to measure with a valve tester.

I'll ask anyway, nothing to loose.

[Diabolical Artificer]

I'm testing some 6BS7's for Ray just now using the test circuit, see attached. That schematic has one mistake, it should -3v at the wiper of the pot. I'm testing them in triode mode and as per the "instructions" on the datasheet, EG triode strapped, HT 100v, g1 -3v. I'm letting them warm up for 10 minutes.

I have 5 NOS 6BS7's, two had wrappings with "card 232" written on them, which might indicate matched valves or valves with certain specs. So far I've got
V1 10pA
V2 0pA ( meter read zero mV)
V3 2.2pA
V4 currently reads 2.4pA.

The readings go down over time. Though the readings are not going to be 100% accurate, I should at least be able to get a valve with with very low grid leakage current or hopefully two that have matched grid leakage. It will be too much to get perfectly matched valves in terms of gm, Ig1 etc with only five valves to choose from.

Andy.

[Cathode Ray]

Great job Andy!

the 2.2 pA and 2.4pA valves seem close and are below the 5pA defined in the Brimar datasheet.
I think you have cracked it.
I've replied to your PM.

Cheers
Ray