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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 30th Aug 2020, 2:56 pm   #21
Bazz4CQJ
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Default Re: Measuring gm of a valve without a valve tester

Yes, I remember now. It really is a pity that the 'Round Robin' Project, which was due to your initiative and hard work, wasn't written up as a paper. I think I'm right that the Project is on record only as the thread on this Forum.

As you said, it was unfortunate that different people began using different software to plot their data, which I know you found a problem.

Is it too late to try and write that Project up as a very brief paper that could be published somewhere (BVWS?) and/or, made a Sticky on here? I'd be very happy to help you do that.

Regarding the ~£1k price of VCM163's, as I've said previously, I was very lucky to have been given mine (I suspect it was very close to going in to a skip). But as I didn't spend money on it, I like to think that I'm fairly impartial about talking about it and 163's in general.

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Old 30th Aug 2020, 4:09 pm   #22
David Simpson
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Default Re: Measuring gm of a valve without a valve tester

Not really wanting to hog someone else's thread, Bazz, but I'll just mention that I will eventually write up a more formal report/paper. I kept reiterating that it wasn't a "my thingy is bigger that your thingy" competition between folk who favour AVO Testers or homebrew DC Testers(like myself) and other folk's testers such as Taylor, Mullard, Hickok, and so on. But sadly, no Taylor, Mullard or Hickok folk joined in. Hence the delay in writing a final report.
Going back to testing ECC valves - it was a basic DC testing rig for them which started me off on homebrew T/Eq ventures. Shortly afterwards I then built a electrolytic tester/reformer. Those ventures were eventually combined with other "repurposed" bits of test gear & odds & sods of components/spares into my eventual big homebrew DC tester. We're talking a timespan of 7 or 8 years, with a relatively low cost outlay.
Hopefully Martin can start out on his project with little expense, then go onto bigger homebrew projects. Wombling in skips, making deals at Swapmeets, scrounging off mates, etc., and a bit of a "Sgt Bilko" attitude, can save folk a fortune & give a lot of satisfaction on completion.

Regards, David
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Old 5th Sep 2020, 12:03 am   #23
martin.m
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Default Re: Measuring gm of a valve without a valve tester

I made a simple tester at zero cost using an old tape recorder power supply and parts from my junk box. At the moment it is only for testing ECC81 82 and 83 double triodes. The HT supply is not stabilised and does vary a little with load current. My method for testing was to set the grid voltage to exactly -1 volt then measure the voltage drop across each anode resistor. Next step change the grid voltage to -1.1 volt then re check anode current. So if the difference is say 0.14mA for a change in grid voltage of 0.1 volt, does this mean the gm is ten times that, ie 1.4? Sorting through a few vintage ECC83s produced gm readings of between 1.1 and 1.5. The grid voltage preset was very fiddly to adjust and I found that the ECC83s cut off completely once the vG got to around -2v. Have I got the maths right with all this and would a stabilised HT supply be better? I was thinking of a high voltage NPN transistor as a series regulator with it's base fed from a resistor and a suitable zener diode to earth. I have noticed that dealers selling used valves on the Internet often quote a 100% spec for the ECC83 as anode current 1.2mA and gm 1.6. Is this the maximum current the valve will pass with the maximum anode voltage and the gm expected under these conditions?
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Old 5th Sep 2020, 1:10 am   #24
Bazz4CQJ
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Default Re: Measuring gm of a valve without a valve tester

It would be much better to test each side on its own; even in a brand new valve, the two sides can vary quite a lot. For reference data, I would not depend on auction site info. Most usefully, use the Avo Data book as that enables you to compare against the most common testers in the UK. Alternative, find the makers datasheet on line.

ECC81 Va 250, vg-2, then Ia =10 and Gm=5.5
ECC83 Va 250 Vg-2 Ia 1.2 Gm 1.6

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Old 5th Sep 2020, 8:18 am   #25
Craig Sawyers
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Default Re: Measuring gm of a valve without a valve tester

The AVO IV valve tester service manual calls for a DC test of a CV491 (AKA ECC82). This is used as a standardised valve to test and calibrate the valve tester.

It suggests a straight line approximation at two different grid voltage and anode currents, but says for a more accurate result to plot over a range of anode currents and grid voltages and measure the slope at the specified values (Va = 200V, Ia = 16mA, gm~4-5mA/V for both triodes in parallel).

But that arrangement can be used for any small-signal valve (or any valve within reason).

https://frank.pocnet.net/instruments...-IVservice.pdf

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Old 5th Sep 2020, 11:21 am   #26
David Simpson
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Default Re: Measuring gm of a valve without a valve tester

Martin, you'll definitely need to maintain a steady Va for your tests. It might seem onerous to monitor steady/smooth DC voltages on Anode & Grid whilst also monitoring Ia. Bit of fannying about with multimeters you might think. However, you're an experienced electronics guy, so could easily work out the multipliers for basic voltage panel mounted mc meters, and likewise the shunts for a wee ammeter for Ia.
You've already started out re-purposing surplus stuff. Have you anything with metering on it ? Or you could see if any Forum folk have any decent 2nd hand panel meters.
A pure steady DC Vg could be acquired from a wee handful of re-chargeable AAA's or AA's, using a Pot or a switch, if you haven't got an LV Bench PSU.
I keep reiterating this - but ECC valves, particularly the high spec(expensive) variants, need to be treated carefully Ia - wise. So accurate monitoring of Vg is essential.

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Old 5th Sep 2020, 9:05 pm   #27
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Default Re: Measuring gm of a valve without a valve tester

Hi Martin - nice to see another homebrew tester being developed. For what it's worth, my first impression is that the little 100k cermet potentiometer (I know you mentioned zero cost parts from your stash) really isn't earning its keep.

Trying to adjust the grid voltage in a 100mV voltage increment from the 6.3Vac (8.8V pk) on a tiny track with a screwdriver must be a bit of a nightmare! Maybe use a potential divider first, with a better potentiometer across the lower resistor to get a more sensitive adjustment? Perhaps even stretch to a zener diode from the stash for an easy improvement too?

Happy tinkering!
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Old 8th Sep 2020, 10:13 pm   #28
martin.m
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Default Re: Measuring gm of a valve without a valve tester

Thank you all for your comments and suggestions. Yes it would be far better to have a variable regulated HT supply. I was thinking of a high voltage transistor as a series regulator with a variable voltage on the base. That 100K preset is certainly a pain to adjust so will be replaced with a full sized potentiometer. For the moment, the HT supply has been set to 250 volts and the grid volts to -2. I have been giving my ECC83s a general health check by measuring the anode current of each section. New spec is 1.2mA and most of mine range from 0.7mA to 1.16 mA. They all sound the same to me though, regardless of make, colour of print, shape of getter etc!
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Old 9th Sep 2020, 9:39 pm   #29
Ed_Dinning
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Default Re: Measuring gm of a valve without a valve tester

Hi Martin, have a look at multi turn pots with turns counting dials; very useful for fine settings

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