Mr.French's Dynamic Mutual Conductance Tester.

[VT FUSE]

French was one of several names used by F.Rayer when writing for Radio Constructor magazine,he did have others as well, I have read issues of Radio Constructor where he published articles under different names in the same issue.

A question on the tester under discussion, presumably if it was directly calibrated in gm, a wirewound potentiometer would be pretty much essential to negate track value drift with age?

I am watching this thread with interest, I am sure many others are as well.

Mike

[TowerRadio]

I have a lovely Colvern precision wire-wound pot(it goes through 360 deg without stopping) but it's only 5k so I went and bought a mil.spec 50k wire-wound. I just need to calibrate it against a suitable scale for a 0 to 10 readout.This may take more time than putting the other bits together.Les

[Bazz4CQJ]

What valves would fill the V1 and V3 slots well? Mr Rayer (?) suggests 'a low-mu triode' and a 'standard high-gain AF pentode', which I guess leaves it open to some degree as to what you have in the your junk box. In terms of valves which are fairly common place and otherwise not in great demand, I wondered about half of an ECC82 for V1 and maybe even an EF91 for V3, leaving your EF86's etc for more demanding roles?

B

[VT FUSE]

How about EC90 for V1 and EL95 for V3?

Should get the job done

[VT FUSE]

I believe that I may have committed a cardinal sin in so far as I cannot corroborate my stating that Mr French and F.G Rayer were one and the same, I stand by my statement that F.G.Rayer wrote under several pen names-however this is not pertinent to this thread.

[stuarth]

W T Cocking has a "Valve Testing Bridge" in the sixth edition of his book "Wireless Service Manual".

Ignoring DC blocking capacitors and biasing, the circuit consists of an AC source connected to the signal grid, a variable resistor from grid to anode, and high impedance headphones from anode to HT positive. The claim is that you adjust the resistor for minimum sound in the headphones. At that point, the signal fed through the resistor exactly matches the inverted signal through the valve, so the mutual conductance of the valve (in mA/V) is given by 1000/R where R is the value of the variable resistor in Ohms. You might need a resistance box rather than a pot for the resistor.

Stuart

[ms660]

The W.T. Cocking valve testing bridge article is also available from the GVR archive courtesy of Joe:

https://golbornevintageradio.co.uk/f...d.php?tid=2758

Lawrence.

[Radio Wrangler]

Only available there if you're a member, it seems, Lawrence

David

[ms660]

That's correct.

Lawrence.

[Bazz4CQJ]

It was posted by Joe way back in 2012 but not one single person has posted any comment on it, good or bad. Judging by the pictures, it must date back to the early 1930's (?). The only valve used is the one under test.

Is Mark Hennessy the person to ask about it?

B

[Ed_Dinning]

Hi Yes, I believe the Cocking valve testing bridge was in one or the mid 30's editions of his book.

Ed

[stuarth]

My copy of Mr Cocking’s book is the sixth edition, printed in 1943, and conforming to wartime economy standards. Small but beautifully formed. Lovely paper.

Was it dropped from later editions because the bridge technique didn’t work well, or because gain testing wasn’t seen as being necessary?

Stuart

[Skywave]

The thread for this has now been closed. However, in one post, Bazz4CQJ, enquired about suitable valves for V1 and V3, but no definite types were stated.
As the cct. design stands, the specific component values listed strongly imply that particular valves were used for V1 and V3, but in the cct. diag. (and supporting text) these are not specified. Has anyone here built this tester with satisfactory results? If anyone has done so, what valves did you use, please?

Also, V2 - the valve under test (VUT) - is shown as a triode. If a pentode was used instead, would it be appropriate to strap g2 to the anode to determine that valve's gm?

Al.

[Radio Wrangler]

Originally Posted by Skywave

Also, V2 - the valve under test (VUT) - is shown as a triode. If a pentode was used instead, would it be appropriate to strap g2 to the anode to determine that valve's gm?

Just spotted this...

No. A tetrode or pentode has TWO Gm values. G1 to Anode and also G1 to G2.

When used as a normal pentode amplifier only the G1->Anode one counts so that's the one you want to measure.

Strap G2 to Anode and you measure the sum of the two mutual conductances, of course. This is the one that's relevant if you intend using the valve triode-strapped.

David

[ms660]

There's often not a lot of difference.

Lawrence.

[Radio Wrangler]

True.

But it lies in wait to catch people out for the ones where it does. Early valves before the fancy work on aligned grids and beam forming were done seem to be where the greatest differences lie.

A lot of people using valves forget or never knew about some of the screen grid 'funnies' like the transitron oscillator.

David

[ms660]

Here's a couple of examples:

http://www.r-type.org/pdfs/el84.pdf

http://www.r-type.org/pdfs/kt88.pdf

Lawrence.

[Radio Wrangler]

But look out for the different Ia Vg1 conditions for the two conditions..

David

[David G4EBT]

Originally Posted by VT FUSE

French was one of several names used by F.Rayer when writing for Radio Constructor magazine, he did have others as well, I have read issues of Radio Constructor where he published articles under different names in the same issue.

A question on the tester under discussion, presumably if it was directly calibrated in gm, a wirewound potentiometer would be pretty much essential to negate track value drift with age?

I am watching this thread with interest, I am sure many others are as well.

Mike

It's true that here was no such person as G.A.Frech in reality - it was a pseudonym, but not of F.G.Rayer.

G.A. French - who wrote the marvellous ‘Suggested Circuits’ series, and the much-loved Dick & Smithy series - were pseudonyms for J. R. Davies who was Technical Editor of Radio Constructor Magazine for many years. He also wrote under the pseudonym W.G. Morley as well as his own name. Assuming that no-on else but he used those names, he must have been quite young when he started writing for Radio Constructor. For example, there were articles in 1949 Radio Constructor by W. G. Morley and J. R Davies at which time he’d have been aged just 25.

More about that topic in this 2019 forum thread:

'Radio Constructor 'G.A. French' and Dick and Smithy unmasked!

https://www.vintage-radio.net/forum/...d.php?t=162080

Hope that's of interest.

[ms660]

Originally Posted by Radio Wrangler

But look out for the different Ia Vg1 conditions for the two conditions..

David

They look about the same to me.

Lawrence.