AVO Valve Testers

[David Simpson]

Has anyone built a CV455 (Standardising Valve) tester from the circuit shown on page 42 of the CT160 manual ?
As I've a handful of CV455's,thought I'd build one & have made a start. Intend to use digital DC LV Bench PSU's for the heater & Vg supplies, & monitor Ia with an AVO8 Mk5, & Vg with an AVO DMM. Have just started to build a 200V HT rectifier from some spare bits, which will be fed from a Variac via a mains isol. t/f.
Any advice or tips would be appreciated.

Regards, David

[retailer]

I did this a 2 or 3 years back, the whole setup was very touchy and I remember thinking multiturn pots to adjust voltages would have been nice. I think that both halves are strapped together in parallel which gives a gm reading for both halves combined. Mine came out at 11 Ma/volt.

[XTC]

There was a thread talking about standardised valves for AVO testers a while back, which you might find interesting.

https://www.vintage-radio.net/forum/...standard+valve

It might be easier to buy one than put together the rig to produce your own.

Pete.

[David Simpson]

Thanks Retailer & XTC, Have downloaded the info refered to in last June's threads.
I'm half way through my wee standardising circuit project, which gets worked on when poor weather drives me inside from our chaotic big garden. Apart from the handful of CV455's(12AT7/ECC81), I've over 30 ECC88 to test,+ many others. My last hands-on testing of valves was undertaken nearly 40 years ago in the RAF. In those days we were spoilt for accuracy as the standardising valves were in pleantiful supply from either the MOD calibration centre at Henlow or direct from Mullard. My recently aquired CT160 is a lovely item, & the manual's voltage tabulation readings seem pretty spot-on using my AVO8 Mk5 & AVO DMM, but shall now also carry out the oscilloscope checks which were described in the www.vintagewireless.net website, just for a final check.

Regards, David

[deco-doctor]

Originally Posted by David Simpson

Have just started to build a 200V HT rectifier from some spare bits, which will be fed from a Variac via a mains isol. t/f.

Hi David

I am in the process of doing this myself. I would say that the 200V HT supply almost certainly needs to be a well regulated regulated supply, otherwise getting everything to balance exactly whist adjusting the HT with the variac and the negative grid volts will be a difficult task.

I have to admit by being a little confused why both triode sections of the valve are strapped. I am assuming you need to plug the calibrated valve into the tester with both triodes still strapped?

I was thinking of making a standardised valve from a single triode of similar spec, 6J5GT for instance.

Martin

[David Simpson]

Hello Martin,The manual doesn't say why it recommends a CV455. I'm assuming(my old grey cells are trying to remember valve theory lessons from back in the early 60's) that using the two halves of the valve strapped together gives a bigger x-sectional area of electrodes & hence a more linear Ia/Vg graph.Whearas a single triode will have a slightly less linear characteristic. I'm sure someone on this site could explain the theory in more detail.
The selection code for a CV455 is :- 741 226 413. However this electrode selection is connected to all 9 pin valve bases in the tester not just the B9A for the CV455.Therefore, I'm assuming that careful fitment of wee jumper leads into the bigger B9 base will enable the electrodes to be parralelled, and just carry out the testing as per para 4. Or prahaps the code could be changed.
I agree that the 200V HT will have to be carefully monitored. I had originally intended building just a half-wave rectifier, but have found a pack of 4 new 1N5404 diodes in a drawer, so shall build a bridge instead.

Regards, David

[David Simpson]

Standard Tester built at last,as today was wet & windy. So far - have tested 4 CV455/ECC81's. Two gave values of 10mA/V, & two were 20mA/V. Readings were tabulated between Ia values of 10 to 20mA in one mA steps. The slope was read between 15 & 17 mA. Shall test a couple more, then use them to check the accuracy of my 2Panel & CT160 valve testers.
Va was monitored with an AVO8/5,Ia with an AVO/Newlec DMM, and Vg & Vhtr was supplied from 2 channels from a BSI PSM3/2 Digital DC PSU.

Regards, David

[stephen]

Is there anyway you could be persuaded to use your rig to produce a standardised valve for me? I've a few ECC81 CV455 i could put in the post.
Regards
Stephen

[David Simpson]

Arrangements made by PM, David

[Richard]

Hi

Just found this thread...

I have made a std valve, few years ago now.

I am lucky enough to have a huge Solortron regulated HT supply ( 0 > 500V @ 350mA in 1V steps) to supply HT and LT, and a small variable supply for the bias.

When you use the std valve in the AVO tester you change 1 setting on the roller switch so you have A1 up twice (instead of A1 and A2) and this puts the two in parallel.

Richard.

[David Simpson]

Richard, Thanks for your info. I just used jumper links in the big B9 valve base, but right enough altering the selector code works just the same. Lucky you having that big Solartron PSU, certainly ideal for the job.
My 3 channel digital LV PSU is ideal for the heater & grid voltages, but I must admit that I have to carefully monitor the 200V from my wee homebrew F/W rectifier and adjust the Variac a smidgin every so often. I've now standardised tested 6 CV455/ECC81's for myself & 5 for Stephen. Its well worth the time drawing up complete graphs from 10mA to 20mA Ia in 1mA steps - to get the best results. Also, its good to be able to monitor Ih on the digital PSU as I'd be worried if it rose much above 0.3A.
Once I've decided on my best CV455, I'll draw up subsequent graphs on a tranparent overlay for it, say once a week if I'm testing quite a few valves on the CT160 & carrying out regular standardising checks, then monitor any change in the slope.

Regards, David

[stephen]

A big thank you to David for standardising some valves and providing graphs for me - an excellent job and by return of post as well!
Re the above comment on A1 and A2 am i correct that it means the set up when using the standardised valve in the B9a socket would be changed from 741226413 to 641226413

[David Simpson]

Stephen, My humble thanks for your kind comments. In the standardising rig I just soldered pins 1 & 6 on the valve base together with a wee jumper. Then when checking the CT160 just used temp jumper in the big B9 base. However, changing the code to 641226413 would be just the same, as Richard suggested. Cathodes & grids(pins 3&8, and pins 2&7) are automatically linked in the code selection.
Now that I've had a chance to put several standardised valves through the 160, and spent a fair bit of time between adjusting RV3,cleaned & re-greased RV2 & RV4, checking ~ indications,dissconnecting & reconnecting the wee link, and setting up RV2 - I've found that by removing the RV2 indicator disc, spraying the blank reverse side with a nice silver-grey paint, then replacing it upside down, then repeating the 1 to 40V measurements - fairly accurate readings can be inked onto the disc. This helps to aleviate the non-linear RV2 indicated voltage problems that folk seem to experience with this 50 year old pot which is now un-obtainable. Most importantly - the CT160 gives a closer green/goodness Ia/Vg reading to the standardised valve value.
Also,as most contributors will already know, its important to constantly monitor the ~ reading, as this actually monitors the voltage from the 55V AC winding of TR1, which when rectified - is the DC source of the measurement bridge, and obviously the source of RV2's 40 odd Volts. Plugging the 160 into the house's ring main, setting it up,start testing valves, then see what happens to Vg when your wife puts the Sunday roast into the main oven & switches it on.Or any other high wattage load for that matter.
Here endeth the lesson. Wish the wind & rain would let up, so I can escape out into the garden & get involved with more mundane tasks.

Regards, David