Free Standardised Valves
 

After kind Forum folk coughed up to buy some surplus working vintage test equipment from me, as a "thank you" gesture I'm giving away some standardised & Gm graphed valves for valve tester/VCM owners. A middle of the road pentode - 6AU5GT is the valve in question. Someone kindly gave me 4 of them some years back.
Simple enough to determine "Green Zone" indications on valve testers & VCM's. Those more adventurous could tabulate a few Vg & Ia readings & draw up their own simple graph. Forum questions and requests on valves & testing seem to have been raised quite a bit over recent months. Repair & calibration aspects can be studied in depth by just looking up "Search".
The first valve I'll post to someone on a "first come first served" PM basis. Postage would be £3.90 or £4.40 for a "signed for"/tracked Royal Mail small parcel. As I always double box posted valves, instead of a flimsy jiffy bag.
The valves will hopefully be just passed on to other folk at various swap meets etc., thus avoiding repeat postage.
We've friends from Sheffield staying with us just now, and another st'd valve will return with them. I hear that there is an enthusiastic VR group in the South Yorkshire locality. Likewise, another valve will head down to Lancashire at the end of the month, I hope. The remaining one can go anywhere.
As valves are passed on, folk could send on blank copies of graphs I've provided via the Forum or email attachments. If some folk would openly share their results, I'm sure the vintage radio fraternity would welcome it. This isn't a " my tester is better than yours" exercise - just a simple sharing of information. Nor do I have any mercenary valve tester/VCM repair &/or calibration service interests.
Why a medium sized robust pentode some folk might ask ? Well - the wee AVO recommended CV455 & 491 ECC types are susceptible to easily being damaged due to faulty or wrongly set Vg pots. Along with tester's meters,(horrendously expensive to replace), whanging dangerously to full fsd.
Folk don't need to use the AVO VDM recommended test voltages for Va, Vs & Vg for a 6AU5GT(or any amplifying valve, in fact). Slightly higher or lower values could produce a family of Gm curves. But quite frankly, I don't have the time to spend hours drawing graphs. Perhaps some experts might well have "curve tracing" equipment &/or software. Their input would be gratefully welcome, please.

Regards, David

Re: Free Standardised Valves
 

What a good and generous idea, in the true spirit.
Sam.

Re: Free Standardised Valves
 

David,

yes - generous offer. I see you used your DC testing system christened the "LeyMarcAvo" - and I read up the 2013 thread all about that.

However confident are you on the calibration of that system - given that your Mk.III VCM is differing by around 10% on the test valve? Which one do you think is out of cal?

Richard

Re: Free Standardised Valves
 

Thanks Sam & Richard.
The issue of pure DC testing v commercial AC mains derived testers has been banged on about quite a bit. Just look at "Search" thread posts by many tester/VCM enthusiasts.
Basically, AC testers cannot compensate for Ia, Va, & Vs loading of their transformer's windings. DC testers, using regulated DC PSU's can, either automatically, or by constant monitoring by a VVM(as in my L/M/A) and manual adjustment of Va & Vs if necessary - for each tabulated reading - as I do. I've tested other VCM's & testers I've owned, and other local folk's testers- and their graphs, if their calibration is OK, fall into that 10% zone in the upper reaches of Ia. I reckon the DC tester is spot-on, and my Mk3 is pretty close for half the graph, but looses it's oomph for the reasons given above for the upper half. If other folk get close enough to the zone between the two curves, then their "green" meter indication should be ok.
Lets get started & see what comes up.

Regards, David

Re: Free Standardised Valves
 

The message above should read: "How confident are you on the calibration of the system?"

[Sorry - admins - I can't be bothered to contact you just for a simple edit when some arbitrary 30 minute time limit expires.....]

Re: Free Standardised Valves
 

David,

Fine - the error on yours does vary - though its actually 10% out at the very lowest current, and then it improves to nearer 5% before returning to near 10% at the higher current levels. The variation in the error is itself "interesting". It suggest some non-linearity somewhere.

Has anyone taken you up on your offer of a standardised valve yet? If not - I will be happy to send you the postage and give it a try on my VCM Mk.IV.

Richard

Re: Free Standardised Valves
 

Great Richard, please PM me with your address & I'll get valve no1 off to you. £3.90 is the RM cost for 2nd class, & £4.40 is the 1st class cost. PayPal would be fine, if that's ok with you - whichever you want.
I see that the Mk3's Maintenance Manual page 15 is generous with acceptable unloaded HT voltage's limits. As is the CT160's I seem to remember. But then we are talking about measurable RMS values of P to P 50Hz voltages used.
All AVO VCM's have a "Set ac" facility, but that's just the 55V winding on the t/f, which is used in the initial setting up. Once you're into reading off Ia, with the valve conducting, one can't fanny about altering it, or the mains I/p tapping, to keep the Va at its maximum when the valve is drawing a decent wack of Ia, and one is steadily lowering Vg to obtain the next required higher Ia reading. That's probably the reason why AVO recommend a wee ECC valve, with its low Ia for standardising calibration. As I said, the 6AU5GT is robust valve for being swopped from one tester to another, and being transported around the UK, as well as being subject to a number of repeated Ia/Vg changes.

Regards, David

Re: Free Standardised Valves
 

That's the first valve posted off to trh01uk. Hopefully it will spark of some interest in the Welsh Borders area down his way amongst Forum valve testing enthusiasts.
"Dekatron" aka Martin, a keen valve testing Forum contributor from Sweden would like one. So with one winging its way to South Yorkshire shortly, and another to Lancashire - that my wee batch of 4 Raytheon 6AU5GT's accounted for. However, I do have a couple of spare used other brands, so if there are further requests - they'll be free also for folk.
The graph for the first valve was already posted as an attachment picture yesterday, and other graphs will be thread posted as they are sent off.

Regards, David

Re: Free Standardised Valves
 

Hi David; this is an topic I've raised previously on the forum. Please reserve one valve for me and I'll PM you with details.

I have a VCM163 and that has been set up very carefully with a "standardised" ECC81 for which I had previoulsy obtained some curves very carefully "by hand".

Having a completely independent comparison valve will be good.

Thanks

B

Re: Free Standardised Valves
 

No problem Bazz. We'll sort something out. Regards, David

Re: Free Standardised Valves
 

This is a very good idea and it will be interesting to see what happens when they are put in a variety of valve testers.

Were these Standardised valves aged before you plotted the characteristics?

Re: Free Standardised Valves
 

Hello Silicon, Valve No1 was left on the 45mA/ -20V Vg setting for a couple of hours only, and no significant changes were seen. If folk have aging concerns, they could perhaps run them for longer. I find that slowly decreasing the -ve Vg to obtain higher values of Ia for graphing purposes, if done slowly - stable Ia readings are obtained. Folk who own AVO VCM's will notice with many valves - that once Ia has been indicated on the meter for "Test", adjusting "backing off" controls often results a wee bit of "creep" above or below zero meter indication just a second or so before switching to "mA/V". This is due mainly to instability in the bridge circuitry which in turn is due to small variations in mains supply. Even though folk have diligently "set ac" at the start of the test procedure.
That's the advantage of DC testing & graphing - steady HT can be constantly monitored, and tired AC bridge balancing circuitry in 60 odd year old testers & VCMs is not an issue.
A while ago, I found an NOS Lissen AC/SG pre-war tetrode in its box along with a factory DC graph. Bingo - my DC tester was pretty spot-on graph-wise, after 80 odd years. Likewise, a couple of years back I DC tested a STC4212D - a fearsome big monster of a triode(there is a Forum post somewhere), and again my DC tester was jolly close to the graph in my old 1941 STC "Standard Valves" book.
I'm not claiming my DC Tester is the Bees knees, I'm sure that some other homebrew DC testers, "Sussex", & VCM163's, and fancy "Curve Tracer" equipment & software, are better. Such advanced testers are ideal for producing standardised valves, and matching valves for Push-Pull or Quad operation. But I suspect that the majority of VR valve testing folk just want to know that their particular tester or VCM is giving a reasonably good "Replace" or "Good" Red/Green indications. AVO VCM's & the thumping great Mullards are just to big & heavy to shift about the UK, but a wee valve weighs next to nothing.

Regards, David

Re: Free Standardised Valves
 

David, congratulations and thank you for your initiative to help others who are maybe having a few doubts with their own tester.
I do occasionally check my CT160 against a dc stabilised psu test jig Vg/Va resulting in "hand crafted" test results.
I also find that whereas I can test with great accuracy to a few (or 10s) of mV this way, the CT160 does leave a lot to be desired on accurate interpretation by eye of actual mechanical dial readings. They are a bit arbitary!
But, I will NOT part with my CT160!
Rob

Re: Free Standardised Valves
 

Just noticed whilst testing the next valve for Bazz, I'm giving Va 250V, not 200V as recommended in the AVO VDM. This is to obtain a higher Ia & consequently a broader Ia/Vg curve for plotting & studying purposes. "The Valve Museum" tells us that these valves can take up to 550V Va. My apologies to 6AU5GT buffs.
Incidentally, Valve No2's results are amazingly close to No1's - Would've made a jolly good matched pair. Next - Valve No3 for Dekatron.

Regards, David

Re: Free Standardised Valves
 

I've got to say what an incredibly generous offer this is.

It takes time to characterise a valve, and doing it in a DC rig where the calibration of the voltmeters and ammeters can be checked provides one of the few routes where a conventional AC valve tester can be verified

David

Re: Free Standardised Valves
 

You ken this David - if it wasn't for this dreich weather we're suffering up here - I wouldn't have had the time just now. Too much outside work to do. Hopefully we're getting a better forecast for the weekend, and by then three valves will be on their way to willing Forum participants. My hope is that out there is a sh*t-hot tester or VCM which could be a benchmark for the rest of us, valve testing-wise.

Regards, David

Re: Free Standardised Valves
 

Right oh, Here is the graph results of Valve No2. The valve will be on its way to Bazz today.
At the moment my graphs & participant's graphs will hopefully remain in this Forum thread. But once there are a few floating about, perhaps some Photoshop or MS Office expert might come up with a superimposing method. i.e. Like the old fashioned OHP method of "Flip Over" acetate sheets. I'm the first to admit that my graphing is a bit "schoolboy-ish". However, as Radio Wrangler says, this process is time consuming. Not that I'm complaining. If the demand is there, I don't mind turning out one or two valves a week, till my wee stack of surplus medium pentodes runs out( I've three or four Motorola 8552's, which I'll try). Lets see what happens anyway.
I've attached graphs in both pdf & jpeg format, so that participant's printers capabilities can choose the best print-off.

Regards, David

PS. Note - the DC Graph is coloured RED, and my Mk3's is pencilled-in in GREY. Just in case the computer/Forum/ Attachment process loses a bit of definition. Dave

Re: Free Standardised Valves
 

Hi David
What a generous offer. Could I have one of the standardised valves to see how my MkII VCM measures up. Quite a lot of the components have been changed, including the meter which has had me wondering just how good it is. Quite happy to pass it on once I've done the check.
Regards
Jeff

Re: Free Standardised Valves
 

That's great; I'll extact the 163 from its cupboard and get it warmed up :). I'll post my results soon as I have them. Thanks David.

B

Re: Free Standardised Valves
 

I thought I had more 6AU5GT's, but cant find them. I've other pentodes surplus, but the 6AU5 gives a jolly good range of Ia for a jolly good range of Vg. Its ideal for plotting on A4 graph paper.
If anyone has any surplus, please get in touch.

Regards, David