Windsor 45B & KT66's

[greenstar]

I found at a car boot sale in a box of bits at the weekend three KT66's, which I plan to sell on, as at the moment this is my main income, and tested them on my Windsor/Taylor valve tester. The valve is in the book with appropriate settings and other different IO valves test as expected. But I find with all three that mutual conductance is on the floor - the first step being to use knob 'A' to move the meter needle to the middle, and then knob B to zero, before reading the value. But even with A at max the needle is still at zero.
It seems, besides being unwelcome, most unlikely that all three are totally dead. I gave them time to warm up and filaments are good. I've never before seen a valve with absolutely nothing.
So is it my tester that's faulty, my procedure (followed exact), or my luck?
Tony

ps, have just tested a 6F6G with the same switch settings, and the tester behaved, giving a good reading - looks like I've answered my own question and have three duds!

[PJL]

Check pins are all clean. Do the heaters check OK and light up? Does the gettering look OK? If they were mine I would want to put them in a DC test rig so I can measure what's going on.

[greenstar]

Yes, all looks clean, and heaters test as good, and they get warm. Gettering looks ok. I think what makes me suspicious is that the mu looks like nothing at all. I just can't see all three being totally dead. PJL, I guess what you mean is to make up a circuit and take measurements there?

[Station X]

I've experienced this with a CT160. It was down to dirty heater pins and holder sockets. The heaters test OK for continuity, but don't reach full temperature.

http://www.vintage-radio.net/forum/s...d.php?t=125701

[greenstar]

Interesting Graham. Maybe it's because the valves draw a bit of current, bearing in mind I've tested a different valve in IO socket 12 and it's tested as good, so either the tester doesn't supply enough juice for a KT66 or there's too much resistance in the way or both. I'll clean socket and valve contacts and see if it changes.

[David Simpson]

Hello Tony,
If its any help, I gave a 45B a good seeing-to back in May/June 2016. Post - 21/5/2016 " Taylor/Windsor 45B". Excellent support from other forum guys.
All the valve holder pin connections needed a jolly good clean.

Regards, David

[PJL]

The benefit in using a DC test rig is you can see exactly what anode current is being drawn and you can try running it at lower grid voltages.

[David Simpson]

PJL is spot-on regarding DC testing of valves. Especially if you are considering selling single or matched pairs of O/P beam tetrodes or pentodes. Accurate Ia/Vg tabulations & graphs can be determined. As well as the static results, DC testers lend themselves to be modified for dynamic testing. I.e. under signal conditions.
Several of us Forum folk have gone down this DC testing path, and heaps of building info can be accessed via "Search".
Right enough, AVO VCM Mk 3's & 4's VCMs, CT160's or VCM163's, can be used to give decent enough Ia/Vg graphs. But Jesus, they are becoming horrendously expensive to buy, and some of their spares are like rocking horse manure. The Taylor 45's are reliable enough for personal use, but they might not "like it up em" Ia-wise if they are constantly used for KT's, EL's, TT's, QQV's, 807's & VT136's, & so on.
A homebrew DC testing rig, capable of handling a 100mA or more, could be put together utilising decent 2nd hand stuff bought from BVWS swapmeets, ARS sales, Forum adverts, & VMARS sales & adverts. For less than the cost of a 45B, and certainly a lot less than the AVO VCM's etc.

Regards, David

[Freya]

Just a thought, are they marked as KT66. An outside possibility they are something else if not marked, hence no emission.

Tried a few KT66`s in my Taylor 47 and 45 testers and they both test as expected.

[David Simpson]

Tony,
The Taylor Valve Chart says that a KT66 requires 100V Va, 100V Vs, and -6V Vg for their test procedure. That's way down the bottom of the expected Ia/Vg (Gm) graph. AVO requires 250V/250V/ -15V for an ideal 65mA Ia & a Gm of 6.5mA/V.
If you like you could post me up a couple of your KT66's & I'll send them back with a DC graph of both valve's Ia/Vg curves. Just for postage costs. Or perhaps a Forum guy nearer you with a DC valve tester,(or a decent AVO Mk3 or 4 or CT160), could arrange tests locally.

Regards, David

[greenstar]

Hi, they are definitely marked as KT66's. David, that is an extremely helpful offer, and I would if you don't mind like to do so, unless anyone nearer responds. I was wondering about the low voltage setting - great big bottles like these surely run on a lot more.

Tony

[David Simpson]

Tony,
By all means, send them up if you don't get a nearer offer. No problem. I'll PM you my address.

Regards, David

[David Simpson]

Whilst waiting for Tony's valves to arrive, thought I'd draw up the Gm graphs for a decent NOS KT66 I have. A standardised reference, so to speak.
The Mk3's & DC Tester's results are pretty close.The DC tester can cope with the higher Ia values. However, when just using the much lower "Taylor"
values, the Mk3 exhibited lower Gm, and the LMA was a bit over optimistic. I noted that at these lower Taylor voltages, the Screen drew no current below -7 volts Vg.

Regards, David

[David Simpson]

Graph has lost its colouring somehow. Will try again. DS

[David Simpson]

Sadly, the two GEC KT66's were duff. One had a cracked glass & the other lit up like a purple firework after drawing just a few mA Ia.
The 3rd, a CV1075 gave pretty good graphs. Although I was a bit worried about a C/H INS reading of a wee bit under a megohm. Despite the Taylor Chart settings of 100/100/-6, even that graph & its resultant Gm gave decent "Green Zone" results. Although, circuitry-wise, one wouldn't use a KT valve in those low voltage regions. Because of its compactness & transformer capabilities, I suspect that Taylors ere on the side of caution, power valve-wise.
I also DC tested a NAVY NR77 & achieved decent KT66 graph results. But I suspect that its inter-electrode capacitance wasn't up to KT66 standards. As it needed my anti-parasitic extension IO valve holder box, even at the lower (-25,-24,-23 Vg) region.
Valves now posted back to Tony

Regards, David

[greenstar]

Thanks to David for testing these valves - at least one usable CV1075. I found all three rattling about in a storage box at a car boot sale, so not surprising - I doubt the damaged occurred on the way to David. I'll have to view the Taylor's readings with caution. As mentioned, I did test another radio valve in the same socket, same settings, that read as ok, so it's odd, and perhaps related to the KT66's being a more demanding valve. I'll certainly clean the contacts.
Thanks to all for their input here.
Tony