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Old 18th Aug 2017, 9:54 am   #101
David Simpson
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Default Re: Free Standardised Valves

Folks, I've now lost track of who has which valve. This is inevitable, and not a worry, seeing as I recommended folk to just contact each other by PM.
There is no time limit on using the valves, as long as inaccuracies highlighted are able to be remedied.
Another recap :- 6AU5GT's nos. 1 & 2 should be in circulation, as should nos. 4 & 5. No 3, which went to Dekatron in Sweden, is u/s, so is out of the loop. No 6 will head home with Mark James this weekend for him to use then pass on to others(possibly at the next Goldborne meet). Also heading down to Lancashire with him will be some 6AQ5's - again for him to pass on.
Hopefully, Dekatron will be sending me a couple of 6AU5GT's, which he has tested on his Roetest & 163. I'll have a graphing session with them, then they(nos. 7 & 8), too will be released into the fray. The 6AQ5's will be given the suffix a), b), c) etc., to avoid confusion.
Please, please post pictures of your results, added to my graphs - just for info. Especially if you add 'before & after' recalibration. Then tell us what needed doing. Once again, I reiterate - don't be shy - this isn't some public toilet where you don't want some big fella next to you to get a glimpse of your thingy.
If folk want to use Exel software, feel free, amongst yourselves. Me - I'm just a vintage fella(who doesn't loiter in public toilets), who built his DC Tester out of other vintage test equipment, and who draws up graphs in a vintage manner.

Regards, David
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Old 18th Aug 2017, 5:55 pm   #102
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Right - three 6AQ5's have been tested & graphed, ready for Mark to take away. That's definitely it for a week or two. DIY, Windows, Garden, etc. beckon.
Keep testing, & graphing please.

Regards, David
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Old 18th Aug 2017, 8:01 pm   #103
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Default Re: Free Standardised Valves

Here's my plot of Valve No.5 on my Sussex. The plot follows the curve, but the numbers are slightly higher throughout which might mean an adjustment error on my machine.
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Old 18th Aug 2017, 8:27 pm   #104
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Default Re: Free Standardised Valves

Many thanks Richard,
It'll be really interesting to other Sussex folk, when you tell us what needed doing on your Sussex to re-calibrate.

Regards, David
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Old 19th Aug 2017, 6:05 am   #105
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Default Re: Free Standardised Valves

Quote:
It'll be really interesting to other Sussex folk, when you tell us what needed doing on your Sussex to re-calibrate.
Looking at the circuit diagram, the anode current is measured across a 1Ω resistor. The resistor in the Sussex is likely to be a 'commercial' tolerance component (I don't remember buying anything special at the time), I will look and see if I can find a close tolerance component.

The other thing I want to check is the accuracy of the anode and g2 voltages since both are set using rotary switches on resistive divider networks. I'm not sure how much effect these voltages have on the Vg/Ia curve.

If those investigations don't bring the anode current measurement into line, I may have to simply add an adjuster at the measurement point to reduce the value shown on the Ia meter.
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Old 19th Aug 2017, 6:48 am   #106
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Default Re: Free Standardised Valves

David,

Just a question. When testing the 6AU5GT Nos. 1 and 5, I followed your lead and used Va=250v, Vs=150v and Vh=6.3v. The AVO book gives Va=200v for this valve and I wondered why you chose 250v.

regards, Nick
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Old 19th Aug 2017, 6:58 am   #107
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Default Re: Free Standardised Valves

This might be interesting, I used the 'book' value of 200V. I might re-test using 250 and see what happens.

I have to go to work first, though. :-(
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Old 19th Aug 2017, 9:52 am   #108
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Default Re: Free Standardised Valves

I guess the Va=250v will take your graph line further north but at least then it will be on the same anode voltage as David and me and so will be directly comparable to our readings which yours isn't at the moment with Va=200v.

regards, Nick
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Old 19th Aug 2017, 9:52 am   #109
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Richard & Nick, I remember mentioning this choice of 250V Va back at the beginning - 'twas to obtain a steeper curve & drive the valve a bit harder. The valve is max rated a fair bit higher than 250V. Running at 60 - 70mA, it also lends itself as a substitute in the AVO VCM's manuals recommendation of a KT33c for final calibration testing.
In a Sussex is it just the Vg pot which controls Ia ?

Regards, David
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Old 19th Aug 2017, 11:55 am   #110
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Default Re: Free Standardised Valves

David,

Thanks for the response re the 250V Va. I must have forgotten your earlier note but it is worth emphasising that we are using 250V for the 6AU5GT to ensure all plots are on the same basis.

regards. Nick
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Old 19th Aug 2017, 12:20 pm   #111
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Default Re: Free Standardised Valves

Quote:
Originally Posted by David Simpson View Post
In a Sussex is it just the Vg pot which controls Ia ?
Yes. Mine has a 10-turn Vg pot, seems to work well.
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Old 20th Aug 2017, 10:24 am   #112
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An update on Valve No.5 tested on my "Sussex". After I'd adjusted the anode volts to 245 (the closest I could switch to 250) and the screen volts to 135 (again, the closest I could switch to 150) I connected my Keithley 175 DVM to check the anode current meter on the "Sussex". The DVM matched the "Sussex" Ia meter all the way.

I wonder if the deviation at the lower Vg is due to the lower Vs that I'm applying? I might run the test again with a higher Vs and see what happens.

I re-checked the valve. The graph is here:


The Exel file is here:
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Old 20th Aug 2017, 10:42 am   #113
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And this, boys and girls, is what happens when you increase the screen supply to 160V:
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Old 20th Aug 2017, 12:36 pm   #114
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Richard, you've got to the heart of the matter. Spot - on. Va & Vs are critical. And to a lesser degree - Is.
Your next to last Sussex graph using the Exel program, came out really clear & defined. And revealed a jolly decent closeness to the DC Tester & 163 results. Then wollop - the 160V on the screen helped push out so much extra Ia ! Perhaps this is why the 6AU5GT was originally called a 'Beam Pentode'. Some folk might say that the GT suffix stands for Gran Tourisimo(fast cars) not 'Glass Tube'.
My whole original idea of this thread was to bring out & discuss all the niggley questions & mystique about valve testers & VCM's. For example, AVO's range select their Va's & Vs's off the same HT transformer secondary winding's tapping - for the same voltage selection.
I suppose if I was regularly DC testing o/p valves for matching - I should also show wee graphs for Is/Vg. I have done it once or twice.
When I've been DC testing the 6AU5GT's & the 6AQ5's, I haven't tabulated the Is values, but I keep a steady eye on Vs & Is, and Vs just need an occasional tweak at the higher end of the graph, when the valves do indeed draw a few mA Is. Maybe, 4,5 7 mA or so. One cannot measure Is on AVO VCMs, but it most probably does have some effect on Ia in the low Vg/high Ia region.

Regards, David

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Old 20th Aug 2017, 3:00 pm   #115
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Default Re: Free Standardised Valves

One thing to think about David, is that those of us with a Sussex have a range of switched values for Va and Vs rather than a fully variable supply. Bearing in mind that although the control for those switched values is regulated, the control is switched using a resistive divider network, so when a Sussex is switched to, say, 250V it might not be producing exactly that voltage.

My tester has been in use for some years now, and so far I haven't had cause to doubt its validity. If I set the controls according to the AVO book for a particular valve type, then I have always seen results in line with my expectations. The most recent was testing a Chinese ECC83 which had zero emissions on one half, the other gave a Gm reading that was better than book.

I am beginning to think that it might be a good idea to improve the voltage selection on the Sussex, or at least to adjust the resistor divider to bring the actual voltages nearer to the dial numbers. The Sussex also uses just one winding for the Va and Vs supplies, but they do have completely seperate voltage dividers.

Comparing results from other testers has been instructive!
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Old 20th Aug 2017, 4:24 pm   #116
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Default Re: Free Standardised Valves

A little bit OT, but about the Sussex.

For those of you using a Sussex it is quite important to get +300V from the voltage source with the MPSA92/LED circuit as that is the voltage present at the top of the resistive voltage divider which sets the voltage for the Anode and Screen voltages. You also need to check the resistors that you are using so that they are within the 1% tolerance to get a good resistor ladder.

Each switch has a resistor chain that should produce voltages within the 1% tolerance of those resistors, so setting it at 200V should show a value of some +/-2V (198-202V).

It is possible to replace the somewhat unstable MPSA92/LED circuit with just a few components using the Supertex (now Microchip) LR8 high voltage regulator. You can see the datasheet and other information on the LR8 here: http://www.microchip.com/wwwproducts/en/lr8.

The LR8 is stable and works very well, I've used it in several applications where I need a small bias current for mosfets in high voltage regulators.

The easiest remedy for the Sussex is to try out different zener diodes in the zener chain to reach the voltage you need, I had to do the same in another design but replaced it with the LR8 since the zener chain wasn't stable enough.
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Old 22nd Aug 2017, 1:07 pm   #117
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Default Re: Free Standardised Valves

I visited David at the weekend. An excellent welcome by him, his wife and the Scots who were so helpful when we broke down.
He has kindly passed on three standardised 6AQ5's with paperwork to pass onto forum members at Harpenden on 24/09/17. So ..... first come first served! PM me. If not enough takers for Harpenden then I hope to be at Golborne and RWB.

Mark
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Old 17th Sep 2017, 5:16 pm   #118
David Simpson
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Just wondering folks - Are the St'd Valves still circulating amongst Forum folk ? Mark has three 6AQ5's, and Dekatron has kindly sent me five 6AU5's which he's standardised - for passing on to Forum folk also. So there are still plenty available. I'll get Dekatron's valves to Mark, and hopefully he'll take them down to the next Swap Meet he attends.

Regards, David
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Old 18th Sep 2017, 12:41 pm   #119
Mark James
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I still have the 3 standardised valves - I should be at Harpendon on Sunday . Anyone interested please PM me.

Mark
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Old 18th Sep 2017, 1:17 pm   #120
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Quote:
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Just wondering folks - Are the St'd Valves still circulating amongst Forum folk ?
Mine, valve No.5, has been passed onto chriswood1900.
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