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Old 3rd Dec 2022, 5:55 pm   #1
waran2005
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Default AVO instructions for creating a standardised valve.

Hi all,

I am in the process of building a dedicated unit for standardising a valve the way AVO describes it in the VCM manuals. AVO doesn't specify the value / type of potentiometer used for adjusting the grid voltage, has anyone built one, lf so, can you please tell me what you've used, what works well?

Kind regards,

Waran.
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Old 3rd Dec 2022, 9:20 pm   #2
Bazz4CQJ
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Default Re: AVO instructions for creating a standardised valve.

I've owned an Avo 163 for a long time, but I've never heard of Avo giving a procedure for creating standardised valves. Indeed, Avo would happily sell you a standardised valve for use with the 163, and very odd valve it was!

I've spent more time engaged in this task than I want to admit to, but did learn quite a lot on long the way.

The very first valve I used for the 163 was an ECL80; two valves in one and nobody else wanted them! Then I think I used EF91's for a while before settling on ECC81's, which are just right (Goldilocks) for the 163 (not sure about other machines).

With a Gm of 5-6ma/V, the ECC81 aspires to be an oscillator, and discovering this, I had to add a generous number of ferrite beads to my "hand-driven" tracer to keep it stable.

I think there are threads by David Simpson on the forum discussing Standard valves, but are a few years old.

Could you post a copy of the Avo procedure?

B
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Old 3rd Dec 2022, 9:53 pm   #3
Tim
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Default Re: AVO instructions for creating a standardised valve.

ISTR with the instructions on my AVO VCM it suggests using ( I think) a new ECC82 and EL34 as “ standard valves”. I have a couple of valves marked up as such and packed away carefully in a very safe place!
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Old 3rd Dec 2022, 10:08 pm   #4
waran2005
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Default Re: AVO instructions for creating a standardised valve.

Hi Bazz,

Sorry, guidance on how to standardise a valve is what I should have said, I'm just following the guidance contained within the Mk3 VCM service manual, but no value is given for the potentiometer used in the circuit as a voltage divider.

As Tim has mentioned, the valve I want to standardise is an ECC82. I am in the processing of making a dedicated unit for this, the 200V supply will be from my varic, rectified and smoothed, the 6.3 heater supply will come from my DC bench power supply, the grid volts will be from a battery contained in the box.

The box contains a 15V meter to monitor the grid volts and a 20 mA meter for the anode current measurement. The switches are there so I can easily connect to external meters to double check readings etc.

I think the potentiometer for adjusting the negative grid may be a case of trial and error.

Thanks all.
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Old 3rd Dec 2022, 10:09 pm   #5
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Default Re: AVO instructions for creating a standardised valve.

The instructions for the 163 call up (on page 1) the use of an E55L, supplied by Avo. Seems like you must be referring to an earlier model.

New E55L's seem to be priced at ~£250 each on eBay, so I'll give those a miss. Very high Gm if I recall.

Avo haven't specified the pot because they haven't specified the PSU for the grid voltage; just needs to be the right pairing to give fine enough control in the right Vg region for the two valves. If you have a decent psu, you may not need a pot but a fixed resistor would do.
Be sure to burn-in a brand new valve for at least 24 hours before standardising; some change l a lot.
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Old 3rd Dec 2022, 10:47 pm   #6
waran2005
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Default Re: AVO instructions for creating a standardised valve.

Hi Bazz,

Great advice, thank you. It is not a new valve, I believe it has done some hours but I will be sure to leave it on the rig as suggested to be sure.

I only mentioned the pot as I thought it would be some standard valve Avo used, no power supply is recommended, Avo just suggest using a 9V battery for the grid voltage. I will have a play with it tomorrow when I power it up and change the pot if I need too. Fingers crossed it will all go to plan and maybe I can standardise a few more ECC82s if anyone needs them.

£250 for a valve, OMG.........

Cheers.
Waran.
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Old 3rd Dec 2022, 11:58 pm   #7
Bazz4CQJ
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Default Re: AVO instructions for creating a standardised valve.

Quote:
Originally Posted by waran2005 View Post
Hi Bazz,
I can standardise a few more ECC82s if anyone needs them.
Cheers.
Waran.
By all means do that, but I'd talk to David Simpson (I think his user name is David Simpson) first; he went down that route a few years ago. He produced a number of valves, circulated around the forum, and I think the outcome suggested that by and large, the different types of Avo tester were generally working well, if they had been maintained. My 163 (currently out of action) agreed with his manually plotted results to within very small percentages.

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Old 4th Dec 2022, 9:21 am   #8
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Default Re: AVO instructions for creating a standardised valve.

My test setup (i have a ct160 also) is a Solartron variable ht bench psu and a Farnell E30-2 for bias. I use a 10k 10 turn pot for bias, with the supply to it around 25% more than the highest grid voltage required. Voltages are monitored with 2 Fluke 77 and a Wavetek dvm for Ia. Tests are done at the Avo CT160 book test voltages, with Vg swung 0.5V either side. This is pretty accurate.
However, you still find that there are people out there who will reckon their Acme mickeymouse tester results are correct and my digital readings must be wrong!
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Old 4th Dec 2022, 11:06 am   #9
David Simpson
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Default Re: AVO instructions for creating a standardised valve.

Hello Waran,
Right enough, I've supplied heaps of standardised valves to Forum folk over the years.
Waran is quite right, folks, AVO do - in their VCM MK3 & 4 & CT160 Manuals - give circuit diagrams for DC standardising ECC81's & 83's or their CV equivalents. They are wee double triodes & AVO require them to be strapped as a single valve. Easy enough done if you have a bench DC HT PSU, fully metered for voltage & Ia current in mA, and a 2 channel LV DC PSU to provide both 6.3V for heaters & a -ve range of voltage for the grid. It's very important to commence obtaining a range of Ia readings by starting with a high -ve voltage on the grid which is slowly reduced in steps to produce a tabulation for drawing up a wee Ia/Vg graph. However, ECC's are prone to lighting up & going "phut" should one inadvertently chose the wrong Vg, or the Vg PSU lead becomes loose. Me, of late have been using 6AQ5's or 6AU5's - sturdy wee pentodes. A much better Ia/Vg graph is obtained. Plus, later on in the Manual, AVO require you to use a power pentode for a final Ia test.
My hand drawing of graphs is jolly outdated, I admit. There are now several Forum valve testing folk who are experts in using modern Curve Tracing software.
There are masses of info available by using "Search", not only my ramblings/banging-on, about AVO VCM's & their calibration. Have a lewk.

Regards, David
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Old 4th Dec 2022, 7:12 pm   #10
Bazz4CQJ
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Default Re: AVO instructions for creating a standardised valve.

Hi David, I've got some 6AQ5's (EL90) but never thought of them for standard valves. They are quite useful output valves and were used in old 2m rigs as modulators, so I guess they sell for appreciable money?

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Old 4th Dec 2022, 8:05 pm   #11
David G4EBT
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Default Re: AVO instructions for creating a standardised valve.

Tesla 6AQ5s are £8.00 each + VAT from Watford Valves:

https://www.watfordvalves.com/produc...hoCtBQQAvD_BwE

Likewise, General Electric 6AU5s are also £8.00 + VAT:

https://www.watfordvalves.com/produc...il.asp?id=2581
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Old 4th Dec 2022, 8:11 pm   #12
Bazz4CQJ
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Default Re: AVO instructions for creating a standardised valve.

Not quite David;

From Watford

6AQ5A/6L31-TESLA x 1 = £8.00
Total Goods £8.00
Total Items = 1Postage = £5.00
VAT = £2.60
Total Cost = £15.60

And David Simpson was supplying standard valves last time from his own stash (with no charge).

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Old 5th Dec 2022, 10:31 am   #13
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Default Re: AVO instructions for creating a standardised valve.

Years ago, I used to supply standardised CV455's & 491's or their ECC eq. for a fiver I think. But then I acquired a heap of decent backsheesh 6AQ5's & 6AU5's. Those I supplied to a heap of Forum folk for just postage, plus Martin Forsberg sent me a few more for free when I started to run out.
They are not prone to PO at their full 40 - 50mA Ia, give a decent Gm slope, and give decent readable Ia readings as long as a steady Va & Vs & Vh are maintained.

Regards, David
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