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Old 27th May 2020, 8:18 pm   #1
G3PIJpeter
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Default Valve testers and E88CC / ECC88 valves

I have a number of ex-equipment but very new-looking E88CCs. I have both an old AVO flat-panel tester and a home-constructed version of the 'Dynamic Mutual Conductance Valve Tester' from the National Valve Museum page at http://www.r-type.org/articles/art-118.htm. I use an oscilloscope as the null indicator and this set-up has proved simple to accurately calibrate over three extended and overlapping scales.

Using the parameters from the (yellow hardback) AVO valve tester book, I obtain a null at around 12.5 for the triode section marked as " i.e. pins 1, 2 and 3. However, transferring to the second triode (marked ') on pins 6, 7 and 8 gives me a very insistinct null at around gm=3. This effect is noted with any and all of the E88CCs tested. I obtain the same sort of results with the flat-panel tester.

Is the problem instability due to the high amplification factor? I do not obtain this sort of result with high-gm valves such as the EF183 - but then these are pentodes containing a screen grid that reduces the grid-anode capacitance. Or have I simply got a bunch of valves that have all been driven badly during their service lifetimes and are not quite so pristine as their external appearance suggests.

Any and all thoughts gratefully received before I consign them to the back of the shed.

Peter G3PIJ
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Old 27th May 2020, 8:31 pm   #2
MotorBikeLes
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Default Re: Valve testers and E88CC / ECC88 valves

I seem to recall a near identical thread very recently, and I remember my response then. One of the entries for ECC8x is WRONG. I THINK it is the ECC88. Check against a valve data sheet.
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Old 27th May 2020, 8:39 pm   #3
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Default Re: Valve testers and E88CC / ECC88 valves

You and I have been here before:-

https://www.vintage-radio.net/forum/....php?p=1177889 Post #12.
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Old 27th May 2020, 9:39 pm   #4
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Default Re: Valve testers and E88CC / ECC88 valves

Hello,

just for comparison: Recently I had to acquire quite a number of ECC88s as I was giving my old Tektronix 585A some tune-up attention. Being surprised by the generally weird pricing of such valves on ebay I ended up buying 50 pcs of 6N23P NOS directly from Russia.
Yes, I had been a little reluctant considering the quality of these valves although all of them looked ok. So I took the EURATELE valve tester for a quick judgement and what should I say - not a single shorted one or showing bad results. In most cases both systems were quite close together. I only made the shorts and emission testing, otherwise I had to fire up the Metrix 661 Gm tester but I was too lazy for that I got to admit.

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Old 28th May 2020, 1:13 pm   #5
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Default Re: Valve testers and E88CC / ECC88 valves

Quote:
Originally Posted by Station X View Post
You and I have been here before:-
https://www.vintage-radio.net/forum/....php?p=1177889 Post #12.
Adding the insights from that thread, I'm now thinking along the following lines:

My yellow AVO Valve Data Manual (17th edition, 1965) lists the selector switch settings for the E88CC as: 641 237 410 i.e. A1 G1 K1 H H+ A2 G2 K2. I have used a hand lens to confirm these connections inside the valve.

The electrical conditions are given in the AVO book as Vg = -1.2V; Va = 90V; Ia = 15mA; gm = 12.5 mA/V. The Flat Panel conditions are given simply as Va = 100V; gm = 13 mA/V. The 1966/67 Brimar Valve Data Book gives a Vg figure of +9V (!) with the other parameters being the same as in the AVO book - but it does specify a cathode resistor of 680 ohms. Similarly, the data sheet for the CV2493 at http://www.r-type.org/pdfs/cv2493.pdf states Va = 100V; Vg = +9V; Rk = 680 ohms. Now, at Ia = 15mA, the cathode will be at + 10.2V (V=IR); if the grid is set at +9V, then the grid-cathode potential difference will be -1.2V, as stated in the AVO book.

The E88CC data sheet at http://www.r-type.org/pdfs/e88cc.pdf confirms the grid voltage of +9V (page 2) but the characteristic curve for Va = 100 (page 8) gives an anode current of 70 mA for this value of grid voltage. With 680 ohms in the cathode, everything will be fine.

All this said, my problem is that I am obtaining similar results from testing on the Flat Panel and on my ‘Dynamic gm tester’, which has clearly labelled rotary switch settings for each electrode / pin combination. Triode number 2 is consistently way down on triode number 1. The actual valves I am testing are CV2493s which are given as equivalent to the E88CC/01. Pin settings for this valve in the AVO Data Book are 741 236 410 i.e. A2 G1 K1 H H+ A1 G2 K2. This arrangement is certainly not what I’m seeing through my hand lens.

Conclusions? I think that adding the 680-ohm cathode resistor into the picture adds some daylight to the data book listings; the AVO 741 switch setting is clearly wrong (for ECC88 / E88CC / CV2439); my measured discrepancies between the two triode sections is due to instability.

However . . . .

Peter G3PIJ
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Old 28th May 2020, 2:04 pm   #6
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Default Re: Valve testers and E88CC / ECC88 valves

Surely the differences between the two sets of rotary switch settings you quote are down to arbitrary numbering as to which of the two triodes within the envelope is designated "1"?

123456789 Pins.
641237410 Settings 1
741236410 Settings 2

Only the anodes pins 1 and 6 have seperate test leads A1 and A2, and presumably they just go to a changeover switch in the valve tester?

Testing using both sets of settings should just result in the dodgy sections swopping designations?
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Old 29th May 2020, 10:27 am   #7
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Default Re: Valve testers and E88CC / ECC88 valves

Quote:
Originally Posted by Station X View Post
123456789 Pins.
641237410 Settings 1
741236410 Settings 2
Only the anodes pins 1 and 6 have seperate test leads A1 and A2,
On my AVO Flat Panel:
641237410 results in one triode section (A) testing good and the other (A2) testing bad.
741236410 results in both triode sections being good.

On my 'Dynamic gm Tester', one side is good and the other bad - consistently from one E88CC to the next. With a gm value of 12 or so, it seems to me that the problem is instability in both testers.

I think the strange Vg = +9V has also been explained away, so I think I shall now turn my attention to a very dead HRO that has been patiently waiting since arrival here.

Many thanks

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Old 29th May 2020, 10:33 am   #8
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Default Re: Valve testers and E88CC / ECC88 valves

Interesting. I have a number of double triodes which test bad on one section using a CT160. I'll try the alternative settings and see what happens.
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Old 29th May 2020, 11:59 am   #9
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Default Re: Valve testers and E88CC / ECC88 valves

Poor contact in switches along the way and also high contact resistance, even in the socket and the pins on some tubes, have been the cause for similar problems for me in many cases when using old AVO VCMs or when re-using sockets in projects.
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Old 29th May 2020, 3:05 pm   #10
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Default Re: Valve testers and E88CC / ECC88 valves

Peter,

I've tested a number of ECC88s and E88CCs on my two panel AVO and get the same result for both settings 1 and 2 so I don't experience your behaviour even for valves with differing gm values for the two halves. I've no idea what is going on with your set up. Yes it may be instability in yours I suppose. I tried slipping a large ferrite toroid over the valve under test but it made no difference.

Interestingly the Avo version 17 data book lists "no data available" for the ECC88 and the E88CC has the funny little square symbol against it which is something to do with voltage breakdown testing and probably doesn't apply to the two panel Avo. Perhaps Avo knew there was something funny about testing the ECC88 on the two panel.

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Old 29th May 2020, 4:40 pm   #11
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Default Re: Valve testers and E88CC / ECC88 valves

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Originally Posted by IanG3XYV View Post
Interestingly the Avo version 17 data book lists "no data available" for the ECC88 and the E88CC has the funny little square symbol against it ... Ian
The AVO data book that I have gives the roller setting 641237410 for the E88CC and 741236410 for the E88CC/01, which is the CV2493 that I have been testing. It's the 641 setting that gives me problems - 741 works fine. As you suggest, maybe AVO knew more than it was letting on.
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Old 29th May 2020, 7:13 pm   #12
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Default Re: Valve testers and E88CC / ECC88 valves

What happens if you use the following four roller selector settings:

000 236 410 Triode on pin 1,2 & 3 floating
000 237 410 -"-
641 230 000 Triode on pin 6,7 & 8 floating
741 230 000 -"-

to test just one triode section at a time? The other triode will have all of its electrodes floating with these roller selector settings.

The square symbol warns you of not testing the tube on inter electrode tests as the voltage is to high in older VCMs (150V peak) but it works ok with the VCM163 (50V peak).
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Old 30th May 2020, 10:27 pm   #13
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Default Re: Valve testers and E88CC / ECC88 valves

Quote:
Originally Posted by Dekatron View Post
What happens if you use the following four roller selector settings:

000 236 410 Triode on pin 1,2 & 3 floating
000 237 410 -"-
641 230 000 Triode on pin 6,7 & 8 floating
741 230 000 -"-

to test just one triode section at a time? The other triode will have all of its electrodes floating with these roller selector settings.
I followed these suggestions and the results are rather interesting. First of all, pin numbers:

ECC88 / E88CC / CV2493 / E88CC/01 (from http://www.r-type.org/exhib/aaa0037.htm )

Pin
1 = a(2)
2 = g1 (2)
3 = k(2)
4 = h
5 = h
6 = a(1)
7 = g1(1)
8 = k(1)
9 = s

Flat-panel roller settings and results:

000 236 410 Triode on pin 1,2 & 3 floating
000 237 410 Triode on pin 1,2 & 3 floating
gm = 8 then falls; shifting zero on return of gm switch to centre position.

641 230 000 Triode on pin 6,7 & 8 floating
741 230 000 Triode on pin 6,7 & 8 floating
gm > 10 (meter f.s.d.); pointer returns to zero on return of gm switch to centre position.

(A2 selected on the valve tester for roller number 7 anode connections)
I have tried looking at the circuit diagram of the AVO Two-Panel tester but go cross-eyed in the attempt. The E88CC is quite clearly two completely separate triodes in one envelope and yet this valve tester returns different results for the two - similar to the result that I obtain with my DIY 'Dynamic gm tester'.

What voltages does the flat panel apply to the electrodes of dual valves? ECC81/2/3s test fine on both sides.

I'm getting more and more mystified.

Peter
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Old 31st May 2020, 12:20 pm   #14
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Default Re: Valve testers and E88CC / ECC88 valves

Could it be that you have poor contact along the wiring for a(1), k(1) or g1(1) which affects the reading of a lower gm-value when you use those pins on the tube?
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Old 31st May 2020, 4:11 pm   #15
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Default Re: Valve testers and E88CC / ECC88 valves

Quote:
Originally Posted by Dekatron View Post
Could it be that you have poor contact along the wiring for a(1), k(1) or g1(1) which affects the reading of a lower gm-value when you use those pins on the tube?
Good examples of ECC81/2/3 (12AT/U/X7) test on spec. and the same on each of the triode sides, so I don't think that the contacts on the base are the problem.

I recently saw elsehwere that a B9A plug tightly wired to a B9A socket would permit the inclusion of ferrite beads etc. in the connecting wiring as parasitic dampers. i.e. plug the valve into the socket end and the plug into the tester. Something to try some time.

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Old 31st May 2020, 5:45 pm   #16
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Default Re: Valve testers and E88CC / ECC88 valves

Can you get hold of a good example of an ECC88/E88CC to test and see if it behaves as well as the ECC81/2/3-series?
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Old 1st Jun 2020, 10:56 am   #17
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Default Re: Valve testers and E88CC / ECC88 valves

Quote:
Originally Posted by Dekatron View Post
Can you get hold of a good example of an ECC88/E88CC to test and see if it behaves as well as the ECC81/2/3-series?
On my twin-panel AVO valve tester set to 641 237 410:

Phillips 'Miniwatt ECC88 - behaves OK (gm greater than 10 on both anode settings; stable zero once warmed up)

JAN 7308 (gold pins) - behaves OK

CV2492 also marked E88CC - behaves OK

CV2492 (from the batch of 10 that started this whole thing off) OK on 'A' setting but not on 'A2', when zero continuously wanders and gm reading is 8 and falling. The valve also gets very hot - all as before.

Conclusion - parasitics (pin 9 screen not earthed, high gm, slight internal physical differences for this batch . . . ?)

Peter
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Old 2nd Jun 2020, 10:02 pm   #18
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Default Re: Valve testers and E88CC / ECC88 valves

Just spotted another error in the 23rd Edition.

12AX7A shown as 741237410.

This will test the same section twice as A2.

Correct setting is 741226413 or 641227413.
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Old 2nd Jun 2020, 10:46 pm   #19
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Default Re: Valve testers and E88CC / ECC88 valves

Having soreed the settings I tested two new 12AX7's on the CT160 using both sets of settings. There was no difference in the results, both sections always testing the same.
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Old 2nd Jun 2020, 11:19 pm   #20
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Default Re: Valve testers and E88CC / ECC88 valves

Quote:
Originally Posted by G3PIJpeter View Post
Quote:
Originally Posted by Dekatron View Post
Can you get hold of a good example of an ECC88/E88CC to test and see if it behaves as well as the ECC81/2/3-series?
CV2492 (from the batch of 10 that started this whole thing off) OK on 'A' setting but not on 'A2', when zero continuously wanders and gm reading is 8 and falling. The valve also gets very hot - all as before.

Conclusion - parasitics (pin 9 screen not earthed, high gm, slight internal physical differences for this batch . . . ?)

Peter
Could that batch be mislabelled?

David
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