ECC88 and Avo CT160

[mikko rintala]

Mikko hi.
I have problem.

I would ask what is the problem when I test the Ecc88 tube?
The first half goes fine but when I turn the second anode the meter goes full scale? Does the same with all Ecc88 tubes. Does it work even if you adjust the grid voltage?

Best regaird Mikko from Finland

[6AL5W-Martin]

there is anything wrong with your connection.
For accurate testing must be connected A, K, G and the filament have to glow.
For System 2: A`, K`, G` and the filament have to glow.

greetings
Martin

[David Simpson]

Hi Mikko,
ECC88 :- AVO Code 741 236 410, Draws 15mA at 90V Va, with a piddling 1.2v -ve Vg. That's two powerful wee high slope triodes with an expectation of 12.6mA/V inside a wee B9A glass valve.
Very little scope of variation of Vg in RV2's 1st quadrant of 0 to -5V will give you a helluva change in Ia for either A1 or A2. They are bound to differ, unless you're exceptionally lucky & have an ideal pair of matched triodes. Hence the backing off controls - Coarse & Fine Ia(Which do not control Ia in any way, only Vg can do that) will need adjusting to counteract the meter's needle whanging to full FSD.
My advice with CT160's has always been to set Vg at a higher value that the Valve Data Manual says (>1.2V) then slowly reduce it to 1.2V to get the safest reading for A1 & A2's Ia's (& protect the meter) with these delicate sma ECC range of valves.
Also, CT160's RV2 are prone(like many of us over 60 years old) to be a bit worn( & hence a bit inaccurate) - particularly in that lower region of -ve Vg.

Regards, David

[Station X]

Originally Posted by David Simpson

ECC88 :- AVO Code 741 236 410

Regards, David

The OP could try 641 237 410 which will swop over the anode connections. That will show whether it's a tester or valve problem. Could be something simple like a dodgy connection on one of the valve holder sockets. I've been caught like that.

[MotorBikeLes]

One of the ECC8x valves is listed INCORRECTLY in the AVO Valve book. I don't remember which, may be ECC88. Have a look in a valve data book to find if it is correct for ECC88 or incorrect.
Les.

[Dekatron]

Try setting the test up as a 6DJ8 and compare results to the ECC88 measurements.

[David Simpson]

My 1965 AVO VDM says that the theta sign next to the ECC91 listing might indicate problems with VCM Mk's 1 & 2. Also, many years ago, I wrote in my manual that an ECC86 needs a 3.3K Resistor in the A1 & A2 links - to reduce the 20V Va selection down to 12V. Apart from that, I would advise that any of those B9A TT's, and their equivalents, need care on CT160's & VCM's if they are high slope/low Vg ones. Particularly the likes of the E288CC. And more particularly because of the cult status cost of some of them - thanks to audiophools & speculative internet valve wheeler/dealers.

Regards, David

[chriswood1900]

Les in my copy of edition 23 it is shown as 641 237 510, edition 17 gives 741 236 410 clearly some scope for confusion.
Mikko what settings are you using?

[MotorBikeLes]

I am nowhere near my Avo book (I think it is the last (reddish front), rather than the earlier (blueish?) one.
However, I have my little Mullard book, 1968 edition in my hand. That gives pins 1,2, 3 as A'',G'' and K'', with pins 6, 7 ,8 as A', G' and K' respectively.
All the selector switches can be decoded backwards (coded?) so anybody with a machine in front of them can check themselves.
In view of CW1900's observations, I think I can safely say it is the LATER books details for ECC88 that are wrong. ECC81,2 and 3 will be correct, probably all the others.
I suggest anybody with the later book amends it in accordance with the Mullard book's data.
Les.

[vidjoman]

I know there are differences shown in one of the AVO manuals for (Ithink) ECC83 and 12AX7. They should be the same but were different although I can't remember which was correct.

[ColinTheAmpMan1]

Originally Posted by vidjoman

I know there are differences shown in one of the AVO manuals for (Ithink) ECC83 and 12AX7. They should be the same but were different although I can't remember which was correct.

Maybe relating to the fact that the ECC83 is usually referred to as having 6.3V heaters (4&5 linked;9) and 12AX7 often referred to as having 12.6V heaters (9 unused: 4;5)? Actually, both connections can be used to check if both filaments have continuity when connected for 12.6V. You can put the 7025 into this group, too; it is a better-specified 12AX7. For that matter, all of the ECC8? types have the same pin-out and can be put into this group, too, as far as I am aware.

It must have been one of the most boring jobs that you can imagine, compiling these valve-data books. It is no surprise that mistakes were made.

Colin.

[Station X]

Originally Posted by MotorBikeLes

One of the ECC8x valves is listed INCORRECTLY in the AVO Valve book. I don't remember which, may be ECC88. Have a look in a valve data book to find if it is correct for ECC88 or incorrect.
Les.

The 23rd edition incorrectly shows 641 237 510 for the ECC88. This is incorrect, as 5 is for a screen grid connection. It should be 641 237 410.