AVO CT160 unable to read 12AX7 valves.

[mark-jacobs]

Never did understand the AVO Valve Data Manual setting for the 12AX7 which is 741 237 410 with a Vh of 6.3V Vg1 2.0V and Va 250V. As outlined the heater center tap is not connected, so the heater volts would need to be 12.6. The other significant error is the Plate 1 is not connected as there is no "6" which is A1. If using the center tap then it would be either 741 226 413 which is the ECC83 setting or 641 227 413 which is the setting for the E83CC, with a Vh of 6.3V Vg1 2.0V and Va 250V.

I just tested some 12AX7 tubes on one of my CT-160 testers, I use 741 236 410 with a Vh of 12.6V Vg1 2.0V and Va 250V, depending on the setting the plates will be flipped.

Also note that the CT160 heater setting voltage can be very high with small tubes. I have modified my CT-160 testers so they have an additional set of heater voltage taps and measure the actual voltage to determine the correct setting.

[Boater Sam]

Surely the heater settings and requirements are the same for ECC81, ECC82 & ECC83 and the 12xx7 equivalents?

[David Simpson]

Folks, the ECC***/ 12A*** range of double triodes "should" be the most compatible & easiest to use valves in CT160's, & AVO VCM's - - - Because they are the very valves which AVO requires to be used as a "Standard" for calibration ! i.e. the CV455 or CV491 as per the Manuals.
However, one note of caution - - go easy on -ve Vg. Better to initially use a higher voltage then ease back to the VDM setting. This avoids the susceptibility of CT160's to "whange" to full fsd on the meter, and also avoids the susceptibility of these wee miniature double triodes to "cook" due to incorrect, or too low a -ve Vg causing excessive Ia.
Out of spec calibration, or a damaged Vg pot - RV2, are the likely culprits. Don't forget - the Ia controls on a CT160 do NOT control the valve's Ia. With Vh & Va correctly set, ONLY Vg(RV2) controls Ia. Furthermore, should someone be tempted to delve inside to fault-find, take extreme care as full P - P 50Hz Anode voltage is exposed on the meter's terminals.

Regards, David

[Dekatron]

Unfortunately the different editions and revisions of the AVO VDMs list the different variants of ECC81, 82 and 83 with different settings depending on which edition and revision you look in. Most changes happened in and after the 19th edition and from reading correspondence with AVO and its customers it is easily seen that the 19th edition contains the most errors, unfortunately many of those errors exist in later editions too even though there were attempts to correct them.

I always go back to check the 17th edition and compare the settings to the ones I am using if they don't work as they should.

Regarding the 12AX7 the settings change a lot between the 17th and 23rd edition and among those changes are not only the heater voltage but also the other voltages (look at the grid voltage which suddenly gets a minus sign and then a dot).

I made a list of the types and have included it here with this message, both as a screenshot and as a PDF-file, they are just taken from a few of the editions and revisions that I have. I only singled out the 12AX7/A from different editions.

I haven't checked any of the settings myself so the list might still contain errors.

[Station X]

Back in post #6 the OP states that he's using thumb wheel settings 741226413 with a 6.3V heater voltage setting. This is correct, so thumbwheel settings isn't the problem.

I believe that the problem with the CT160 (assuming there is one) could easily be identified by taking some measurements as I describe in post #15.

[David Simpson]

Don't forget Guys :- Full P - P AC on the Anode, 1/2 Wave AC on a Screen, and a combination of two separate -ve rectified pulses on the grid(from the 55V & 66V T/F windings). Hence the somewhat misleading RMS & DC meter readings required to be taken.

Regards, David

[mark-jacobs]

By using 741 226 413 and then 641 227 413 setting you are testing either side of the triode on A1 and A2, this might point to a selector problem. You need to test the voltages on the pins of the tube to see if there is an issue in the connections/switch gear. Also recheck the A2 link as sometimes there is a bad connection or try to check both links with a mA meter.

[vidjoman]

I've just looked at the settings in the manual I downloaded for use with my "Sussex" and the settings for the ECC81/2/3 are correct but the setting for the 12AX7, although different, looks to be good except the heater centre tap (pin 9) is open circuit so the heaters, connected to pins 4 & 5 should read 12.6 and not 6.3 as shown. I have tested 12AX7 valves but used the ECC83 settings (as equivalent) and not had a problem.

[ColinTheAmpMan1]

Isn't this what I said in messages #2 and #19?
Colin.

[usradcoll1]

My B&K Dyna Jet Mutual Conductance tube checker is one that uses multiple sockets for the most common valves. The socket for the 12AX7 is the same one as the ECC81/2/3, 12AU7, 12BH7 etc. The Sensitivity control is set for the various types that use the same socket, where the heater setting is set at 12.6 volts.
On the setup chart, the 12AX7, 7025, and the ECC83 is the only one from that group that has a notation, that states if the reading is 22 or higher, tube is good!
Normally, the reading is in the replace zone.

[Station X]

We're going round in circles here.

I've consulted the 23rd edition (1981) of The AVO Valve Data Manual and the settings given for a 12AX7 are 741 237 410 6.3V. This is clearly wrong as there's no connection to the heater centre tap. It would be correct if the heater voltage were set to 12.6V. These settings have the advantage that an open circuit in one half of the heater can be detected when using the heater continuity test. The anodes and grids are also connected together, so it's not possible to test half the valve.

An earlier version of the manual gives settings 741 226 413 6.3V which is correct. However an open circuit in one half of the heater won't be detected when using the heater continuity test.

[old_shuck]

My money's on a dodgy B9A valveholder or switch contact. BTW - if you need to lift the panel from the lid assembly, there's a rubber sealing gasket that will have decayed after 50-odd years and may be holding the said panel in a death grip!

Attached pic shows yours truly testing CV4014s (IIRC) in the late 60s.