AVO CT160 unable to read 12AX7 valves.

[django01]

Hi everyone,

I have recently acquired a AVO CT160 valve tester and have been going through my valve collection getting used to testing various valve types and rectifiers.
But!!!!!! I cannot get any M/av readings fo any 12ax7 preamp valves.
I have all the controls and thumbwheel set to the AVO specs for this particular valve along with the neg grid/anode voltage/anode current and unable to see any rise in the M/av.

KT66/6l6/GZ34 all test ok with transconductance but not a movement on the meter for any 12AX7.

Please could anyone offer a reason why this is so-

Cheers guys and good to be on the forum

[ColinTheAmpMan1]

Just a thought. Are you testing these with the settings for 12AX7 or for ECC83? The reason I ask is because while the ECC83 settings might specify 6.3 V heaters connected to pins 4&5 and 9, the 12AX7 is a US designation and might specify 12.6 V heaters connected to pins 4 and 5. While it shouldn't make any difference if both parts of the heaters are ok, the series connection 12.6 V will result in no heater continuity if one part of the heater is open-circuit. Check to see if the heaters light up in both configurations to verify.
Good Luck, Colin.

[django01]

Hi Colin-thanks for your reply.
I am using the ECC83 settings in the AVO book -I set the heater voltage to 6.3v but absolutely no movement on the GM wheel. The valves are fine as they are in use in 3 of my valve amps and test fine on my orange VT1000 valve tester.
Just strange that I cannot get a M/av reading-this is when testing mullards/brimars/harmas ecc83

[TonyDuell]

Now I have never used a CT160... I have a VCM Mk IV though which is somewhat similar I think. But perhaps I am talking nonsense.

What number have you set on the thumbwheels? Let's see if it makes sense.

Do you get the same results for both triodes in the 12AX7?

Do you get any anode current at all?

[Station X]

The CT160 heater settings for 12AX7's and ECC83's are such that pins 4 and 5 are connected together with 6.3V connected between there and pin 9. (741226413). This means that if half the heater is open circuit it won't show up on a continuity test. Have you checked heater continuity with a meter?

I assume that you can zero the bridge by adjusting the grid voltage or anode current settings, but nothing happens when you rotate the spring loaded dial?

[django01]

Hi Graham-I will check heater continuity with a meter and yes the bridge is zeroed by having the grid voltage and anode settings at the settings in the AVO valve book-I get nothing when rotating the spring loaded dial-all these valves are ok though and work fine in my fender twins/marshall jtm amps
thumbwheel setting is as tony 741226413

[paulsherwin]

Can you see the heaters glowing?

[django01]

Also to add I get the same issue on both triodes. Anode voltage is set to 250V, neg grid 2V, heater voltage 6.3V anode current 1.2mA.

[Station X]

I hope I'm not teaching egg sucking, but it's not enough just to set the grid voltage and anode current settings as per the book. Invariably you have to rotate the mA/V dial into the black zone and adjust one or other setting until the meter reads zero. Both sets of controls should vary the meter position. Once the meter is zeroed, the mA/V dial is further rotated to the specified gm figure and the result read from the meter.

The anode current setting dials don't actually vary the anode current. They just allow the bridge to be balanced.

[django01]

The problem I have is that the meter reads zero with the grid voltage and anode set -but when the mA/V dial is rotated into the black zone or any other area on the scale the meter doesn't move at all. testing kt66/rectifiers/6l6 the mA/V control does what it should and gives a good GM reading with the ability to zero the meter.
I can see both triodes heating up and H/cont and insulation tests pass as ok-

[Station X]

Does varying the grid volts or anode current controls cause meter to move away from zero?

[django01]

yes -if i move the anode current to 20 amps the meter moves hard to the left of zero on the scale-the grid volts does nothing.

[Station X]

I can see I'm going to have to find a double triode and test it on my own CT160 to see what happens.

[django01]

Cheers graham

[Station X]

I managed to find several ECC83's and was able to test them without problems, although the performance of some of the "pulls" left a bit to be desired.

As expected the grid volts and anode current controls could be used to unbalance the bridge.

I broke each anode link in turn on the top panel and connected a 0-2.5mA ANALOGUE meter in its place. With the CT160's controls set to TEST A1 or TEST A2 the meter indicated half the anode current actually flowing. This is because the CT160 uses half wave rectified unsmoothed DC for testing purposes.

Changing the grid bias or anode volts resulted in a change in anode current as did rotating the mA/V dial. The anode current controls didn't change the anode current at all, as their function is just to enable the bridge to be balanced, at which point they indicate ACTUAL anode current.

So I don't know what the problem is with your CT160 or testing method, but I suggest you try the above.

[django01]

hi-I think my AVO CT160 has a fault as today I tried the same ecc83's on a AVO m4 and got the required results.
just odd as KT66 and rectifiers test ok and the m/Av wheel does its job in reading the expected values on the CT160.
Thnx for your checks though

[Station X]

Have you checked the calibration as described in the manual? You can do quite a lot of checks without the need for a standard valve.

[django01]

Good point graham- i will try that

[ColinTheAmpMan1]

It might also be of some use to try other valves of the ECC?? and 12A?7 families, as they all share the same pin-outs. The 7025 ruggedised valve has the same pin-out, too.
Colin.

[Colinaps]

I'd second the other Colin's comment - and are you actually getting anode volts on the socket? Dirty/loose socket? Dirty switch wafers? With the tester unplugged, it may be worth testing continuity between pins of two sockets - say B9A and Noval.

Cheers,

Colin.