Have looked for 6/12AU7 and their equivalents but don't have any.

Took out the tester to measure the voltage at the various pins for the given ABC selector positions. However the filament voltage selector shaft is very loose so will have to investigate that. So can't do any tests until I have time to pull the tester front off.

Have worked through the wiring chart for B9A base valves with pins for 12AU7 and I come up with ABC switch positions for the two triodes of 10, 9, 12 and 2, 17, 3 which differs considerably from their spec of 2, 8, 3 and 5, 9, 0.

So these could be tried, with anode and screen of 100V and grid of 0V, measuring the voltage at the various pins to see what comes up. Pin 9, the filament centre tap should have half the filament volts of 13V. If this works out with my settings, try a valve.

Good luck - had to go through the procedure a few times before I could come up with the same result twice - a bit mind bending and easy to make a mistake. So no guarantees.

Thank you STC830 for the time you have put into this. Switch positions 10 9 12 work well. Switch positions 2 17 3
did not work. Same result, knob A will not advance the meter.
Regards Jimb.

STC830.
IF you email me your address I will send you a NOS. 12AX7 for your collection.
Regards Jimb.

Don't worry about the valve Jim.
Will have another look at the second triode.
My immediate thought is switch B. It is on 17 to keep it out of the system as the A and C circuits feed anode, cathode and grid. 15 and 16 should work too.

Hello GTC 830.
I appreciate the time you have put into this. However you must be getting sick of it.
I actually lost it working as well on the 10 9 12 settings there was some slop on the C swich I found this was due to a loose grub screw on the segment closest to the front segment. I tightened it up and these is now no slop on the shaft and now the 10,9 12 positions now work again however still not the other. Looking at that chart I cannot see how the second triode section is configured. Position A ok for anode however pins 7and 8 grid and cathode lie in the switch c position. Nothing corresponds to switch B.
Regards Jim

"Nothing corresponds to switch B."
That's why B is put on position 17 which has no circuits connected and should take B out of the second triode's control - 15 & 16 could also be used. So it all has to be done with A and C. Haven't reconsidered this as it is bin day tomorrow so the grass had to be mowed.

I don't mind helping out because I know what a coot these testers are to deal with as far as working out settings for unknown valves or valves for which the settings don't work. I have tried to work out a rational system but so far I have had to just look over the possible settings and decide which looks best and try it.

Decided to do a search at the 45C's land of its fathers. Used Taylor 45C 12AU7 as the search term and came up with these:

https://www.vintage-radio.net/forum/showthread.php?t=131934

This has a more complete wiring chart (circuit selector switch positions), and more to the point indicates that positions 16 and 17 are "occupied" for switch B, so that switch position 15 should be used for B switch for the second triode, ie A, B, C at 2, 15, 3.

In the case of switch A I recommended position 2 to put the cathode circuit on the grid of triode 1 with triode 2 under test. You might as well test with switch position 1 which would put the grid circuit on the grid of triode 1, ie A, B, C at 1, 15, 3.

There are indications on the thread that there could be problems with the centre tap of the filament getting grounded. Might be worthwhile looking to see if that might be happening. Wouldn't be good with 13V on it in that case. Might have fritzed it on you.

https://www.vintage-radio.net/forum/showthread.php?t=51844

This thread also discusses issues with 12AU7 and its "flavors", meaning close equivalents. It infers that the settings in the manual are correct, but that doesn't gel with us working out a much different working setting for at least one of the triodes of the 12AU7. This assumes that for triode 1 your measured mutual conductance is around 1.3 as advertised, ie it is actually working correctly.

https://www.vintage-radio.net/forum/showthread.php?t=103086&page=3

This thread has another wiring chart with more blank spots filled and with extra valve types, 45A-B-C etc.pdf. Might be useful for future reference.

It might be worthwhile joining this forum and putting the question to them. There is a lot on this tester there, and the valve has European equivalents. We could thrash around in the dark forever when someone there might nail it straight away.

Hi again GTC 830.
Tried 1,15,3 and 2,15,3 again no positive result. I am now prepared to accept some valves it tests ok and others not.
Some of the settings you came up with I also had tried and yes some upset the centre tap and did put 13 volts across one side I turned it off quickly. One of the settings I came up with was very exciting! I can't remember what those settings were because I had to find some clean underwear rather quickly. The valve filaments lit up ok did not operate any switches at all and it suddenly lit up like a Roman Candel on a 4 of July celebration and it was the end of my 12AU7. The annoying thing was I had Several 12AX7's and only one 12AU7 and I had accidentally used the 12AU7. I will now shelve it for the time being unless anything else comes up.
Will go out tomorrow and restock up my underwear.
Regards Jimb

Hi again GTC 830

Make that STC830

OK when I get my 45C running will try the various settings and see what voltages appear at the valve socket and go from there.

Sorry GTC and thank you once again STC830.

Sorry GTC and thank you once again STC830.

Finally got this job done. Sorry it took so long but had to fix the tester, and other life things happened.

Filament voltage selection multi position switch came loose. The switch is held in position by a large star washer - there is no flat on the thing to hold it in position in the face of the tester. So it is necessary to carefully locate it and tighten the flat nut which is inset into the plastic face of the front. Had to get myself the right tube spanner. Be aware that the other setting knobs are the same - no positive location and held by a star washer. So if they come loose be careful that they are in the right position with respect with the lettering on the front face. Usually this is easy because of stops at the fully clockwise and anticlockwise positions. In my case switch B has the stop broken off internally, so it is necessary to take the front off and make sure that the switch is in the correct position before tightening.

I have written out my full procedure for working out settings for A B C switches, the circuit selector switches for the 12AU7. This is very long winded and difficult to post here because of formatting issues, so have put it into a pdf to submit to Brad for consideration for including in this post.

12AU7 Settings

I assume that you are aware that a silicon rectifier is one hell of a lot more efficient than the metal? That will cause an increase in voltage, which may be a problem?

Most of those test instruments were made using the same components as the radios; Therefore the same rules apply to wax Paper caps. Most of them & many oil filled types leak like sieves. Unlike radios, where you might get away with a leaky cap (bin here) you won't in a test instrument, as it can throw it out of calibration.

When researching the replacement of the metal rectifiers I came across a post (in the British vintage-radio forum I think) that said that silicon rectifiers can be used in the 45C without further modification. I wondered about it at the time; however the tester does work OK, though the mutual conductance readings are changed on known valve somewhat - but a while since I have looked at this. I was just happy to have it going again to check such things as internal shorts in valves, after a saga of collapsed switches, l shorts and broken connections as a result of manipulations to repair the collapsed switches.

After all the final test is, does it work in a radio?

There are only three caps in the tester, and only one paper cap which has been replaced, so that issue is OK. (By the way, this cap was located between two wire wound resistors so not ideally located with respect to heat - shifted to a cooler location.)

That said I would be not at all surprised if I have made a mistake fixing the broken connections. I very carefully followed the circuit diagram to see if the repairs were correct, but it is such a rats nest in there that I could have missed something, especially in between closely located multiple position switches where I have had one known short. And most of the wiring is single conductors inside spaghetti and so fragile.

Getting back to rectifiers, the voltage drop through a silicon diode is 0.7 volts; can you give advice on what would be expected through a metal rectifier? Or give advice on what modifications are needed to more properly simulate a metal rectifier with silicon rectifier?