1U5 - not OC but heater won't heat

[David Alan Roden]

I'm pretty new here - hope this doesn't seem too obvious.

I recently finished my uTracer3+ and it works fine on the small "fleet" of valves I have. I have a little 1 valve regenerative battery powered receiver and I thought I'd test its valve - a 1U5. First I thought I would test the spare 1U5 I have for it. First thing I did was check not open circuit on the filament - 8.3 ohms - which is slightly lower than the one in the receiver at 9.2ohms. I have a 1.5V battery pack for the testing. So I wired it all in for a pentode test on the uTracer - I put in the battery filament circuit my multimeter set to mA. It used 125mA (it's supposed to use 50mA) and there was no sign of glow. I put this down to the lighting in my workshop. Ran the standard test - total flatline for all values of Vg.

I then tried the one in the regen set and it worked fine - the heater used 50mA as per spec and the curves looked near enough exactly as per spec sheet. 

I am a little puzzled why the first 1U5 would not heat - what I expected to happen was a spike - 125mA seemed reasonable considering the 8.3 ohms cold - then current should go down to 50mA - as it did in no time on the 1U5 in the regen set.

I went and tested to see if there was any short to one of the other electrodes - perhaps it was cooling the filament down somehow - none found.

Might be gassed but would still expect the filament to heat up and then burn out. There is no "getter blacking" in the top of the valve as there is on the one that works fine. Is this a clue?

Anybody any ideas?

Thanks in anticipation

Dave Roden

Esbjerg, Denmark

[theold...@gmail.com]

Hi Dave,

 

I have a “flat” of 12AX7 tubes, they are / were intended for OEM use who bought in large quantities. This in itself is not significant, except that for one tube looked different. Its top was clear, whereas all the others were the typical silver color. I tested the tube, and it also drew too much heater current, and measured dead as a doornail. I suspect that in the manufacturing process the getter did not fire. I am not surprised this could happen, but I am very surprised it made it out of the factory. These tubes were branded Rodgers, and I suspect they were manufactured by Philips. Only difference with your tube is that mine did get hot, way hotter than the good ones.

 

Is your tube new, or is it used? If used, I would suspect it is simply gassy. The filament in your tube is very small, and even at 125mA at 1.5V it only generates less than 0.2 Watt of heat. I am not surprised you can’t feel that.

 

Bill v.

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[David Alan Roden]

That would make sense - not sure about where the valve was sourced - but as the getter marks are not there it makes sense it was a fault at manufacture like you say.  I suppose what I have now is a little keepsake from a factory quality issue example.

As for the make it's an RCA "Radiotron" 

Thanks for the response

Regards, 

Dave.

[David Alan Roden]

I suppose I should have looked a little closer at the two - the internal construction is different - and I'm not sure the non-functional 1U5 is actually RCA - the markings are scrubbed off - the 1U5 is still there though, and it came in an RCA box. Anyway, just for fun, here are a few pictures of the two valves - which on reflection perhaps I should have posted at the very start.