Is the 26 valve the same as a 45 under the glass? - UK Vintage Radio Repair and Restoration Discussion Forum

LCTorana · 3rd Feb 2017, 1:33 pm

I have a ST14, UX-based triode valve, whose filament measures roughly 1.1 ohms cold. I've breathed on the glass, done all the tricks, but there's no markings at all.

I've been looking at data books, and trawling websites like the "National Valve Museum", and it could be either a 26 or a 45.

I've read discussions where folks have speculated that some 12.6V car radio valves might actually be just 12.6V ordinary valves, as the electrode structure looks similar.

In the same vein, is it possible that the 26 actually is a 45, just with an under-run filament?

I know that sounds strange, but stay with me. The physical construction of the two valves looks identical to my eyes, same base and glass envelope and mica supports. Inter-electrode capacitances are too close to tell apart with home equipment.

The 45 filament runs at 2.5V/1.5A, or 1.666 ohms hot, the 26 runs at 1.5V/1.05A = 1.429 ohms hot. Since valve filaments are slightly non-linear, it is not outside the bounds of possibility that it is the same filament in both.

Same with mutual conductance, plate resistance - they depend closely on filament current.

So all this raises three questions:
1) How could I tell, without a mutual conductance tester, whether it is a 26 or a 45?
2) Would a 45 behave like a 26 at 1.5V on the filament?
3) Are they actually the same valve anyway?

ms660 · 3rd Feb 2017, 2:06 pm

My guess is that if your valve filament measures 1.1 ohms cold then it's resistance when hot would be much higher than that indicated by the 26 or 45 valve when those filaments are hot.

Lawrence.

short wave · 3rd Feb 2017, 5:45 pm

If you can see the heater apply 1 1/2 volts to it and see how it glows... if very dim try upping it to 2 1/2 volts.
regards S-W

3rd Feb 2017, 1:33 pm	#1
LCTorana Triode Join Date: Mar 2009 Location: Melbourne, Australia Posts: 17	Is the 26 valve the same as a 45 under the glass? I have a ST14, UX-based triode valve, whose filament measures roughly 1.1 ohms cold. I've breathed on the glass, done all the tricks, but there's no markings at all. I've been looking at data books, and trawling websites like the "National Valve Museum", and it could be either a 26 or a 45. I've read discussions where folks have speculated that some 12.6V car radio valves might actually be just 12.6V ordinary valves, as the electrode structure looks similar. In the same vein, is it possible that the 26 actually is a 45, just with an under-run filament? I know that sounds strange, but stay with me. The physical construction of the two valves looks identical to my eyes, same base and glass envelope and mica supports. Inter-electrode capacitances are too close to tell apart with home equipment. The 45 filament runs at 2.5V/1.5A, or 1.666 ohms hot, the 26 runs at 1.5V/1.05A = 1.429 ohms hot. Since valve filaments are slightly non-linear, it is not outside the bounds of possibility that it is the same filament in both. Same with mutual conductance, plate resistance - they depend closely on filament current. So all this raises three questions: 1) How could I tell, without a mutual conductance tester, whether it is a 26 or a 45? 2) Would a 45 behave like a 26 at 1.5V on the filament? 3) Are they actually the same valve anyway?

3rd Feb 2017, 2:06 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Is the 26 valve the same as a 45 under the glass? My guess is that if your valve filament measures 1.1 ohms cold then it's resistance when hot would be much higher than that indicated by the 26 or 45 valve when those filaments are hot. Lawrence.

3rd Feb 2017, 5:45 pm	#3
short wave Pentode Join Date: Mar 2013 Location: Leeds, West Yorkshire, UK. Posts: 246	Re: Is the 26 valve the same as a 45 under the glass? If you can see the heater apply 1 1/2 volts to it and see how it glows... if very dim try upping it to 2 1/2 volts. regards S-W