[GreenKeys] tube?

[Bruce Gentry]

On 1/6/13 6:29 PM, Duncan Brown wrote:
> Tim,
>
> Yes, the tube is a 25L6GT, designed to work with military 24-28VDC 
> supplies.
>
> The first time I put power to our TT-30 (military, 26VDC, version of 
> the M31), the motor ran fine, but then it started to slow down. 
> Thinking something was binding or there was a bad bearing, I shut the 
> power off quickly.  After checking for anything overheating, I tried 
> it again and got the same result: motor slowed down after a few 
> seconds, but then maintained the new speed.  Then I realized that the 
> filament had to warm up before it would start regulating!
>
> Duncan,
> K2OEQ
>
> On 06-Jan-13 16:57, Jim Haynes wrote:
>> Don't know for sure, but I would suspect a 25L6. You'd want a 25 volt
>> tube so you could run 26V on the heater.
>>
>>
>> jhhaynes at earthlink dot net
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If it's a 25L6, it's almost certainly controlling the shunt field in the 
motor.  The motor is likely compound wound and designed to run a bit 
fast, the regulation circuit then increases the shunt field current to 
bring the speed down to normal. I have a 400 cycle MG set that uses this 
method for frequency control, and the current sensitive diode to 
regulate the output voltage.  This older gear was very clever, and far 
too many younger people don't know or care anything about it.  On a 
related topic, 25 and 50L6s will work surprisingly well on low  plate 
voltages. The Stromberg Carlson telephone exchanges used multiple 50L6s 
in push-pull paralell to generate dial  and busy tone from the central 
48 volt supply.

       Bruce Gentry, KA2IVY