[Collins] Transformers, was 32S-3 wire

[Dr. Gerald N. Johnson]

On 8/26/2013 6:48 PM, antqradio at sbcglobal.net wrote:
> Definitely easier but not a great idea for a receiver or transmitter.
> If the resistor is chosen to lower the voltage for the operating
> condition, it won't do nearly as much for the standby condition and
> varying supply voltage heats up the tube heaters more on standby and
> raises the unregulated power supply voltages so you get more tube
> cathode temperature changes that can induce delamination of the
> cathode coating and will cause drift of unregulated oscillators when
> turned on from standby.
>
>
> Really?  The ohmic losses in a power transformer won't produce any
> voltage change in the heater circuit as you go from standby to
> operate and vice versa?  Most power transformers are supplying
> approximately half of their rated power to heat filaments and the
> other half supplying B+.  Since the power transformer is going from
> approximately half to fully loaded, there has to be an affect on the
> filament voltage which you are overlooking.

In the transmitter the heater load is considerably less than the plate 
load. 60 or 70 watts heater, 30 or 40 watts low b+ and 180 to 200 watts 
high b+. Transformer impedance 2 to 4% of full load spread between the 
primary and the three main secondary loads. Dropping 10 volts in a 
resistor is more like 8% change, probably three times as much as seen at 
the heaters.
>
> Wasn't cathode delamination the cause of sleeping sickness when
> vacuum tubes once powered computers?  Caused by running the tube in
> cutoff for long periods of time, much like standby operation.
> Speaking of standby, I am under the impression that standby was a
> temporary expedient and not to be used for extended periods of time.
> As for oscillator drift, any voltage change or time itself could
> cause what you describe.

Transmitters often spent many more hours in standby than operating in 
communications service. One WW2 ships sparks claimed he transmitted just 
once during the whole war, yet he had to keep the transmitter ready to 
transmit. Those days the transmitter tubes probable were all using 
thoriated tungsten filaments, no coated cathodes.
>
> Tube filament voltages are (or were when the tube manuals actually
> published this data) + or - 10% and line voltage changes are now
> specified as 120 vac + or - 5% so I don't think this is much of a
> problem for vacuum tubes to handle.  Compared to a transformer near
> saturation and producing excess heat, what is the harm in using a
> series resistor to drop approximately seven volts? Jim

The harm is in the poorer regulation avoided with the bucking 
transformer and many an early Collins rig is rated at 115 volts, not 117.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>
>
>
>
>