[Collins] Transformers, was 32S-3 wire

Tue Aug 27 11:26:46 EDT 2013

"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."

As I understand it, the only way to get any degree of voltage regulation in a power transformer is to operate it near saturation.  Otherwise, the transformer's secondary voltage is determined solely by turns ratio.  Near saturation, with the flux density in the core at maximum, any increase in input power just heats copper and core material because there can be no increase in flux density.  If the transformer was designed to operate at these higher temperatures then no harm is expected.  But if this is not so, then the transformer will just cook itself to destruction, most commonly resulting in a shorted turn.    

Any of the two methods already mentioned to back off from saturation by reducing the input voltage, will obviously reduce the inherent voltage regulation that comes from operation near saturation.  I have no problem with the bucking transformer method.  It is just that finding room for it in the equipment is problematic.

Cost is also an issue, new filament transformers are not cheap.  And finding one with a non standard secondary voltage is next to impossible, so one is stuck with a compromise unless you can rewind the bucking transformer to suit your needs.  That said, power resistors are obviously less expensive then a bucking transformer and can be easily sized according to need and they can be made to fit in smaller spaces by series connecting several to arrive at the optimum value.

Saving power transformers from self destruction is the issue, not the best method for doing so.  Power transformers will benefit from operating at a lower temperature, regardless of the method used to get there.
Jim

________________________________
 From: Dr. Gerald N. Johnson <geraldj at netins.net>
To: collins at mailman.qth.net 
Sent: Monday, August 26, 2013 9:08 PM
Subject: Re: [Collins] Transformers, was 32S-3 wire

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.
> 
> 
> 
> 
> 

______________________________________________________________
Collins mailing list
Home: http://mailman.qth.net/mailman/listinfo/collins
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Collins at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html