[Collins] Transformers, was 32S-3 wire

Mon Aug 26 19:48:16 EDT 2013

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.  

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.

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

________________________________
 From: Dr. Gerald N. Johnson <geraldj at netins.net>
To: collins at mailman.qth.net 
Sent: Monday, August 26, 2013 4:18 PM
Subject: Re: [Collins] Transformers, was 32S-3 wire

On 8/26/2013 3:36 PM, antqradio at sbcglobal.net wrote:
> Doctor J's comment about higher then design line voltage made me
> think a bit out of the box.
> 
> I was always under the impression, mistaken or not, that 117 vac was
> just the geometric mean for the line voltage limits of 110 to 125
> vac.  Designers specifying 117 vac instead of the high limit of 125
> vac may have inadvertently created a problem for the rest of us.

117 IS the geometric mean between 110 and 125 volts. But many devices were specified at 115 with little range in the specifications. I used to have a 1/3 hp electric motor that said 115 on the nameplate and it drew rated current with no shaft load at 122 volts. It was on my metal lathe, then I moved it to an air compressor and burned it up when the piston rings froze in the winter when I neglected to unplug its cord.

Power companies tend to prefer voltage running high at light load, then they get fewer complaints about low voltage. Each standard transformer tends to have about 4% impedance which means if they regulate the 34.5 KV area distribution voltage, there's up to 4% drop when the substation step down transformers to 13.8 KV distribution and then another 4% drop in the distribution transformers down to 120/240. If they regulate 8 or 10% high (to allow for some conductor voltage drop) then the inherent transformer drop won't need a lower voltage regulator. Some tend to run even higher when the utilities board requires a minimum of 108 or 110 and they have designed the power system to occasionally (as on hot days) load distribution and substation transformers to twice nameplate currents. A classic reference manual says doing that to the transformers may cut their life in half, from 40 years to 20 years. So the power companies run high voltage which makes
 resistive loads, like electric water heaters, electric heaters, electric ranges, and incandescent lamps draw more power, hence more revenue. Also induction motors draw less current unless the core goes into saturation at higher voltages, improving the customer voltage regulation by keeping currents a little lower for motor loads.
> 
> I would hope that any well designed power transformer would not
> saturate at 125 vac on the primary and thus operate at higher then
> ambient temperatures.  Obviously any transformer with primary taps
> for 115, 120, 125 is in the well designed category.

In power transformer design, the most economical design operates as close to saturation on peaks as possible to save on both iron and copper. It may not be the most economical for operation or the coolest operating transformer but its the cheapest to build. In documents for 10 million a year fluorescent ballasts I've seen changes of one layer of lamination more or less which I believe were in response to changing costs of transformer iron and copper. In that production quantity if they didn't cause overheating and saved a penny a unit, the engineer made his annual salary for the company in one change. Besides the lamination count they changed the turns count and I don't know what that did for the performance or efficiency where they expect the core to saturate to regulate the secondary current.
> 
> That said, if anyone notices that the power transformer is running
> too hot to touch in any piece of electronic equipment and that
> transformer does not have primary taps; I would recommend that you
> install a power resistor in the primary lead to drop a few volts and
> run the transformer with 115 vac on the primary.  Much easier to do
> this then to add a bucking transformer, especially when space is at a
> premium.

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.

I much prefer the bucking transformer, it can easily be a filament transformer properly wired and the regulation is very close to that of the line even with varying loads.

A variac works well, but unless its hidden some visitor will turn the knob to see what it does and a variac these days can be costly compared to a filament or low voltage DC supply transformer.
> 
> The power transformer is, for the most part, the single most
> expensive component in just about any piece of equipment.  I am sure
> that cost pressures have resulted in the design of power transformers
> with too little iron and copper in them so that they are now close to
> magnetic saturation at present day line voltages.

Some that Radio Shack used to sell had a very high operating temperature as if designed for 100 instead of 120 but was labeled 120 volts. I don't think they sell transformers anymore.
> 
> Much better to prolong transformer life by reducing the internal heat
> then trying to find a replacement transformer for your favorite
> receiver. Jim
> 
Very true.
> 
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

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