Re: {Collins} Preparing a Hum-Bucker (sp?) for an old S-Line

I would imagine if one's 516F-2 has been converted to solid state rectifiers you could use the existing now unused 5 volt filament windings to buck a combined 10 volts, no? (keeping it all within the same real estate and putting the now unused windings to good use) Just have to work out the phasing.


On 11/17/2014 1:20 PM, Roy Morgan wrote:
On Nov 17, 2014, at 11:28 AM, Bill Carns <wcarns@xxxxxxxxxxxxx> wrote:

Hi Doug...  I don't think that you mean a "Hum-Bucker”..

Thanks you for your comments.  May I add a couple:

“Hum Bucker” is a term from the olden days of radios, when filter capacitors were expensive and the speaker magnetic field coil acted as the radio’s choke.  A separate coil in the speaker picked up the filter capacitor ripple current and opposed the signal from the final amplifier tubes to reduce the hum.  Tapped filter reactors and output transformers were also made to solve the same or similar hum problem.

I think what you
are talking about is a bucking transformer set-up.
Indeed: a line voltage bucking arrangement.  A low voltage transformer is arranged with the secondary in series with the output feeding the equipment to reduce the voltage.

What current you set up for depends on what equipment is in your S-Line. The
best way to quickly scope out what you need is to take all of the AC fuse
ratings and add them up.
This will result in plenty of reserve current capacity.  The fuses operate somewhat below their rating.

The voltage at the time the S-Line was developed was around 115 Vac.
My manuals for the 516F-2 and 75S-3 show 115 vols.
The 30L-1 diagrams show 110 volts but the specifications show 115 volts +/- 10 percent (up to 126.5 volts).

I would look for a nice fat Variac at the local swaps, or you can go the
bucking route. Bucking is less flexible.
Here I have a caution:

I have at hand an unmodified Powerstat model 116. It is rated at 115 volts input, 10 amps at up to 135 volts output.  The input connection is at the “115” spot on the dial.  My line voltage at the moment is 121 volts.  I measured the maximum output voltage at 140 volts.  This will fry your S-line. Here’s how:

Imagine you are lucky enough to have a big Golden Retriever dog as shack assistant, who likes to sleep under the operating table next to the warm humming variac.  The dog in her contentment rolls onto her back, stretches out her legs and moves the variac from 115 volts output to 140.  (With some of them it is  very easy to move the knob.)  It takes you some time to realized there are popping sounds from your radios and to figure out what’s wrong.  Alternatively you are monitoring 10 meters one night and the band suddenly opens up to east Tanganyika, which you desparately need for WCC.  You hurriedly reach for the mike and inadvertently move the variac knob up to full output.

If you have a variac, you may find it easy to rewire the thing for output up to the incoming line voltage maximum.  The incoming line goes to the end of the winding not to a tap as is normal to provide over voltage output. There is little danger of seriously over-volting your radios with this arrangement,   If your variac dial plate is calibrated for over voltage (as is the Powerstat here) and meant for 115 volts input, you can move the input tap on the winding so the dial indicates correctly.  Then you’ll know just how much over voltage you fed your S-line since you last paid attention to the setting.

If your line is pretty stable at say 125 Vac, then bucking down 10 Vac would
work well and I recommend it or the Variac.
I recommend both: arrange a bucking transformer to be run at variable line voltages from a variac.  A small 2-amp rated variac will happily run a 12 volt 10 amp secondary transformer to buck down your line.  These two combined in a nice box with an outlet and output voltage meter makes a fine bench accessory.  A DC milliammeter with a 0-25 scale and run against a zener diode to reference the zero point to 100 volts ac gives you quite a nice indication of your output voltage, 100 to 125.  Shall we call this “The Under-Volter”?

But if all you do is mount a 12 volt filament transformer in a deep electrical box and wire it up in the middle of an extension cord for fixed line voltage reduction, it’s simple and satisfying project.

To buck the line down, you need say a 10 VAC filament transformer designed
to work from your current line voltage
If you have a very old filament transformer (one here is a Thordarson 7.5 volt 7 amp one from the 1930’s meant for 115 volts input), you can run the transformer primary from the reduced output voltage of the arrangement and all is well.  Doing this with a bucking transformer of old or new primary voltage rating lets you increase or decrease the secondary voltage and thus the amount of bucking you get, within a small variation.

and then put the secondary in series
with the incoming line on the hot side. ... opposing and
not adding…
To find many descriptions on how to do this, use your favorite search engine with:
"line voltage bucking transformer"

Hope this helps.
It certainly does, and I hope my comments inspire some folks to be kinder to their radios, and even to build the “Under-Volter” adjustable bucking system I suggest.


1) Many variacs have a fuse in the input only. I suggest you put one in the output.  It’s easy to burn the first few turns of a variac if the output is shorted and you start at 0 and crank up slowly,
2) A three wire line cord and outlet are, of course, the safe thing to use.
3) The term “Variac” has been a registered trademark of the General Radio Company.  I think they are not made anymore, and certainly the term has come into common use to mean any variable autotransformer.  the Superior Electric Company still makes all sorts of Powerstat products.
4) I have a “Diatribe” on variacs - glad to send it to anyone interested.


Roy Morgan
K1LKY Since 1958

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