A Universal Shop Speaker and Dummy Field Coil
Special Note: You may have to do some digging to find the parts for this project, as the state of electronics parts suppliers has changed greatly in the years since this project was built.
I have a small workshop and often find it a nuisance to align and test the chassis of a set with an electrodynamic speaker. The problem is that such a chassis needs the field coil, output transformer and speaker contained in the cabinet in order to operate. The cabinet takes up valuable bench or floor space and can be cumbersome to work around. I have constructed a universal shop speaker and dummy field coil that can be used with the chassis of almost any set with an electrodynamic speaker. The system has three parts - a speaker, an output transformer box, and a dummy field coil box.
Because I often work on consoles I selected a large speaker that can handle a fair amount of power. I found a 12" Quam speaker, rated at 20 watts continuous power, along with a matching wall-mounted baffle, at a local electronic parts house. The pair was marked down to about $50.00 but originally sold for about $100.00. You may choose a smaller speaker, especially if you normally work on smaller sets, but be aware that it is very easy to damage a small speaker if you are working on a powerful console. A reasonable alternative is an inexpensive stereo speaker.
The Output Transformer Box
The universal output transformer has been available for over 30 years. It was developed for repair work, so that a small number of universal transformers can be used to replace a wide variety of output transformers. Mine is a Stancor A-3830 which I found marked down to less than $20.00 (originally about $35.00). It can handle 20 watts, primary impedances from 1500 to 20,000 ohms, a variety of voice coil impedances, and is usable with single-ended or push-pull output stages. Similar transformers, with various power-handling levels, have also been made by Triad and Hammond. Some are available from A.E.S. (Note 1).
The schematic of the transformer box is shown in Figure 1 below. The transformer (T1) has a center-tapped primary and a secondary with 6 taps. For push-pull outputs the primary brown and blue (end) terminals are connected to the plates and the red (center tap) terminal to B+. Single-ended outputs use either brown (plate) and red (B+) or brown (plate) and blue (B+). Which pair of secondary taps is used depends on the desired plate load and speaker impedance. The transformer comes with an instruction sheet which shows, for each combination of load and speaker, which taps to use. S1 and S2 are single-pole six-position rotary switches. Although the schematic doesn't show it I used two-pole six-position switches, tying each pair of poles in parallel, doubling the current-handling capability of the switches. The first position on each is not connected to the transformer. The remaining positions of S1 are connected to taps 1 through 5, and on S2 taps 2 through 6. That allows the speaker or dummy load to be connected to any pair of taps or to be disconnected from the transformer.
S3 is a double-pole double-throw center-off toggle switch. It selects one of three loads for the circuit:
The "external" terminals can also be used to connect a VTVM or other instrument across the speaker or dummy load for alignment, frequency response or noise and distortion measurements, or other tests.
A front view of the transformer box is shown in Figure 2 below. The terminals for the shop speaker are on the back of the cabinet. All of the parts needed, other than the transformer, are available at Radio Shack (Note 2). I used three-way binding posts for the "external" terminals, and banana jacks for the transformer primary connections. The jacks are painted to match the R.M.A. color code for output transformer primaries - brown, red and blue. I have a number of cables that I use with the transformer and field coil boxes. One has alligator clips to connect to any chassis. The other cables have plugs that duplicate common speaker plugs. The other ends of all of the cables have banana plugs, colored to match the appropriate jacks. I used test-probe wire for the cables. The alligator clips are tagged with colored tape in the R.M.A. colors, matching the banana plugs and jacks.
The Dummy Field Coil Box
A substitute for a field coil requires inductance and resistance in series. The DC resistance of a field coil is much higher than the DC resistance of a filter choke. The total DC resistance of our substitute is more critical than the value of the inductance used. When the field coil is used as a filter choke its resistance (and voltage drop) determines the output voltage of the power supply - too low a resistance means too high a plate voltage on the tubes. A shunt coil of too low a resistance may overload the power supply or reduce the plate voltage to some tubes. This means that the field coil box must be able to provide a variety of resistances, providing a close match to the original coil.
The schematic of the field coil box is shown in Figure 3 below. L1 is an 8 Henry choke with a DC resistance of 145 ohms. It is rated for 150 ma continuous current, 200 ma peak. It is a Thordarson 20C54 which I puchased at closeout for about $2.00 - the original price was about $20.00. Any choke of about 8 or 10 Henries will do, as long as it can handle enough current for the chassis you will be working on. A choke rated at 150 ma is sufficient for even the 10- and 12-tube Zenith consoles with large speakers. A.E.S. stocks filter chokes which can be used (Note 1). The rest of the parts are available from Radio Shack (Note 2). All of the resistors are made up from identical 100 ohm 10 watt power resistors. The 200, 400 and 800 ohm resistors each consist of the appropriate number of 100 ohm resistors in series. The 50 ohm resistor consists of two 100 ohm units in parallel. Each switch is installed so that when the handle is up the resistor for that switch is in the circuit. The banana jacks are colored yellow and black, approximating the R.M.A. color codes for field coils (start - black w/ red tracer, finish - yellow w/ red tracer).
A front view of the field coil box is shown in Figure 4 below. To use it you will need to know the DC resistance of your field coil, either by measuring it or from the value on the schematic. Let's assume that you are working on a 1938 Zenith 12-S-265, chassis 1204 (the 12-tube "Walton's" robot dial chassis in a console cabinet). The field coil has a resistance of 670 ohms. My choke has 145 ohms resistance - let's call it 150 ohms. As you can see from the schematic the box can provide an additional 50 to 1550 ohms, yielding a range from 150 to 1700 ohms in 50 ohm steps. The appropriate substitute value for the field coil is 650 ohms, achieved by setting the 100 and 400 switches up and the others down.
As described this system works only for single, untapped field coils. I plan to add a second field coil box for tapped and split field coils (see speaker plugs). The additional resistance provided by the second box will also be useful for field coils that have a DC resistance greater than 1700 ohms.
1) A.E.S. stocks the following parts appropriate for this project, as listed in their 1998 catalog:
Universal output transformer - part. no. P-T125E - $33.11. This Hammond transformer is rated at 15 watts and provides single-ended and push-pull plate loads from 1,200 to 25,000 ohms.
Filter choke - part no. P-T158M - $18.35. This Hammond choke is a 10 Henry unit rated at 100 ma. If you need greater current capability for large consoles, use two 5 Henry 150 ma chokes in series instead (part no. P-T158Q - $18.59 each).
Antique Electronic Supply, 6221 South Maple Ave., Tempe, AZ 85283; Phone (602) 820-5411, FAX (602) 706-6789; Website TubesAndMore.Com.
2) Parts available at Radio Shack as of April, 1998 (Find your nearest store at RadioShack.Com).
||R1||1||271-120||8 Ohm 20 Watt Resistor||$1.49
||S1,S2||2||275-1386||2-Pole 6-Position Rotary Switch||1.69
||S3||1||275-1533||DPDT Center-Off Toggle Switch||1.99
||2||274-725||Pair of Banana Jacks||.99
||1||274-662||Pair of Binding Posts||1.99
||1||274-632||Speaker Terminal Plate||2.99
||1||274-688||Pack of 4 Tie-Point Strips||1.29
||1||274-415||Pack of 4 Knobs||1.89
||R?||9||271-135||Pack of 2 100 Ohm 10 Watt Resistors||$ .99
||S1-5||5||275-602||SPST Toggle Switch||1.49
||1||274-725||Pair of Banana Jacks||.99
||2||274-688||Pack of 4 Tie-Point Strips||1.29
||270-356||Pack of 4 Alligator Clips (As Needed)||$1.99
||274-730||Pack of 2 Banana Plugs (As Needed)||1.99
||278-563||Spool of Red Test Probe Wire||2.99
||278-564||Spool of Black Test Probe Wire||2.99
Other items needed are brown, blue and yellow plastic model paints for banana plugs and jacks, and brown, blue and yellow colored tape or other means for tagging the alligator clips. Also useful are plugs matching the speaker plugs of sets that you often work on. Since I often work on pre-WWII Zeniths I have two cables made up to match the two most common Zenith plugs of the period. Each uses a standard 5-pin tube base salvaged from a dead tube (see speaker plugs).
Up to Service Information
Up to Star City Antique Radios top page
Email comments to Dale H. Cook
I do not sell radios, test equipment, parts, or anything else so please don't ask!
Copyright © 2003-2011 by Dale H. Cook. All rights reserved.
Text, images and files at this site may be downloaded for personal, non-commercial use only, and may not be
used on any web site or published in electronic or physical form without prior permission from Dale H. Cook.