The group met at our venue of years past on Senter St. in Irving – the Garden Center and Arts building. That’s where most of our meetings will be held this year. Jim filled us in on plans for the new year, including auctions, meetings and events. The April meeting will feature Richard Kearley with his history with WSM, of Grand Ole Oprey fame. It’s always interesting to hear about the broadcasting end of the radio business and what our antique radios were picking up in their day. Jim welcomed Chris Ripley (back) and new visitor Tim Quinn. Jim reminded us of our Houston, Texas sister group’s upcoming Convention. A good-size group is going (we’ll soon need a bus to take everybody down there). Note: the Houston website is at hvra.org. Our group shares a lot of interest and memberships, too, with them and our closer neighbors, in Arkansas and Oklahoma. Jim asked us to think about display spots at libraries, antique malls, etc. where we can display items and tout our organization and gain new members. Those who have radios in malls might add some displays with information about our club.

Our program director, Mike Grimes, noted that our website roster has been updated. He also announced that, after the spring auction, the April program topic will be about I.C.’s and the history of their development into the modern age of CHIPS, and how they are made. He put forth a plea for help in getting a new lamp for our digital projector. We have really enjoyed our programs with the advent of that device. We’re finding out that the lamps are a big percentage of the cost of those things.

Mike introduced our returning program presenter Mike McCarty. He presented a program about impedance matching and making measurements to evaluate transformers as candidates to replace bad ones.

                   MIKE’S MAKE ON MATCHING (IMPEDANCE, THAT IS).

Mike first explained the difference between resistance and impedance, using the analogies of a water wheel, with its inertia, and a "springy" container, (balloon?) analogous to capacitance. The water wheel keeps the flow going, but the current flow lags the feeding of the water to the wheel (analogous to inductance). The springy container builds up pressure, but the pressure lags behind the water flow. These factors affect A.C. systems because the voltage and current are swinging positive and negative (for our power systems, at 60Hz), and the time, or phase, relationship between them varies. He pointed out that the symbols used for voltage and current are lower-case "v" and "i" for A.C and upper-case V and I for D.C. Impedance is "Z" instead of "R". Mike then went into the question of why impedance matching is important and therefore the reason we find output transformers in our vacuum tube radios and amplifiers. He pointed out the properties of vacuum tubes that require consideration in order to get the most power from the amplifier into a speaker with minimum distortion (a lot of power isn’t good if the sound is distorted). Because loudspeakers have very low impedance, compared to a vacuum tube, a transformer is needed to provide a match. The matching is created by the magnetic coupling between coils having different numbers of turns of wire – each coil carrying the same power (or volts-times-amps) except for small losses. Since volts-divided-by-amps equals impedance, we have an impedance "transformation", one winding having more voltage but fewer amps than the other one. Mike described properties of transformers especially of the iron-core type used for radio speaker impedance matching. As long as the core is not saturating magnetically, the transform functions as it should, with the ratio of voltages matching the number of turns of wire in each winding. His description of the core iron saturation phenomenon was interesting – as the magnetizing force increases, more and more atoms of iron (which are little magnets) line up with the field as the force increases, until they are all lined up. Then no more flux change will happen in the core as the force is increased!

Mike set up some test equipment and demonstrated methods for testing transformers to see if they might be usable replacements for an output transformer. His setup includes a Variac feeding a step-down transformer that serves to excite one of the windings on a candidate transformer. This excitation voltage is brought up to just enough for good readings to be taken. First, you use an ohmmeter to find out which transformer leads are associated with each other for each winding. If there are three associated leads, it’s probably a center-tapped winding with equal resistance readings from the common wire to each end. Relatively low resistance implies low impedance, e.g. under one ohm. Then you connect your excitation source (transformer) to one of the higher resistance windings of the transformer. Using an AC voltmeter, measure and tabulate your excitation voltage and that appearing on the other windings. Then compute the voltage ratios. Squaring the different ratios will tell you the impedance ratios. Then you’re ready to refer to Mike’s analysis below. Mike pointed out other factors affecting suitability of a given transformer – about the same size (same power handling capacity), not an inter-stage transformer (two high-impedance windings). Candidates do include some power transformers that have filament windings – sometimes tapped and even the lowly wall-wart.

Mike wrote up the following discussion about impedance matching and associated analysis and process.

          K E Y  P O I N T S  A N D  E Q U A T I O N S  by MIKE McCARTY

For maximum power transfer, one needs the load resistance to be equal to the source resistance. The optimum load resistance for minimum distortion is usually approximately, but not the same as, the maximum power transfer load resistance. The recommended load resistance is some compromise between optimum power transfer and minimum distortion. The exact best power and best distortion values are, however, fairly strongly dependent on the exact operating point of the tube, so they are not critical, so long as the effective load resistance is not markedly different from the recommended load resistance.

To find the recommended load resistance for a given tube, look in the data sheet for the tube, and look for a symbol Rl or Rload or similar. It is likely to be a resistance in the range of 2000 to 5000 ohms. If you are dealing with a push-pull circuit, then look for the Plate to Plate resistance, or P-P resistance.

To find the impedance of a speaker, simply measure the resistance. The resistance is close to, but not the same as, the rated impedance. For example, an 8 ohm speaker may have a voice coil resistance of 7.6 ohms.

To achieve a match, one uses a transformer. A transformer is two (or more) coils of wire so arranged that a changing magnetic field generated by current in one of the coils induces a voltage on the other coils. The voltage on each coil is proportional to the number of turns in the coil, while the current available (neglecting wire size limitations) is inversely proportional to the number of turns. In mathematical terms, letting Vp be the primary (energized) coil voltage, Np be the number of turns in the primary coil, Ip be the current in the primary coil, and Vs, Ns, and Is be the corresponding voltage, number of turns, and current in the secondary (load) coil, we have

Vp Vs Vp Np

-- = -- or equivalently, -- = --

Np Ns Vs Ns

Ip Ns Vs

Ip x Np = Is x Ns or -- = -- = --

Is Np Vp

If we use a load resistance of Rl in the secondary, then the effective load resistance in the primary is Re, given by

Re = Vp / Ip

Rl = Vs / Is

so

Re Vp / Ip Vp

-- = ------- = (--)^2

Rl Vs / Is Vs

That is, the impedance transformation ratio is the square of the voltage transformation ratio. The voltage transformation ratio of a transformer may simply be measured, by injecting a few volts AC into the primary, and measuring the voltage on the secondary.

So, to find a transformer which provides an adequate match, one needs to calculate the square root of the impedance ratio of the recommended plate load resistance to the speaker impedance, and check that against the measured voltage ratios of transformers in stock. For push-pull circuits, one uses the recommended plate to plate resistance and uses that to apply to the entire primary coil.

As an example, suppose we wish to load a 50L6GT with an 8 ohm speaker. The recommended load resistance of the 50L6GT, from the data sheet, is 2000 ohms with a plate voltage of 110 VDC, and 4000 ohms with a plate voltage of 200 VDC. If we are working with an AC/DC transformerless set (typical for the 50L6GT) then we want a load resistance of 2000 ohms. So, the impedance ratio is 2000/8 = 250. The square root of 250 is 15.8 or so. We need a transformer with a turns ratio of 15.8 or so, or equivalently a voltage transformation ratio of 15.8. If one considers using a small "wall wart" style AC transformer, then a voltage ratio of 15.8 results in a secondary voltage of 120/15.8 = 7.6 VAC. A 6.3 VAC transformer might provide an adequate match. Generally, it is better to err on the side of too high a load resistance, rather than too low, as the power transfer curve is steeper on the low side than it is on the high side.Author’s Notes

Wall-warts are "impedance-protected" to meet the Underwriter’s Laboratories requirement that they must tolerate a short-circuit load indefinitely without catching on fire or producing any hazard. Therefore they have an inherently high impedance and poor regulation of the output voltage. The lower-voltage ones should be the best choice. You might measure the AC voltage on the speaker voice coil and across the primary with a steady tone coming out of the radio – to find out the real voltage ratio.

Years ago, using the formulas contained in Mike’s analysis, I re-wound an output transformer to match an 8 ohm speaker, because the radio came with a 3.2 ohm speaker. 3.2 ohm speakers are hard to find, except for those taken from "parts sets", i.e. "junkers".

A fun experiment is to take a transistor radio that has an earphone jack and connect it to an old horn speaker, using an output transformer hooked up in reverse to its normal arrangement. This provides a fairly good impedance match between the transistor radio output and the old high impedance speaker. An old ACDC radio transformer works fine. The voice coil winding is connected to a mating transistor radio earphone plug and the high impedance (plate) winding is connected to the horn speaker. It’s amazing how the speaker can sound even better than when it’s played with an old 20’s radio. (I used an old Radio Shack "Flavoradio".)

Mike pointed out that the "plate resistance" of a vacuum tube is where all the real heat comes from that must be dissipated. Data sheets talk about "maximum power dissipation". That’s the power Mike is referring to. If you don’t connect a load to a power amp, the plate voltage swing will increase and the tube can burn up, if driven, because there is no transfer of load back into the tube. Solid-state amplifiers do not suffer the same problem, if they do not have an output transformer. If they have an output transformer (most don’t) they can suffer transistor failure due to voltage spikes from transformer inductance.

Anyone know a good source of 4" 3.2 ohm speakers?