For example, if a voice coil measures around 6-ohms DC, it's most likely an 8-ohm speaker. Due to the fact that voice coils have very little inductance, you can determine this by measuring the DC resistance and multiplying by 1.25 (a 'rule-of-thumb' number). Next thing you need to know is the impedance of the speaker voice coil. You have to be careful to use the lowest voltages that still provide usable measurements to avoid saturating the transformer-which really throws everything out the window-and it is best to determine the voltage ratio in both directions and average the results.įor practical purposes, voltage ratio is equal to turns ratio, and this is equal to the square root of the impedance ratio: 400-Hz is a good frequency to use for audio transformers. The easiest way to determine the load impedance an output transformer presents to a tube is to measure the voltage ratio of the transformer using an audio oscillator and an AC voltmeter. Using a transformer that presents a different load impedance than what a tube calls for will still work, but distortion will be higher. Problem is, beam power tubes have lower load resistances (4,000 to 5,000 ohms typical) than power pentodes like the 42 or 6F6, which are up around 7,000 to 8,000 ohms. As a consequence, most junkbox output transformers you'll find were made for beam power tubes.

So they introduced and pushed beam power tubes like 6V6, 6L6, 6K6, and all their 12, 25, 35, and 50-volt cousins as alternatives, and that's what most manufacturers used on this side of the Atlantic from the mid-1930s onward.

Pentodes were first commercially developed by Philips, and RCA was on somewhat shaky ground, patent-wise. It is a power pentode, which is somewhat uncommon in later American and Canadian radios. The 42 tube is same as a 6F6, except for the base.

Universal output transformer a 3880