I've been characterizing some transformers that came from old console ampliers, reportedly Magnavox. One pair is SE, the other P-P. The size tends to indicate thay were used with 6BQ5 or similar tube types. I characterize transformers by driving the primary with a signal from a sine wave power source at 50V RMS, 300Hz. The 50V amplitude gives me a good signal at the transformer output and enough voltage to properly excite the core so that I can get readings for primary inductance under large-signal conditions. I also monitor the input current using a Yokogawa WD210 power analyzer, so that I can calculate large-signal primary inductance.
Anyway, to the chase. The SE transformers measured 1.24V out for 50V input. This is a turns ratio of 40.3 to one. This is squared for the impedance transformation. Assuming 8 ohm load, this is a 13k primary impedance, which seems rather high, unless the speaker load is really 4 ohms. Primary inductance was ~30H, which is respectable.
The P-P transformers measured 1.15V out for 50V in. This is a turns ratio of 43.5:1, or a primary impedance of 15k for an 8 ohm load - again, a 4 ohm load sounds more plausible. Primary inductance was 60H, again respectable.
Anyway, can anyone with experience parting these consoles out tell me wheter you have encountered 4 ohm speakers?
Anyway, to the chase. The SE transformers measured 1.24V out for 50V input. This is a turns ratio of 40.3 to one. This is squared for the impedance transformation. Assuming 8 ohm load, this is a 13k primary impedance, which seems rather high, unless the speaker load is really 4 ohms. Primary inductance was ~30H, which is respectable.
The P-P transformers measured 1.15V out for 50V in. This is a turns ratio of 43.5:1, or a primary impedance of 15k for an 8 ohm load - again, a 4 ohm load sounds more plausible. Primary inductance was 60H, again respectable.
Anyway, can anyone with experience parting these consoles out tell me wheter you have encountered 4 ohm speakers?
You might want to confirm that sec out voltage down at 40 Hz too. Most commercial core is all done by 250 Hz for passing power and above that you are getting into antenna event coupling only. This is especially true of the beloved 26 gauge M19, called audio core, that most audio transformer manufacturers used.
I think most of those ancient alnico speakers were actually 16 ohm, unless they were a weird variant, like 12 ohm. Magnavox could have anything they wanted after all.
Bud
I think most of those ancient alnico speakers were actually 16 ohm, unless they were a weird variant, like 12 ohm. Magnavox could have anything they wanted after all.
Bud
I just went back in the lab and drove one of the SE transformers again at 50V RMS, varying the frequency from 60Hz upward. I also placed a 10 ohm load at the transformer output. Output voltage under these conditions was pretty constant with frequency (a little down at 60Hz, though) at 1.14V out for 50V in. This is still a rather high turns ratio, and tends to indicate a 4 ohm load rather than 8 ohms. I have some organ transformers that when subjected to this test, indicate that they were intended for 16 ohm speakers.
Those are 4 ohm transformers IMHO. I have had around twenty different magnavox consoles pass through my hands. Almost all of their outputs were 4 ohms as far as i remember. Post any numbers that are on the trans. and i can check the sams for maggies and probably give you all the specs. I seem to remember the some of the pp trans had 3.2 ohm secondaries
I may just say to heck with the turns ratios and find alternate uses for the transformers, such as a plate choke in the case of the SE transformers (like maybe on top of the husky triode half of a 6GF7), or in the case of the P-P transformers, use the primaries in an autotransformer phase splitter.
The brand and tube type can be confirmed. I kept the power transformers and the tubes from the amps. I did have a pair of 3.2 ohm 10 inch speakers with 8 ohm horns from an old console in the warehouse, so 3.2 ohms is a definite possibility.
Most wax coated transformers from old radios, TV's and HiFi sets that have been in south Florida for 50+ years look like that, but they usually work. I know the SE amp was working when I parted it out. I had it at work for a while connected to a PC and a set of plastic speakers. It sounded pretty good but the safety people really didn't like it and it had to go.
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Actually, the coil sections look ok, but the frames were kind of spotty - below decks duty for sure. Can't complain about the price, though... I'm more than half serious about using the push-pull units as autoformer interstage/phase splitters in my "Bursar" push-pull amp. The size is right. The thing that's holding me back starting that amp is the prospect of punching/drilling socket holes in the thick steel that Hammond uses for their utility boxes. Maybe I should invest in a hydraulic punch set - the one sold by Harbor Freight isn't all that expensive.
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