I hauled in a bunch of push-pull audio transformers to test on my AC sine wave source at work. I generally test using a 50V, 300Hz sine wave. I monitor the output with a DVM set on AC, and look at the exciting current with a Yokogawa WT210 power analyzer that is also hooked up to the sine source. That way, I can determine turns ratio and from the exciting current, I can calculate primary inductance. I had some unknown transformers that looked good for ~40W (on the hefty side) that tested out at around 6k:8, a pair of "7k:8" transformers that tested at around 6.6k:8, some Baldwin organ transformers that tested out at ~3k:8 (probably wound for 16 ohm speakers), and some bitty transformers that tested at around 10k:8, with not a lot of primary inductance. Next weekend, I'll bring in some Wurlitzer P-P transformers that came off of an amplifier that used 7868 outputs. Fun times ahead, as I figure out how to purpose all this iron...
I think I'm not the only one to have had a semi-failure with this one and I think it's a good case study.
I know we won't make a racehorse out of a donkey but it's a very good learning ground.
I know we won't make a racehorse out of a donkey but it's a very good learning ground.
I just found my pair of Electra-Print 3k P-P transformers (yes, I tested one and it is pretty close to 3k:8 impedance, with 59H primary inductance). I wonder what to pair with these - matched 300Bs would be a bit rich for my blood, but I do have some 6C4C triodes that might be suitable. Matching the 6C4Cs might be a bit of a pain, and I wonder if I have enough to come up with two matched pairs. I bought them back when they were still cheap, and the prices have gone way up since then, though not anywhere close to 300B territory.
The Wurlitzer P-P transformers tested out at 5482:8, with ~ 65H of primary inductance. They may be a nice fit for a 6LR8 P-P amp I'm building, with proprietary autobias/balance and a folded cascode front end.
With the Electra-Print 3k:8 transformer, I may try some trioded 18GB5s. If you take the spec seriously, they require a pretty hefty screen resistor to prevent the screen from being damaged while the amp is warming up. For 230V plate voltage, the required screen resistor is 2k2. I may try 250V B+ trioded with a 2k7 screen resistor.