HBP do you happen to remember where to find PRR's bit about using a Champ for testing tubes other than what it comes with? I'd certainly be interested in reading that.
If you enter "Champ tube tester" in the Search field, and search the entire forum, you come up with a bunch of threads talking about the concept:
What tube tester to buy?preamp tube testing chassis"Champ Checker" built - need input about performance12AX7 A/BWhats a good tube tester thats affordable... and why?A word on Tube TestersQuestion about an EICO 667/tube testerI'm not sure anyone's put together a "How-To" manual for interpreting readings in it, but the better approach is for me to ask you what you need to know about the tube's you'd be testing.
As to impedance, you're right, I haven't absolutely needed to find one yet, I've been able to use a workaround. It would make a quick way to decide what to build with some of these old transformers I have. Most of my I/O 'impedance matching' is guesswork ...
First "tool" to use is good documentation of what you took the transformer out of, and what leads where connected where (like which marked output taps, etc). Labels on every wire marking where it went (like "output tube plate" or "4Ω Tap") would be a good plan.
Second tool is to use fundamental transformer properties and measure primary voltage when a known voltage is applied to the secondary.
The transformer primary and secondary have a set ratio of turns of wire, like "X turns primary to Y turns secondary." The transformer's
volts ratio is directly proportional to the turns ratio; the transformer's
impedance ratio is proportional to the
square of the turns ratio. Knowing this, if you know the intended load for a secondary tap, you could calculate the primary impedance of the transformer.
Say you have a transformer of unknown primary impedance, but you know one secondary tap is for an 8Ω load. Let's also say you have 6.3vac handy to apply to the secondary. Connect a voltmeter's leads to the outer ends of the primary (this hypothetical
push-pull OT also has a center-tap, but you documented that when you yanked it from the donor chassis). Attach the 6.3vac to the 8Ω and Common (aka Ground, etc) tap of the secondary. Apply power.
Let's say after doing this you get ~200vac measured on the primary. Now it's math time. 200v/6.3v = ~31.75 to 1 (or 31.75v on the primary per 1v on the secondary); this is our Turns Ratio, and also our Volts Ratio. Now square each term to get the Impedance Ratio: 31.75
2 : 1
2 = 1008 : 1. Multiply each term by the correct load for that secondary tap: 1008*8 = 8064, 1*8 = 8 -> 8064Ω:8Ω.
The correct answer is 8kΩ:8Ω, because there was probably slight measurement and/or rounding error. So now you would know this OT is probably suitable for 2x 6V6's or EL84's with an 8Ω load attached to the secondary. Other conclusions are possible, but rely on your experience somewhat (like "this OT is as-heavy as a 50-60w OT; maybe I guessed 8Ω incorrectly and it's really 4kΩ:4Ω for a pair of 6L6's or EL34's").
Directly measuring impedance of an OT primary would give confusing results. That's because while their is a characteristic impedance due to primary inductance (measured with the secondary open and not connected to anything), the transformer really uses the reflected impedance due to a load connected to the secondary. So the voltage measurement approach above actually yields the useful result you want to know.
Topic: Princeton Reverb Turret Board problem (Read 19993 times)