OT Test - a Happy Accident

NTC

Tele-Afflicted
Joined
Mar 17, 2003
Posts
1,203
Back in about 1999 or 2000, I built my 5E3 clone. I made it with transformers I found online that were supposed to be for a Deluxe Reverb. Conventional wisdom on OT's at the time was "6V6 amps require twice the plate-to-plate load impedance as 6L6's" and "The transformers for a 5E3 Tweed Deluxe and a Deluxe Reverb are the same." At least, that was the case in the circles I was traveling in. The voltages came out very close to the schematic values, and the amp sounds good and breaks up very early. Of course, we now have better information and a Deluxe Reverb uses a 6.6K primary, while a tweed Deluxe is supposed to have an 8K primary, both with 8 ohm loads. I have worried (well thought about) this ever since. I even bought a Hammond 1750E OT, though balked at making the replacement due to the size of the Hammond - it is MUCH smaller than the OT in the amp.

Yesterday, the alarm system in my house was updated with a new controller. The power supply for the old alarm is a wall-wart with a rated output of 16.5Vac and screw terminals on the output - perfect for making transformer turns ratio tests. I tore into the 5E3 today and measured the input and output voltages of the OT with the tubes and speaker removed. The calculated impedance ratio came out to 8.77Kohm:8ohms - certainly in the tweed ballpark.

While I was at it, I measure one of the two SE OT's I bought back then that were advertised as being for a "Tweed Princeton" that came out to about 5.7K:8ohms. Again ,just about perfect. These are probably too small for a 6L6 amp, though they were advertised as being appropriate for that. They are about the size (and close the impedance of) a Hammond 1750B, which supposedly is a reverb driver.

Thank you to 2LMan for repeatedly suggesting this method. I have put that little wall-wart aside with a label stating it is for OT testing.
 

King Fan

Poster Extraordinaire
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Joined
Jan 1, 2013
Posts
8,164
Location
Salt Lake City
We see *so* many threads where people got a transformer that ended up being wrong, it's refreshing to see one where some 'wrong' transformers ended up being right.
 




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