separks wrote:I'm planning on making a 1625 ST70 which I'm dubbing the JAN70. I was thinking the 4.3K impedance would be too low for 1625s, so I was going to use the 4 ohm tap (for 8 ohm loads) to get a better match. At almost 6K, this seems much better. Obviously I will test at the top level and compare there, but I'm trying to do a better job of evaluating iron.
For 1625s and 807s, what has been claimed for a long time to be the optimal input impedance is 10K p-p.
http://www.retrovox.com.au/A515.pdfFor UL use, an audio transformer with a tertiary screen tap winding ratio of 50% is said to be the best for 1625s or 807s.
http://oestex.com/tubes/ulo.htmlseparks wrote:FWIW, it certainly seems as though the cloth lead and plastic lead trannies are functionally identical. My cloth leaded ones are definitely early - like around 1960 - due to the style of the original kit.
The cloth ones (part number: 464006) were hand-wound in Philly; The plastic ones (part numbers: 454326 and Z-326) were machine-wound in Japan.
The st-70 had the same amp design from its birth to its death, so I expect the engineers wanted the outputs to be "functionally identical", but maybe the switch in fab processes altered some of the characteristics of the outputs (differences in input impedance).