michael said:
I've got three of these, two with open primaries in the Peerless output transformers. This seems to be a common problem with the first version of this amp; the second, I believe, used a more robust design, also from Peerless, but these seem few and far between. Prohibitively expensive, too; I've seen a pair of these for not much less than a pair of amps! Your comments, please, on some good-souding alternatives, i.e., Chicago Transformer, Stancor, Thordarson, etc. as well as some of the Hammonds that are still in production. Something that won't require additional holes in the chassis--maybe a spacer--would be ideal.
Thanks,
Michael
The 16309 is the smaller physical version and the one I've seen most likely NG over the years. I even blew one up. The 16458 is noticeably larger.
IIRC, the primary for the 16309 is ~10K and that puts it right in the wheelhouse w/ the classic Williamson approach for triode-connected tetrodes such as the KT66 on the W5M except, the KT66 are not connected as triodes. They are connected Ultra-Linear (or non-Haflerese, ultralinear). That always struck me as odd. You would not want to match to this primary.
The later 16458 has a lower primary Z but I cannot recall it off-hand. I do remember that when I measured it the Z made sense in context of an ultralinear Williamson. I'd bet it was between 6.6K-7.6K. This is the Z-range I would try to approximate.
There are a lot of choices in the 25W range. You'd have trouble fitting anything larger. As it is, the 16458 is a snug fit on the W-5M chassis. A couple that should work are as follows. One is new and the other vintage.
The first is the Hammond 1650F (end-bell, 7.6K Z, 25W). I've never used these and I would solicit comments on their SQ. I've heard opinions on Hammond's OTs all over the map & really don't know what to believe. They do have the advantage of availability.
The second, is a real gem and a sleeper. The Stancor A-8072 is a nearly unknown, yet fabulous transformer. They are either 7.2K or 7.6K, ultralinear and 25W. They should just fit. If you can find them, it is the direction I'd go. I'd be hard pressed to tell any difference between them and the Peerless in this application.
There are also equally good OTs from Thordarson, Merit, Thermador, Halldorson, and so on but they are much rarer. You could wait years before you see a mate to the one you buy off eBay next week. Not so w/ the A-8072. Stancor must have sold a fair number of them. I've owned at least 8 pairs or so over the years. You should be able to snag a set for ~$120 if you are patient. No one looks for them (except me) the way people glom onto UTC so the demand is low.
Good Luck!
