Okay, tube mavens, be gentle :).
I have been rehabilitating an old (1959) Zenith console, SFD288. To date I have replaced all the capacitors in the amplifier and preamplifier/tuner, as well as resistors that had drifted well out of specification, using the following Sam's photofacts as my guide:
https://app.box.com/s/jnjt1ahxrap1nqb9nx00
I am not an engineer, but I can follow a schematic (given enough time :) ), am handy with a soldering iron, and have built a few components (Foreplay preamp, several headphone amplifiers) and speakers over the years. I fell in love with the Zenith and I'm going to bring this fantastic old MCM console back to life (with a few updates here and there, most likely).
The amplifier is now performing well, and is surprisingly quiet (the tuner is another matter, currently hunting down a hum in one channel).
However, its design is a bit of a puzzle. If you look on page 9 of the link above, one interesting "feature" is that while it has a 2 channel PP topography, with ECC83 drivers and EL84s, the output transformers are "tied" together (note the "yellow" wire connecting the coils of the two). As a result, input from either channel is sent to both speakers. I'm a little flummoxed about why anyone would design the circuit this way... There are probably folks who recognize it instantly (and the intentions behind the design). The additional taps on the second transformer feed additional speakers that could be purchased (surround sound ), see page 4, perhaps that has something to do with it.
I am considering replacing one or more of the output transformers and making the channels independent-- doesn't seem to make sense to have stereo input otherwise?
I am also wondering if there are simple modifications I could make to bring its performance up a bit. There is room in the chassis to do so, though I'd like to keep the changes manageable considering my skill level and my limited knowledge of how these circuits actually work :).
I'm looking for advice on how to do both. This seemed like a crowd that might have a thought or two on the topic...!
thanks!
-vince