pushing the "every other EL84" envelope

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pushing the "every other EL84" envelope

Postby vnowinski » Wed Feb 11, 2015 8:55 pm

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 (y) ), 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
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Re: pushing the "every other EL84" envelope

Postby cdmawolf » Thu Feb 12, 2015 11:43 am

While the transformers do have a wire in common the other end of the coil is not connected to anything. Some of the older amps were 3 channel(this was how recordings were actually made) and the third channel is not hooked up(or even 4). That is the way I see it anyway. [:)
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Re: pushing the "every other EL84" envelope

Postby vnowinski » Sat Feb 14, 2015 7:46 am

Thanks, Wolf. I have seen a few other console amps with a third channel (often for a bass unit). Haven't seen a Zenith with that configuration.

Curious what you're seeing when you say the coil is not connected to anything. There are two wires coming off the transformer with two windings: yellow (yel) and white (white). The yellow joins the middle winding on the other transformer, whose leads connect to the left ("white" tap) and right ("red" tap) taps. The white joins the third winding, which connects to the taps for the satellite speakers (yellow and grey). I've tested for continuity between red and white-- they are continuous. I've also put a signal on the inputs (labeled 10 and 12 on the power socket to the left of the schematic). Whether the signal is applied to 10 or 12, both the white and red taps from transformer T3 will drive an attached speaker.

The ST35 and other circuits with a similar topography seem to have independent, symmetric output transformers...?
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Re: pushing the "every other EL84" envelope

Postby paart » Tue Feb 17, 2015 7:29 pm

I’ve worked on a couple of these Zenith and Magnavox consoles a long time ago to help some folks get good sound from them. My son had a real nice one that he picked up for pocket change, but as of a couple years ago, I think it was placed in storage. From my experience, even though the speaker system is not sealed, most of them were quite good. Some of the consoles even used nice horns for midrange and HF drivers.

Many manufacturers (even Dynaco) tried to keep up with latest fads, and one of those fads was “more than” two channel systems. It appears you have one of those short-lived designs.

I think you are on the right track when you mentioned replacing the output transformers and turning the system into a more conventional design. All these consoles that I’ve seen have OTs that were of marginal quality anyway. The circuit is “not bad” but doesn’t compare to the Dyna ST-35, so if you really want to do a “super” job on it, I’d find a way to install Shannon’s DIY-35 board. The specs on the Zenith power transformer seem as if it’s a perfect fit. Short of that---decent pentode OT’s shouldn’t be hard to find.
If you haven’t done so, I’d replace the crossover caps with polypropylenes or mylars of the same value.

Something I find puzzling is that the 3 ½” speakers, which I would guess to be tweeters, don’t have series xover caps. It looks as if SP1 and SP3 (along with SP4 and SP6) are mislabeled on the diagram.
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Re: pushing the "every other EL84" envelope

Postby EWBrown » Tue Feb 17, 2015 7:33 pm

he labeling of the 12 inch and 3 1/2 inch speakers should be swapped. Else there would be a 3 1/2 inch woofer and a12 inch tweeter. *) =:o
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Re: pushing the "every other EL84" envelope

Postby vnowinski » Wed Feb 25, 2015 3:31 pm

You guys are correct :). There is a correct schematic for the speakers elsewhere in the photofacts-- although interestingly, the crossovers are not the same... one cabinet has twin 12" drivers, and in that one, there is no .22mfd on the "midrange;" instead, it shares the series 3mfd cap on the tweeter. Haven't looked at that speaker to see if that's accurate...!

Anyway, some additional investigation online and in the schematic reveals that the amplifier is designed to accept "sum and difference" signals to produce a stereo signal at the speakers. The preamp circuit does in fact blend the left and right channels before sending to the amplifier, and furthermore has an "extended stereo" control that varies the amount of out-of-phase signal incorporated, as near as I can tell. Wild stuff.

I'm likely to remove that from the preamp circuit. I found an interesting suggestion on how to leverage the existing transformers for a more traditional two channel stereo output from the amplifier; I'll post those changes after I've thought them through for comments here...!

-v
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Re: pushing the "every other EL84" envelope

Postby EWBrown » Wed Feb 25, 2015 6:02 pm

It's interesting to note that the two OPTs are different, not only the secondaries, but the primary of T2 is 7600 ohms A-A, and T3 is 6800 ohms. (???) *)
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