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1961 Motorola Console SK56MB-1A Rebuild

now you're into the realm of needing hands on experience with these, and thats something I do not have. I'm working purely on theory and guesswork here. I don't know precisely what the channels are capable of.

sticking a bigger transformer in without doing anything else will do nothing for max power output at mid band. It may improve performance at the bottom end if the original transformer was saturating.

Power isn't quite directly related to transformer size but there is some relationship. More power needs more current and/or more voltage. More current needs fatter wire, which takes up more space. More voltage needs more insulation which also eats space. It is possible to make a lot of power with fairly small transformers, but they will be frequency limited. Low frequency performance needs a fair bit of inductance, which tends to mean more iron in the core so it doesn't saturate. Single ended amps need even more iron because they don't have the benefit of a self-cancelling magnetic field.

so the left and right transformers on the Motorola can be small mostly because they don't have to deal with any bass.
 
gadget73 said:
now you're into the realm of needing hands on experience with these, and thats something I do not have. I'm working purely on theory and guesswork here. I don't know precisely what the channels are capable of.

sticking a bigger transformer in without doing anything else will do nothing for max power output at mid band. It may improve performance at the bottom end if the original transformer was saturating.

Power isn't quite directly related to transformer size but there is some relationship. More power needs more current and/or more voltage. More current needs fatter wire, which takes up more space. More voltage needs more insulation which also eats space. It is possible to make a lot of power with fairly small transformers, but they will be frequency limited. Low frequency performance needs a fair bit of inductance, which tends to mean more iron in the core so it doesn't saturate. Single ended amps need even more iron because they don't have the benefit of a self-cancelling magnetic field.

so the left and right transformers on the Motorola can be small mostly because they don't have to deal with any bass.
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So if I want to do something like I did with my 185, tear out everything but the tubes, transformers, and choke, I'd need either a circuit that could deliver full-spectrum sound with the existing OPTs. Alternatively, I'd need to also replace the OPTs.

At some point I did a Google search for how to increase the power output of a tube amp. The short answer is that I found no short answer, so I'm still kind of in the dark. Solid state is, on this point at least, more straightforward: jack up B+ and B-, sub in hunkier output transistors, and get a bigger heat sink.

But your assessment is helpful, so thanks for that. And for not pretending to know what you don't know! That's always pretty awesome.
 
yeah, all you need to do is take everything, throw it away, and do something different :) The basic circuit is probably OK, its just the filtering upstream of it. If you've got a schematic I can be more specific, but without having something to look at I don't know precisely what parts are there. I've seen them before though and its not incredibly complicated. You still just have the limitations of the transformers.

Kinda why I never messed with these. They're really designed to be what they were used for originally, it just doesn't make a lot of sense (IMO) to try and make a 3 channel amp into a 2 channel. Easier to start with a 2 channel thats already well on the way to doing what you want. By the time you replace all of the parts and plug a bunch of chassis holes you could have spent a little more money and saved a lot of effort.

the way you make more power with a tube amp is kinda the same as with solid state, more voltage and/or more current. Watts are still volts * amps. Hanging with a 6V6 for example, practically speaking you're not getting much more than about a dozen watts from a pair without exceeding their operational limits. You can parallel a set though. Keeps the same voltage, half the plate to plate impedance on the transformer, and you'll be at 2x the power at the speaker jack because the parallel tubes are delivering double the current. Assuming the transformer can handle it of course.
 
gadget73 said:
yeah, all you need to do is take everything, throw it away, and do something different :) The basic circuit is probably OK, its just the filtering upstream of it. If you've got a schematic I can be more specific, but without having something to look at I don't know precisely what parts are there. I've seen them before though and its not incredibly complicated. You still just have the limitations of the transformers.

Kinda why I never messed with these. They're really designed to be what they were used for originally, it just doesn't make a lot of sense (IMO) to try and make a 3 channel amp into a 2 channel. Easier to start with a 2 channel thats already well on the way to doing what you want. By the time you replace all of the parts and plug a bunch of chassis holes you could have spent a little more money and saved a lot of effort.

the way you make more power with a tube amp is kinda the same as with solid state, more voltage and/or more current. Watts are still volts * amps. Hanging with a 6V6 for example, practically speaking you're not getting much more than about a dozen watts from a pair without exceeding their operational limits. You can parallel a set though. Keeps the same voltage, half the plate to plate impedance on the transformer, and you'll be at 2x the power at the speaker jack because the parallel tubes are delivering double the current. Assuming the transformer can handle it of course.
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Wise words RE: the Motorola chassis. Explains why you see a fair number of overhauled Magnavox amps but rarely Motorola. I've already got a second Magnavox chassis waiting in the wings. And I'm still working through my pre-amp design. No need to rush to get another chassis that's not really what I want. I could go on about the pre-amp, but I should probably start a new thread for that.
And thanks for helping me to understand tube amp power. Makes sense now when I think about an old Fender with a row of 6V6's for one channel.
 
Geomorphdog said:
Anything to report? I'm going to look at one of these in a few days. I like @gadget73 's idea (as I typically do) of running it as a 2.1, whether inside the console or not. Did you decide to restore the console and keep it intact? I think I'm more inclined to want to gut the console. Not much demand for consoles these days if I want to sell it at some point. I'm also inclined to gut the circuitry, make the L/R channels full range, and keep the center channel with a low pass, although maybe lower as appropriate for a subwoofer.

And you mentioned schematics. Any chance you could share those or point me to where you got them?
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I went on vacation, but restoring it. I got the schematics at https://www.radiomuseum.org/. I had used just the amp with two channels a dac and external speakers and it sounded great, but I plan on fixing issue with pre-amp and vibra sonic. I’ll update once I get back to things. Have some spring items to take care of for the wife.
Eric
 
Eric Butler said:
I went on vacation, but restoring it. I got the schematics at https://www.radiomuseum.org/. I had used just the amp with two channels a dac and external speakers and it sounded great, but I plan on fixing issue with pre-amp and vibra sonic. I’ll update once I get back to things. Have some spring items to take care of for the wife.
Eric
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Hey Eric,
Any updates? I’ve got one of these 3ch amps and I’m try to decide whether to invest the money into it or sell it. I’ve got the original “bass woofer” but the hf speaker were not salvageable. I do have a pair of Magnavox and zenith full range that I think might work for that. Anyway it’s probably not something I should mess with, I don’t really even understand how to get signal to the “center/sub” channel without using the original preamp which I don’t think I’d want to do. Just curious if you got it all worked out and the sound was….GLORIOUS!

Mark
 
need a schematic to see how they do it. Some of these have the crossover on the amp chassis and its just got an L and R input. This one looks to have 3 input jacks though? If so my guess is the crossover is on the preamp. Likely not a big issue to duplicate that circuit in the power amp. Others I've seen just involved a few resistors and capacitors to get it all done, or sometimes it may have involved a coil. None of it is impossible to reproduce.
 
From what I have seen, I think the one with the 3 input jacks incorporates the reverb unit called vibrasonic. My 3 channel console amp just has the standard 2 (left & right) input jacks and no vibrasonic. It's all stock as it is used in the original console.
 
Yes, the one I have had 3 inputs and one was running to the reverb unit. That output transformer is larger than the other two.
 
Amp schematic attached. The bulbs limit (compress) the reverb input, reducing the effect at higher levels.
 

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so the amp is designed basically like the others. If you just feed in the L and R, the bass channel does sum them and runs through a low pass filter. The center channel input bypasses the filters, guess thats where the reverb comes in. If you want to run it stand-alone, just connecting the L and R inputs ought to get you proper output with all 3 channels like the other Motorola 3 channel rigs get.
 
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