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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Thu 05, 2018 5:20 pm 
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questions:

since this is a stereo unit, why did they pull all the cathodes thru one resistor that became mighty hot ?

why didn't they use a cathode resistor for EACH channel (two PP tubes) instead of using one for both channels (two pairs of PP [4] tubes) ?

i know "stereowise" it does not matter.

all i can think of is that it was a cost saving matter by not using one more resistor and a capacitor.

also, why the 0.47 ohm resistor on the fils to the preamp box ?

steve


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 Post subject: Re: Motorola SK14M HiFi Stereo Restore Questions
PostPosted: Jul Sun 08, 2018 3:25 am 
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any answers to my questions above ?

in the meantime, I have another.

when the companion speaker is not plugged into the main unit via the 1/4 inch phone jack, the phone jack SHORTS OUT THE OUTPUT for the companion speaker RIGHT ACROSS THE OUTPUT TRANSFORMER !

there is a wire to the "switch" in the 1/4 inch female jack that definitely shorts out the right channel output when the companion speaker 1/4 jack not plugged in.

WHY would Motorola put a direct short across the output transformer when the companion speaker is not connected ? I understand that an amp needs a load on the output transformer to prevent all the technicals i don't need to list... BUT WHY A SHORT ?

installed an 8 ohm resistor so when the phone jack for the companion speaker is disconnected, the amp on that channel sees an 8 ohm load, NOT A SHORT across the output that Motorola designed... :|

I metered it all out and it definitely is a direct short when the companion speaker is not connected. it is also on the schematic as shown.

someone please tell me why Motorola shorted the output and did not put an 8 ohm load on the output like I did.

...terribly confused...

steve


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 Post subject: Re: Motorola SK14M HiFi Stereo Restore Questions
PostPosted: Jul Sun 08, 2018 3:48 am 
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Dutch Rabbit wrote:
any answers to my questions above ?

in the meantime, I have another.

when the companion speaker is not plugged into the main unit via the 1/4 inch phone jack, the phone jack SHORTS OUT THE OUTPUT for the companion speaker RIGHT ACROSS THE OUTPUT TRANSFORMER !

there is a wire to the "switch" in the 1/4 inch female jack that definitely shorts out the right channel output when the companion speaker 1/4 jack not plugged in.

WHY would Motorola put a direct short across the output transformer when the companion speaker is not connected ? I understand that an amp needs a load on the output transformer to prevent all the technicals i don't need to list... BUT WHY A SHORT ?

installed an 8 ohm resistor so when the phone jack for the companion speaker is disconnected, the amp on that channel sees an 8 ohm load, NOT A SHORT across the output that Motorola designed... :|

I metered it all out and it definitely is a direct short when the companion speaker is not connected. it is also on the schematic as shown.

someone please tell me why Motorola shorted the output and did not put an 8 ohm load on the output like I did.

...terribly confused...

steve


My Guess is that when the the console was new, it included the companion speaker. i guess that Motorola didn't anticipate that the end user was going to run it without the companion speaker.
also, it is better to short the secondary then leave it open circuit, because the spikes can cause more damage to the output tube & O. transformer, then if it had a load on it.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Sun 08, 2018 4:08 am 
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hi robert,

i have the companion speaker and i completely understand why one should not run any amp with no load, but a direct short instead of a load ?

never in my life in nearly 4 decades of this have i ever seen a shorted output be used as a "load" to prevent the spikes, transformer issues, and output tube issues.

i've seen loads put in place and understand why, but not a direct short.

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Sun 08, 2018 4:29 am 
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Dutch Rabbit wrote:
hi robert,

i have the companion speaker and i completely understand why one should not run any amp with no load, but a direct short instead of a load ?

never in my life in nearly 4 decades of this have i ever seen a shorted output be used as a "load" to prevent the spikes, transformer issues, and output tube issues.

i've seen loads put in place and understand why, but not a direct short.

steve


I know, it doesn't make sense why they did that, after all, it wouldn't have cost them much to add the resistor at the time of manufacture. This was a good question that is kinda impossible to get the answer for.

B.T.W. i got one of those Motorola amp's like yours, except that it does not have a jack for the external speaker, it has screw terminals. it also uses four 6BM8's (two for each L+R channel) & a pair of 6BQ5's for the bass channel.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Sun 08, 2018 2:15 pm 
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from the technical standpoint, i just can't, won't, and don't understand why they would short the output on an amp rather than giving it a load. that can't make things happy from the outputs to the transformer.

it just can't.

steve

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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jul Tue 10, 2018 2:37 pm 
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Dutch Rabbit wrote:
i must say, motorola had it over zenith, magnavox, and rca of that era. i've heard, owned, repaired many zenith, magnavox, and rca hifi sets. they sound good and i like them, but any motorola tube amp from this era was waaayy over the top of them in comparison.

steve


That's because Motorola used Jensen speakers.



Concerning the shorting of the output transformer it might just be a protection if the cord got accidentally unplugged and not meant to be run at all for any length of time without the speaker plugged in.

For the speakers I would suggest figuring out the impedance of the woofer and tweeters then seeing the frequency the tweeters are crossed over at and figuring what value of choke should be in series with the woofer so that it is properly crossed over.

Might make it sound even better as with the way it currently is the woofer will reproduce up to whatever maximum frequency it can and if the tweeters are crossed over below that frequency then the output will be louder in the range that both the woofers and tweeetrs are covering.

Of course they could have used the inductance of the woofer to determine the crossover point and found the 5uF cap is close enough to make the tweeters be crossed over at the right frequency.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 3:44 am 
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i beg to differ if i may :) .

in my opinon, it is about the jensen speakers. zenith, rca, and magnavox used excellent speakers as well. i love RCA's curved cone 12 inchers and the infamous zenith 12 inchers from the same style/era.

it is the motorola amplifiers what gives this the extra kick over the competition. the output transformers are much larger than average too.

the motorola 6V6 amps and this 6BQ5 amp has a lot more over the other brands of the era no matter what speaker system i have used them on, that being my allegro 3000's in the shop or the realistic mach ones in the shop.

calculating the xover frequency on the hf drivers is around 5k doing the math in my head. i'm sure the woof does not go much over that, and if it does, the woof's rolloff itself will be the xover point.

it sounds wonderful even without the companion speaker as i am working on its cabinet now. i never connected it yet.

i am not going to alter the design with a choke on the woofer. it will barely make any difference. i've tried it before on other vintage systems and it wasn't worth a 50 cent coil.

if i were to do so, i'd go second order, but i don't want to see "all that" in the cabinet.

this is such a nice sounding unit, it must stay as the original design intended.

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 3:51 am 
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does anyone know why Motorola pulled all four cathodes (two tubes per channel) thru one scorching hot 82 ohm resistor ?

couldn't they have pulled each channel thru a 39 ohm one ?

cost saving or was there another reason ?

I installed two 39 ten watters and a 4.7 ohm in series for the new "82 ohm cathode resistor" they are much cooler and everyone is much happier.

steve


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 4:53 am 
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Using two separate resistors would be an extra part.

Also if one were to separate the channels to their own cathode resistors which would be two 164 ohm resistors that would require a second bypass cap.

Reason I said the Jensen speakers is they often looked a little beefier than a lot of the vintage speakers I've seen from that era.

Yes the output transformers do make a huge difference.

Found that out with my RCA K-80 console when it sounded a little bass shy with its tiny push pull output transformer until I installed a larger one from a mid 50's Zenith console. Now it sounds like a console.

Often the smaller output transformers can be used when the speakers used can't reproduce the lower bass well under 50Hz far as frequency response goes. Using larger transformers there would be a waste.

Once had a pair of Magnavox 12" field coil speakers from a Magnavox AMP-101 push pull parallel 20 watt 6V6 amp which I used with a single ended pentode 6BQ5 stereo amp at 5WPC.

I upgraded the output transformers and while the amp could go loud with those efficient speakers the output transformer upgrade was pretty much a waste as the speakers could not reproduce good under 40Hz.


You'd be surprised at how much a choke in series with the woofer can make the sound better.

I have a 15" Jensen and three 5" Jensens from an Emerson console and the crossover is 6,284Hz which is determined by the 4uF cap. I calculated that I needed a 130.6uH choke for the woofer and using an inductance tester I made one by unwinding wire from another higher value choke.

It actually improved the sound.

I do agree with originality and most likely I would leave it original if it were mine if it sounded good as is.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 5:22 am 
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hi tube,

you have some great points, but i don't want to alter this :P .

as i''ve learned, a lot has to do with the "iron".

aah, double up on the cathode resistor instead of divide by 2. my mistake :oops: .

yes, all i can think of is that it was an extra resistor and perhaps an extra can b/c the 200 mfd cathode bypass was the fourth capacitor in the big FP can. perhaps that would have been a physical design problem that added more money.

i guess they had to skimp somewhere after using all that "iron" in the OPTs.

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 11:19 am 
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Yes would have meant another can.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 4:49 pm 
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Honestly I'd just leave it be. That Motorola designed it the way they did clearly works. I've never had an issue out of the 4-5 of these that I've owned or use. So instead of worrying it to death just assume that the engineers knew what they were doing.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 8:55 pm 
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not worrying, not altering, just curious

8)

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Wed 11, 2018 10:29 pm 
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You know what they say about curiosity and cats right?


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Thu 12, 2018 3:29 am 
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yep, they have nine lives :P

well, it is all done except for the turntable. both cabinets have been touched up, rubbed down, and look fantastic. the chassis have been shined up and so have the speaker baskets.

i hooked it all up tonight to the computer and separtated the main box and the companion speaker by about 8 feet in the shop.

the sound is very full, rich, robust, clear, and smooth. i played some deano thru it and it was like he was right here in the shop.

https://www.youtube.com/watch?v=ZbKhXEVrRko

it is top notch.

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Thu 12, 2018 11:52 am 
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Awesome.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Thu 12, 2018 2:52 pm 
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It was my understanding that sharing the cathode resistor would not only save parts, but would insure that all cathodes were at the same voltage, even if the tubes were not perfectly balanced. Could that have anything to do with it?


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Thu 12, 2018 7:01 pm 
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That is true, but for a stereo amp it is better to use two seaprate resistors one for each channel.


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 Post subject: Re: Motorola SK14M HiFi Stereo Questions
PostPosted: Jul Thu 12, 2018 9:36 pm 
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i installed a fuse on the primary of the power transformer just for extra protection to help with any catastrophic failure.

the entire unit, which would include the phono motor, is not fused, just the power transformer's primary AC input is fused.

at power up, this draws 800 mA, it drops to about 600 mA while warming up, and then slowly climbs to 800-850 mA.

when i installed a fast blow 1A fuse, i got a few days worth of "turn ons" with the 1 amp fuse until it blew.

shall i install a 1.25A fast blow, a 1.5A fast blow, or a 1.0A slow blow to give me the best protection ?

i know there is much controversy about how much a fuse will protect in instances like this and that is not an argument i want to start, but this is nothing more than a extra mode on saving a power transformer if something messes the bed.

many amps that i've seen/had used a fuse in the primary to the transformer.

so, by the looks of my numbers above, what fuse style/value will provide the most protection, but at the same time will not unnecessairly "blow" at (after a few days of) turn on ?

thanks.

steve

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