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Magnavox 1956 Provincial Serenade?

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Post by curbsidefinds Tue Apr 14, 2015 10:21 pm

My fall find was a 1956 Magnavox Provincial Serenade console (278 B). The power amplifier is the AMP-149. This is NOT a biamplified device, as the same model was in 1957 (uses a different amplifier), but it does have separate output transformers for the 12" woofers and for the horn tweeter. Both are connected to the same 4 output tubes, 6V6s in parallel push-pull.

My problem is that the horn wiring plug has three wires, and the horn has (of course) two terminals. The third wire was disconnected. The black and green wires were connected to the horn. They go directly to the output transformer. The third wire, yellow, is a 12 ohm, 1 watt device connected to the green wire under the chassis. If I connected it to the black wire terminal of the horn, it puts the resistor in parallel with the horn's voice coil. If I connected it to the green wire, it's shorted out and does nothing.

If Magnavox wanted to parallel the horn tweeter with a resistor, why didn't they just do it under the chassis and avoid one extra wire running to the horn? On the other hand, connecting the yellow wire to the black one at the horn is the only action that makes sense. The Sams schematic for the AMP 149 only shows wiring up to the speaker plug on the chassis. I need somebody with a 1956 Provincial Serenade at look at how the wiring from your tweeter plug is directed and then to tell me about it, please.

Thank you,
Chris Campbell

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Post by Wildcat445 Tue Apr 14, 2015 11:05 pm

I am not as familiar with mono Magnavox instruments as I am with the stereo ones, but I will offer my thoughts, for what they may be worth.

The statements "It does have seperate output transformers for the 12" woofers and for the horn tweeter", and "This is NOT a biamplified device" are not totally congruous in concept.  It sounds to me like this is a dual-frequency amplifier of some kind with three 6V6's for the woofer and one 6V6 for the horn. This would be borne out by the fact that Magnvox utilized dual frequency amplifiers in its topline instruments and a "normal" amplifier would not have seperate output transformers otherwise.  My guess, based on Magnavox pratice with which I am familiar would be that the yellow disconnected wire might possibly go the the external speaker wiring somehow.  There should be a wire from the positive on the horn, a wire from the positive on the woofer and a common negative for a total of three wires for the external speaker.  I will try to come up with a schematic in the effort of offering more positive comments.   I hope this somewhat helps and good luck.

Edit.....It is a good thing that I don't mind being all wet occasionally.  I did not come up with a schematic, but I did find out that there may have been a stereo gizmo of some kind available for this instrument.  It is possible that the stray wire may be a part of that setup.  I don't see an external speaker connection, per se, so I have no idea for certain what the stray wire is for.  I still feel you have some kind of dual-frequency amplifier, but I have been wrong before.  Good luck.

WC


Last edited by Wildcat445 on Tue Apr 14, 2015 11:23 pm; edited 1 time in total

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Post by curbsidefinds Tue Apr 14, 2015 11:22 pm

I've got the Sams schematic, and the AMP-149 clearly has all four 6V6 output tubes in parallel push-pull, and both output transformers connected to all 4 of them. The woofer transformer has two iron-core chokes in series with its primary winding. It's not clear why Magnavox chose to use two transformers when it seems that one wold have been just fine. Maybe it's so they could use two big 12" speakers in parallel--a pretty low impedance--and the Jensen horn tweeter, which is probably 8 or 16 ohms.

The 6V6s on each side have little air-core chokes--those curled wires you'll see in photos--between the plates. Wish I knew more about audio engineering.

Chris Campbell

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Post by Wildcat445 Tue Apr 14, 2015 11:37 pm

Would you be good enough to post a schematic of the AMP-149?

The iron core chokes (at least in the stereo amps) are referred to as "tone chokes" and serve the basic function as capacitors in a typical crossover network.  The air-core chokes on the plates of the 6V6's are also on my Concert Grand.  I have yet to figure out what they do.  The tone chokes are not in series with the output transformer on the 8201 chassis on the CG, however.  This is interesting from the standpoint of some of the research I am currently engaged in.  The AMP-149 chassis and its bretheren are renowned for robust bass response.  The stereo dual-frequency amps are criticized for a LACK of bass response.  I would like to see how yours is wired and compare it to a stereo amp.  I know squat about audio engineering, but there are guys out there who do.  I am just trying to make the silly things perform like they should, trying to figure out why mine does not.  I would like to see how the transformers are wired.

WC

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Post by curbsidefinds Wed Apr 15, 2015 1:44 am

You can see the schematic for the AMP-149 here: http://vintagehifi.net/index.php?action=dldir;sa=details;lid=163. I scanned the Sams schematic. If that doesn't work, I'll try to figure out how to post it here. Let me know.

I've also got a 1956 Magnasonic, but it's in storage and I don't have the amplifier number at hand. That one uses a single output transformer but uses the good ol' four 6V6 configuration that Magnavox loved. It has the air-core chokes between the plates. I'm guessing that they were there to suppress oscillation or something.

Chris Campbell

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Post by 75X11 Wed Apr 15, 2015 5:46 am

Here is a short guide for posting pics and images that might be of help. Our system is a little different but pretty simple.

https://www.tuberadioforum.com/t4163-hosting-then-posting-pictures
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Post by Wildcat445 Wed Apr 15, 2015 4:25 pm

I got an error message.  It might be best to scan the document onto your computer than post it per 75's instruction.

WC

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Post by Wildcat445 Wed Apr 15, 2015 7:01 pm

I downloaded the file.  Let me see if I can post it.

NOpe.  My "file type is not allowed".  Goody.

I looked at the schematic.  The AMP-149 is a strange setup, among myriad strange setups by Magnavox.  Referring to memory, it appears that all six 6V6's are paralleled for a push-pull arrangment.  The input is transformer coupled to a 6C4 preamp.  The transformer serves as the phase inverter for the push pull stage.  Both the bass and treble channels are connected to the same six tubes.  It appears that C10 and C11 serve as the only crossover network that I see.  The "tone chokes" in series with the audio output transformers serve the same purpose as those in stereo amps, only the stereo ones are ahead of the 6V6's rather than after.  I still believe that this amp is a dual frequency, compound frequency, divided frequency or whatever.  The schematic did not show the wires on the speakers, per se.  It only shows up to the speaker plugs.  I am certainly willing to admit to being wrong if anybody has a different or better interpretation of the schematic.  The AMP-149 is very similar to the later stereo amps, only some of the components are located differently and it is mono.  I'm not sure if this helps the OP any, but it brings up an interesting topic.

WC

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Post by Wildcat445 Wed Apr 15, 2015 7:29 pm

This system will not allow me to post a PDf file.  What am I doing wrong?  I am almost sure I have done this before, but you know me...... Rolling Eyes

WC

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Post by 75X11 Wed Apr 15, 2015 7:36 pm

Allowed formats : bmp, jpg, png, gif, tif < 2 Mo

So says the hosting service. Did you post a link to the PDF file?
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Post by Bill Cahill Wed Apr 15, 2015 8:35 pm

curbsidefinds wrote:I've got the Sams schematic, and the AMP-149 clearly has all four 6V6 output tubes in parallel push-pull, and both output transformers connected to all 4 of them.  The woofer transformer has two iron-core chokes in series with its primary winding.  It's not clear why Magnavox chose to use two transformers when it seems that one wold have been just fine.  Maybe it's so they could use two big 12" speakers in parallel--a pretty low impedance--and the Jensen horn tweeter, which is probably 8 or 16 ohms.

The 6V6s on each side have little air-core chokes--those curled wires you'll see in photos--between the plates.  Wish I knew more about audio engineering.

Chris Campbell

Those chokes were used on the tweeters. Not the woofer. And, there should be a capacitor block somewhere in there that the horn would connect with.
I have the 1947 version with a 15" woofer, a four inch mid range, and, a 4" tweeter. This is in a large "Belvedere" style cabinet with a two speed changer, a 12" round tube tv, and, four 6V6 output tubes in push pull parallel.

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Post by Bill Cahill Wed Apr 15, 2015 8:40 pm

Hmm. Didn't see your post. I'll shut up here. Too many things done by Magnavox for me to understand. And, yes, my unit also has an am fm sw radio. Modern fm. But, get this. NO eye tube!!!!
6J6 FM oscillator.

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Post by curbsidefinds Wed Apr 15, 2015 8:51 pm

OK, I'm looking at the Sams AMP 149 schematic. The iron-core chokes are clearly in series with the bass speaker output transformer primary (its secondary goes to the "bass speaker" plug). The tweeter output transformer has C 11, a .01 mfd. 600 V capacitor in series with its primary. There are 4 6V6 output tubes, not 6. The 6-tube model was the 1957 version that used a different amplifier (two more output tubes for biamplification--separate output tubes for tweeter and woofer). The biamped models also had a pot on the amplifier chassis to adjust output levels.

My AMP 149 is sitting on the kitchen floor, about 20 feet from me, waiting for me to replace the electrolytic caps.

Chris Campbell

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Post by 75X11 Wed Apr 15, 2015 9:05 pm

So you use multitasking rooms for your electronic work, too? Smile
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Post by curbsidefinds Wed Apr 15, 2015 9:19 pm

When I moved here from Lansing, Michigan's capital, there was an interesting price differential for real estate. I found my house after a year and called it "the small house with the large price." It's still small--I don't have a nice electronics bench, just floor space here and there or the dining room table with cardboard over it. When I retire I will work on the unfinished back room and include some bench space so the collection of old radios and such can get the attention it deserves.

The dining room floor has an old RME 84 communications receiver waiting for diagnosis. Now winter is ending and summer projects take over.

Chris Campbell

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Post by 75X11 Wed Apr 15, 2015 9:29 pm

I know real estate along the shore and nearby comes mighty dear. It's beautiful country!
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Post by Wildcat445 Wed Apr 15, 2015 9:39 pm

I mis-typed.  I put 6 instead of four.  My bad.  Rolling Eyes

I have worked on so many different instruments lately that it is sometimes hard to keep each feature straight.  I apologize for the confusion.

I'm not 100% certain just how this thing works, but I believe it goes something like this.  All FOUR 6V6's are amplifiers.  The divided frequency part comes after the output tubes.  The two chokes in series with the bass channel output transformer and C10 and C11 are essentially the crossover network, seperating the bass amplifier from the treble.  Instead of using totally different tubes ala the later stereo versions, the treble channel is split off part of the output.  The RF chokes in series with the 6V6 plates serve to eliminate feedback.

I'm not sure what your question is, so I'm equally unsure whether I helped or not.

Bill, I think your instrument is similar but still different in certain features yet.  Magnavox instruments are not easily diagnosed.  They are very similar, but different in detail, sometimes remarkedly so.  I believe that it is fun to learn and discuss the different features.

75, how do I post a link to the PDF file?

WC

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Post by Bill Cahill Wed Apr 15, 2015 9:52 pm

I like the extended bass, treble, and, loudness switches. Also has ARRL scratch filter..... The sound is fantastic!!!!

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Post by Wildcat445 Wed Apr 15, 2015 9:58 pm

That has been my impression as well.  The later stereo models like I collect are not afflicted with that malady.  Most of them sound like transistor radios, considering the exotic electronics they use.  I'm trying to figure out why.  This pertains to the dual frequency, compound frequency, divided frequency amplifiers in top line instruments.  The 8800 and 9300 series amps in the "lesser" instruments live up to the hype.  Just like the older ones did, like the one you've got.  I am simply not impressed with the performance of the top line Magnavox stereo instruments with divided frequency amplifiers.

WC

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Post by 75X11 Wed Apr 15, 2015 10:05 pm

WC, are you looking to post a pdf you created or one from another source? I have only done so with files I've created. There is a separate pdf hosting service that works somewhat like the host here.
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Post by Wildcat445 Thu Apr 16, 2015 12:30 am

One I got from somewhere else.  I could print it, then scan it, then send it as an attachment.  Wow!  What a pain for a schematic.  Is this something what you had in mind?  I need to figure this out.

WC

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Post by 75X11 Thu Apr 16, 2015 1:50 am

If you have a hard copy, you should be able to scan it and have the scan as a .gif image then you could handle it like any other picture.
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Post by neali Sat Apr 18, 2015 4:05 pm

https://app.box.com/s/ww5alq8qge8ocrnksu7faamoqzpwbsmh

That should clear up the confusion.

It is a single PPP amplifier with a low pass filter to the bass driver and a high pass filter to horn. Very weird departure from their typical biamp stuff.

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Post by Wildcat445 Sat Apr 18, 2015 5:01 pm

Thanks, Neali. I hadn't seen anything like it before.

WC

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