1959 Magnavox mini Trapezoid - Videokarma.org TV - Video

Teron · #1 05-11-2008, 09:50 PM

Acquired this sometime ago.

Looking to use it as an iPod amp or otherwise. Legs are no longer attached. They did not survive storage well. Help wasn't aware stacking heavy boxes not welcome on this...

Unlugged the record player hooked up iPod shuffle. Had to turn on record player to turn on amp. Sounds pretty decent.

Diagram on back seems to me that the red/yellow/black wire goes to the record player start/stop switch. Haven't investigated further.

Best suggestion to put a switch on the amp directly?

Red/Yellow connected turns it on, Red/Yellow disconnected stops?

Thinking about keeping the grill boards intact with speakers and using them open baffle.

Also thinking the speakers are coaxials?

I could in theory use a crossover and drivers from a dismantled box set and just run the outputs to speaker from amp to my crossovers?
I'm assuming there is no crossover in this model? Haven't torn it apart completely yet. But will soon.

Suggestions welcome to properly get this amp to work without the record player.
Number hidden behind red/yellow/black wires is a 9 so it's 6 50 9 other number is 80-02-00

Speakers are decent? Jensens or no?

Best guess on model number is 180261E or 180261F or 1?0261 E 1?0261 F where ? = cannot identify.

Channel 1 is L, Channel 2 is R? Black is Left, Green is Right?

If so I've got it backwards...

Thanks!

Captain Scary · #2 05-11-2008, 09:56 PM

Sorry I can't answer your questions,
But that is way cool........

truetone36 · #3 05-11-2008, 10:21 PM

Trace the wires that lead to the switch on the bottom of the turntable. You should be able to cut them and put a switch on them. I've seen toggle switches used in this way. BTW, what are you gonna do with the TT?

Teron · #4 05-11-2008, 11:34 PM

Quote:

Originally Posted by truetone36

Trace the wires that lead to the switch on the bottom of the turntable. You should be able to cut them and put a switch on them. I've seen toggle switches used in this way. BTW, what are you gonna do with the TT?

Good suggestion on the wires. I know they go elsewhere and the switch moves lever/gears to do something in another location. Have to finish dismantling it all. I sent you a PM on my plans for the TT - can't break AK rules.

Jeffhs · #5 05-12-2008, 12:54 AM

Your Magnavox console should sound great with those 50EH5s in the output stages. My Zenith MJ1035 FM stereo radio also has 50EH5s in its output stages (one per channel), with a 12AX7 preamp, and sounds absolutely fantastic, even though the volume control is intermittent as all get out (if I pull outward on the control shaft I get more volume than I'll ever need, although the radio plays at normal listening volume with the control set where it is at the moment--so I won't do any more with it or to it until or unless something goes wrong, like a tube filament burning out).

In case you ever need to replace one or both of the output tubes (or any tube, for that matter) in your console, be sure to replace them with the same type Magnavox put in there in the first place, when they designed the unit. The 50C5 may work in a pinch as a substitute, but I wouldn't count on it; it is best to use 50EH5s to replace 50EH5s. Another good thing about the 50EH5 is that it is actually cheaper to replace than a 50C5, as I found out from an AK member (electroking, in Montreal) recently, in response to an inquiry I had made about substituting 50C5s for 50EH5s. I found this rather odd, as I had thought it would be the other way around; that is, the 50C5 would be cheaper than the 50EH5.

GordonW · #6 05-12-2008, 06:25 AM

I also have one of those!

I haven't gotten around to doing anything with it yet... but one concern is that this is one of those unit that has a LIVE CHASSIS... i.e, no power transformer. You need to exercise extreme care in connecting other out-board things to this... the danger for electrical shock is pretty high!

I was thinking of modding mine, with a power transformer... depending on which one I pick, it might wind up "boosting" the output a bit. Would be interesting to see what I could get out of it, changing over to 6AQ5 output tubes... but, that's a project that I will have to get around to, when I get around to it...

Regards,
Gordon.

Teron · #7 05-12-2008, 02:18 PM

Quote:

Originally Posted by GordonW

I also have one of those!

I haven't gotten around to doing anything with it yet... but one concern is that this is one of those unit that has a LIVE CHASSIS... i.e, no power transformer. You need to exercise extreme care in connecting other out-board things to this... the danger for electrical shock is pretty high!

I was thinking of modding mine, with a power transformer... depending on which one I pick, it might wind up "boosting" the output a bit. Would be interesting to see what I could get out of it, changing over to 6AQ5 output tubes... but, that's a project that I will have to get around to, when I get around to it...

Regards,
Gordon.

Now that's some useful information. Is there a way to remedy that live chassis problem easily? Is it safe to assume that many of these Console tube units are live as well? Suggestions to remedy this are welcome.

Luckily I've learned long ago connections/tinkering is best done with power off, and also with devices not plugged in for further assurances that all is relatively safe.

radiotvnut · #8 05-12-2008, 02:39 PM

Quote:

Originally Posted by Teron

Now that's some useful information. Is there a way to remedy that live chassis problem easily? Is it safe to assume that many of these Console tube units are live as well? Suggestions to remedy this are welcome.

Luckily I've learned long ago connections/tinkering is best done with power off, and also with devices not plugged in for further assurances that all is relatively safe.

The best way to remedy the live chassis problem is to use an isolation transformer between the wall plug and the AC input of the amp. Antique Electronics Supply sells these.

I have seen a few cheaper consoles that had a transformerless hot chassis. These were usually record player only units or low end AM or AM/FM/phono combinations. I just gave away a BOTL Zenith tube AM/FM/phono console from the mid '60's with a hot chassis. All of the better consoles used a power transformer chassis.

Most power transformer sets used capacitors either across the AC line or connected between one side of the AC line and the chassis. If an "across the line" cap becomes leaky or shorts, it can explode and/or blow the fuse in your house. If a "line to ground" cap shorts, you'll have AC on the chassis! It's best to replace these caps with modern "safety capacitors" that are designed solely for AC line bypass. www.radiodaze.com sells these types of caps as well as just about any other type of capacitor that you might need for tube electronics.

Teron · #9 05-12-2008, 03:50 PM

Hmmm...

So an isolation transformer would mean a new housing for this amp would be a good idea?

This might end up being too much of a project for me at the moment. I was looking for a quick remove the amp and use it kind of thing.
Not keen on potential BZZZZT scenarios...

What if I mount it to a block of wood, and put a plexi vented cage around it?

Reece · #10 05-19-2008, 07:23 AM

With an isolation transformer, you just plug your power cord from your Maggie into the transformer and then the transformer into the wall receptacle. You don't have to modify your amplifier if you don't want to. The isolation transformer can be "outboard". There is probably room inside the Maggie cabinet to mount the transformer. Some isolation transfomers may be intended for hard wiring rather than plug-in applications.

Rather than remove the speakers and speaker boards from the cabinet, you are going to have better sound by leaving them in there the way they are. Speakers need acoustic isolation between the front and back of the cone in order to produce bass notes, and generally speaking, the longer the acoustic path between the front and the back of the cone, the better. A speaker sitting out by itself with no baffle, or only a small board baffle, and no enclosure, won't have much bass.

I would say that these speakers aren't coaxial, which means that there is a separate tweeter mounted coaxially inside the woofer, with its own voice coil; but that they may be dual cone with either a small cone or an aluminum or hard paper dome over the center of the main cone, for the higher frequencies, but all actuated by the single main voice coil. I don't see any second voice coil connections on the speaker terminals.

Reece

kenbarbdae · #11 09-18-2008, 07:24 PM

I've got a Magnavox similar to the Trapezoid but in a portable suitcase. It has an amplifier number 8001-10. I'm in the process of recapping the amp and ran across a component that I'm unsure of. I believe it may be Wima cap. It is in an orange flat pack, two leads with a W and 22 stamped on it. Anyone know if it is a cap and is the value in uf or pf? I'll try to attach a picture of it.

Tom Bavis · #12 09-18-2008, 07:53 PM

That looks to me like a wire-wound power resistor in series with the silicon rectifier. Check it with an ohmmeter - if it's anywhere close to 22, leave it be...

Jeffhs · #13 09-18-2008, 07:57 PM

Quote:

Originally Posted by kenbarbdae

I've got a Magnavox similar to the Trapezoid but in a portable suitcase. It has an amplifier number 8001-10. I'm in the process of recapping the amp and ran across a component that I'm unsure of. I believe it may be Wima cap. It is in an orange flat pack, two leads with a W and 22 stamped on it. Anyone know if it is a cap and is the value in uf or pf? I'll try to attach a picture of it.

It could be a capacitor, but on the other hand it could also be an encapsulated component combination unit, also known as a couplate. I have a Zenith radio with two components that appear similar to what you describe, one for the stereo FM decoder and the other for another function that escapes me at the moment. However, since the flat pack component in your Magnavox amp has only two leads, I'm hesitant to even guess what it is. I have never seen a capacitor in a squarish flat package--round, yes, but never square. The W and 22 may be value markings, but I'm not sure.

If there is a little diagram taped or pasted onto the underside of the chassis, your mystery component is probably a couplate.

Note: Ooops. I just read Tom Bavis's post, in which he described the component you're seeing as possibly being a wirewound resistor, very possibly a fusible resistor. If it's in the power supply, it may very well be a fusible "sand resistor" (another name for a fusible, probably named that because of its sandy appearance), designed to open in a hurry under a severe current overload. Many series-string televisions of the 1950s had these; I remember my folks' second TV having a 33-ohm sand resistor in the power supply, near the selenium rectifiers. My Zenith MJ1035 stereo FM radio has a 33-ohm fusible in its power supply, only that one is much smaller than most sand resistors and is enclosed in a ceramic tube. Seeing all the sand on the component shown in your attached thumbnail almost convinces me as well that the thing is probably a wirewound fuse resistor.

kenbarbdae · #14 09-18-2008, 09:11 PM

Just ohmed it out and it measures around 24 ohms. I think I'll take the leave it be for now advice and research it further. Thanks for all your help! Also, thanks for the safety advice about the lack of isolation. Does anyone know if these amps are listed in Sams?

wa2ise · #15 09-18-2008, 10:23 PM

Quote:

Originally Posted by kenbarbdae

Just ohmed it out and it measures around 24 ohms.

That's a specially designed "fusistor". Combination fuse and resistor. If it does burn out, just replace it with a regular power resistor and an in line fuse holder with fuse. 1/4 amp slo blo should be about right.