90 year old Magnavox Amp

[P Batty]

Had a chance to buy this vintage amp today but it had some damage inside to the output transformer and the filaments were open on the tubes (301A) so I passed on it.

It had a pair of knobs on threaded posts for input, output, "A" battery and "B" battery connections, but also had a pair for "Field".  Would that have been for an electromagnet of an external speaker, or was that the "C" connection?



I left the seller with a low bid; I might still get it. The tubes aren't too hard to find, and not too expensive (although the original 216A 'tennis ball' tubes are).  I could probably figure out something for an output transformer. There is a hand drawn schematic inside the case. It's almost worth it just for the cool logo and nameplate:

[eleventeen]

Really cool looking! The tubes are obtainable, with some effort. But I (in your place) would be buying it for an "exhibit" rather than a working amp. Still, you did very well checking the OT winding.

The "field" is intended to power the field of the loudspeaker. It's in essence the input and output terminals of a/the power supply choke.

You ask, why would I need a choke when I'm powering the thing with smooth DC?

Because that's how the speaker gets its "magnet"---by running the power supply thru a coil of wire.  

It's interesting that such an amp would appeal, esthetically, to fans of the modern tube-based ultra-gourmet power amps that sell for ridiculous prices.

[P Batty]

Yeah, I'm sure it would be for show, but it would be nice if the tubes would light up!

I kind of thought that was what the Field terminals were for.

It's appearance is really stunning, the photo looks grungy because of the dust, but it is in very good shape for its age.

[eleventeen]

Did you have an appropriate power supply such that the tubes COULD light up? Or did you measure the heaters as open as they sit?

[P Batty]

I measured the filaments ("A" supply) and they read open. If I did get it, I would do some research before I applied any voltages.

[billcreller]

The 301A are the same as 201A, and 01A I believe, and have 5 volt filament.  I had some of those but gave them away a few years ago.....

[P Batty]

When they first came out the first digit was the manufacturer, Cunningham was 301A, RCA were 201A,  Arcturus (blue glass!) were 101A, DeForest were 401A.  Cunningham was made by RCA. Eventually most triodes of this type became  01As. There was also a 00A which was the same tube with a less efficient filament.

[billcreller]

The 00A was a better detector than the 01A, for the one tube regen radios I built as a kid.

 The other tube I liked back then was a 30.

[PRR]

Worth $80. Popular Science Monthly, November 1922

(But sells for much more now.)

[P Batty]

Leave it to PRR to find an original ad! 

$80 was big money in 1922.

I told the guy selling it that if he didn't get any takers I'd buy it for $50.

They sell for over $400 on eBay, but this one had some "issues".

 

[PRR]

The maximum output of an '01A is about 50mW (0.050 Watts). You won't get "anywhere" with modern speakers, you want the vintage horn-speaker too.

With another '01A as driver, you need about 1V signal to get the full output.

It's an incredibly poor amplifier by modern expectations. I suppose someone is proud to have one that runs; I'd be pleased to just have it on the shelf display-only.

[P Batty]

There's a guy who is using two UX201A as an interstage "sound processor" to warm up sterile-sounding CDs:
http://vinylsavor.blogspot.com/search/label/01A

[P Batty]

If I did get it, I would probably rig up a 5VDC supply for the heaters, just so it would light up. The transformer is very strange, looks to be  some kind of air core (the windings wrap around a tube about 3 inches long and about 1 1/2 inches wide). Several windings are broken, it would need a real expert in the old stuff to repair it.

The 01A tubes are fairly easy to get, about $15 to $30 dollars for average samples. The original tube for this is a Western Electric 216A which run about $40 for non-working ("display") to over $200 for a good specimen! The 216A heaters are rated at 6 volts, I suspect that might be why the 5 volt heaters on the 301A tubes are open. It was retubed in 1930, the date is on the service label.

I'll be talking to the guy again this weekend, if I get it I'll post the schematic and some gut shots.

[P Batty]

It was sold yesterday.

A crystal radio could be used to drive it. I found this PDF showing hookup:

http://www.shinkukan-jidai.com/additional_radio_pages/page_2_files/download/Magnavox_AC2_AC3_Operation_Instruction_Manual.pdf

300 volts on  the B+!

Here are gut shots, the transformer is the big gray tube in the center of the third picture:

http://www.radiomuseum.org/r/magnavox_ac_2_cac2_amplifier.html

[PRR]

> I believe the transformer is an o/t

Actually it appears to be the interstage transformer.

Magnavox didn't include all the transformers in the amplifier.

On the full-system hook-up, the box at left is a several-tube radio with headphone output. Just to get from here into the AC2 you have to supply a transformer. Then tube transformer tube. The Output Transformer is actually on the board with the loudspeaker.

> 300 volts on  the B+!

And the '01 only has numbers to 135V.

The WE 216A *appears* to be a MUCH bigger tube. 550V 65mA, but that's 35 Watts which seems unlikely. Another source suggests 130V and maybe 20mA, which fits the bottle-size, but does not really agree with the 300V batter pack.

Another in the family is WE 101, which is NOT the same as the "01" used in nearly every pre-1928 radio. This could make as much as 230 milliWatts output.

[eleventeen]

More often than not.....it is (or should be) a relief (to you) to have it gone! Now you can stop wondering about it and it can go clutter up somebody else' shelf for the next coupla decades.

[P Batty]

The elderly gentleman who was selling it said he wished he had another one to sell me. 

Oh well, live and learn, that is learn about 1920s amps.

A matched pair of 216A recently sold on eBay:

http://www.ebay.com/itm/WESTERN-ELECTRIC-216-A-PREMIUM-MATCH-PAIR-VINTAGE-BALL-TUBE-SONIC-EXCELLENCE-/161151385656?pt=Vintage_Electronics_R2&hash=item25855f0038

Thanks PRR, for helping clear things up.