Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: eleventeen on August 19, 2010, 01:07:56 pm
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I kind of dislike tearing apart complete gear (for parts) that could be useful to somebody, even more with ebay and various forums which simplify finding a potential user. I don't mind tearing apart something that's homebrew or some bizarre piece of single-purpose industrial control gear. Here's an item that came from the magic WW2 garage of doom that will obviously yield more value in useful parts than it will fetch selling it, but I am (only somewhat) reluctant to destroy it.
(http://i172.photobucket.com/albums/w32/ttm4/radio_fiesta061.jpg)
(http://i172.photobucket.com/albums/w32/ttm4/radio_fiesta063.jpg)
Weird kind of transmitter, huh? Built in a BUD style hinge-top cabinet, but it appears one step up from a homebrew item.
FAT power transformer (430 volts under load w/5R4 or 5U4 rectifier, yeah!)
FAT choke
Biggish modulation transformer HERE'S MY QUESTION: What's the general nature of secondary impedances on modulation transformers? Any ham operators out there?
3 chicken head knobs. Jewel pilot lamp holder. Goofy cabinet that would be good for making a battery charger or a rodent trap, LOL.
Switches. Nice meter.
Came with 2x 6L6 GA JAN Sylvania. 807 final.
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Hey E-Teen,
If you can pull some numbers and a brand off of the iron, here is a reference on the most popular brands of mod trannies. Shows both wiring and capacities. Hope it helps!
http://www.amwindow.org/tech/htm/modtran/modtran.htm
Jim
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Many thanks, Ritchie! I think if that piece of iron has speaker-type impedance on the output, Mr. Lettine is headed for the screwdriver/wrench-based operational theatre!
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If it has a mic input, then it's got a guitar output! :grin:
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I found this info on it.... nice old radio made around 1950
http://www.qsl.net/la5ki/org/le/le.htm (http://www.qsl.net/la5ki/org/le/le.htm)
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If it has a mic input, then it's got a guitar output! :grin:
+1 :laugh:
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> What's the general nature of secondary impedances on modulation transformers?
V/I of the 807 stage.
You got 430V. The meter reads to 150mA, which is an odd number, so is probably just a bit higher than normal running current. The Mazda Belvu 807 sheet on Frank's site shows (page 3) shows Class C telephony as 400V and 80mA. 400V/0.080A= 5,000 ohms. That's the secondary impedance.
What is the audio power stage made to do? RF power output is maybe 2/3rd of 807 DC input. 430V at 80mA is say 33W input, 22W output (sheet says 22.5W). The audio modulation power is half of that plus a margin, say 12W. It could run 6V6 but not (wisely) at 430V; safer to run 6L6 at 360V (rated max) and load lightly. A pair 6L6 at 360V makes over 20W in 6K. It could be working 6K load with ample over-modulation, or 10K load to get just the 12W and lower supply current.
The killer is that you do not have 5,000 ohm speakers. Also the 807 side in-effect works "single-ended", the iron is over-size to carry the DC. And if you abandon the 807, you got another 80mA of spare current to drop on the 6L6es, which would allow a much lower plate load and higher audio power. It could pull 5KCT or 4KCT and make 25W-35W. All of which suggests re-selling the modulation iron and dropping a Bassman OT in. The rest of a perfectly good audio path is in there.
One wild thought. Keep the 6L6es to mod-iron as stock. Re-wire the 807 as an SE audio power stage loaded in the mod-iron "secondary" (the impedance is correct-enough). Add another transformer (cheap P-P is fine) to convert the many-K impedances down to speaker impedance. Have separate controls for P-P stage and SE stage. One way is 10W SE, the other is 15W P-P. If you get polarity right, in the middle they combine for a SE/P-P rig. It won't make 25W without impedance re-tap, but it could be an interesting 15+W sound.
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The killer is that you do not have 5,000 ohm speakers.
That, in a nutshell, is precisely what I suspected would be the snagola. Thanks as usual for your insight!