Junkbox Transformers

Discussion in 'Shock Brother's DIY Amps' started by bad_rudy, Nov 19, 2019.

  1. bad_rudy

    bad_rudy TDPRI Member

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    The topic of snagging old vintage radios, etc for amp components came up in a recent thread. It inspired me to reach out to a few Craigslist ads, and through those I found some local antique radio enthusiasts (and veritable pack rats) who have large stashes of great stuff for sale cheap.

    I am curious, however, about vetting transformers from these units. How do you all go about this? A few friendly folks here talked me through assessing voltages in the PT secondaries. I am curious how one moves from that information to determining what the unit could be used for. Is measuring current across the secondaries a necessary next step?

    I snagged a nice old radio chassis today for $10. It is running a 6V6-GT power tube with a 5Y3-GT rectifier and a 6AV6 (I understand this to be basically 1/2 of a 12ax7) preamp. These are in addition to 5 other tubes that I think are specific to its radio receiver. Output in an old manual is listed as 3.5 watt undistorted (I love how they specified this) and 5 watts maximum. Given the tube compliment and power range, is it basically a foregone conclusion that the PT and OT (if functional) are 5f1 or 5f2a worthy, or are there other considerations?
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Yup. That's probably how Leo started - buying shelf radio transformer sets for his K&F amps and Champion 600s..

    If it runs a 5Y3 and 6V6 it's got enough oomph to run a pair of 12AX7s which are very low draw both on heater and plate by comparison.

    I built a 5E3 using a 50s radiogram pt. No problemo getting within a few volts - tubes aren't that critical and early Leo amps had 15% tolerance on specs.

    You might find the pots usable too. Junk the electrolytic caps and think about the wax paper and oil in paper types. They might be OK or might be leaky (as in leak DC voltage which is nasty sounding).
     
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  3. bad_rudy

    bad_rudy TDPRI Member

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    Ok, so I did some testing on the PT out of this chassis. Looks like it is about a 315-0-315. The high current secondaries are coming in at about 5.5VAC and 6.9VAC. My original plan was to use these parts for a champ micro. The HV secondaries here, however, are just above what rob recommends as a max on his site. At the same time, they seem a little low compared to what many people use in a standard 5W tweed.

    So my question is: how much variability around a specified "preferred" value of PT output voltage is acceptable? I guess it could be phrased alternatively as how much play in B+ is needed before there is a noticeable effect on tone?
     
  4. muchxs

    muchxs Doctor of Teleocity

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    It's such a perfect Champ transformer. It ran most of the correct tubes to begin with. Keep diggin' for a Micro PT. Hint: I have an old Eico FM receiver with a small PT that would work. Old reel to reel PTs will work. I'm thinkin' the last version Webcors. I've got two with PTs and one transformerless.


    Considering the Micro was reverse engineered from the reverb circuit of a '60s Fender amp I took a quick look at a Princeton Reverb. The plate supply on a Princeton Reverb reverb circuit is over 300v. That's considered to be severe duty for "preamp" tubes. Late Princetons tend to eat 12AT7 reverb drivers and reverb transformers.

    A quick check with various tube data sheets shows 300v max for 12AT7, 12AU7, 12BH7 and 6SN7 tubes. In other words if you're doing it "by the book" 300v max is what you want.

    "Preamp" tubes will function with 100v on their plates albeit at reduced output. Reduced output is what we want, isn't it? I recently worked on an old timer with 50v on its (grid leak biased) preamp plates and 150v on its driver stage plate.

    I'd say 250v is ideal, 300v max, less than 250v is o.k. if you're working with free junk box transformers. Smoke 'em if ya got 'em!


    The advanced program would be to build it up with real live Kevin O'Conner Power Scaling. Then you can dial your B+ down until you have DC on the pots or dial it back up until your tubes launch like little bottle rockets.

    :D :D :D
     
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  5. bad_rudy

    bad_rudy TDPRI Member

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    Thanks muchxs, that is good info.

    I also came to the conclusion late last night that this is likely more appropriate for a 5W build or similar. No problem there, I'd be glad to build another. Just debating what exactly it will be. I am in the process of converting my 5f1 to a 5f2a + tremolo. Perhaps I will just go with a standard 5f2a (with addition of NFB switch and maybe a tone-stack lift) for now and then use this new (to me) PT for a proper vibrochamp in a few weeks? Not sure. Would be open to suggestions on what other builds might be cool. I have no particular "needs", just having fun really. Are the old gibson ga-5 circuits worth playing with?
     
  6. muchxs

    muchxs Doctor of Teleocity

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    5F2A has a low loss tone control. There is no point lifting it.

    Your "new" transformers would make a nice Vibro Champ. Save some substantial dough compared to buying new.

    :cool:

    I have a couple builds cued in the pipeline. One is going to be a non- reverb Princeton stuffed in a real '68 Vibro Champ chassis. Another is the coolest under the radar über rare you've probably never seen one and for sure you've never heard one Bargain Bin Special. Imagine a tweed Deluxe with trem, fixed bias to facilitate the trem and EL84s. It was originally an inscrutable rat's nest point to point layout and no two were the same. I went through a month of migraines working out a tag board layout. Shelved it for a while. Its time has (almost) come.

    All these 5F1s and 5E3s get bo- ring after a few decades.

    :rolleyes:

    A GA-5 is practically a 5F1 with 220k resistors on the preamp plates.
     
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  7. muchxs

    muchxs Doctor of Teleocity

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    Just participated in a house cleanout. The folks who lived there had been there forever and they saved everything.

    How 'bout 150 year old Jamaican rum bottles still sporting their original labels?

    The obligatory fat tire Columbia bicycle under the stairs?

    But... what I was really there for was the phonograph they bought when the dude returned from WWII. It's a 1947 Bendix console, essentially an American five tube with a turntable. They tucked all the original paperwork inside including the loan from the bank to buy it.

    How 'bout $379 1947 dollars?

    Three hundred seventy- nine dollars. In 1947.

    I mention this because it's just about what the OP has, "Champ" PT, 5Y3 rectifier, 6V6 audio output. I think the Bendix has a 6SC7 added to the American five tube to accommodate the phonograph.

    It has a sticker from the defunct local Bendix avionics service station over at the airport.

    It stopped working in the early '60s. They carefully carried to the basement and put it on a pallet. They covered it with a quilt.

    The portable Motorola phonograph they replaced it with is across the room. That one still works.
     
  8. Mongo Park

    Mongo Park Tele-Holic

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    Yikes the inflation calculator in USA dollars said the $379 is equal to $4373. If it worked to say 1967 it cost them $218 dollars a year not including the loan interest.
     
  9. Mongo Park

    Mongo Park Tele-Holic

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    Go with the Fenders first and branch out from there..
    5F2a with some mods is a good start.
     
  10. printer2

    printer2 Poster Extraordinaire

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    Already has a 5F2, I say a BF.
     
  11. bad_rudy

    bad_rudy TDPRI Member

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    Yes, currently in the process of taking my gutted 5f1 over to 5f2a. The plan is to leave that more or less stock (with the addition of NFB switch) for a while, but eventually add a simple reverb. The junkbox PT and OT I think are destined to be used in a vibrochamp. Always wanted one.
     
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