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Transformerless tube amps?

Discussion in 'Shock Brother's DIY Amps' started by wnorcott, Feb 23, 2009.

  1. wnorcott

    wnorcott Tele-Holic

    862
    Sep 30, 2008
    New Hampshire, USA
    I know there are some tube amplifiers for hifi that are "output transformer-less" (OTL) that use for example capacitor coupling instead of transformer though I do not know of a guitar amp that does this ( but then again I haven't looked for one either :lol: )

    But has anybody built a tube amplifier without a power transformer? I found there was an old guitar amplifier company called Gregory whose amps had no power transformer but did have an output transformer.

    So I would like to know if anyone has built a guitar amp that has no transformers?

    I know there are signal pentodes like EF86 that will operate at 110 Volts supply voltage but are there power pentodes that will do the same? It seems to me you could save quite a bit of money by omitting both power- and output transformers from the design. The transformers seem to be by far the most expensive components of tube amplifiers.

    Bill
     
  2. Natstrat79

    Natstrat79 Tele-Meister

    339
    Jul 28, 2007
    Iowa
    There are quite a few guitar amps from the 50's and 60's that didn't use power transformers. They are mainly small department store amps such as danelectro, kay, valco and the like and they are typically single ended power amp designs.

    As a point of reference here's the schematic for the kay 703 which does not have a power transformer.

    To be entirely safe however most of these amps should really be used with an isolation transformer.
     
  3. markw51

    markw51 Tele-Afflicted

    The OTL hi-fi amps I know of have many vacuum tubes to get the impedance down to speaker values. Not very practical for a guitar amp or ANY amp for that matter. Here a photo of one. Now you know why very few tube amps are made this way.
     

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  5. celeste

    celeste Friend of Leo's

    Age:
    55
    Jun 24, 2006
    Maryland
  6. celeste

    celeste Friend of Leo's

    Age:
    55
    Jun 24, 2006
    Maryland
    It depends on the tubes. Mine uses 2 6C33C-b's for 26 watts into 16 ohms. It is not so much the number of tubes as the high output impedance that means it sounds different with different speakers, so it is a real crap shoot if it will sound good with what you might pair it with. http://s35.photobucket.com/albums/d179/CeCeHall/?action=view&current=CycloFront.jpg
     
  7. FiddlinJim

    FiddlinJim Tele-Holic

    830
    Mar 28, 2006
    Seee-attle
    A buddy of mine has an old Epiphone Electar (pre-war) that he restored that has no PT. As pointed out in describing one similar to this (Electar): "The plug was non-polarized. Meaning, every time the amp was plugged in, there was a 50% chance the chassis would have 120 volts on it."

    So, isolation transformers might be a good idea...
     
  8. EdMax

    EdMax Tele-Meister

    273
    Mar 18, 2008
    Northern Illinois
    On most audiophile forums I belong to, topics on amps without power transformers are locked and removed almost instantly and for good reason.

    Basically you are risking your life every time you turn it on, REGARDLESS of plug orientation. All of those old Kay, Wards, and other el-cheapo line based amps can only be made safe with a proper isolation transformer.

    and yes, musicians have indeed died from such devices.
     
  9. wnorcott

    wnorcott Tele-Holic

    862
    Sep 30, 2008
    New Hampshire, USA
    Yes I saw that exact amp design and that is the one I was thinking of now that you mention it :lol:

    Bill
     
  10. 6942

    6942 Poster Extraordinaire

    Age:
    63
    Apr 15, 2006
    Santa Fe, NM
    I have an old mid 60's Harmony H400 amp at home.
    And YES.....and isolation Xformer......is mandatory with this amp.

    Steve
     
  11. ThermionicScott

    ThermionicScott Poster Extraordinaire

    Age:
    36
    Dec 3, 2005
    SW CR IA US NA PE
    Batteries are another way to avoid power transformers. (Makes for squeaky-clean B+, too.) To solve the output impedance problem, you could use a lot of 12AU7's in parallel push-pull, driving several small high-impedance speakers in series. I bet it could be done! :cool:

    - Scott
     
  12. wnorcott

    wnorcott Tele-Holic

    862
    Sep 30, 2008
    New Hampshire, USA
    Thanks for the information it is a good thing this is TDPRI where there is no such thing as forbidden knowledge. ;)

    Seems to me investing say thirty bucks for an good used isolation transformer could open up a whole world of possibilities of cheap and easy to build tube amps. By the way some of those old cheap amps sound fantastic and probably deserve to be resurrected.

    What if you install a proper grounded AC cord on the chassis and then test the plug every time with one of those $5 plug in AC plug testers like this before you plug it in? That tests for crossed hot and neutral etc on the plug.

    [​IMG]

    Just saying.

    Bill
     
  13. Dan German

    Dan German Poster Extraordinaire

    The H303 (or at least H303A and H303C) from the same era DO have power xformers, and are safe to use, asuming you do a three prong conversion. Wonder why the H400 is otherwise? (My H303C is a 1964, BTW.)
     
  14. wnorcott

    wnorcott Tele-Holic

    862
    Sep 30, 2008
    New Hampshire, USA
    I wonder what kind of impedence we are talking about here? 16Ω speakers are real easy to come by, say you get 4 tens in series that would be 64Ω impedance or 5 would be 80Ω that seems to be in the range of some triodes output impedence. Of course this is just pure speculation on my part. :twisted:

    Bill
     
  15. ThermionicScott

    ThermionicScott Poster Extraordinaire

    Age:
    36
    Dec 3, 2005
    SW CR IA US NA PE
    That's what I was thinking -- get the plate resistance down to a hundred ohms or so, then make a hundred ohms out of little 8Ω or 16Ω speakers. I have absolutely NO experience building OTL amps, but hey, with a beginner's mind, anything is possible! :lol:

    BTW, you have a PM. ;)

    - Scott
     
  16. Crawfish

    Crawfish Tele-Afflicted

    Apr 21, 2008
    Maryland, USA
    I have quite a few old radios that are, as we say in the old radio business, "AC/DC" - aka 'transformerless.' So for working on them, an iso transformer is a must. Same with cheap geetar amps.

    BTW, the "insulation" on most of these was literally the cabinet - plastic or bakelite or catalin or wood, so in general there was no danger of shock. Not saying it couldn't happen, but as long as you don't touch the chassis you're ok.

    A couple of random thoughts (it's early in the am, and the tea hasn't quite kicked in yet so bear with me if I don't make sense).

    - You could just build an iso transformer into one of these amps - or if you're homebrewing, build one into that. Here are some:

    http://www.hammondmfg.com/169.htm

    - You could use two transformers removed from "wall wort" power supplies coupled front-to-back. For example, if you have two 120 to, say, 12v transformers with the same ratings, connect the two 12 v outputs together and you have an isolation transformer on the cheap. One 120 v side goes to the mains, and the other is your 120v to be rectified as B+ in the amp. You just need the transformers rated for enough current for your circuit. I sort of collect 'em from dead printers and stuff around the office so I have a "free" supply of 'em. Many will be 'odd' voltages - 9v, 15v, whatever, but the concept is still the same.

    I'm eyeing up the Kay 703 schematic as we speak and am about to brew one up.

    -Kevin
     
  17. Dan German

    Dan German Poster Extraordinaire

    I got my first lesson in why they call those old amps "widowmakers" when I played a neat little thrift store find. I pugged it in, hooked up a guitar, and it sounded pretty good, but... there's a good reason why they usually put bakelite or plastic knobs on those things instead of aluminum. Touching the knobs was the same as touching the chassis, and that was a bad idea. Very bad. Anyway, I survived, and I am much more careful now. I also dismantled that amp for parts. Once they get a taste for blood...
     
  18. 6942

    6942 Poster Extraordinaire

    Age:
    63
    Apr 15, 2006
    Santa Fe, NM
    Dan,

    I think that Harmony.....with that H400 amp.....was going for the lowest price point possible?
    Parts = Money.
    Anyway.....I got my isolation Xformer for FREE.....when they gutted the local hospital.
    Problem is.....being the size of a bowling ball.....and weighing as much as a male German Shepard.....it ain't exactly portable?

    Steve
     
  19. Crawfish

    Crawfish Tele-Afflicted

    Apr 21, 2008
    Maryland, USA
    Here's a good piece with info on making these things a little safer:

    http://antiqueradio.org/safety.htm

    -Kevin
     
  20. Crawfish

    Crawfish Tele-Afflicted

    Apr 21, 2008
    Maryland, USA
    It was all about money - the PT is the most expensive piece of hardware.


    -Kevin
     
  21. celeste

    celeste Friend of Leo's

    Age:
    55
    Jun 24, 2006
    Maryland
    I am going to suggest that if you are going to play wiht old amps that might not have PT's, that you invest in a stand alone isolation transformer. OneAC units are available on Ebay all the time. A five amp unit will handle anything I can think of. plug it into the wall and plug your amp into it and you are isolated from anything but the amp.
     
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