1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice

bearfootIII base tube amp ...transformerless!!!!read on(pic)

Discussion in 'Amplifiers' started by fnkycoldmedina, Jun 15, 2006.

  1. fnkycoldmedina

    fnkycoldmedina Well-Known Member

    Joined:
    May 18, 2005
    Messages:
    2,266
    Likes Received:
    89
    thats right this thing is transformerless.i never knew this existed?when i went to pick this amp up i nearly threw it in the air...lol....i had to look twice inside only to find no transformer....wow...cool.....big fat caps in it ...3 of them...............31js6 tubes if i remember right.3 of those .swings 500.........i could swear i seen the tube cap leads soldered on the tube caps.the fan pulls air across the tubes instead of blowing on them.coolest little amp i seen yet....no transformer in a base tube amp.....sweet.

    anyone ever see such a thing?

    -------------------------




    Report this to a Moderator Reply Quote Top Bottom Edit
     

  2. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,911
    Likes Received:
    1,336
    Never heard of this one, but SBE made a no-transformer sweep-tube amplifier in the early sixties. Well, it had a small transformer for the tubes' heaters, but used a voltage tripler circuit, direct from the 120 Volt AC line cord. The negative side of the power supply was NOT grounded to the metal chassis. A bunch of disc capacitors would make the chassis an effective "RF" ground, since you are NOT permitted to hook either side fo the 120 Volt AC line circuit to anything on the outside of a cabinet. Table radios and televisions used this trick, with the antenna connections isolated with small capacitors and/or transformers to isolate the external connections from the "live", or "hot-chassis" ground circuits inside. Cabinets and knobs would be made of insulating plastic.

    The filter capacitors have to be large to keep the DC voltage from dropping under load. Filters used with a transformer-type supply need not be so large.

    Sounds like this guy hooked the four 31-Volt tube heaters in series, and runs them directly from the 120-Volt line.

    Ken-Rich used the heaters-in-series trick to keep from having to use a heater transformer in some models. Worked okay with barefoot drive, but larger boxes would experience breakdown between the heater and the tube's cathode. But only in the final stage. Seems the cathode was insulated from the heater well enough for 120 Volts AC, but not for that PLUS the driver's RF voltage.

    A barefoot-drive box probably won't have that bad habit.

    Just make VERY, VERY sure you have a VERY good ground on this box, and a proper-size fuse in it. For my money, any "transformerless" design like this should have a fuse in BOTH hot and neutral sides of the AC line cord. The third-prong safety ground should NEVER have a a fuse in line with it. The risk is that if one of those disc capacitors should short inside, the full 120 line voltage could appear on the outside of the cabinet. Very dangerous, and creates a NASTY hazard. Making sure the case has a separate (solid) ground will at least cause the fuse to blow before it can create that kind of shock hazard.

    Had to wonder, how much cheaper the enormous filter caps really are, in comparison to a transformer plus smaller filters? I suppose if you have the filters already, and don't have a transformer this would make it a lot cheaper.

    73
     
  3. fnkycoldmedina

    fnkycoldmedina Well-Known Member

    Joined:
    May 18, 2005
    Messages:
    2,266
    Likes Received:
    89
    bearfoot

    [​IMG]
     
  4. fnkycoldmedina

    fnkycoldmedina Well-Known Member

    Joined:
    May 18, 2005
    Messages:
    2,266
    Likes Received:
    89
    hey nomad radio ...heres a picture of the amp since you never heard of this one.i didnt even see a small transformer at all ,none.......just a thick yellow 3 prong cord leading up to the electronics inside and you can see 3 big fat tall caps inside.with 5 watts in i get 100 watt key on high swinging to 4-500......on low....50watt key swinging 250..........not a great big keyer but a nice little 4-500 watt pep amp.there is a fuse inside that you can see very easy.i,ll take a shot of the insides later when i have more time.so this would be a rare amp huh?i knew i had something rare here.how would you say ground it??run a wire from the chassis to where??no actual good ground in my room here other than the electrical outlet ground.i imagine if you hammered away at this amp the caps could drain enough that the power could suffer??
     
  5. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,911
    Likes Received:
    1,336
    Hey, way cool!. Must have been mostly a west-coast phenomenon. Being in the sticks means I see mostly products that were marketed nationwide. Gotta fiigure there were a lot of "local" products here and there around the country that were never made in large quantities.

    Sure, more pics would be cool. It didn't need a transformer for the tubes' heaters, since they add up to nearly 120 Volts with four of them wired in series.

    Gotta wonder how they provided power for the changeover relay. Wouldn't require more than a tiny transformer for that alone, but I'm not sure how they'd do it without one. Or how I would, for that matter.

    The "KLM" meter is a humorous touch. I'm sure the folks who ran that company would be mortified to see it on a sweep-tube "CB" leen-yar. They were stritctly ham, and sorta high-end when they were in business. Makes me suspect the meters were bought at a bankruptcy or a hamfest flea market after the company was long gone.

    For about ten years, meters that had the "Dentron" name on the scale were plentiful at the Dayton Hamvention flea market. The company was located in Ohio, and the parts that got liquidated when they closed kept circulating for years.

    Yeah, more pics would be cool. The 3-prong line cord is a safety measure. Meant to blow the fuse if a short between the incoming line volage and the chassis occurs. You really don't want the metal cabinet live with 120 Volts AC on it.

    Using only one fuse implies that you know which of the line cord's (other) two wires is the "hot" side. You really want the fuse wired into the "hot" side.

    Trouble is, telling which of the flat blades on the 120-Volt outlet is "hot" and which one is "neutral" is error-prone. In real life, some outlets are wired in reverse. In this case, the UNFUSED side of your line cord COULD become the "hot" side of the 120-Volt circuit. That's the reason for suggesting a fuse in EACH of the two wires going to the line plug's flat blades. No matter which one of the (other) two wires on your line cord is the hot side, it's protected from overload with its own fuse.

    Makes me wonder if it was sold with "Slow-Blow"-type fuses. Those enormous filters should draw a large surge of current when they are turned on, and fill up from empty. You can keep a normal fast-blow type from popping at turn-on by using one with a current rating larger than is really safe.

    If the HV filter caps are the computer-grade type with screw terminals, they may have a four-digit date code printed on them. Typically, they'll have a three-digit number that identifies who made the cap. Pretty sure Mallory was "235". Next would come four digits, two for the year, and two for the week 01 to 52. A cap marked "235-5274" would have been made by Mallory, the last week of 1974. Different manufacturers will print this info laid out without the dash, or with just the 4-digit code alone, or with those four digits at the end of an arm-s length string of digits.

    If they appear to be over 30 years old, keep an eye on the small rubber plug that serves as the safety vent. When that kind of cap breaks down, that rubber plug pops out to prevent a steam explosion. Sometimes spews a foul-smelling geyser of white fog. Any outward swelling of that vent, or brown/tan powder around it is a danger sign.

    Good news is that this type filter tends to last longer than the smaller ones found in most tube equipment. Longer, like 25 years instead of 15 or so. Around 40 years, they just go bad. And around 30 years, they will soon.

    73
     
  6. Motor Doctor

    Motor Doctor Member

    Joined:
    Dec 14, 2005
    Messages:
    9
    Likes Received:
    0
    my bearfoot

    hi guys i have a bearfoot 3 also nice little amp
    swings to 500 watts easily!!!! runs 3 35lr6 tubes
    however i have a problem with the pre-amp, when you
    turn it on the receive gets weaker. any ideas what might
    cause this?
    a guy told me the relay may need cleaning how can you clean these? they are sealed in clear plastic
    can Justin repair this type of amp??
    we dont have a good tech in my area
    thanks for any help you can give!!!!
    charlie aka motordoctor
     
  7. Motor Doctor

    Motor Doctor Member

    Joined:
    Dec 14, 2005
    Messages:
    9
    Likes Received:
    0
    hey fnkycoldmedina do you have instructions or a
    schematic for your bearfoot?
     
  8. Motor Doctor

    Motor Doctor Member

    Joined:
    Dec 14, 2005
    Messages:
    9
    Likes Received:
    0
    bump
     
  9. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,911
    Likes Received:
    1,336
    Hey, M.D.,

    Preamps tend to blow out, unless the design has some serious surge protection. When you key the radio, there is a short delay before the relay can respond, and take the preamp out of line. During this delay, your radio drive is pumping into the preamp's output side. Typical designs will include a couple of diodes that 'catch' this brief upstream surge, and protect the preamp transistor. If it's a lame design, they leave those parts out, and the preamp doesn't last long. Even if it's a good design, sooner or later someone tries it on SSB mode. The same delay that keeps the amplifier's relay from chattering on SSB mode also delays the relay's "pull-in" as well. During that delay, the radio's SSB power pumps into the preamp transistor, probably long enough to smack the protection diodes, if it has any.

    Usually the first time they try the preamp on SSB is the last time the preamp works.

    Or, the preamp's relay could be at fault. Or both?

    If you really need a preamp, try an external. The "built-in" types are almost never worth squat, even before they break. An external will usually have some tuned circuits in it, which reduces interference from shortwave broadcasters. A built-in is nearly always "broadband". They are also famous for bringing in Radio Moscow or Radio Havana Cuba when the conditions are right.

    73
     
  10. fnkycoldmedina

    fnkycoldmedina Well-Known Member

    Joined:
    May 18, 2005
    Messages:
    2,266
    Likes Received:
    89
    hey nomad,i am gonna try n have those inside pics up by thursday .....i should have some time that day ......my pre amp still works .......doubt this box was used much .all the equiptment we bought that day was mint....barely used or kept very well.i am actually looking forward to cracking it open myself....lol.....
     
  11. robogator

    robogator Member

    Joined:
    Jan 19, 2006
    Messages:
    43
    Likes Received:
    0
    Here in Florida on a 3 wire 110 volt single phase ac wall socket,it should be wired with the round connector to ground(Green),the larger (wider spade) connector is tied to white (which is another Ground for safety),the smaller spade is tied to black which is (hot/110volts ac).I have never seen them wired backwards by a licensed electriction but I have seen them wired wrong by the average joe...it is best to check with a volt meter if you are not sure or let someone who knows check the circuit for you just to make sure..I myself do not like ungrounded base linears...
     
  12. Dave Berry

    Dave Berry New Member

    Joined:
    Sep 8, 2018
    Messages:
    4
    Likes Received:
    0
    Should the signal meter swing all the way to max on this box? I just got one and am not sure on how to load and tune it
     
  13. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,911
    Likes Received:
    1,336
    The meter on the box is set from an internal adjustment trimmer. It shows whatever that adjustment was set to show. It's commonly called a 'relative' meter. Means that a reading farther to the right is more power, to the left is less.

    You really need a wattmeter in the coax line to tell what it's doing.

    The internal meter is mostly decorative. Not a wattmeter.

    Had one of these come in the shop a week ago. Told the customer I wouldn't work on it. Too much product liability.

    He was amused that we call that one the "widowmaker".

    73
     
    Dmans likes this.
  14. Dave Berry

    Dave Berry New Member

    Joined:
    Sep 8, 2018
    Messages:
    4
    Likes Received:
    0
    Thanks for the info. The more I hear about this box the more I think maybe it will make a good paperweight. I wish I could find a operators manual but I’ve only found dead ends in my search of one.
     
  15. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,911
    Likes Received:
    1,336
    Never have seen a schematic. Nobody has ever offered to pay the labor to draw one up.

    No good reason to do it for free.

    73
     

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!