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1941 Radio Conversion

Discussion in 'Shock Brother's DIY Amps' started by tweedman2001, Dec 5, 2015.

  1. tweedman2001

    tweedman2001 Tele-Afflicted

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    Ok Boys,

    I just obtained this 1941 (best guess) Sparton 1081 console radio. After opening it up I've decided that it's going to be the host for a guitar amp. I've included the pdf with the schematic. The layout is for 10 tubes not the 12 tube version. It has one 5Y3 rectifier. It is supposed to have 6F6G power tubes, but one was a coke bottle 6L6. Hey, I don't know. The owner said that it has sat in their entrance way for 30 years. I did not attempt to power it up.

    Obviously the first thing is to test the power transformer. If that is good (or even not), I'm looking for suggestions on what to end up with. My current thinking is to eliminate the entire radio section and just go with a 6F6G or 6V6 powered amp in the vein of a tweed or brownface amp. It has a fieldcoil 12 in speaker. I will probably upgrade that and go with a different output transformer and speaker, but I'm looking for ideas.

    I'm not looking for a how to but more of which way should I go? :D
     

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  2. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    If the PT/OT are still good, I'd keep 'em rather than gutting everything. Just leave the output section wired as-is and build a 5E3 preamp onto the front. 6F6's are easy to come by and cheap since no one else is using them. :cool:
     
  3. ejb222

    ejb222 Tele-Holic

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    Cool! can't wait to see what comes of this.
     
  4. tweedman2001

    tweedman2001 Tele-Afflicted

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    I was actually thinking of using a hybrid Harvard 5F10 preamp with a 6SQ7 in place of the 6AT6/6AV6 and a 12AX7. And trying to use the existing output tube section and field coil speaker. I'll know more after I test the transformers and speaker. Wondering if the original speaker would hold up.

    I want to use only a single input with a volume and tone. The existing tone pot has the on/off switch built in. Not sure what the value of the pot is yet.

    Anyone see any problems here before I go off the rails? Here's a few more pix too. Last one is a different restored radio.
     

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  5. jonrpick

    jonrpick Friend of Leo's

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    I was in a thread about this a while back. My thinking was... everyone talks about "bedroom amps". I want a "living room amp". Something that doubles as a decorative piece of furniture. This could be awesome. ;)
     
  6. tweedman2001

    tweedman2001 Tele-Afflicted

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    I pulled and cleaned the speaker. Looks to be in great shape. Hopefully tomorrow I'll get to put a meter on the transformers and the speaker components.
     
  7. bparnell57

    bparnell57 Poster Extraordinaire

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    I bet it would be a great platform for an EH-125 amp build.
     
  8. tweedman2001

    tweedman2001 Tele-Afflicted

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    That's a good idea too.

    Thanks and keep those ideas flowing please.
     
  9. tweedman2001

    tweedman2001 Tele-Afflicted

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    That's exactly what I'm going for. Actually that thread got the creative juices flowing for me. :cool:
     
  10. Jeru

    Jeru Tele-Holic

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    YES..! I love these. My 1937 Philco Tombstone champ is one of my favorite things I've ever done. (See below photobucket links, and Link for full build pics)

    Likely you'll need a new speaker and OT. Assuming the PT is still good you'll definitely be using it. Tube sockets will likely need replacing because they're ancient.
    Gut the radio stuff but LEAVE THE TUNING MECHANISM AND THE LIGHTS THAT LIGHT IT ALONE. Turing the lit-up dial is cool, even if it doesn't do anything but
    turn the dial. Those lights should (hopefully do) run on the same 6.3v as the filaments (my Philco was wired this way).

    Keep us updated on this, or post again to keep throwing around ideas. You have the opportunity to do something really cool here..!

    http://s177.photobucket.com/user/joshuarosie/media/Philco%20Build%20-%2038_zpshsztozj3.jpg.html

    http://s177.photobucket.com/user/joshuarosie/media/Philco%20Build%20-%2032_zpsm9nnscgt.jpg.html
     
  11. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Unless the old tube sockets are cracked or have seen arcing, why replace them? If you do, I'll take 'em.
     
  12. tweedman2001

    tweedman2001 Tele-Afflicted

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    Here's some pix of the speaker. It looks to be in great shape. Tomorrow the meter comes out.
     

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  13. Jeru

    Jeru Tele-Holic

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    If you end up keeping and using the FC speaker and/or OT from the radio, please educate me/us (some of us, at least)
    a little bit as to what needs to be done to use a FC speaker in the types of circuits we associate with instrument
    amps -- I confess near-total ignorance.
     
  14. bparnell57

    bparnell57 Poster Extraordinaire

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    A field coil speaker simply uses an electromagnet instead of a permanent magnet. This was very common prior to the late 40's as AlNiCo magnets were quite expensive. If I recall, all 3 of those elements were strategic materials in WWII.

    Most utilized quite high voltage, and had a resistance in the several hundred ohm to several thousand ohm range. They were basically really big filter chokes.

    Many required very high voltage due to their high voltage drop (takes a lot of power to make that strong of an electromagnet) and this means that the iron in radios and amps of the era showed these signs.

    Lowering voltage before the rectifier with a variac or afterwards with VVR will lower the speaker's magnet strength, this reducing efficiency. Flux-Tone speakers do this.

    The issues with them are limited output wattage, high value to radio restoration/theatre hifi guys, fragile often irreplaceable cones, aging coil wires, shock hazard due to having the B+ leave the chassis, and two major ones:

    They will hum if almost any power supply ripple is present, or they will modulate your guitar signal at 60 or 120Hz depending upon the rectifier setup. This can be overcome with a bridge rectifier and large filter caps.

    With power supply sag they will lower in efficiency due to the lowered voltage, making for (what many modern guitarists would call) odd compression and sag.

    Look up YouTube videos of Gibson EH type amps, or early lap steel amps.

    They are almost never cranked for the demos.
     
  15. tweedman2001

    tweedman2001 Tele-Afflicted

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    I tested the OT and the speaker components today. They're good so it should work if I decide to use it. I may try it first and upgrade if needed.

    Here's a little more info on Field Coil Speakers and replacement.

    http://www.radioremembered.org/edspeaker.htm
     
  16. motor_city_tele

    motor_city_tele Tele-Afflicted

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    I just added another item to my garage sale lookouts. I think any 18 watt p/p deluxe type amp would be cool as heck in an old radio cabinet. I'm betting one could get one of those old cabs for 20 bucks. I have three completed 5e3 chassis and a couple of champs that would just pop right into sommat like that.
     
  17. Wally

    Wally Telefied Ad Free Member

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    This may be of no importance to anyone, but that radio is worth some fair money in working condition. I admit that the cabinet is sharp in a very old school way.
     
  18. Jeru

    Jeru Tele-Holic

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    Thank you for the explanation and link -- much appreciated.
     
  19. tweedman2001

    tweedman2001 Tele-Afflicted

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    I agree Wally and I am attempting to make it work as a guitar amp without destroying the radio section, just in case I or someone else wants to restore to original later. Right now I want a "COOL" living room guitar amp. :D

    I value your input. Any suggestions about this becoming a guitar amp?
     
  20. Wally

    Wally Telefied Ad Free Member

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    I would suggest that if you have a significant other who is 'style sensitive' that you make sure that the radio fits with the decor that exists. (;^)
    I am sure that there are better sources than I for what needs to be done in order to have a guitar amp built in that chassis.
     
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