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Kay 703 help

Discussion in 'Shock Brother's DIY Amps' started by tpaul, Aug 20, 2010.

  1. tpaul

    tpaul Poster Extraordinaire

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    I picked up a little Kay 703. The tubes light up but there's no sound.

    I have two questions:

    1) what's it going to cost to get this amp running? It looks pretty clean, and I can't imagine it's very complicated to diagnose, since it has only three tubes and two controls. What range am I looking at for repairs?

    2) How much would it take to make this a safer amp? I mean, installing an isolation transformer and converting to a three-pin plug.

    I can solder but I'm no amp tech, so I'm looking for informed opinions on what I'm looking at paying to have this work done.


    Thanks for helping!
     
  2. burnt rope

    burnt rope Tele-Meister

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    it cost me $150 to get my Kay running @ Savage Audio, but it completely just depends on what is wrong with your amp and the cost of the labor whereever you take it.
     
  3. tpaul

    tpaul Poster Extraordinaire

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    Right... I was just hoping to get an idea of what the likely scenarios would be.

    Anyone installed an isolation transformer?
     
  4. celeste

    celeste Friend of Leo's

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    All the 703's I can find a schematic for have transformer isolated B+, so the isolation transformer may be superfluous. Frankly, I would look for a stand alone iso transformer, like a OneAC. There are others, but that is the one I use.

    As for getting it going, it depends on what is wrong. What tubes does it have so we can tell which one it is
     
  5. tpaul

    tpaul Poster Extraordinaire

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    Thanks for your help. The tubes are one 18GD6A, one 60FX5 and one 36AM3. They all light up but I don't know if they are working properly. They are all old RCA tubes. I guess I should replace them all first, before I do anything else, to eliminate the possibility that one or more are not good.

    What does "transformer isolated B+" mean?
     
  6. Crawfish

    Crawfish Tele-Afflicted Silver Supporter

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    To answer your last question, your amp doesn't have a power transformer. In the 1940s and 50s, some manufacturers built amps (and radios...) that used the line voltage directly to heat the filaments and run the B+ (high voltage for plates). The upsides of this arrangement were that a lot of money was saved in building the gear since an expensive power transformer wasn't needed. The equipment could also run off a DC power line - some areas (NYC and Boston) had DC power for a time before AC became the standard.

    If you total the filament voltages in those tubes, 18+60+36 equals 114 volts, which is right around the 110-115 volt wall voltage. This is a dead giveaway that the amp doesn't have a power transformer.

    The downside is that this type of gear is dangerous to use off an AC line. You can be electrocuted if you touch a metal part while it's plugged in (not even turned on in most cases). To make it safe, the line voltages need to be isolated from the amp and its chassis.

    I hate to see old amps get trashed...I suggest you look into one of these alternatives:

    If you're going to use the amp on a regular basis, you should have an isolation transformer to run it off of. If you're going the use the amp for practice or recording, that's the easiest alternative. The other alternative is to wire a power transformer into it - that's more work but is feasible. You'd need a transformer that is 120 volts on the primary and about 110-120 volts on the secondary side. You would then connect the secondary B+ to the existing wiring.

    It's not a project for a newbie (and I'm not suggesting you are), but it's not too difficult to do.

    EDIT: Sorry, I just reread all the responses that were coming in as I was typing...these are all good suggestions. Clearly you know what you're getting in to. I have a vintage RCA iso trans on my workbench - you can get a basic one cheaply online.

    -Kevin
     
  7. Wally

    Wally Telefied Ad Free Member

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    IIRC, some versions of the 703 have an isolation tranny and some don't. IT will be easy to see if there is one there or not.
     
  8. celeste

    celeste Friend of Leo's

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    Crawfish, In every case of 703's I can find, while the heaters are series string connected directly to the mains, the B+ is through a Power transformer, so the sum of the heater voltages is not a dead give away that it is a non isolated amp
     
  9. celeste

    celeste Friend of Leo's

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    That tube compliment would be a 703C, schematics here http://www.schematicheaven.com/bargainbin/kay703c.pdf and here
    http://www.schematicheaven.com/bargainbin/kay703c_2nd.pdf

    If you are up to taking some voltages (all the usual cautionary warnings apply here) take the voltage at the cathode of the 36AM3, pin 7, The plate of the 60FX5 also pin 7 and the plate of the 18GD6 which is pin 5.
     
  10. Crawfish

    Crawfish Tele-Afflicted Silver Supporter

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    I eat my words, and thanks for the correction. That's a rare case! Maybe I should say "in almost every case that's a hint that you have an amp with no PT." :)

    Thanks for the schematic, btw. I've got tons of these types of tubes and have been wanting to build a couple of amps around them.

    -Kevin
     
  11. Wally

    Wally Telefied Ad Free Member

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    I stand corrected, also.....it seems that even the first version of the 703 has an islation transformer. Apoogies.
     
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