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Elkin 4 tube any help is definatly appreciated.

Discussion in 'Amplifiers' started by EchoKilo, Nov 16, 2018.

  1. EchoKilo

    EchoKilo New Member

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    I'll try and be as direct and to the point, was talking on my 4 tube and everything was fine just as it always has been nothing was changed. Transmit just dies mid key. Tried to key up again thinking it was my mic cord, but the relay was just clacking. Decided to open the bottom see if anything got warm and 3 diodes pop out onto my desk. In the picture included I soldered them together so I wouldn't loose them I am assuming they went together because they fell out together but they weren't found like that. The diodes are 1n4753, 1n4738a 714, and a signal diode (?) 4753a (?) Not sure if that's correct that One is very hard to tell. Any info anybody has I appreciate it. I know schematics on these are very hard to find. If anybody has pictures of under sides of these I can use for reference would be greatly appreciated as well. 73s
    1542411428830-427429257.jpg 15424116438161486863110.jpg


     

  2. kopcicle

    kopcicle Sr. Member

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    Okay let's get so old news out of the way . That's Ray Felts work. (believe that's Raymond Edgar Felts Jr of Elkin NC but I've heard it other ways). When monochrome gave way to color television tubes like the 12DQ6 could be had for pennies.
    Ray was something of a mad genius and as such may have kept notes but none that he has shared in 5 or more decades.

    What you are looking at is a grid driven amplifier. Without going into all the theory I can take a stab at where the parts came from. Start at the center tap of the transformer on one side and pin one of the tubes on the other. Somewhere in between will be two TIP 50's , the base of one is fed by several (two or more) 90V zener diodes in series . That is what you are looking at in your hand. No screen voltage? No amplification.

    I'll out Nomad Radio as a self taught Elkin mechanic. (we all are actually, since Ray didn't provide a syllabus) I happen to know he's written a bit about them over the years. I'll even include the schematic that provided a hint to what is contained in the dark recesses of my memory.
    4_tube_base_12dq6_power_supply_sch.gif
     
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  3. kopcicle

    kopcicle Sr. Member

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    Screenshot at 2018-11-16 20-36-16.png

    You'll have to do some research . Find the pin out for the TIP50 and the (6)12DQ tube.
     
  4. nomadradio

    nomadradio Analog Retentive

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    The diodes that failed are zener diodes. They serve to regulate the grid bias, I think.

    If the banded (cathode) end of the series (end-to-end) string of these diodes was soldered to a ground lug, that settles what they were doing. This would tell you that they were regulating a negative-polarity DC voltage.

    The tubes' screen-grid voltage is also regulated, but it's a positive voltage. A zener or string of them used for this part of the circuit will have the unbanded (anode) end connected to ground.

    Odds are that one tube 'flashed' or had an internal arc from the high voltage to the tube's control grid.

    Inside the tube. Ray didn't include any protection circuits for this. A tube that has enough gas contaminating the vacuum to cause an internal arc fault usually checks bad on a tube tester.

    Usually. But tube testers are not all created equal. The tube that caused the damage may check as if it's just fine.

    If a tester says a tube is bad you can usually believe it.

    If it says a tube is good, it MIGHT be.

    The amplifier puts higher voltages on the tubes than any tube tester will do. As a result, a tube with gas in the vacuum may break down in a linear and check good on a tester.

    May.

    Oh, and the transistor in the pic looks like the keying transistor, not a regulator.

    The 1N4753 zener has a 36-Volt rating. Two of them in a series string add up to 72 Volts in the circuit. The third diode's voltage rating (whatever it is) will add to the other two.

    Pretty sure they are (were) the negative-polarity control-grid bias regulators, not the positive screen-grid side of the circuit.

    The two 1N4763A zener diodes in RadioTech's drawing above are 91 Volts each for a total of 182 Volts. This feeds to the screen grids on the tubes.

    73
     
  5. Radio Tech

    Radio Tech Radio Operator

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    Funny seeing a schematic I drew many years ago here on this.
    Nomad is spot on. I have worked on tons of these years ago. No I do not anymore.
    Since you amp is tube regulated (0A2 or 0B2 the two small tubes) I have attached a picture of the location of the diodes that have fell out. Later amps I worked on I switched this to solid state using TIP 50 transistors. It actually created a more stable bias circuit.

    The one thing I do not see in your picture is the bias control pot. Perhaps it is still lying inside the chassis or fell out on the floor when you opened it as this circuit requires an adjustable bias, not fixed. So check around real good.

    Something had to go really wrong for it to heat the solder enough for these to just fall out. When techs worked on these they would change out the zeners until they had the correct voltage to obtain correct bias. This meant just tacking the diodes together without using "j-hooks" to hold them in place.

    The next problem is you rarely find two of these amplifiers that are identical. Really depends on which group he had assemble them and what stock pile of components he found as a bundle.

    This is the only 4 tube elkin I have here. Was dropped off 16 years ago and never picked up. Been sitting on a shelf since.

    I also wanted to comment on the filter capacitors. Looks like someone added a couple of caps across the old ones. Not a good idea and may have contributed to your failure.

    Good luck on your repair.
     

    Attached Files:

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  6. kopcicle

    kopcicle Sr. Member

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    See what I know :) I knew it was your schematic. I was hoping you would drop in Buddy .
     
    #6 kopcicle, Nov 17, 2018
    Last edited: Nov 17, 2018
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  7. Radio Tech

    Radio Tech Radio Operator

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    LOL. Yep, sort of a give a way huh. Seeing that schematic reminded me of many sleepless nights.
     
  8. kopcicle

    kopcicle Sr. Member

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    Left you an update over "there" . Some Siltronix humor.
    We've been doing this a long time Buddy. contesting.com/pipermail/amps still wonder who I am :)
     
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  9. Radio Tech

    Radio Tech Radio Operator

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    Ok, now I am lost lol
     

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