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Thread: 6J7, 6F6, 5W4 single ended amp

  1. #1
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    6J7, 6F6, 5W4 single ended amp

    I recently picked up 3 Webster Electric intercoms from the late 30's (I believe)
    They're really heavy for their size and have big power transformers. They're all the same model: 212 CS

    The tube complement is 6J7, 6F6, and 5W4 rectifier. I'd like to minimally change the circuit and turn them into guitar amps. I traced the schematic out by hand and was able to find the repair manual online which helped confirm that my schematic was right. It also told me that the internal speaker is 50 ohms which means that the output transformer will likely be all wrong for guitar.

    I wrote the schematic out in the style of the Fender champ 5C1 because I wanted to easily see how it compared. It differs in some ways I find confusing. On the schematic I simplified some things like removing all the switches and relays for the intercom. there was standby switch which I disconnected for the time being

    I removed the microphone transformer and connected the guitar input directly to the grid of of V1 (6J7). The ground on the jack connects to the bottom of where the mic transformer was (the .1 cap, 150K resistor)

    This circuit is really confusing me. Any time I try to change the value of any resistor or capacitor following V1 I get a lot of distortion, high frequecy ringing and loud hum. I'm also getting a confusing grounding problem where when I hold my hands on the strings of the guitar it actually hums louder.

    I'm considering taking all the components out and starting from scratch because the wiring is a real web and the solder connections are poor. In this case I would wire up basically the Champ 5C1 circuit. I definitely want to keep the tubes that I have in there? I don't know how to change the values for the 6SJ7 and the 6V6 for 6J7 and 6F6 although I've read that they are similar tubes.

    I think I've gotten in a bit over my head, but the circuit seems simple enough I think I just need a push in the right direction.

    thanks!
    Attached Thumbnails Attached Thumbnails receipts004.jpg  

  2. #2
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    In addition to removing the mic x-former I would also remove the resistor and cap from the input circuit. Connect the jack (signal pin) to the grid thru a 22k stopper resistor and ground the grid thru a 470k (grid to ground). Connect the jack ground pin to the input circuit ground.

  3. #3
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    The problem I'm having when I try to remove that capacitor and resistor is that I then get a very distorted, lower power signal with high frequencies. Also there is a grounding issue because whenever I touch my hands to the strings I get louder hum.

    My instinct is to rip everything out and start from scratch but I can't find a suitable circuit with this tube complement and I've never tried such a thing. Still it seems like maybe the best option.

    I'm thinking that the best thing to do would be to rebuild the power supply and work from there. I haven't done this so some advice would be welcome. Would this be the process?
    1. ) measure secondaries of OT with my voltmeter

    2. ) add 3 filter caps like on Champ 5C1 and see what the voltage drop would be at A (rectifier), B (OT), & C (plate of 6j7, grid of 6V6)

    3.) based on voltage at C (going to plate of 6j7) change values of resistors and caps to reflect page 2 of http://www.r-type.org/pdfs/6j7.pdf

    I was planning to use the choke from the intercom but I'm not sure how much this will drop the voltage. How should I calculate the value of the other resistor? Once I get the preamp wired up should I just leave the 6F6 as is with the 470 ohm resistor on the cathode?

  4. #4
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    The input transformer primary needs a ground connection otherwise your guitar will float away.

  5. #5
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    For now I've disconnected the input transformer and soldered the connections to the secondaries to the input jack.

  6. #6
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    The x-former secondary provided the ground reference for V1 grid. By removing it the guitar PU becomes the ground reference. If you unplug the guitar then you lose the reference. This is why I recommended adding the grid resistor to ground in my earlier post.

    I would remove the mic circuit (x-former, resistor and cap) and replace with a more standard guitar amp circuit before doing anything else. As for the power supply I would leave it alone unless you know it is faulty.

    The 6J7 appears to be a very high gain device. Assuming that the original mic had a much lower signal than a guitar PU you might need to add some NFB to the input stage.

    DG

  7. #7
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    Great, that fixed the grounding problem. I removed the 500k resistor going from the .004 coupling cap to the grid of the 6F6 which gave me a bit more gain. I'm sure the 6J7 is capable of more gain though

    I tried changing the 250K resistor to 1M coming off the grid of the 6J7 but got a distorted, lower output sound and removing the 100K resistor had a similar effect. Any ideas to get more gain?

    I'm pretty sure that the tubes are good. I have 3 of these intercoms in all so I've been trying all of the tubes at one point or another

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