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Old Teisco running... almost. Hmmmmm.

Discussion in 'Shock Brother's DIY Amps' started by FiddlinJim, Apr 26, 2009.

  1. FiddlinJim

    FiddlinJim Tele-Holic

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    Well, I got the Weber power transformer transplanted in the old Teisco and I get decent tones, but a loud 60Hz hum which increases with volume. Since there's only one tube (a 6C6) before the volume pot, it's got to be 'round there someplace. Of course, that's the part of the amp I touched the least.

    I've replaced the PT, filter caps, cord, power switch, fuse holder, and removed some mojo, I suspect. I did remember to ground the center taps of the PT.

    The 6C6 tube is one of those with the grid connector on top with a shielded wire. I s'pose I could check it, but I suspect shielding only would make a difference with RF interference, right? The input jacks are flimsy and a mess, so I can check grounding there. The heater wires don't seem to be near the grid, so I don't think the hum comes from there.

    Hints about tracking this down?
     
  2. celeste

    celeste Friend of Leo's

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    If you are sure it is 60hz and not 120hz, then it is mains and not lack of filtering on the B+. That points to heaters. Is the heater winding referenced to ground in some way?
     
  3. FiddlinJim

    FiddlinJim Tele-Holic

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    I'm pretty sure it's 60Hz, and it increases directly with volume, so I suspect something with the first preamp tube. The heater windings have a center tap which is referenced to ground.

    The filter caps had been replaced once, but to be sure, I replaced them again. The original (disconnected) cap can was 10/10, but probably had a direct cap off the rectifier that was 20 or so. The replacements were 22 - 22 - 8, and I replaced them with 22 - 22 - 10 (500V F&Ts).

    Voltages look good (B+ around 360).

    From the thread about diagnosing hum you (Celeste) sez: The two major sources of 60hz are heater to cathode coupling and magnetic coupling between the PT and OT

    Since the OT is on the speaker frame and the hum is volume dependent, I'm not too worried about it.

    Would heater/cathode coupling be a tube-internal thing? If so, I've got a couple of replacement 6C6's on order. I don't see any physical "nearness" of the heaters and the cathode leads. The 10uF bypass cap is old and I haven't replaced it yet, but I would expect the only thing there would be low-end response, right?

    The input jacks are pretty dodgey, so I'll play with them and maybe try the 6V battery on the heaters trick.
     
  4. celeste

    celeste Friend of Leo's

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    yes, if the volume control effects, it has to be before the control unless the control is the grid reference for the following stage.
    If it is 60hz, then it is not a B+ issue because full wave rectification creates 120hz ripple, not 60hz
    When I wrote those words I was making an assumption that I did not say and should have. I assumed the layout was not Bozoed. That means lead dress to prevent capacitive coupling, a grounding scheme that prevents large ground currents from modulating the input stages and one and only one ground connection to the chassis so it can act as a shield. I also assume that all contact connections are free of dirt and corrosion.
    Yes, heater to cathode coupling is inside the tube, This is where heater elevation comes into play. Any winding in use on a transformer need to be referenced to ground in some way or it floats and you have no way of controlling where it floats to. If you just ground one side of the heater winding, then that side is locked at ground and so even if you twist the heater wiring you do not have balanced voltages so the electric fields do not cancel out. If you ground the center tap, your heater swings 3.15v above and below ground 180 degrees out of phase so magnetic and electric fields tend to cancel. Inside the tube things get tricky. The heater is insulated with a porous ceramic, usually aluminum oxide. It is just intended to keep the heater and cathode from touching. What it does not do is keep the heater from developing it's won electron cloud when heated, just like the cathode does. Because it is hotter then the cathode it can develop a really dense cloud. If the cathode is more positive then the heater, then the electrons in the cloud are drawn to the cathode and you get heater noise injected into the cathode. The electrons of the cathode cloud are never drawn to the heater because the heater is so much hotter and the it's cloud is so dense. Now, if you bias the heater so it is more positive then any cathode will ever swing, then the heater electrons will never be drawn to the cathode. That is called reverse biasing the heater/cathode virtual diode.

    Sorry if that is too long winded. It is in chapter 1 or 2 or RDH4.

    Running the heaters from a battery will tell you about heater noise pretty quick. It does not stop the heater/cathode coupling, but there is no noise on the heater to inject.

    On an older amp, I would look for corrosion. Input jack, it is possible the chassis in connected to ground through an uninsulated input jack, so take the jack out and if it is not insulated, make sure there is a good connection.
     
  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    In my tinkerings, I've noticed that if I disconnect one plate of the rectifier tube (forming a half-wave rectifier), I'll get 60 Hz hum instead of 120 Hz buzz. Might be worth trying another rectifier tube or checking all your connections there.

    - Scott
     
  6. celeste

    celeste Friend of Leo's

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    Yes, but the voltage across the the rectifier would double. He says the B+ is in the right range, so I discounted the half wave idea.
     
  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Yeah, it was kind of an outside shot. Still, on my amps, when I disconnected one of the plates, the B+ didn't halve -- it went down by about 30-50V instead.

    - Scott
     
  8. FiddlinJim

    FiddlinJim Tele-Holic

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    First of all, Celeste, you Rock! Never cease to amaze... Thanks for providing that kind of background.

    Since this layout (although with cheezy hookup wire) looks obsessively neat, any potential "Bozoing" would have been done by me, which is highly likely.

    I've got a couple of 6C6s coming which will probably rule out the heater cathode coupling, but hey, worth a shot. I've got a few #80 rectifiers, so I can swap them and rule out issues there.

    Thanks again; I'll keep ya posted.
     
  9. FiddlinJim

    FiddlinJim Tele-Holic

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    PBC (Potential Bozo Confession): On my way to work, I was thinking about this 6C6 tube and remembered that someone (I guess that'd be me) powered up the amp with the 6C6 rotated by one pin. All I was getting was Buzz (no signal) and I tracked it down to that. In my defense, there's a huge tube shield around V1 and although the heater pins are a little larger in diameter, it is pretty easy to put the tube in wrong and I did. I had powered it up "softly" (light-bulb current limiter) and these old tubes are probably pretty tough, but I suppose putting plate voltage on one heater or heater voltage on a cathode could have damaged the 6C6.

    I'll test this theory when AES sends me my 6C6 tubes.
     
  10. FiddlinJim

    FiddlinJim Tele-Holic

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    Teisco-San Lives!

    Well, I found a loose heater wire, not at V1, but at one of the power tubes, so I had an open heater circuit. Bad trip to buzz town.

    So, a new PT, new e-caps, new power cord and switch, a bunch of spare tubes, and much, much labor from me and help from you guys and the dinosaur lives. I learned a lot, which is what it's all about for me.

    It sounds kinda like I expected it would. It's probably sort of close in volume to a what I think a TV-front deluxe would be (push-pull, paraphase inverter, 2 6V6-like power tubes and a 5Y3-like rectifier). It's pretty squishy as you might imagine.

    Thanks again for your help guys and gals.

    Jim
     
  11. jh45gun

    jh45gun Banned

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    Congrats on getting fixed and finding your hum issue
     
  12. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Hooray! Another ancient device brought back from the dead! :lol:

    FWIW, there are octal versions of some of those tubes: 42=6F6 and 6C6=6J7, in case that helps you find useful info. The 76 was a really low-gain (Av=13.8) tube, and it doesn't look like they bothered to repackage it in octal (or other) form.

    - Scott

    P.S. Do you have any pics (or sound clips) as it exists now?
     
  13. FiddlinJim

    FiddlinJim Tele-Holic

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    Thanks for the congrats...

    I think most of the signal gain must come from the 6c6 V1. The first 76 tube must do a little post-tone stack gain make-up and the other two do the phase splitting.

    I still need to clean the amp up a bit for my friend, and I'll take some glamour shots and post them.
     
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