Startup woes with a 5C3

Discussion in 'Shock Brother's DIY Amps' started by Prophetsnake, Mar 28, 2020.

  1. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Just completed a 5C3 replica for a friend of mine. I fired it up last night and nothing, so I had a good look and found a stray strand of wire shorting the grid stopper to a filament pin. Snipped that and tried again. The amp worked and sounds quite good - from the two quick strums of a guitar I did before shutting down.
    The current limiter wasn't full on bright, but it was brighter than I'd expect, so I had a look around and tried again, this time measuring the B+, which was around 280V. The transformer is a Hammond 290BEX and with no load it's delivering 450V with only the rectifier tube installed. With either or both of the preamp tubes it's about 430 - so they don't seem to be the problem. But with either power tube (6V6) in either socket I'm back down under 300V. I've just spent a good hour and a half tracing, checking the circuit and I'm not finding an issue. The tubes were good, I robbed them from another amp, so.

    Any ideas?
     
    Last edited: Mar 28, 2020
  2. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    What is the voltage across the 6v6 bias resistor?
    What is the plate to cathode voltage 6v6 pin 3 to pin 8?

    Please post pictures of the 6v6 wiring and of the power filter caps/resistors.
     
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  3. D'tar

    D'tar Friend of Leo's

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    And what is the measured value of the power tube cathode resistor? And 6v6 p8 to ground measure?
     
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  4. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Thank you.

    Across the cathode resistor the voltage is 12.5
    The voltage from plate to cathode is 245.
    Now, the current limiter is still in line, but I can't see it lowering the voltage by 120 volts or thereabouts. On any previous build, I generally removed it straight away, but in the case of those amps the glow was negligible, in this amp, not so much.

    5c3n.jpg 5c3m.jpg
     
  5. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    The resistance across the cathode resistor is 248 ohm Voltage across the resistor is 12.43V
    Ground to plate is 257V
    Ground to first power node 265V

    All with the current limiter in.
     
  6. D'tar

    D'tar Friend of Leo's

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    Once you fire up the amp without issues such as direct short to ground/chassis or gross miswiring, the limiter is no longer needed and will only drive you crazy!!!
     
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  7. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Heh, I figured it was probably just that, but I didn't want to chance it without. You reckon it's okay to plug it straight in? the amp did make nice sounds when I plugged a guitar in!
     
  8. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    A couple things I cant tell from the pic.
    The 470R resistor touching the OT mounting bolt?
    The soldering job on the rectifier?

    (I think it is just the pic angle)
     
  9. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    The resistor is well away from the bolt, over an inch. It's just the pic angle. Yeah, not my best work, but I had to snip out the diodes I had in as they were the wrong type and I am out of the correct ones. I had a difficult time getting in there to make a pretty joint, but it's solid.

    So, plugged it in and you were right, I was worried about nothing. B+ is 373 and the plates are around 360, which couldn't be better. I played through it while it warmed up and it sounds fantastic. A little him which might be a bad ground as it seems to come and go, but success!
    thanks for your time and help!
     
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  10. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    I don't see your artificial center tap. Hmmmm:D
     
  11. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    It's at the far end of the circuit, on the last tube. I was thinking of adding a variable CT if I can't get the amp quiet, but I'll give it a chance before I do that.
     
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  12. D'tar

    D'tar Friend of Leo's

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  13. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    Check out what @D'tar posted.
    I want to put this delicately. Err... Umm...Your lead dress needs attention. Get the heater wires away from the rest of the wires. Try to get the rest of the wires to lay on the chassis. If wires have to cross, have them cross at right angles.
     
  14. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    OK. I thought it really didn't mater where you put the artificial CT, and I have used the elevated tap before, to good effect, but I usually only do it if I need to. What you say about V1 makes sense, though. I'll move it.

    I have to increase the value of the cathode resistor in any case. My friend has NOS 6V6 and dissapation is at 13.6 Watts, so I will crank that down just a little.
     
  15. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Okay, thanks. I don't mind criticism.
    I have a reasonably thick skin and an even thicker skull. I was going for as short as possible, but I have seen that originals of the period have them flat to the chassis. I don't know if I will do the lot, but I will move them around and see if any are giving me issues.
     
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  16. RollingBender

    RollingBender Tele-Afflicted

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    This is concerning...
    DEB69434-DEE9-47FD-B99D-63DD85B8F86E.png
     
  17. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Yeh, I saw that ok. Those particular 10Ks do that very easily. I was in and out of that joint very quickly and on the other side of that browning as well, so the coating must be very sensitive indeed. Had a clip on the capacitor lead too. I'm happy enough to leave them in for the time being.
     
  18. D'tar

    D'tar Friend of Leo's

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    That sign of over heating/over current shows in your first set of photos. This could possibly be the result of your stray wire shorting mentioned earlier.? You can use the voltage drop across the resistor and the value of the resistor to confirm the current through that node to troubleshoot possible issue or not. You should have screen current and whatever is typical for your pre amp tubes. Your plate dissipation is likely right where "I" would like it best:)

    I bet it sounds fantastic!!!
     
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  19. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Hmm, never thought of an actual overheat. as soon as I saw the lamp go bright, I killed it, but yeah, maybe. In fact I've already changed up the resistor so the dissipation is just under 94%.
    And it does indeed sound fantastic! I like it better than my 5E3, and I love my 5E3!
    The pre-amp tubes are 6SL7s with the cathode split on V1. I've kept one input as per original, and replaced two others with 5F1 Champ style inputs. It has a NFB switch with a 330k replacing the original 1M. It works perfectly. Super clean jazz tone with it engaged and easy dirt with it open.
     
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  20. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    Many thanks for all the help gents.
    I've been tinkering with the amp and have ironed out almost all of the problems. Almost dead quiet with just a tiny hiss with all three knobs up at max. The tone is really sweet as well. Voltages are all in order as well.
    Just one thing left. When I switch off I get a fairly load 'snap'. Nothing scary or startling, but more than I am accustomed to.

    Any suggestions?
     
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