6V6 JCM800 ?’s

Discussion in 'Shock Brother's DIY Amps' started by ZMurph, Jan 17, 2020.

  1. ZMurph

    ZMurph TDPRI Member

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    Hey guys,
    This is my third amp build. First was a Boothill 5F1 kit and the second was a Two-stroke. I’m slowly learning more and more as I go, and having fun.
    So the amp successfully powers up and makes proper sounds. I do have a few issues though.
    1st: my voltages are high. I’m using the Allen TP25 and TO26 transformers that Rob recommends on his website. My B1+ is 448VDC. If I’m looking at the schematic right, I should be around 395V. The TP25 spec sheet shows the hv lines at 310VAC each. I’m measuring around 340VAC each. My line voltage is 122-123VAC. I’m using some “matched” Groove Tube 6V6GT power tubes that I happened to have. I’m reading plate voltage on both at 447V (from pin 3 to ground.)
    2nd: when I turn the MV up past halfway, I get a high pitched (not loud) squeal. I’ve determined that it is affected by the presence control. The higher the presence is turned, the higher the MV has to be before I hear the squeal. I thought maybe it is the big orange drop capacitor (104J) I used on the presence pot.
    3rd: the amp sounds good using the low input. When I use the high gain input, it sounds really thin and treble-y, kinda weak.
    I know my soldering is not that great, but I did check that I’ve got good connections. I also tried to use a heat sink as much as possible. Could my wire routing be causing issues?
    I’ll try and post some pictures below.

    Thanks in advance for any insights.
    Zack
    3DA340F7-8128-4E0D-AFF7-1FF64B398302.jpeg 29C00D87-7F66-4D0F-B8DA-EB853193AC5C.jpeg
     
  2. elpico

    elpico Tele-Holic

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    What's the idle current on the 6V6?

    Have you tried the usual "swap the OT primaries" thing for the squeal?
     
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  3. ZMurph

    ZMurph TDPRI Member

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    The primaries or the secondaries?
     
  4. EmWeAh

    EmWeAh TDPRI Member Ad Free Member

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    The primaries. Sometimes your negative feedback turns into positive feedback.
     
  5. ZMurph

    ZMurph TDPRI Member

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    I moved the negative feedback line to the other speaker jack, where the other OT secondary wire is connected. It made the squeal happen as soon as the MV is turned from full down, so I swapped it back.
     
  6. ZMurph

    ZMurph TDPRI Member

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    Should I still try swapping the primaries?
     
  7. Nickfl

    Nickfl Tele-Afflicted

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    Yes
     
  8. muchxs

    muchxs Doctor of Teleocity

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    Transformer performance under load is a widely misunderstood topic.

    You're using Allen transformers? Talk to David Allen.

    Or... punch a hole in the chassis. Add a 5V4 rectifier.


    The TP25 is a horse. Conservatively rated at 150ma, you know it has sack if it will support a pair of EL34s. It will support a pair of EL34s.

    The more current available the less the voltage will sag under load.


    A TP24 may have been a batter choice. Or a Classic Tone 40-18027.


    Lotta guys suggest zeners or bucking transformers. Don't need any of that if you use the right transformer.

    As other people have said, flip your OT primaries.

    Bill_the_Cat-jpg.png
    Shoulda led with that one. It only takes on bad connection to screw up the whole operation.

    Good solder joints are essential.
     
  9. ZMurph

    ZMurph TDPRI Member

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    ;) Yeah, I shoulda started with that. Thanks everyone for all the help. I’ll swap the OT primaries first chance I get. Unfortunately it’s gonna be a few days before I can do that and give you an update.
     
  10. ZMurph

    ZMurph TDPRI Member

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    Correction. I ended up having a little more time than I thought. I swapped the OT primaries and that fixed the squeal. Thanks for help guys!

    As for the other stuff, I’m still getting around 446-447 plate voltage. That’s measuring with my meter from pin 3 to ground and also using my old Weber bias-rite probe. Also, it’s showing idle current on each tube at 15.8mA and 16.2mA respectively. That’s the most I can get with bias trim pot.
    I measured the resistance of the OT from the plate connection at each tube to the center tap and measured 117 and 130 Ohms respectively.
     
    Last edited: Jan 17, 2020
  11. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    I'm very interested in sound clips (/videos). I'm planning to build one when I have a bit more time...
     
  12. ZMurph

    ZMurph TDPRI Member

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    I’m still trying to figure out the voltage problems. My negative bias voltage is too high even with the bias adjust maxed out, so the tubes are running super cold. I guess that’s why the plate voltage is running so high.
     
  13. elpico

    elpico Tele-Holic

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    Verify the screen voltage isn't low because that 6V6 current is low. If the screen voltage is also high then I'd want to do two things, reduce the B+ voltage and adjust the bias circuit to bring the current up. Bringing the current up will probably not by itself pull the B+ down into the range you want.

    MuchXS mentioned two easy ways to do that though, and actually it looks like you have an unused hole for the rectifier socket right next to the PT with the yellow rectifier leads taped off right there and ready to go, so very easy to go that route. A zener works too if you prefer that.
     
  14. KT89

    KT89 Tele-Meister

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    The Allen is a 310-0-310 at 150mA. A quick 310*1.41 calculation gives 437, and the extra few volts can be accounted for by mains variations. As muchxs pointed out, you used a transformer rated for multiple times what you're asking of it so it's not even breaking a sweat to deliver 440. The ways you can remedy this have also been pointed out in the thread. It sucks, but if you want a lower voltage then your power supply needs some redesign in all likelihood.
     
  15. ZMurph

    ZMurph TDPRI Member

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    What about if I just use the EL34 tubes? I used transformers and choke that Rob lists on the JCM800 mini site. The only discrepancy I’ve found is at the bias circuit. His schematic shows a 7.5k resistor going to the bias trim pot and the layout shows a 10k. Right now I can’t get the bias voltage above -45mV (with each leg balanced.) The tubes are pretty much at cutoff.
     
  16. radiocaster

    radiocaster Poster Extraordinaire

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    I don't know about bias, but EL34s have a slightly different pinout, if you want to pop them in you need to add a connection from the cathode to the suppressor grid, which is connected internally in 6V6s, 6L6, etc.

    [​IMG]

    [​IMG]

    When you add this, you can put any of them in, as long as the bias works for what you're putting in.
     
  17. ZMurph

    ZMurph TDPRI Member

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    It’s already there. Rob had this on the layout so you can use 6V6, 6L6, or EL34 tubes.
     
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  18. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

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    Are you using shielded wiring in high gain/grid areas?

    High gain amps are also a lot less forgiving of long and sloppy lead dress. I learned that the hard way.
     
  19. ZMurph

    ZMurph TDPRI Member

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    No. I though shielded wire was only needed from the input Jack to the grid stopper at V1A...?
     
  20. ZMurph

    ZMurph TDPRI Member

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    I’ve taken the bias trim pot and balance pot out and checked them and doubled checked the resistors. Everything checks good. I put everything back and my negative bias voltage is -66mV before the bias controls and, at each leg after the balance pot, the most I can bring it up to -45mV.
     
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