5E8A Low Powered Twin Build Thread

Discussion in 'Shock Brother's DIY Amps' started by guitarbuyer, Dec 18, 2013.

  1. guitarbuyer

    guitarbuyer Tele-Meister

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    Ok, logged some time last night and got most of it wired up. Still waiting on the backordered power transformer but everything else is done. Any glaring mistakes, shout it out -

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    A couple of questions -

    I grounded the two power tube sockets through the ground on the speaker jack, I assume that is cool?

    Just want to make sure that the bias pot is wired correctly. That is the only thing I haven't been able to verify completely. It is wired like this:

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  2. guitarbuyer

    guitarbuyer Tele-Meister

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    About to wire up the rectifier sockets and curious about the differences in the various layouts. Specifically the connection of pin to each socket.

    The Weber layout states to not common them. But on the Fender layout and the Ceriatone they are connected. What is going on with that?

    Also intrigued with the Ceriatone method. Those diodes are there to protect the transformer? Can't think of a downside to adding them. They are 1N4007? Those .01 caps. They spec 3K volts. Do they need to be that high? I see them around 630v but not 3k.

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  3. mysticwhiskey

    mysticwhiskey TDPRI Member

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    The W022798 power transformer from Weber is rated at 3A for the 5V filament winding, which is only enough to power one 5U4GB rectifier - hence the warning. I assume the power transformer supplied by Ceriatone has a 5V winding sufficient to handle two rectifiers.

    I'm not 100% sure about the capacitor voltage ratings, but my guess is that they need to be rated high enough to handle the initial transients (voltage spikes from the surge current) on startup.
     
  4. sjhusting

    sjhusting Tele-Afflicted

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    I have found that just using 1 rectifier socket and using a 5ar4 is close enough for me, and when I want more sag or whatever, I can switch to a 5u4. Just my opinion, though, I was never anal about building 100% clones. The diodes are cheap insurance and I put them on all my tube rectifier builds. They saved me once when a modern GZ34 shorted out on the bench. Sure was a pretty thing to watch.

    If the layout allows, I try to use 2 350V for each of the first two filter stages (where the second stage feeds the screens). It really depends on if/how/where your standby is (I don't use standbys anymore) and if your first stage is strictly a reservoir or if you are feeding the plates from it. I suspect most good quality modern electros would survive the power on peaks, but caps are cheap. I also put a current inrush thermistor on mains live. I have no idea if it helps but it can't hurt.

    Steven
     
  5. guitarbuyer

    guitarbuyer Tele-Meister

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    So, so for the most part, very good news. I have what amounts to a working amplifier. Though it took a bit of wrangling before I got to plug in a guitar. Have an issue I need to sort out.

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    So, upon going through the start up procedure all went well until the power tubes were added. With all knobs at zero, there is an ear piercing squeal. Intolerable. So I shut it off straight away and began investigating. On a side note, I did find a pair of wires on the first preamp tubes that needed switched. I was optimistic that was the issue. It was not. Seemingly unrelated.

    Anyway, what I have discovered, with the Treble control on 12, squeal is gone at which time I plugged in a guitar and made wonderful music (currently just plugged into an 8 inch 8 ohm speaker on my work bench).

    So I have inspected all the solder joints on all the wires on and around that circuit. No change. I did discover that with the presence and bass controls engaged, that does change the situation. As I increase those two, the amount I can turn down the treble increases before the squealing resumes. I chopsticked everything and I moved all the wires that surround it and that does not change the sound.

    I did swap preamp tubes around with no change.

    I did make sure that those three caps were grounded well. With the issue changing with the interaction of those three controls, my gut is leaning to those caps. Do you think there is a problem having the grounded so close together? Here is a closer picture of that particular cluster:

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    I'm not really sure what I should pursue at this point?? So I turn to you for help.

    The second ground you see here on the output jack was moved. It was the pin 8 ground for the power tubes. That has since been grounded to the power tube socket mount screw.

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  6. guitarbuyer

    guitarbuyer Tele-Meister

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    deleted to avoid confusing the issue.
     
    Last edited: Jan 24, 2014
  7. Glen Smith

    Glen Smith RIP

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    Have you tried inverting the red and blue wires to the output transformer?
     
  8. guitarbuyer

    guitarbuyer Tele-Meister

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    Just swap them? Red wire to pin three and the blue replacing the red on the board? I can give that a try.
     
  9. tubeswell

    tubeswell Friend of Leo's

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    "Low-powered twin" makes me chuckle. The squeal is probably coming from positive feedback in the NFB loop. Disconnecting the NFB will confirm it, and if that's what's causing the squeal then swapping the OT primary winding (OR secondary winding) ends around will fix it. (Its to do with the phasing of the feedback signal at the pre-amp gain stage NFB-insertion point viz-a-viz the phasing of the output transformer signal - from whence the NFB loop originates).
     
  10. Glen Smith

    Glen Smith RIP

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    Yes.
     
  11. mysticwhiskey

    mysticwhiskey TDPRI Member

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    Wait - aren't you meant to swap the primary wires to the power tubes (blue and brown), and leave the red wire alone, the one going to the board?
     
  12. Glen Smith

    Glen Smith RIP

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    Oops! I think you are right.
     
  13. guitarbuyer

    guitarbuyer Tele-Meister

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    Yikes, I just made the swap (blue and red). It worked. Quiet as a church mouse. That is the good news. But, it definitely lost something. Hard to really tell since it isn't connected to a proper speaker yet. In fact, I just was thinking I hurt that speaker since it is not rated for that kind of power.

    Should I put the red back and swap the blue and brown?
     
  14. Glen Smith

    Glen Smith RIP

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    I think you should try it.
     
  15. guitarbuyer

    guitarbuyer Tele-Meister

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    Worked like a charm. Swapping the blue and brown (red back to the board) and the amp is super quiet with almost zero noise at any volume at idle.

    Thanks a lot for the quick solution. Sounds pretty damn good even through that little speaker. Just waiting on the cabinet now.
     
  16. guitarbuyer

    guitarbuyer Tele-Meister

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    No kidding. Nothing low-powered about it. Thanks for the explanation. Makes sense and jives with what I looked up quickly when it was first suggested.

    Now I am off to learn to properly set the bias.
     
  17. guitarbuyer

    guitarbuyer Tele-Meister

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    Bias pot worked like a charm. Used the Uncle Doug method found here: http://www.youtube.com/watch?v=pjKYiSr497w (that is part 2 - part 1 explains the theory). I dialed it right into 16.2W PD which is just about dead on 70% for grid biased 5881s.

    Thanks to everybody for answering questions and to hackworth for getting some of the parts for me and encouraging me to go for it.

    This has been a fun build and can't wait for the cabinet so I can give it a proper workout.
     
  18. 0018g

    0018g Tele-Meister

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    I've only been around one of these(mine), but I don't think it's loud at all for a 40 watt amp. Speaker selection has a lot to do with it, however.
     
  19. guitarbuyer

    guitarbuyer Tele-Meister

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    Can we chat about the bias? I don't think I have it right.

    After playing the amp a while, I checked voltages to make sure everything was good. But I discovered I have 425 volts on the plates. The 5881 has a max voltage of 400.

    If I measure the resistance across the output transformer (half for each tube) I am at 185.4 ohms. If I measure the voltage drop across that same link, I am at 7.5 vdc (this is the lowest I can go with this pot incidentally).

    Simple math tells me that I can divide that voltage drop by the resistance (v/r) to get the plate current. In my case it is: .0404. I then can multiply the plate current by the plate voltage to get the plate dissipation (PV)(PC)=PD. In my case, that is 17.21 watts.

    As I increase the bias pot, sure I can get the voltage to go down but the plate dissipation increases and I am already a watt over where I should be.

    I can't figure out what I can do to get this within the parameters of the 5881. Can I even do it with this setup or should I go to 6l6GCs?

    Max plate current is 400vdc and plate dissipation is 23w (using 70% is 16.1 for grid bias)

    Messing around with the math, best path may just to use the 6l6GC-STR which can have up to 500vdc on the plates and 30 watts.
     
    Last edited: Jan 26, 2014
  20. guitarbuyer

    guitarbuyer Tele-Meister

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    So, I have a pair of 6l6gc-str on the way. Which I think will solve my problem as I will get a higher safe plate dissipation which will lower the plate voltage assuming I understand the math above.

    For my own education, are there any changes I could make to the bias circuit to make 5881s work? I have some really nice NOS tubes I would have liked to use.
     
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