EF80 Small Pentode Push-Pull Power Amp

Discussion in 'Shock Brother's DIY Amps' started by robrob, Dec 13, 2020.

  1. tazwolf

    tazwolf TDPRI Member

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    I have read about putting silicon o rings between the valve socket base / mounting screw holes and the chassis and you dont need to desolder the socket if its needed just unbolt it. A useful mitigation if microphics turn out to be a problem.
     
  2. Paul-T

    Paul-T Tele-Holic

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    I wonder about the simple rings as you still have a mechanical connection via the bolts. I think I will investigte buying the silicon and cutting out a gasket. I see now there are oven tray liners that look promising. I'll try those. I'll probably add some kinds of O-rings to the mounting screws for the chassis too.

    existing ones seem ludicrously overpriced, although perusing this kind of 'review' I can see why. They obviously have mystical properties.

    Edit: now I've seen the oven trays yes, I'll try and shockmount the sockets and bolts, and do a basic shockmount for the chassis (which sits on the bottom of the cabinet), like a reverb pan.
     
    Last edited: Jan 30, 2021
  3. tazwolf

    tazwolf TDPRI Member

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    Gasket seems to be a good option. With the o rings you use a big one for the socket base and smaller ones for the bolt holes with washers.
     
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  4. NSB_Chris

    NSB_Chris Tele-Holic

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    I have been thinking about this also for an amp with a 5879 preamp tube. My thought is that you need a good flexible support like rubber to isolate from chassis vibration, but you also need firm stops for putting the tubes in and out. Seems to me mounting the tube to a good size plate and then supporting that plate with the resilient mounts in the chassis would be the way to go. Could be as simple as supporting the plate like reverb tanks are often mounted.
     
    Last edited: Jan 30, 2021
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  5. NSB_Chris

    NSB_Chris Tele-Holic

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    One option I see could be accomplished with just some tubing and typical standoffs. See sketch below. Smaller tubing on the stadoff maintains lateral isolation between the socket plate and the standoffs, and larger tubing around that maintains vertical isolation between the socket plate and the chassis. This is sort of like how some pickups are mounted with tubing instead of springs.

    If the edges of the socket plate are bent over with a small gap to the chassis, tubes can be pushed in without damaging the isolation tubing. Taking the tubes out, you would just have to brace one hand on the socket plate and pull the tube out with the other.

    I would try to do it such that only round holes are cut in the chassis. Just easier and cleaner finish. I think isolating a plate holding two power tubes would just need 3 isolation mounts.

    res mount.jpg
     
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  6. dougsta

    dougsta Tele-Meister

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    I use the vintage B9A rubber skirted sockets, get them from ebay for £5-10
    Screenshot 2021-01-30 at 16.52.28.png
    Doug
     
  7. Paul-T

    Paul-T Tele-Holic

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    Ah, great advice Doug. I was short one McMurdo socket for this build in any case.
     
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The EF80 is known for microphonics so in a combo you will need to use rubber washers to isolate the sockets and I would put silicone damping rings on the tubes too. You may have to try a few extra tubes to find some that will work.

    All of the Hammond 125 output transformers will work with an EF80 or 12A*7 power tubes. The A is rated at 3 watts and is plenty.
     
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  9. Paul-T

    Paul-T Tele-Holic

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    @robrob Micro EF80: probably academic, but 270R, or 330R for the power tube cathode resistor and is wirewound ok?
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Paul, I'd go 330R for the cathode resistor for a set of two EF80s in push-pull. 300 will work with most EF80s as long as the plate voltage isn't above 300v. A 270R will work with a plate voltage around 270v.
     
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  11. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    And Paul, I’m guessing wirewound would be fine, or metal oxide. And 2W would be way plenty, if I’m thinking straight.
     
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  12. Paul-T

    Paul-T Tele-Holic

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    Thanks folk. PT , OT, most bits are here, mapping out the board right now to try and leave half a tube free for a trem, hoping to place the turrets tomorrow before picking up some more hardware Monday.

    Then I'll post a thread, likely with lots of requests for hand-holding.

    David Bowie used to say you need to swim out to the deep end a few meters away from where you can feel the bottom and that's where I'm heading,
     
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  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Great, Paul. Did you say you'll be building a 5E3 micro, or was it something else?
     
  14. Paul-T

    Paul-T Tele-Holic

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    Deluxe micro PT 1.jpg Yup, Deluxe Micro: I'm hoping to lay out the turrets tomorrow.

    I've eliminated V1b, the normal channel, with the plan that a few months down the line I can add a tremolo (if I can change the 12AU7/12AX7 V1 and V2 for 12AX7/7025). I've added a B+4 for a 1-tube reverb that I'll add if i can get the basic thing working. And I've left space for the reverb components.

    The whole thing looks feasible, as long as it's OK to mount the board on 10mm standoffs, with the PT underneath (or, actually, above). I think the OT will be inside the chassis. Then there are other little issues, for instance I think this design has 1.5k grid stoppers, it could be super-hard to fit those on the tiny valve bases - especially as they'll be a little trickier to get to in the rubber-mounted version
     
    Last edited: Feb 13, 2021
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  15. dan40

    dan40 Friend of Leo's

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    You may want to reconsider mounting the PT that close to the circuit board and sensitive preamp components. It may create a noise issues that will be very difficult to eliminate. Try to mount the PT as far as you can from the preamp section of the board.
     
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  16. Paul-T

    Paul-T Tele-Holic

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    }
    Yes, I'm worried about that.

    However, the PT would be in the normal position by the filter capactitors but partly on top. Other end from the preamp. It's a standup so overall it's not that much closer than a typical PT. But space is going to be cramped.

    I can make the board physically shorter (probably cutting it today) but then I'd have more components off the board which in itself will be more confusing and could induce noise.
     
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  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Paul, darn it, now you’re making me curious to know more. Is it time to start a planning thread? :):)
     
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  18. Blister

    Blister Tele-Meister

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    So when it's all said and done cuz all that is way over my head how much would you charge for an amp?
     
  19. reggiepe

    reggiepe TDPRI Member

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    Love the idea of a low watt pentode power section...just thinking that this may work in a Brownface Deluxe (6G3) as the tone of that amp (I believe) is from driving that output section hard. So, to incorporate that, I am thinking that instead of cathode biasing, a fixed bias output section could be incorporated and then have the ability to keep the bias wiggle tremolo. Most all P/P EF80 output sections I have seen are cathode biased. Is that the hard and fast rule for these tubes, or can fixed bias be applied?
     
  20. Len058

    Len058 Tele-Meister

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    I can't find the grid stopper resistors in the Deluxe version. The Bassman has both grid leak and grid stop but it has no cathode bypass cap. Why the difference?

    I'm trying to understand the circuits so I can replace two el84 with EF80's. So far the Deluxe resembles the amp I want to change the most but it has a cathode bypass cap and grid stoppers. Sorry my bad it has a LTP so it doesn't resemble the deluxe.
     
    Last edited: Feb 28, 2021
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