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EF80 Small Pentode Push-Pull Power Amp

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

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

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    @dougsta used a pair of EF80 small, novel pentodes in his push-pull power amp design and after some discussion in that thread I decided to use the EF80 datasheet characteristics curve to design a power amp to use with my micro amp designs. New Old Stock EF80s (also called 6BX6) are inexpensive at around $6 and the supply is plentiful. The EF80 uses standard 9-pin sockets and 6.3v heaters so they are easy to implement into a modern amp build.

    [​IMG]

    I decided to use 300 plate volts even though the datasheet lists the max plate and screen voltage at 250v since most great tube amps exceed these limits ;) The Hammond 125B output transformer wired for 22.5K:8 ohm is a near perfect match for a pair of EF80 in push-pull. A 240 ohm cathode resistor is also a perfect match for a 2xEF80 amp and yields the desired 100% max plate dissipation at ide for cathode biased power tubes.

    [​IMG]

    The EF80 version of the Bassman Micro LTP uses two EF80 pentodes in push-pull for true pentode power tube overdrive with 2 watts of output. Pentodes overdrive differently than triodes due to their screen so this is a way to get pentode overdrive in a 2 wattish power amp. The power amp shown below can be adapted to pretty much any preamp, including high gain amps like the Soldano SLO-100 and 50. For best results keep the plate voltage between 220 to 300 volts (design max is 250v for plate and screen).

    The Bassman Micro LTP EF80 below uses 295 plate and 290 screen volts using a Hammond 269AAX 200-0-200V power transformer. The 269AAX is a good choice for any amp using an EF80 power amp but it does not include a 5v secondary so use an EZ81 (6.3v) tube or solid state rectification.

    [​IMG]
    [​IMG]
    Bill of Materials
    [​IMG]

    The Hammond 125B output transformer works well with the EF80. Use the orange and yellow secondary wires for 22.5K:8 ohms and 5 watts. Standard Fender reverb transformers (22.5K:8 ohms) also work well as output transformers for the EF80.

    EF80s tend to be microphonic so they are best used in head amps and not combo cabs. If you do use them in a combo cab expect to have to try several tubes to find a pair that are not microphonic. Using rubber insulated sockets and silicone damping rings on the tubes may help with microphonics.

    Typical screen current at 250 plate and screen volts is shown as 2.8ma in the EF80 datasheet. Using a 1k 1/2 watt screen resistor will add some screen sag distortion and emphasize the difference between triode and pentode overdrive. Since the EF80 pin 6 is not connected to anything we can use it as an anchor point for a screen resistor.

    The EF80 can also be used in the preamp like the EF86 in many early tube amplifiers.

    More info and the latest schematic & layouts can be found here:
    https://robrobinette.com/Bassman_Micro.htm#Bassman_Micro_LTP_EF80

    Rob
     
    Last edited: Dec 13, 2020
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Your mention of the desired B+ for the EF80 brought this question to mind.

    Besides a rectifier tube dropping voltage to be able to use a higher voltage PT to attain a desired B+, would a rectifier tube in such a circuit contribute any sag to speak of? The current demand would be low compared to a *big* tube amp. So am I wrong in thinking neither a SS rectifier or any tube rectifier would contribute any noticeable sag for these small tubes?
     
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  3. dougsta

    dougsta Tele-Meister

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    Exemplary work as usual.

    Doug.
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, with such a small power amp current demand I doubt anyone could hear any rectifier sag so solid state is just fine. Of course you could use a B+ voltage dropping resistor of good size to drop voltage and possibly add some sag.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hey Rob, isn't the Hammond 269AAX 200VCT a 100-0-100v? It is not 200-0-200v or 400VCT.

    EDIT: Maybe a 269EX for 190-0-190v 75mA
     
    Last edited: Dec 13, 2020
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  6. Nickfl

    Nickfl Friend of Leo's

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    Very nice! The pentode option is a nice addition to your micro amp designs and it looks like you've found a good tube to base it around.

    I've done a "micro" build with 6BM8 power tubes when I wanted to have a true pentode power section and while they sounded good, they put out probably about 10W in push pull and volume wise are closer to an EL-84 amp than a real "micro" amp like your 12AU7/12BH7 designs. Sounds like the EF80 will be a much better choice for a low power pentode.

    Your schematic seems like it has an error, it looks like you have the screens tied to the plate on V5 instead of being supplied by the node after the choke.

    I am also wondering if you will be ok with using the same power supply node for both the PI plates and the power tube Screens. I wonder if there is a chance of oscillation without decoupling those?

    I'm in the process of rebuilding my old Bassman micro (built using your earlier design with a cathodyne PI) as a JCM 800 micro and I may now consider doing it with EF80s instead of a 12BH7.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    It sure is LLC, I'll make the fix.

    Yep, that is a schematic error. I'm hoping the miniscule screen current combined with relatively large filter caps plus the 1k screen resistors will isolate them enough to prevent oscillation but I may have to add another power node.
     
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    LudwigvonBirk and alexhael like this.
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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  10. NSB_Chris

    NSB_Chris Tele-Meister

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    Isn't a 269EX (190-0-190v) going to give you closer to 268 VDC after SS rectification and even lower with tube rectification? I would think you would want something like 450VAC C.T. to end up closer to 300VDC after the rectifier. Something like the 369HX.
    Or did I confuse something?

    Edit: I was going off of the 5F6A-EF80 micro schematic which listed B+1 at 300. Maybe you are shooting for something much closer to the 250VDC.
     
    Last edited: Dec 22, 2020
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The current draw is so low from this amp that voltages are higher than typical.
     
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  12. brommers

    brommers TDPRI Member

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    I have recently put together a frankenstein micro-amp using the robrobinette 12AU7 power stage, AB763 preamp and Vox-style LTPI (inspired by the Divided by 13 FTR37).

    I'm really impressed with it - it has a LOT of gain, the volume can be tamed by the PPIMV, but I'm only using the first 1/4 of the pot on a hair-trigger! The phase inverter is the first to give up in this design.

    Looking at the output on a scope, I think I'm using the master volume at a point where it's adding too much "character"...

    I am going to give the EF80 configuration a try, if I understand well it should be a fraction quieter than the 12AU7 (2w vs 3/4w)?

    I think I can get away with keeping the same power transformer, shaving 50V off the B+ with zener diodes.

    It almost goes without saying, but thank you Rob for the amazing wealth of resources, development and knowledge shared here and on your site.
     
  13. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Would the ef80 power amp work well with the blackvibe micro as well?
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes. I'm planning to do a Blackvibe Micro EF86 in the near future.
     
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  15. Mark the Moose

    Mark the Moose Tele-Afflicted

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    I’ll hold my horses until someone smarter than me gets it figured out!
     
  16. chas.wahl

    chas.wahl Tele-Meister

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    Wha' happened to all of Robrob's images?
     
  17. David Barnett

    David Barnett Doctor of Teleocity

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    So it's not just me then.
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I don't know what happened to my images here on tdpri. I'm guessing they changed some policy. Maybe my images are too large or something.
     
  19. pmacaula

    pmacaula Tele-Meister

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    Rob - I had a similar issue with too-large images. Started embedding at 1/4 the source file resolution and no problems since then.
     
  20. Mongo Park

    Mongo Park Tele-Afflicted

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    Is the schematic on the website, I am looking but only see layouts. I figured it must be there but haven’t found them yet. Cheers Ron.
     
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