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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    tazwolf, The 470k is the grid stopper, that's all that is needed but you can add another 1.5k on the socket itself.
     
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  2. turbobork

    turbobork NEW MEMBER!

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    Wotcha all, what a fantastic project this is. I've only just happened upon the Rob's great website and the Bassman Micro, after looking at various lower powered amps based on subminiature tubes.

    I've previously build a Gerhart Gilmore Jr amp with a 6n1p in push-pull. This is rated at around 1/2W and I still found it too loud. Are there any tweaks that could be made to the output stage in the EF80 Bassman to bring down volume?

    One idea I was kicking around was a subminiature pentode like the 5672 that Thomas has used in his ultra low wattage amp. Is there any reason that this might not work?
     
    Last edited: Apr 13, 2021
  3. Gijs

    Gijs TDPRI Member

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    You could try a push pull amp with 5672 tubes. Looking at the datasheet, it should have around 200mW output. You would need an output transformer with high primary impedance, about 30 kohms I would think.
     
    Last edited: Apr 14, 2021
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  4. Gijs

    Gijs TDPRI Member

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    In the first post of this thread it is said that a 22,5k transformer works well with the ef80 in push pull mode. However, looking at the graph, it is obvious that the tubes are being seriously overloaded almost all the time.
    It is generally considered ok for the class B loadline to partly surpass the maximum dissipation curve, but in this case the class A loadline also does. I feel that can't be good for the tubes.
    22,5k would seem to be fine with a plate voltage of 200-250 volt, but with 300 volt I think 30k or a bit more would be a better match. The tubes wil run much cooler, at the cost of a bit less output power (with 30k, 3.4 in stead of 3.8 Watts).
     
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  5. Hyakuya

    Hyakuya TDPRI Member

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    Hello folks.

    I'm slowly gathering the bits I need to build the Bassman Micro LTP EF80 and I was wondering about posible alternatives to the 12AX7 and 12AY7 tubes in the layout. EF80 and EZ81 are plentiful and I can pick them up cheaply but the 12A*7's not so much....

    I noticed a comment on Robs webpage about "The EF80 can also be used in the preamp". Does this mean I can use an EF80 instead of the 12AY7 ?

    Or maybe 12AT7 (ECC81) instead of the 12AX7 ?
     
    Last edited: May 11, 2021
  6. dougsta

    dougsta Tele-Meister

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    Yes you can use them but you will need 2 as there is only the eqiv of 1 triode stage (can also be used in pentode mode for more gain)

    Here's a clip where I switch from triode mode to pentode mode. I used 5 EF80 valves in total, 2 in preamp, 1 for the phase inverter and 2 for the output push-pull pair:

    Triode mode:


    And pentode mode:

     
    Last edited: May 11, 2021
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  7. dougsta

    dougsta Tele-Meister

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    You could try the Russian equivalent to the 12AX7 the 6N2P, you can get 4 for less than the price of 1 Chinese 12AX7.

    https://www.ebay.co.uk/itm/184623364639?hash=item2afc68fa1f:g:69sAAOSwIihgAo78

    They are not drop in replacements, just need to wire pins (4) & (5) for the filament supply instead of (4,5) & (9), actually a bit simpler and neater. Same filament wiring as the EF80.
     
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  8. Hyakuya

    Hyakuya TDPRI Member

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    Thanks @dougsta , I've got a pair of 6N2P on the way from eastern europe and I'll give them a try... maybe fix up the valve base with a switch of some kind to change the filament wiring between 6N2P and 12AX7.
     
  9. dougsta

    dougsta Tele-Meister

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    Aye, the switch works well, I've used it on my 1st Champ build, though that was to also use some old '60s Mullard ECC85 valves, dual triodes but 4&5 heaters
     
  10. dougsta

    dougsta Tele-Meister

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    Something to note with the 6NxP valves, the filaments suck down a bit more juice.

    The 6N2P is only an extra 50mA but the 6N1P and 6N5P take an eyebrow raising 600mA.
     
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  11. Hyakuya

    Hyakuya TDPRI Member

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    Lots of options for V1 then (with a switch on the filament wiring).... 12AY7, 6N2P, ECC85

    What about the 12AX7 in V2 and V3.... I've seen mention of 6N4P being close but I don't have enough juice on the 6.3v secondary to satisfy 600mA if that's what they want.

    I'm interested in your mention of using EF80's but I'm nervous about deviating too much from Robs design. Did you replace all the 12A*7 tubes with EF80s and did you need to make many changes to the design to accommodate these ?
     
  12. Hyakuya

    Hyakuya TDPRI Member

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    Also just found this comment from @robrob on his 5F6A Mods page:

    Consider using 1 watt resistors instead of the standard 1/2 watt. The larger resistors will run cooler which will make them generate less hiss. The amp will also be more reliable in the long, long run. Bump all the plate load resistors up to 2 watts for the same reason except as noted in the next paragraph.

    Consider using metal film resistors if you don't care if the circuit board does not look period correct. Metal film resistors generate 1/10th the noise of carbon comp. R.G. Keen recommends metal film for all signal resistors except for the plate load resistor of the stage just before the phase inverter (it's inside the feedback loop) so the V2A 100k plate load resistor and V2B 100k cathode resistors would be a good place to use 1/2 watt carbon composition resistors to ensure "vintage" tone without generating excess noise.

    Looks like we're all on the same page.... hooray...
     
  13. dougsta

    dougsta Tele-Meister

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    Best stick to Robs design.
    Last December I was looking at using a bunch of old '50s Mullard EF80 (and its younger bro from the '60s EF184) pentodes I had gathering dust. I came up with an all EF80/EF184 push pull low watt amp and the preamp using the EF80 triode strapped. It hadn't been done before and inspired Rob to make a better design.
     
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  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes you did.
     
  15. tazwolf

    tazwolf TDPRI Member

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    Just wanted to thank Rob and Dougsta. My amp was powered up for the first time and it works. I am using russian 6n2p valves in the preamp and its a modified marshall circuit ( Friedman dsm) into robs ef80 power stage. ,It sounds amazing,
    But i have only had an hours playing time on it. Preamp voltages are a bit low so i will look into perhaps lowering the 10k dropping resistor. Pics and clips to come.
    pics and clips to come.
     
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  16. wangdaning

    wangdaning Tele-Meister

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    What effect, if any, should I expect if I try to use a 5y3 rectifier instead of the EZ81? I ask because I procured a 190-0-190 PT with a 5v heater tap and have a 5y3 on hand. I am planning to follow the layout/schem of the 5F6A micro with the addition only of a second input (one hi one lo). It seemed from reading that the EZ81 was chosen because it has 6.3v heaters. I have priced them and it seems easy enough to get one if it is a bigger issue.

    *Just bought a lot of eight EF80s for about 30USD and waiting on my output transformer, choke and chassis to be made. Getting excited for this summer build.*
     
  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Look at voltage drop. Near the top of his his Bassman micro page, Rob says, “I recommend you use an EZ81 9-pin rectifier tube because it's low output will suit the low current needs of the Bassman Micro so you'll get a little voltage sag. The EZ81 will drop about 20 volts compared to a solid state rectifier. It uses a standard 9-pin socket and 6.3V filament heat so no 5V power transformer output is needed. If you want to use a full size 8-pin rectifier tube like a 5Y3GT which drops about 60 volts compared to a solid state rectifier then I recommend the higher voltage output Hammond 270CAX power transformer to compensate for the extra voltage drop.”
     
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  18. Gijs

    Gijs TDPRI Member

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    I think you could also use the EZ80, it has lower current rating than EZ81. Don't know if it has the same voltage drop. It is easier to find and somewhat cheaper.
     
  19. wangdaning

    wangdaning Tele-Meister

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    Thanks for pointing that out. Must have missed it. I will go with an EZ81 just to keep things simple.
     
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