6AF11 Percolator Style Build

Discussion in 'Shock Brother's DIY Amps' started by tvas22, Apr 15, 2020.

  1. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Hi everyone,

    I bought a 6AF11 for a few £s, with the intention of building a low cost, low powered, single tube amp.

    I was looking at a few existing designs, and came across a schematic on EL34 world for the Zeppelin Percolator, which I think sounds like what I'm after. I've attached the schematic listed here to this thread.

    I had a few questions though - first of all, I wondered how the rectified heater circuit worked? I'm guessing that the capacitor smooths out ripple, does the LED regulate voltage? I was also a little confused about the grounding of the filament circuit - does this all look right?

    The Squirrel Monkey (another 6AF11 build) has a treble control built into the NFB loop, could I essentially cut and paste this onto the Percolator?

    I was also thinking the Hammond 1750B transformer (5.5k primary, 8 ohm secondary) would be a good fit for the output transformer, and this power transformer.

    I should be able to do the entire build for about £80-100, I think. I could probably get that down further too if I can source some cheaper transformers.

    Thanks!
     

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  2. BigDaddy23

    BigDaddy23 Tele-Holic

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    I think it is running DC heaters by the looks of that Percolator schematic. I reckon the LED is simply a pilot light.

    On the squirrel monkey schematic, the x2 220ohm resistors grounding the filament circuit are an artificial center tap for hum control. I'm not certain how you could factor that presence control into your build though. The bias of that 2nd triode is totally different.
     
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  3. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Haha, yeah of course it’s a pilot light with a current limiting resistor! I don’t know what I was thinking.

    Does pin 12 still need referencing to ground though, given that there is already an artificial centre tap in the diode rectifier?
     
  4. BigDaddy23

    BigDaddy23 Tele-Holic

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    Yes, look at FIL12 leg on the heater schematic. It has a ground reference shared with the 100uf cap.
     
  5. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Thanks, I guess it just looked weird to me compared to a normal heater circuit
     
  6. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Also, I was looking at the GA-1RT and was wondering if implementing something like that tremolo would work in this build. I took the phase shifting oscillator tremolo from here.

    If it would work, how would I go about adding in a depth control?
     

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  7. tvas22

    tvas22 Tele-Meister Silver Supporter

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    I had a little more of look around and found the Marshall 18-Watt tremolo schematic, which implements phase shifting tremolo with a depth control. I had a go at adding that in and then putting together a layout. I kept roughly the same insertion point as on the GA1RT schematic.

    My understanding is that if I get the oscillator working, I can play around with the insertion point afterwards? I suppose it can only really go after one of the two gain stages...
     

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  8. zippofan

    zippofan Tele-Afflicted

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    I built a Percolator as my first tube amp kit, it's a lot of fun! In addition to the vendor supplied National tube, I raided my father-in-law's stash of old television tubes and scored an additional Sylvania and a GE 6AF11's. He was amused I was building an amplifier with it. I know they are DC heaters, the manual from Zeppelin is excellent, took me back to my Heathkit days.

    To test it, I plugged it into my old original Crate practice amp speaker (the wooden one, cool cabinet, awful SS amp) which has a Jensen 12C12Q replacement for the original metal dust plug Eminence. It's been set up like that ever since, a great match for some raunchy rock n roll :)
     
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  9. tvas22

    tvas22 Tele-Meister Silver Supporter

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    That's encouraging to hear! Does the Zeppelin manual happen to give any info about the primary impedance of the output transformer or the power transformer's VAC secondary? I'm pretty sure I'm trying to get down to a B+ of about 200vdc, but it would be good to know.
     
  10. drf64

    drf64 Poster Extraordinaire

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    I've somehow never heard of zeppelin before. Thanks for that.
     
  11. zippofan

    zippofan Tele-Afflicted

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    I'll get out the manual and check, I can't remember offhand right now. It's a fun little amp, turned down it's got some nice hairy cleans, but halfway up it starts to roar.
     
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  12. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Thank you - I managed to find it online. There's no info about the output transformer other than it being 4W. I think the Hammond 1750B should be fine although it's overrated.

    B+ is given at 170, which is about what I thought. So their transformer must have 120 secondaries - so I think this one should work http://primarywindings.com/product/pwgp25-120v/
     
  13. Paul-T

    Paul-T Tele-Meister

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    I'll be following this. I love seeing funky stuff without expensive transformers. Will you have a separate transformer for the heaters?
     
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  14. zippofan

    zippofan Tele-Afflicted

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    You beat me to it!
    I thought there was a schematic diagram, I must have been thinking of their test point graphic. The tube spec sheet lists 200V max on the plate, I'm guessing Zeppelin ran it lower to save tube life. I've found 6AF11's online that aren't too expensive yet, but I doubt they'll ever be made again!

    Forgot the photos;

    percolator_complete.JPG percolator.jpg
     
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  15. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Those shields are interesting! Something to hear in mind as I move forward.


    That is a good point - I would have to with that transformer. In which case perhaps it would be better to use a different transformer that gives 6.3v and a higher b+ and drop it a little more. I’ll have a play with dropping resistor values later, but I’m not sure what current draw I’m looking at for the 6AF11... I’ll have a read about.

    Is it plate current I’m interested in? On the datasheet it gives an amplifier example, and lists 24ma for the pentode and 7 and 9ma for the triodes. So should I use 40ma when calculating dropping resistor values?
     
  16. tvas22

    tvas22 Tele-Meister Silver Supporter

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  17. Paul-T

    Paul-T Tele-Meister

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    Last edited: Apr 19, 2020
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  18. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Yes that looks great! I’ll add it to my list. I’ll put an order in for parts after payday at the end of the month.
     
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  19. tvas22

    tvas22 Tele-Meister Silver Supporter

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    Moved the oscillator input to V1's cathode as per the Marshall schematic I posted above.

    I read a post earlier about how the oscillations in the GA-1RT passed straight through into the speaker, making it thump away.
     

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  20. Paul-T

    Paul-T Tele-Meister

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    You might have noticed this already, but I was searching for information and saw this Bluebell page. It's the most easy-to-understand summary of which Hammond has which secondary voltage etc etc.
     
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