Low watt pentode powered amp?

Discussion in 'Shock Brother's DIY Amps' started by JohnnyCrash, May 13, 2019.

  1. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I have some left over parts and I like to challenge myself.

    I’m looking to build a super tiny all tube amp (except rectification) using a capacitor doghouse for a chassis. I built a Firefly once and wasn’t impressed with the triode output stage.

    Any circuit suggestions (and associated schematics) for an amp that uses a single 12AX7 pre and a low watt pentode output?

    I’m aiming for something lower powered than a Champ if possible (I already have two Champs - a tweed I built and a ‘78 VibroChamp :)
     
  2. scottser

    scottser Tele-Afflicted

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    Sort of like the zvex nano?
     
  3. Nickfl

    Nickfl Tele-Afflicted

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    Take a look at 6BM8 / ECL82 tubes. They are a pentode and a triode in the same envelope, so they could fill your need with one tube socket. I've used them as the inverter and power section in a push-pull amp and was very pleased with them and I'd definitely use them again. They might be a little higher power than your thinking of though, I believe the pentode is pretty close to being equivalent in output to an el84.
     
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  4. Finck

    Finck Tele-Afflicted

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    There is a fantastic circuit designed by a Brazilian guy that meet your requirements.

    Here's the link (don't worry about instructions in Portuguese, schematics are self explanatory, and I can help you if you need):

    http://www.altanatubes.com.br/downloads/HB-1 V1.1.pdf

    If you really prefer 12AX7 in preamp, look at the schematics below:

    http://www.handmades.com.br/forum/index.php?topic=9592.0

    What's the idea: the guy has designed a circuit that uses tubes whose filaments draw similar current, so it was possible to connect them in series. That allows the use of only a cheap 30+30V tranny (60V with center tap) to feed all the filaments AND B+ (through a voltage multiplier).

    As OT, any SE unit with 1,7kohms (for 25L6 tube) or 2,5kohms (for 25C5 tube) primary impedance will work.

    The 25L6 is a tube that has the same characteristics of a 6V6, only difference is that its filament uses 25V. 25C5 is a bit different, but final result is almost the same. Both are very cheap and easy to find.

    All circuits were tested and work very well. The project with 12AX7 is very crunchy, with a voice that could be described as a mix of Marshall and Vox. Output power is about 2W with any of the power tubes.

    Some guys have built the circuit using a Hammond 1590BB enclosure as chassis, so I guess the caps doghouse could be used with no issues.
     
    Last edited: May 13, 2019
  5. TequilaCaster

    TequilaCaster Tele-Holic

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    What is your definition of low power?
    An EL84 / 6BQ5 pentode sounds great in an AC4 or an AX84.com P1 style build.
     
  6. Snfoilhat

    Snfoilhat Tele-Afflicted

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    6DL5/EL95 - it's like half of a 6BQ5/EL84, made for car radios. Pentode, 7-pin socket, very low heater current, 6W plate dissipation. My most recent amp used this and I think it came out well.
     
  7. printer2

    printer2 Poster Extraordinaire

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  8. dkevin

    dkevin Tele-Meister

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    Too big?
     

    Attached Files:

  9. printer2

    printer2 Poster Extraordinaire

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    Actually thought of it, no need for the first mosfet, enough gain for a 6AK6 with two triodes. I put it in because the values of the tone stack works with a lower supply impedance. A Tweed volume and tone control would work well. The splitter mosfet could be an IRF820.
     
  10. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I’ve still got my last build on the bench for troubleshooting... but I’m about to start a low watt pentode powered amp.

    Still taking suggestions.

    I’d like to build it for my brother’s home studio, so I’m looking for something that can do both clean and dirty for him. I’m hoping it can sound like an actual amp, rather than a triode powered anemic thing.
     
  11. tubelectron

    tubelectron Tele-Meister

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    Hi JohnnyCrash

    The ECL86/6GW8 tube would be an excellent choice : it can give circa 2W rms in clean power. It's what I used with success in my little standalone Radio-Tone amplifier :

    [​IMG]

    [​IMG]

    My Radio-Tone is inspired from the MagnaTone 411 amp :

    [​IMG]

    Minus the tremolo section (I did not have the room for it !).

    -tbln
     
  12. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    How does it sound when cranked up?
     
  13. tubelectron

    tubelectron Tele-Meister

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    Judge by yourself : ;)

    Here is a sample recorded with a Squier Std Strat, a EHX Cathedral Reverb, via a Roland R-05 pocket Digital Recorder :



    Here is a dry sample showing tone comparison between a Fender Greta (1st heard) and the Radio-Tone (2nd heard). Still recorded with the R-05, both amp cranked :



    Matching photo - notice the difference in size between the 2 amps :

    [​IMG]

    -tbln
     
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