Low Wattage Amps

Discussion in 'Shock Brother's DIY Amps' started by dunner84, Aug 14, 2019.

  1. dunner84

    dunner84 Tele-Meister

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    Hello all,

    I have been wanting to build a low watt amp for quite some time, but I don't have any experience with any of the circuits. Ive done some reading, and listened to youtube clips, but you never know until you have one in front of you. I have lots of big amps, no micro amps...

    I have a big collection of power and PP output transformers, but almost all of them 300 - 0 -300 and higher, so not conducive to 12au7,12bh7 or el84s. I was organizing things today, and I noticed some of the larger 6.3V, and 12V transformers also have 120v secondaries (I never noticed before)...So, no iron in the 250-0-250 range which seems to be ideal for these small bottles.

    I also have a few of these "cute" hammond enclosures with 2 9pin sockets and room to add a 3rd if I need to, and they are screaming to be turned into something cool.

    My questions to people who have built micro amps: what is the best way to build a power section with what I have? I can do 12au7/12bh7 in PP, and add a PI, OR 12au7/12bh7 parallel SE, OR el84 SE.
    I have one SE OT. It is one of those 3.5W Hammond reverb transformers, and I it could work for this.

    The 120V tranny... am I dreaming, or can I get a good sounding amp with lower voltages? Voltage doubler maybe?

    Open to suggestions and insight
     
  2. dunner84

    dunner84 Tele-Meister

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    The Hammond enclosures..
     

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  3. magicfingers99

    magicfingers99 Friend of Leo's

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  4. dunner84

    dunner84 Tele-Meister

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    , and I suppose I could power the heaters just for effect!
    I have experimented with the lm386 in the past, and I was never happy with it. Although, I think one of my kids is still using it for her iPod dock...
     
  5. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Robrob's site has a growing library of info on micro amps. I'd recommend going to the source, but my recollection is that his designs take into account how many good, inexpensive fender-style power transformers with higher B+ voltages are available, and makes them work with the tubes you mentioned.

    I've built one amp that qualifies as micro around an EL95 power tube, and I took the not very elegant route of running the entire dc voltage supply through a big dropping resistor to get my plate and screen voltages down to the 300ish maximums.
     
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  6. dunner84

    dunner84 Tele-Meister

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    Thanks, I've definitely spent some time on Rob's site. It's an amazing resource. I know he mentioned preferring 12au7 SE vs PP, possibly more power?
    I really don't want to source new iron if I don't have to, which is why I haven't built a micro amp yet.. more just curious which power type you all prefer, and why, and also if anyone has had good experiences with lower voltages..
     
  7. printer2

    printer2 Poster Extraordinaire

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    I used a voltage doubler on 120V to feed two 6AK5' P-P. Think I did it also for some 6AQ5's. They can run off 120V normally rectified (aprox. 160V) SE. A lot of ways to skin a cat.
     
  8. dunner84

    dunner84 Tele-Meister

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    Great! So next question is SE el84 vs SE 12au7/12bh7?? Which has better headroom, and can anyone comment on the overdrive characteristics between the two?

    Cheers
     
  9. Nickfl

    Nickfl Tele-Afflicted

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    El84 is a pentode and the other two are triodes. Because of the lack of a screen grid, the triodes will have more linearity and what i would overall describe as less crunch (darker and less aggressive). There are good sounding amps that use triode power tubes (like Rob's micro bassman), put triode operation is significantly different than the pentode power tubes we are used to in virtually all classic guitar amps.

    IMO an amp with a triode power tube needs to rely more on the preamp for distortion and harmonic content.

    Basically, the EL84 pentode is going to act more like a standard power tube because it is one.

    Also keep in mind that an EL84 is very similar electrically to a 6v6. They need basically the same primary impedance and they output about the same power level, but the 6v6 can tolerate higher plate voltages. They do sound a bit different but probably the bigger difference is the difference in the circuits that they are commonly used in. So a 6v6 might be a better match for that 300-0-300 power transformer you have on hand.

    A single-ended 6v6 or el84 is going to be a roughly five watt amp, like a champ or ac4. Is that still a micro amp? That's up to you, but I would just point out that a 5 watt amp and a 1-2 watt amp (12bh7/12au7) are going to be very similarly loud through the same speaker. Personally I would very much prefer the amp using the pentode or tetrode valve.
     
    Last edited: Aug 16, 2019
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  10. Asmith

    Asmith Friend of Leo's

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    I assume you mean SE el84 vs PP 12au7/12bh7. The different output sections will have the usual differences mainly symmetrical vs asymmetrical clipping and as nick said the triode will be a touch darker than a pentode output. Id assume the SE EL84 will have more headroom, 5 watts of clean vs the power rating of the triode output less than 2 watts I believe.
     
  11. dunner84

    dunner84 Tele-Meister

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    I would likely run the 12au7/12bh7 Parallel SE, not PP to avoid adding a PI.. I read somewhere on Rob's site that he preferred the SE arrangement to PP for these tubes. I have been thinking about this project for so long, and I think I have finally made up my mind. The best way to find out what I want is to dive in, and try some things... I already have a 5f2a that I can roll different 8pin tubes through, and I have never loved any of my el84 amps. EL84 only seemed an option because the chassis I intend to use has two 9pin sockets...
    I think the plan is to build a parallel SE 12au7 power section first using the 120v tranny. I think it will net me around 200v to play with, and if its too low, or sounds like trash I will add a voltage doubler. I will feed it a few different preamps, and see what sounds best. Hopefully the best sounding preamp is a small one, and I can build it in.
     
  12. JuneauMike

    JuneauMike Friend of Leo's

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    I used zener diodes. (Because, in my ignorance, I tried this in a fixed bias amp my TDPRI thread went on for pages and pages while helpful folks talked me through defusing the bomb).
     
  13. Asmith

    Asmith Friend of Leo's

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    The EL84 will definitely has the potential for more headroom than a triode output. But at 120V on the plates youll be looking at less power than 5 watts. For example the el84 has a max dissipation of 12Watts, from a 120V transformer youll get about 165V after rectification, so about ~160V will be across your plate to cathode going by the plate dissipation you could bias at 70mA and be comfortably under 12 watts.

    But then 70mA is probably way over the max cathode current so you'd have to bias it colder to get under whatever the max safe value is and this means you wouldnt be able to get the full power potential of that tube. There would be other symptoms I assume, the distortion would be different I imagine as the gain would be a different value and the cathode voltage would be larger percentage of the plate voltage. I dont think it would mean you couldnt build a nice sounding amp its just that if you were to copy a design and then cut the voltage supply by over half you would end up with something with less headroom that might be good or bad. It would be a similar story for the triode output.

    Im far from an expert and Im very sleep deprived today (I got 3 hours last night and that was 17 hours ago) so if Im wrong can someone correct me.

    I would personally go with the triode output section if your after something which you can crank at lower volumes. Even 1 watt can be too loud for home use if your pushing it.

    Another thing I think you should look into (if your not a purist) are high voltage depletion mode mosfets, they're pretty good cheats in valve amps. I have 2 LND150s in my DIY amp but all the distortion comes from a 12ax7 and the el84 se output. I replaced the input triode with one and I have another after the tone stack as a recovery, niether add any distortion and I dont see it any different than someone using a clean boost to push their tube amp.

    If you were to build an amp based on a dual input/channel amp with a cathodyne phase inverter (say a 5e3), then you could ditch one of the channels in favour of your prefered (or even put the option on a switch, its only 1 cap) and then replace that input triode with a LND150. That would cut a 12AY7 out of the schematic and the rest would be all valve.

    Im sure you can figure something out no matter what you choose though.
     
  14. Dacious

    Dacious Poster Extraordinaire

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    For a HT source, use two 6.3 volt Filament transformers back-back with a voltage doubler.

    I built one with a 6BM8 tube - triode and pentode in the same Tube. Sounded awesome.

    There's tons of single tube amp schematics - often vintage stereos used them. I just put a 1 meg resistor to ground on the input and 68k inline grid resistor like a Champ. Added a volume control inline - easy peasy.

    I used a line voltage as the output transformer.

    250 volts - screamed like a little Vox. About 1 watt.
     
    Last edited: Aug 16, 2019
  15. Nickfl

    Nickfl Tele-Afflicted

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    +1 on the 6BM8. It's a great sounding pentode power tube and at about half the power of an el84 or 6v6 it works nicely for lower powered designs. Plus it comes with a bonus triode!

    They can be kind of hard to find at a reasonable price, but Russian 6f3p are equivalent and you can still find "winged c" OTK 6f3p (which are excellent tubes) cheap on eBay.
     
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