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Thread: Low power push pull ideas?

  1. #1
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    Low power push pull ideas?

    The plan is for a complete amp with all the goodies. Volume treble, bass, trem, reverb, phase inverter, push-pull output, the works. No inverse feedback.
    The special characteristic is a 5-watt maximum output, with 1 watt being more like it, and an 8-inch speaker.
    I don't want to reinvent the wheel, so, at the risk of pounding a topic that may have already been done to death here...
    What are some suggestions for a really low-watt but completely true pushpull output section? I still want everything the big amps do when they are sweating, so no class-A and no single-ended configurations will do.
    I do know about Zachary Vex's nano amp and it is a monster, just great, but I want to take it to the next level and make a complete amp in a cabinet with a handle.

  2. #2
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    Search "utube 1W guitar amp", 6SN7 in push-pull output. This might give you a starting point.

    12BH7 could also be a candidate?

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    Member riscado's Avatar
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    glad you decided to include a phase inverter as a feature

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    More shameless promotion...

    I have about 10 or 11 smallish combo cabinets (about the size of a Fender Vibrochamp but with a 10" speaker cutout) and some matching chassis' left from our old Starlite project... one of which was a mini amp with a low power, push pull 6SN7 PA.
    No reverb in the Starlite series but a multi-spring 9" reverb tank will fit in the cabinet and the chassis is very universal. It has plenty of tube socket holes for stuff like this as it was a chassis design used for small amp experimentation anyhow.

    Since we moved, we've found all kinds of stuff we are going to stick it all on Ebay for the do it yourself builders.... but, if a few of you are interested, I'll whip something up, shoot some pictures and list the cabinets and chassis in our "garage sale" page at Mission Amps.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    Member riscado's Avatar
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    Bruce's sugestion is a good one...

    I also saw an amp some time ago, which was around 1w, but I'm not sure wether it was SE or push pull, I'm talking about the emery microbaby amp, you could try and check a schematic for that one (if there is one on the web of course).

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    I picked my front end; it will be a variation on my '65 Cambridge Reverb 18 watt.
    Now I need to modify the power section. I've seen a very expensive stereo amp that uses ten 12AX7s as the power section, maybe I could investigate a reduced version of that as a possibility. On the other hand, what 9-pin small-bottle power tubes are available? Is there a "baby" version of the EL84? Any shortcuts here before I go on a research mission?

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    how low wattage do you want? Look at the 6HU8 (ELL80). It has two power pentodes in one bottle capable of three watts push pull. I've no experience with this tube but the low wattage possibilities may be worth investigationg.

    The 12AU7 would make a better low watt power triode than a 12AX7.

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    Thanks for the heads-up.
    Regarding "12"'s in general, how about the 12AT7? I see it is used as a reverb driver in Fenders, which is essentially a miniature power amp anyway, complete with output transformer. The only catch is that it is not a push-pull pair. My understanding is that the AT7 is intended as a current amplifier more than a voltage amplifier, similar to power tubes.
    Power - yes, about two or three watts is about right for this.
    Since I am looking to reproduce as much of the familiar sound as possible, I am expecting to look at low power tetrodes or pentodes. I don't know - but I suspect - that simply underpowering an EL84 will not give the same effect as a much smaller tube being pushed to nearly its limits. So it's a suggestion of a low-power output tube that I am looking for, something designed to produce a watt or so each, then I want to drive them hard.

  9. #9
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Fretts View Post
    ... how about the 12AT7? I see it is used as a reverb driver in Fenders, which is essentially a miniature power amp anyway...
    Jeff,
    The 12AT7 would work. In his book TUT2, Kevin O'Connor's talks about converting the single ended Fender 12AT7 reverb driver to PP. He shows a "transformer loaded Schmitt splitter" configuration which is an interesting way to get a PP output section without a separate phase inverter circuit. I don't see any reason that you couldn't design a phase splitter feeding the 12AT7 in PP. There is no mention of the transformer used. maybe he sells them or uses something like a Hammond 125A.
    Tom

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    The Hammond 125A will work - Angela had the 125C for a better price, so that's what I used for my push-pull 12AU7 build.

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    Quote Originally Posted by Fretts View Post
    I picked my front end; it will be a variation on my '65 Cambridge Reverb 18 watt.
    Now I need to modify the power section. I've seen a very expensive stereo amp that uses ten 12AX7s as the power section, maybe I could investigate a reduced version of that as a possibility.
    You're probavly thinking of the E.A.R. V20

    Quote Originally Posted by Fretts View Post
    On the other hand, what 9-pin small-bottle power tubes are available?
    Look at 12BH7s. They have 3.5 watt anode dissipation per section.

    Quote Originally Posted by Fretts View Post
    Is there a "baby" version of the EL84? Any shortcuts here before I go on a research mission?
    There are triode/pentode TV tubes that would work. You can try a cascode output stage if you are so inclined.

  12. #12
    Junior Member Mr. dB's Avatar
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    Quote Originally Posted by quayhog View Post
    how low wattage do you want? Look at the 6HU8 (ELL80). It has two power pentodes in one bottle capable of three watts push pull. I've no experience with this tube but the low wattage possibilities may be worth investigationg.
    I vote for this solution, if only because I like the harmonic spectrum of tetrode or pentode outputs for guitar. Triodes are a little too "pretty" and just don't seem "interesting" enough when you abuse them.

    Triodes are for preamps and hifi.

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    Quote Originally Posted by Mr. dB View Post
    I vote for this solution, if only because I like the harmonic spectrum of tetrode or pentode outputs for guitar. Triodes are a little too "pretty" and just don't seem "interesting" enough when you abuse them.

    Triodes are for preamps and hifi.
    Mr. dB: Exactly, we are on the same page. I am seeking the reverse of the usual. Typically I have said, why can't I have a big amp that does everything my little amp does? Now I am zagging; I want a little amp that does *everything* my big amps do --except vibrate my clothes--.

    riscado: For my goals, a phase inverter IS a feature; I want All of it. The nasal honk, all of it.

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    [QUOTE=Arthur B.;6718]You're probavly thinking of the E.A.R. V20

    Look at 12BH7s. They have 3.5 watt anode dissipation per section.

    Arthur B., 12BH7 sounds like a candidate, low power by nature, not weird or esoteric, not hard to find. Now if the tone is acceptable, we are off and running.

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    Quote Originally Posted by Mr. dB View Post
    Triodes are for preamps and hifi.
    I guess you have never played or heard a Moonlight... Uses a 6SN7 in push pull for the power stage.

    Nothing pretty or HiFi about it... Do a Google search for Moonlight clips.

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    [QUOTE=Fretts;6728]
    Quote Originally Posted by Arthur B. View Post
    You're probavly thinking of the E.A.R. V20

    Look at 12BH7s. They have 3.5 watt anode dissipation per section.

    Arthur B., 12BH7 sounds like a candidate, low power by nature, not weird or esoteric, not hard to find. Now if the tone is acceptable, we are off and running.
    6CL6 is rated for around a watt. Anyone have experience with this one?

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    [QUOTE=Fretts;7698]
    Quote Originally Posted by Fretts View Post

    6CL6 is rated for around a watt. Anyone have experience with this one?
    I'm quite sure the 6CL6 as more power than that, but it should work just fine.

  18. #18
    Better Tone thru Mathematics bob p's Avatar
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    it would be a whole lot easier for average guys (me, for example) to find a good tube for this application if push-pull plate loads were published for these amps in the tube data sheets. invariably, it seems that they aren't.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by bob p View Post
    it would be a whole lot easier for average guys (me, for example) to find a good tube for this application if push-pull plate loads were published for these amps in the tube data sheets. invariably, it seems that they aren't.
    I assume that you mean that the data is not in the old RCA receiving tube manual. That's probably because the tube was never intended to be used in push pull. That doesn't mean that it wouldn't work, just that with so many tube types available in the old days, no one was using the particular tube that way.

  20. #20
    Better Tone thru Mathematics bob p's Avatar
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    Yes -- for tubes that were not specifically designed for P-P applications, Ra-a is not published in the tube data sheets. Yes -- that's definitely the case for the old RCA receiving tube manuals. It also seems to be the case for every other data source I've looked at, including every factory tube data sheet that I've ever been able to find. If this is just a case of me just looking for information in the wrong place, please let me know!

    Taking a look at the archives at a place like Frank's Electron Tube Pages, there are plenty of tube data sheets from a variety of manufacturers. Invariably, it seems that plate to plate load resistances aren't published for these types of tubes. I could be wrong about this -- there could be a data sheet in a language that I can't read that contains this information -- but I've read every one of them for most of the "common" mini triodes and pentodes and I haven't found good data.

    The problem isn't that small triodes or pentodes can't work in P-P applications. The problem is that the load data for that application doesn't seem to be available anywhere. That lack of published information is the biggest impediment toward the creative use of these tubes in new applications. At least that's the biggest impediment for me.

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    There is a way to calculate load resistance from plate resistance, or at least come very close, but the formula escapes me at the moment. That is what to search for instead of a published value.

  22. #22
    Better Tone thru Mathematics bob p's Avatar
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    fwiw, i am familair with the 10% rule, which isn't all its cracked up to be.

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    Lifetime Member Ray Ivers's Avatar
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    Single 12BH7, push-pull, fixed bias, 300V B+, 15K plate-plate load (courtesy of GlassWare's P-P Calculator):



    This was just from five minutes dicking around - I really didn't try a whole bunch of different loads, B+ voltages, etc.

    Ray
    Attached Thumbnails Attached Thumbnails 12bh7-p-p-2.bmp  

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    I see no reason to even go push-pull for this type of project. I would use the first gain stage of a 12ax7 as the pre-amp, and then the second gain stage into a 10K primary impedance of a small O/T. All can be done with one tube, and have a nice warm SE tone.

    -g
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    Junior Member pbreyfogle's Avatar
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    So, what was the verdict?

    So, Fretts, what did you finally decide to go with?
    Sounds like you're after just the thing I need...

  26. #26
    Lifetime Member Rob Mercure's Avatar
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    Hey Guys,

    Hate to come in late but I like the 6G6 - an "ST" bottle small output pentode that was used to power the headphones in WWII aircraft, etc. The basing is 7AC so it's "plug and play" with the 6V6, etc. and the plate dissipation is only 2.75 W. An attractive good sounding tube that usually goes for cheap (and I've got a whole box of about 40 WWII - new in box - military surplus tubes).

    Rob

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