Building a Lightweight Tube Amp

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by aortizjr, Jun 28, 2006.

  1. aortizjr

    aortizjr Member

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    So I am working on a personal project and am figuring out that I just don't know where to look for some stuff.

    I am trying to build a tube amp combo/head with the primary focus being light weight. But I am having trouble finding parts and understanding some specs. I have only built small amps from kits, or direct from schematics.

    But anyway...

    So how could you build a pretty loud amp lightly? Where would you shave off weight and still have decent reliability and sound?

    So first, use toroidal transformers. More expensive but could be worht it. Who do you buy from? Plitron is the only source i know of that even has audio specs. But they use strange impedances.

    But looking at weights of some hammond transformers, you can shave off up to 10 lbs.

    Then the chassis. Maybe go to 18 Ga steel? Or what about Aluminum? It is really hard to find those specs. Would 18Ga be strong enough? Would Aluminum be too light? Would Aluminum/Titanium be too expensive? Where do you find this info?

    Then the wood itself. In a head you are looking for mostly strength. What type of wood would be best? What about carbon fiber, or molded plastic, or completely aluminum? Where can you find weight and manufacturing information?

    In a combo you have to worry about tone. The speaker is fairly easy, go Neodymium, shave off 10lbs best case. More like 5-7lbs. But the wood or other material?

    Anyway if someone steals my idea (which based on the Rules, I think it is blown anyway) no big deal. I would probably buy this if it was reasonably priced from someone else.

    But imagine a 20-100 Watt amp that weighs under say 30 lbs or so and is tube, with great tone. I would buy one.
     
  2. 908SSP

    908SSP Supporting Member

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    Use a transistor power amp. If you build a Mesa/Dumble style where all the over drive comes from the pre-amp carrying a heavy power amp seems a bit pointless. A transistor power amp would save you a lot of weight. Aluminum is lighter then steel for a given strength if the chassis is designed right. If you do run transformers for a power amp a couple legs extending from the transformer mounting screws can add enormous strength. The cab really doesn't have to do much if you don't put a handle on top it just protects the amp from spills and dust. Only if you put a handle on top does it have to be strong. If it is a combo put the handles on the sides screwed to the chassis. Of course you need some strength for the cab around the speaker, but not much with an open back cab.

    I build ultra light weight motorcycles so if you have questions feel free to email me.
     
  3. TwinandTwang

    TwinandTwang Member

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    Here around DC Dave Chappell has what looks to be a BF Twin reverb head -only it has 2-8"s in it. It looks pretty managable as far as size and weight. I am shocked nobody is taking this idea and running with it. Brilliant! Guitar in one hand , amp in the other.
     
  4. PFCG

    PFCG Member

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    Or you can just buy an acoustic immage? the thing weighs 5 pounds and is 1000 watts. all you need is a speaker, they even make 250 watt versions
     
  5. aortizjr

    aortizjr Member

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    Thanks for the responses.

    Yeah I have considering building a "hybrid" amp basically. Tube pre and SS power amp. You can really do some cool stuff with Class-D. However, in the past, hybrids haven't been too successful. Plus I don't think I could design an amp better than say... Marshall or Fender who have both had semi-failing (arguably of course) hybrid designs.

    Randall has done some interesting stuff with Bruce Egnator using a MOSFET power amp with what looks like a tube driving stage.

    Anyway I checked out Acoustic Image. Very very cool stuff. I think I know where my next bass amp and acoustic amp will be coming from. But they seem very Hi-Fi and the jazz guys love them from looking at reviews. But I am sure that unless you have some killer pedals, distortion through those would be fairly painful.

    I guess my goal isn't Hi-Fi at all. I am thinking more of a rocker amp. Plus all tube, you can enter into higher gain arenas and have the feel of a typical tube guitar amp.

    This may end up being more a theoretical discussion.... but all tube I think would be neat if you could do it light weight.

    Some additional thoughts though, the only reason metal chassis are even used is for shielding. But you could use say... plastic and coat the inside. Then use legs or plates for the transformers.

    I finally found a site, Online Metals that gives lb/ft2 for different materials and their prices. It is really close between steel and aluminum, especially considering strength. But I really don't see that much weight loss there unless you go down a guage.

    Unless maybe 16Ga Aluminum (.06") would be fine. But it seems that it takes .09" of Aluminum to equal the same strength of 16Ga steel. Is that true?

    The nice thing about Toroids is that you can place them closer together. So assuming you can deal with the heat, you can really make an amp much more compact. Taking off even more weight. Sorta going with a THD like design almost. But use say molded plastic throughout...

    Hmm... thoughts on that?

    Anywhere I can look for plastic enclosures or custom made ones? (I honestly haven't started looking yet, but will post as I find stuff).
     
  6. soulohio

    soulohio Member

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    take a look at a Reverend Goblin...that is an excellent point to start. The designer used lightweight transformers and a Jensen Neo speaker. You might be able to shave off a pound or so from the cabinet, too.
     
  7. donnyjaguar

    donnyjaguar Member

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    If you like to live life a little on the edge you can get rid of the power transformer. Run the filaments in series with appropriate power bleeder resistors. Now put a voltage doubler on the power line and this will net you about 300Volts when loaded by a pair of output tubes. The output transformer will keep the high voltage off the speaker leads, but you'll need some isolation on the input jack. A 1:1 line/isolation transformer could work here. Use big plastic knobs. You'll need to run separate earth and ground circuits. Negative feedback and getting rid of low level hum will be a challenge.

    As exceedingly dangerous as this sounds, many consumer electronics from the 1950s were designed this way.

    Another, much more complicated, way would be to use a switching power supply on the input. You'll still need a transformer but it will be much smaller.

    DJ
     
  8. aortizjr

    aortizjr Member

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    Yeah that sounds a bit scary.

    I will look into the switching power supply though. See if I can find some design information. Anywhere you know of that I can look?

    Of course going toroid will be much simpler. Switching supplies at the voltages for power tubes could be really interesting and may end up weighing more than a toroid trans.
     
  9. donnyjaguar

    donnyjaguar Member

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    Actually, the weight of the iron in a switcher is largely dependent on the speed at which it is switched. For example, the new computer power supplies for desktops weigh next to nothing yet produce 400Watts. If you have 400Watts at 400Vdc that would be 1A. Enough for even the most ambitious amplifier projects. I don't know that a computer power supply is the best starting point for a high-voltage switcher, but technically it could be done.

    I've seen 12Vdc input tube amplifier supplies. The only difference with a 120Vac input would be the addition of a bridge rectifier and consideration for the higher voltages with respect to semiconductor and inductor choices.

    Now what would be really slick is if one of these amplifier parts houses could make a power supply like this that could be integrated into tube amp projects. Just think, perfectly regulated HT and filament voltages with built-in soft turn-on.

    Hmmm.....that gives me an idea... ;)
     
  10. Matt H

    Matt H Guest

    toroids and 1/2" ply for the head construction would shave weight off compared to a traditional transformer set/ 3/4" ply setup...

    give up reverb (figure the pan, chassis real estate, and transformer that it requires...)
     
  11. billdurham

    billdurham Member

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    A,

    Don't get me wrong, I'm all for doing things differently and thinking outside the box.. but I think that there are some laws of physics that you are going up against with regard to tone that might not be attainable using non-traditional components. If you get away from IE transformers, I'm just not sure it will sound right. I am not a EE, so maybe I'm full of it, but I think that since toroids have been around for a long time, somebody would have tried it by now.

    BD
     
  12. da-boogieman

    da-boogieman Member

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    Check out Linear Technology. They have a free version of SPICE to help design stable (phase/gain) regulators. Lots of good info there and I've had very good results. I think I've run across some low-voltage linear regulators configured to operate at high voltage using pseudo grounds lifted appropriately. A switcher should be able to be configured the same way. I've given some thought to using this in a power-scaling like circuit, but I work all day in electronics and have too many hobbies and sports to start any more projects!
     
  13. aortizjr

    aortizjr Member

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    Well there are actually quite a few amps that have used Toroids in the past. Recently I was just checking out the new Ibanez Thermion that has toroids. The specs are the same, I am sure there is some difference in sound, just like the differences between Mercury Magnetics and Hammond.

    But I am sure it is a workable sound. You may not be able to build a perfect Marshall clone or something, but I am sure you can get something sounding great, perhaps a bit more unique.

    The only reason I can find is expense, and particulars with tone. An equally spec'd traditional iron transformer is considerably less expensive and much easier to find than a toroid. Weight is often of little to no regard in head building as it seems with most companies....
     
  14. baald

    baald Member

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    i know there are some hifi cats who've used switchers for tube amps, but can't remember who off the top of my head. you could also use an OTL output stage, with say 6as7g's. for a shell, you could go composite, if necessary, line it with foil or use conductive paint.

    verb isn't going to add more than a pound. shouldn't be a deal breaker....
     
  15. Jon C

    Jon C Silver Supporting Member

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    I know that amp, Dave, and the amp's builder very well, and have one like it.

    They're called "Stumpy" for the funny look, Dave's is actually a SF Bandmaster Reverb head converted to a BF Twin circuit w/ the 2-8s.

    My "Stumpy" is a SFBMR head turned into a blueprinted BF Super circuit w/ 2-8s. THey just *kill*, even w/ "only" the 2 high powered Emi 8"s in there (bass speakers so they have good bottom end), thru an extension cab it's outside gig stage volume (loud). There's a multi-tap xformer so you can run internal speakers only, internal & external, or external only.

    They are heavy: there is no sub for big iron if you want good clean tone. I don't recall what Dave's Stumpy Twin weighs but my Stumpy Super is probably close to 60 lbs. Not light, but small. I LOVE it.

    Here it is (with the original SF BMR front plate, I now have a repro BF Super faceplate) with the matching low-profile 2-10 cab (open/closed back).

    [​IMG]

    Jon
     
  16. donnyjaguar

    donnyjaguar Member

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    Just wanted to add that I've heard hi-fi amps with toroidal output transformers and they sounded just fine. Lots of bandwidth and dynamics (300Watts) so I can confirm its a viable option.
     
  17. TieDyedDevil

    TieDyedDevil Member

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    IIUC, toroids don't tolerate unbalanced DC very well, so you'd have to to get pretty fussy about biasing and be prepared to either control the DC balance via a servo or be prepared to tweak it frequently as the tubes drift apart.

    Assuming that you can actually find a way to shed the heavy components, an aluminum chassis should work just fine. You just want to avoid having the chassis deformed when you drop the amp...

    Poplar plywood is lighter than birch, but softer. You can also save some weight by using thinner plywood and backing it up with some cleats in the corners and stiffeners on the longer panels. Keep in mind that you can use a thinner baffle board, too, given the light weight of a Neo speaker.


    I wish you luck. I've gone down this road, too. On my first attempt I built a 15W amp with a 15" speaker in a 19" by 26" by 10" cab at 36 pounds. And that was with Hammond iron that's oversized for the amp. A thirty pound amp should be attainable by using slightly smaller iron, switching to an aluminum chassis, and going to a 12" Neo speaker in a smaller cabinet.
     
  18. voodoovalve

    voodoovalve Member

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    Yes it can be done and it sounds great!Using 8 el-34's ,cathode follower driven + twin differential ,direct coupled,tube voltage amps.Power is two seperate switching supplies(220w core for fil. & bias /470w for h.v.)Aluminum rack case and chassis with plitron output trans.Easily puts out 280 watts of real/loud/great sounding tube power...and only wieghs 35 lbs!(mostly that plitron!)...thats the only real weight in it!..the tubes/plitron/speaker don't care where the power comes from,as long as its solid,low impeadance power(only using 50khz switch,cold be a bit lighter at higher switch frequency,say 200khz...harder to make though.Took a year to make and tune ouy the bugs,working on a simlar one now,but 8x kt-88 & 435watts(estimate less than 40 lbs)...and no, it doesn't electricute you,perfectly safe.Also has timed start up for high voltage to save tube life.
     
  19. voodoovalve

    voodoovalve Member

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    The toroid does require more strict balance of currents in core,but I think that this is alot less of a problem with bass & guitar ,as most amps are way into AB1 or even AB2(bass amps),so this means lower idle currents than hi-fi,so any slight imbalances would be alot less critical.I put 440 screws with TO-220 insulator bushings on back of chassis...each one hooks to a cathode bank(each bank grounds through 1 ohm resistor)..easy/quick bias reading with V.O.M. from each screw head to chassis(ground),(balance error is read between screws)...easy!
     
  20. Trout

    Trout Member

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    There is a reason this type of thing is no longer allowable in the electronics world. Its called electrocution.
    You are a brilliant Guy & Tech, and honestly I love 90% of your posts, but you should never ever suggest a circuit that is this potentially deadly.:nono
     
    Last edited: Jun 10, 2009

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