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oh no.. not another DIY tube amp thread...

Discussion in 'Amps and Cabs [BG]' started by Milothicus, Aug 31, 2005.


  1. I've got the bug. i want to solder something, and i want it to be a tube amp... or some variation of one.

    I've bought The Tube Amp Book, and am flipping through it, and there's a lot of great info...

    so... i'm not sure what my goal should be...

    Do i start with just a preamp? If i build just a preamp, can i just use it as an effect before my current Combo, or do i have to use it directly into a power amp?

    I have an old tube guitar amp (non working) i haven't played with at all sitting at home in canada that i may be able to butcher for parts. will the parts from a guitar amp (transformers, mainly) be useful for a bass amp? is there anything fundamentally different?

    i'm not wanting to do this to save money, but to have something i've built myself... and i know how expensive it's likely to become.

    If i were to build a full amp, or even a preamp, what do i model it after? i've found schematics for the fender bassman, and a few others, but i'm not sure which to start with...

    I know i have more questions, but i can't think of them right now.

    can anyone point me in the right direction?
     
  2. Passinwind

    Passinwind I Know Nothing Supporting Member

    www.firebottle.com/ampage
    Follow as many of the links as you can stand. :cool:

    Since you're Canadian, maybe check out the London Power website and books as well. ;)

    I'd strongly recommend that you start with a simple solid state preamp while you get all the tools together you're going to need to build tube gear successfully. I say this stricly because blowing up cheap low voltage stuff is better than blowing up more expensive high voltage stuff. It'll be safer too! Tube gear is arguably easier to work on once you have the knowledge though, FWIW.

    Personally, I'd try an Ampeg style preamp and a Fender style one once you get to the tube phase. The real fun is in not duplicating existing designs on purpose though, in my opinion. If you acquire the scary knowledge, you can probably install a power amp input on your combo, although hacking working stock gear has plenty of potential downside.

    What's your tube old guitar amp? Some work fine for bass projects, IME. Output transformers are often the weak link for bass amp conversion, which won't matter if you're only shooting for building a preamp out of the parts.

    luck,

    ---------Charlie E
     
  3. Do yourself a favor and ignore everything but the schematics. The rest is pretty much BS and GT ads.

    Dave Funk's (yes, the Thunderfunk guy) book is EXCELLENT I recently got another copy off eBay. I had forgotten how good it is!
     
  4. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Like passinwind said, you can learn a lot by starting with solid state. Indeed, a Class-A JFET preamp will teach you most of what you need to know about tube preamp design, without the danger or expense. You can hook up JFET circuits on a breadboard to your heart's content, and mess with tone curves etc.

    With both tubes and SS, it's fun to learn how to simulate a circuit using LTSpice, which is free. It comes with JFET models, but there are tube models that you can find on the Web.

    Yes, you can play your DIY preamp through your existing combo. Just pad the preamp output, to bring it back down to instrument level, and set the tone curve flat on the combo.

    Another thing about solid-state is that we all eventually succumb to the temptation and start taking our DIY gear to gigs. You would do yourself and your band mates a favor by leaving any homemade tube gear at home.

    In terms of your tube guitar amp, I say leave it alone until you are confident enough with your skills to diagnose and repair it without getting killed. Since debugging is part of the DIY experience, I think your amp is more valuable as a learning tool in its present state than if it is stripped for parts.
     
  5. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Thor = Subscribed :cool:
     
  6. Some great advice. i really appreciate it.

    I've done some soldering. i made myself a compressor pedal a couple of years back, and made a small practice amp out of a 7w amp kit bought at maplin (UK equivalent of radio shack...actually, better than RS).

    I understand the dangers, and i'm sure i can keep myself safe while working on it.

    as for the old amp, it doesn't have any branding on it at all. i haven't actually looked at it in about 2 years, but i was never able to find anything on it. my grandfather bought it at a yard sale, but never did anything with it, so he gave it to me.

    it's got a strange setup, as i remember. one large speaker (maybe 12") and a couple of smaller ones (2x4?). i was able to get some sound out of it, but not much at all. there's a switch that seems to select wether or not the large speaker is working. very strange, i thought. it also has a tremolo knob, but at the moment, it doesn't do anything at all.

    i don't have pictures, so i can't post anything else about it, but even if i don't want to butcher it, i'd like to have it as a guitar amp.

    I know that the speakers are different for guitar and bass amps, but how much difference is there in the electronics?

    i'm guessing some values need to be changed in the tone stack to use a bass's range, and i've read something about input capacitors, but i don't know exactly how to change their values...

    i guess these are some of the 'more questions' i have...

    keep the opinions coming, though!
     
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    If you do your homework on Ampage, you'll get all those answers fairly quickly, including the math. But briefly: you can change frequency response in a tube amp by changing coupling caps in various stages (including the input stage), the tone stack caps, the cathode bypass caps, and sometimes the feedback loop caps (if present). You may then need to adjust resistances to prevent overloading whatever stage you changed the cap value in, and you'll find that those resistances in turn change the frequency response again.

    Comparing Fender values between several 50 watters will give you some idea of the magnitude of changes that work well. Many times, no change is actually necessary for decent results if you use a suitable bass speaker, but the fun with a lot of tube amps is the ease with which you can swap out components.

    As I said before, many guitar output transfomers have pretty wimpy bass response, but fortunately more suitable low powered transformers don't cost a fortune. Expect substantial voodoo to be involved once you start swapping transformers though. ;) Another part of the circuitry that you may want to change is the power supply filtering and rectification, where sag = good for guitar, and maybe not so good for bass. Not all guitar amps are built to sag, but many with tube rectifiers are, or at at least do.
     
  8. couple more questions...

    This Fender Bassman... is it a guitar amp or bass amp?

    which ampegs are bass amps?

    why can't companies make these things clearer!?!
     
  9. Passinwind

    Passinwind I Know Nothing Supporting Member

    Which model? The open back combos with 4 tens are one of the Holy Grail guitar amps, but don't float many bassists' boats. The later 100 and 135 watt heads with ultralinear transformers are nearly worthless for guitar, but are nice sounding bass amps if the volume is enough for your needs. In general, I'd say the Bassman models with solid state rectifiers are more desirable bass amps. However, if you just building a preamp, I think tube rectification has interesting possibilities for making your overall sound "tubier".

    Schematics here:http://www.ampwares.com/ffg/index.html

    Maybe start by looking at the AB165 schemo, that's a classic model. Many folks like the Dual Showman head for bass too, be sure to check that schematic out as well. That's the tone stack arrangement Alembic uses in their preamps, I believe.

    which ampegs are bass amps?

    SVT, V4B, B-15 are the classics, IMHO.

    Schematics here:http://users.aol.com/bluemuse/ampeg.html

    why can't companies make these things clearer!?!

    C'mon! :cool: Leo Fender had no way of knowing how people would use his products at the time he designed them, and Everette Hull was into accordion (although he was a bass player), and hated rock and roll.


    ----------Charlie E
     
  10. ok. i'm back in Canada B+)

    took another look at the amp, and found out it's a Lifco brand amp, made in Montreal, Canada.

    4 preamp tubes (6AV6, which is apparently half a 12AX7?)

    2 power tubes (6BQ5, which is another name for EL84?)

    rectifier is tube, 5Y3GT...

    i wish i had a working tube amp handy to know what's normal. when i turn it on, it's silent, and then slowly starts to crackle a bit. moving any of the knobs leads to more crackling.

    is this normal noise?

    i didn't plug anything into it (no patch cords in sight after move), so i can't report on instrument output.

    tubes all glow orange near the tip.

    what's next?
     
  11. tadawson

    tadawson

    Aug 24, 2005
    Lewisville, TX
    That sounds pretty typical of a normal powerup of an older amp, with older, dirty pots. Try setting the volume low, and touching the tip of a cord going into the amp - do you get the hum in the output? If so, it's alive and well . . . . .

    - Tim