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DR. DIMENTO'S MICRO TUBE AMP EXPERIMENT

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


  1. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Having pretty much maxed out on pups, I'm now looking to tube tone for experimentation. At this point, I've formulated a plan, done some homework, and I'm moving to bring it to reality. But before actually buying the stuff, I'd like some feedback from anybody who might have some kind of direct experience along these lines.

    In a nutshell, the basic plan is to take about a 15 watt tube amp and run it to a Demeter-like silent cab and mic it for needed amplification. The cab and speakers however I plan to be smaller and the speakers to be hopefully hi-fi or car speakers. The amp itself will have it's own preamp but I intend to try at least some independent pre's with it as well (if only mic tube pre's).

    Who is it that possesses me? The goal is the capacity to generate a variety of quality tube tones at low volume in a compact package that can be amplified if it makes the grade. Versatility of tone will be had via a variety of front end pre's, tube swaps, speaker swaps, versatility within construction of the cab design itself, mic's and mic location.
     
  2. BassJunkie730

    BassJunkie730

    Feb 3, 2005
    Brooklyn
    I'm am right with you on this I have thought about running a small wattage tube amp through my avalon many times (I just need a load that I can run in parallel)

    But since you're using different preamps - I would go for a Single Ended Mono block tube power amp design with a 9 pin and an octal socket (or an octal with a converter) that way you can switch between all types of power tubes. Try to make the circuit as flat as possible that way you can hear the tube differences (and a nice beefy power supply can help too :D ) I think if you got just a combo - you'll be at the mercy of the particular circuit (think fender bassman). Maybe look into overhauling an old hifi tube power amp that is modified to run an assortment of power tubes.

    Along the same lines - I've been waiting for EHX to make a power tube overdrive pedal. There is such a demand for the rich ampegish overdrive out there!

    I say more power to you!

    Experiment on!

    ~Michael
     
  3. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    In deed you are right with me on this cuase that's exactly what's been going on and all the above have been perused and alternatives determined.

    The big question for me at the moment is the amp to speaker mix. That part I've just started a serious look at once the amp setup was pretty much nailed down. I checked around enough to know that the plan has sufficient merit. The details just aren't there cuase, if it's been done for bass, it's hasn't been done much.
     
  4. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    ok, first apparent glitch - speaker options. Looks like tough, low wattage, low frequency speakers are what's needed - not a commonplace combo for the most part.

    Anybody got any ideas here?
     
  5. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    little update. Maybe it will generate a little input. Thanks to bgavin for assistance on the speaker deal - which is still in the works.

    I just got a speaker in today to try out and the amp arrived as well. I decided on a THD univalve since it was basically already setup for what I have in mind. Goes low enough (LP MM pup/GS 112 cab) and output is way more than enough for my intended purposes. In fact, through the high gain input the output is suprisingly loud - I'd estimate enough for band practice at modest levels, cafe' gigs, and the like.

    Also tried the Art tube mic pre on the front end and confirmed that experimenting with pre's is a viable tone altering option. Darkened and fatten up tone noticeably and also boosted output noticeably through the clean lo gain input.

    Haven't got around to building the cab yet cause the speaker aspect hasn't progress sufficiently. But I'm guessing that will come to pass in the next week as well as more tubes and speakers soon to follow. Power tube in it at the moment is a Groovetube 6550 with 6L6, KT88, and 5881 waiting their turn. THD recommended a KT66/6550 power and 12at7/12au7 pre for bass. Tube amps are a different world.

    That would only leave mics but boosting volume to serious levels won't be needed for a while. Who knows, maybe not all.
     
  6. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    The Univalve seems like a good choice for an experimenter. Being single ended the distortion spectrum will probably be fairly monotonic (ie distortion will increase farily linearly with power output) so you'll be able to get a linear overdrive function in the amp.

    WRT speakers, why do you need low wattage types? Are you looking for the breakup from the cone/ voicecoil exiting the gap that guitar speakers have and use for a large part of their tone in many cases? The amp itself will give a far amount of distortion when driven hard.

    Once you have a box built, try anything you can borrow or get cheap, especially drivers from radiograms, older hifi and MI speakers. There are a myriad of them out there, and only wanting a single driver makes choices a lot easier (also true wrt tubes)

    Not sure if I mentioned via email before, that I'd make the rear chamber volume of the cab much larger than needed as iirc, portability isn't much of an issue. You can always fill it with bricks to reduce it. The front chamber I'd make with golden mean dimensions to minimise additive effects of standing waves between walls.

    I'd also consider adding a big wirewound pot (maybe 5 ohms / 20W) in series with the amp output (switchable/patchable) as this will add to the output impedance of the amp and increase the Qts to allow you to 'tune in' the low end response of various speakers in the cab

    Looking forward to hearing how this goes.
     
  7. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Brett,

    good to hear from you and reminds me that I've been meaning to email. But since we're here.

    You have to understand that 1) I don't know what I'm doing but 2) I've ran the idea by guys like you who do have a clue and got the go for it.

    But I concluded that the Univalve was a good choice for a whole string of reasons with the singled ended class A just being a couple. Not much downside except cost but historically resale and demand is serious on these units. Even if I were capable, highly unlikely I'd be able to build an amp with similar features for any loss I might take on the Uni. Even assuming I could pull off building such a thing, the time for experimenting is now not months from now. I've already experienced more impatience than I'm accustomed to.

    My logic behind low watt has been smaller and cheaper speakers in a smaller enclosure. I learned from Shpeakenshtien (roughly 2' x 3' x 4' 6-15 test-lab cab) that next time would be different. Also the considiration that tone changes in tubes as output increases so I figured I'd need lower watt speakers to keep from having to crank a small tube amp and driving it into distortion just to get the volume level desired. From what I've gathered with tubes there's a too little and too much range to steer clear of. In that regards, another good thing about the Uni is I've got two different channels to work with.

    Initially I was thinking along the lines of 5 watts or less though and the Univalve is 15. From running that Uni through the Aggie, higher watt speakers dont appear to be an issue in terms of sufficient volume or distortion. Verdict on the size, cost, and enclosure is not in yet. But a major part of an ideal outcome is small. I know it can be done cause I've read some accounts. It would be a plus to be able to toss in mixes of distortion to add to variability but that's about the last thing on the wants list - and I don't expect that to change. I've been more concerned about getting distortion I didn't want.

    Bgavin directed me to the infinite baffle idea as well (which sounds like what your saying) and that appears the obvious choice for starters.

    You're going to have to explain that radiogram thing cause that sounds like AM/FM email to me. And MF sounds all too familiar but MI I can't place.

    Also a bud mentioned a way to rig up some kind of shock absording barrier that fools the speaker into thinking the enclosure is larger. Can't recall the layout but it sounded plausible and something he's done and had success with. Does that ring any bells?

    Hmm.........very interesting. What is this wirewound pot thing? First I've heard of that - at least that I know of.
     
  8. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    MI = Musical Instrument. Mainly my thinking was small, budget or trashed combos with a speaker that still works may be worth trying.

    Radiogram is one of the all-in-one console type hifi cabinets that were common from the late 40's to the 60's and are still today to some extent. Some of the earlier ones had high quality drivers in them, and because they were often open backed or had 'leaky' ie not sealed box, but not deliberately ported the drivers often work well in smaller sealed emclosure. Jensen, Rola, Magnavox, Utah, Goodmans are the first names of a long list (but all I can recall ATM).
    Car driver will probably work well too in a lot of cases. At least technically - no idea if the sonics will be suitable, but probably....

    Agreed. Building amps if you're not set up for it is a time consuming and dangerous chore, and one that I wasn't real keen on you starting without help as the voltages inside can be lethal. In my next email I was going to suggest a secondhand low power amp from an old console or from a MI application like a Hammond.

    The power that a driver can take and it's efficiency or how much noise it makes with those watts are two different things. High power speakers may work fine.

    Not OTTOMH.

    Modern speakers are designed with the assumption that the poweramp has a very low output resistance, which they usually do. Tube amps, especially those without feedback like most single ended type have an output Z of a couple of ohms. When you're designing an enclosure and use the T/S equations and parameters for the driver, the assumption (unless stated otherwise) is that the amp having a trivially small output Z, it can be factored out of the design. Because you don't have the driver parameters, and there will be a number of different types and designs using the same box, a series resistance, may allow you to make some significant changes to the transient response (Q) and the low frequency response of the system. A series resistance, whether the amps output Z, or an added series resistance will affect Qes and Qts of the system.

    If you can't find a suitable pot, some resistors on tagstrips on a board, and use crocodile (alligator in the US) clip leads to select the values. Couple of dollars and a half hour total.
    This sort of arrangement will work in low power rigs like this, but not higher power stage gear unless you want to dissipate a lot of the power amp's watt in heat in the resistors. In your little rig here it'll be a few watts max.

    I was thinking as I typed this, that if both sides of the box are well sealed, the load on either side of the cone will be fairly uniform and it may be possible to get the drivers to operate to down near their resonance.
     
  9. luknfur

    luknfur

    Jan 14, 2004
    DIXIE

    MI - of course. Been a while. Suprisingly, I'll bet I've only ran across that a couple times lately in looking at probably a hundred speakers easy. Most of them car audio though.

    The radiogram is a first but console I know - although it's been A LLLLLLLLong time. And that's a good point about those speakers in sloppy enclosures and something to look into.

    As a matter of fact, initially I was looking into small 6V6 and Hammond units. I realized almost instantly I wasn't up for some corroded old Thomas Edison looking rig but I'd considered picking up something from the 50's or 60's in prestine working order and modifying it. Even with that I quickly learned you need to know your transformers and all and I realized I needed a more realistic approach. So I'd concluded the Uni saved me some grief and probable failure but I'd forgot all about when I was a kid and the old man blew up the TV set.

    Man that wirewound pot sounds rrrrrrrrrrreal appealing. I don't recall ever running across mention of anything like it. Assistance would definetly be in order for that though cause from the explanation I got the words but I didn't get the music.

    In regards to sealing both sides of the speaker, would doing so by chance make it possible to make the enclosure smaller? And isn't free-air resonance where all hell breaks loose? Or is that a different resonance than what you're referring to?

    At any rate, speakers are proving to be the challenge. Even if you get one with all the right numbers and it functions as designed in the enclosure you want, the fact is, the tone can still suck and tone is what it's all about.
     
  10. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    A little experimentation and results from a power tube change.

    This tube variation thing already looks real promising cause I just swapped from the 6550 to the 6L6 and the tone changed significantly to a deeper, bigger, darker, dirtier (mild distortion), warmer, bluesy, EBO, sub-woofer kind of surround-sound. The 6550 was signficantly louder, clearer, cleaner, and brighter.

    Kind of floored me a bit cause one of the things I was hoping for optimally was what I call Onkyo Tone. The bass tone my Onkyo surround-sound puts out is what really intiated the motivation to do this whole tube exploration. That deep, burring, sub-sonic kind of rumble that lays below typical bass tones. Anyway, the 6L6 caught a bit of that rumble. Even with the lower apparent output of the 6L6 on a volume level of about 4 the lower frequencies were rattling the grill on the combo amp next to the GS 112 and I could feel the tone as much as hear it.

    I'd heard that power tubes are where to get the tone variation from a tube amp and not so much preamp tubes (one reason I wanted to check out a tube amp not just tube preamps). What little messing I've done with each so far supports that.

    Anyway, I don't know if this one swap is reflective or an exception but when you consider the dozen or so tubes that can be used with this unit for both power and pre sections, all the combinations available through mixing them up, and you add up all the makes and variations for a given model tube, the inherent variations that exist within the same exact model tube, and variations that occur as a result of wear, there's probably as much tone variation available through tube change as varying pups.
     
  11. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    got the first speaker in which was a dual coil prototype that I don't even have the specs for. Distorts but a consistent, pleasant, even distortion (at all volumes) much like the Stones Satisfaction tone but not as fuzzy with distortion not so out front. Sound fundamentals and complimentary harmonics that project well so that the distortion is background sound. Covers the bass frequency range well. In all, different, but quite appealing and interesting to play to popular tunes in general - a nice useful twist to some (like Queens Another One Bites The Dust). Adds an aggressive, driving edge to tunes but not in an in your face way.

    So another promising indicator for some tone shaping if this speaker is any indication of versatility to be had. Ideally, I hoping to be able to mount various speakers so I can just slip them in and out and of the demeter type cab and swap speakers in a couple of minutes to get a different tone.
     
  12. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    little more progress. That distorting speaker was defective, shipped it back, and got the replacement. Think I liked the defective one better and didn't spend much time messing with this one.

    Got the initial experimential micro cab built and can adjust the cab volume from 1 - 3 cubic feet. External dimension of 14 x 15 x 25" with an internal open back speaker box of 12 x 13 x 11. Therefore the internal box pushed all the way back yields right at 0.993 cubic feet and when telescoped 6" 2.994 cubic feet. 1/2" MDF makes for a smooth, tight, sliding fit between the speaker box and surrounding cab, and is easy to work with but is on the heavy side at 35 lbs loaded with an 8" pioneer (about 5 lbs I'd guess).

    More narrow, taller dimensions would be more convenient for transport but I wanted to be able to use up to a 10" speaker, have some space around it, yet be able to go down to 1 cubic foot volume so I could check out the smallest possible speakers which are of primary interest to me.

    This is just a prototype for experimentation and should point the way to more optimal alterations. This cab's not going anywhere so portability is not so much an immediate current concern as a long-term goal. For all I know, this idea may not pan out in the long run at all and that would be the end of it.

    No mic yet so that aspect I haven't addressed. But playing through the cab itself was interesting to hear tone variations by varying cab volume alone. Don't know enough about speakers to know what's actually going on but varying cab volume I could hear such differences as an overall frequency drop with larger volume, to the bottom frequencies dropping off and upper half frequencies becoming more pronounced, to boxie/honky upper half, to tightening of frequencies across the band with an eveness in response. Not a great tone but I've heard a lot worse and it's a start. Mic'ing I'm guessing will add a whole new dimension to tone. Heres this speaker's specs. It was like $16 shipped.

    PIONEER B20GR30-51F 8" BUTYL SURR WOOFER Power handling: 80 watts RMS/115 watts max * Voice coil diameter: 2" * Impedance: 8 ohms * Frequency response: 31-7,000 Hz * Magnet weight: 30 oz.. * SPL: 91 dB 1W/1m * Vas: 2.44 cu. ft. * Xmax: 2.8mm * Qms: 6.51 * Qes: .27 * Qts: .26 * Fs: 31 Hz.
     
  13. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    another update. Guess I'll condense it all at some point so it flows reading wise and all the info is together instead of scattered in chunks. But it's a work in progress and chunk time for now.

    Got some US 12au7 and 12at7 tubes. One a black plate which I've read are more desireable (fore whatever reason), and this one is the best of the lot including a grey plate Tung-sol so maybe something to it. The AU's are consistently darker in tone while AT's bring out the upper mids and top end nicely.

    Also have some 12AY7's and AV7's headed this way.

    I've ran a GEB-7 EQ up front of the THD and into that pioneer and it permits a lot more control over the response of the speaker than just the amp. Allows for honk or whatever to be cut and bass or whatever to be boosted. Suprising how little tweaks make a significant difference and in general how much more control there is over the tone emitted by the speaker. I haven't adjusted the micro cab into some wierd frequency response and attempted to straighten it out with the EQ but seems EQ could be used to work around some slop in a cab design.

    No more speakers or a mic yet but that is forthcoming. At any rate, I don't see small bass instrument speakers cutting it cause the FS on 99% of them is in the upper 50's and 60's at best. I don't know what they used to do when the multi-small speaker cabs were in vogue. Use them in conjunction with 15's I guess.
     
  14. Rvl

    Rvl

    Dec 23, 2003
    Aomori Japan
    I am having a mini amp made
    Chose Fab Amp to make it
    http://fab.stuff.home.comcast.net/index.html

    Trying to decide whether to go for a single KT88/6550 or the 6L6?

    Very interested in all of your experimentation
    So keep up the good work
    And keep on breaking new ground
     
  15. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Didn't run across Fab amp in early searches but here's another guy I was checking into initially:

    http://upsidedownandbackwards.com/FrenzelElectronics/LittleJOE.htm

    You really need at least 15 watts of bass frequency for anything not mic'd to hear it though. The THD will get loud enough to play with an unmic'd drummer who has some respect but that's on the guitar channel (lead/hi gain) - which works for some bass but I haven't used it at all really. The low gain, clean bass channel has lower frequency output and a fraction of the volume, even with the KT88. The guy at THD recommended 88's or 66's for cleaner tone. Seems there's a disadvantage to 6L6's but can't remember what it is but I know someone who does know and I'll ask.

    The THD was steep pricewise but they have great resale and it's ideally setup for experimentation and has lots of little pluses. Taking virtually any power or preamp tube without rebiasing. Has the line out and a dummy load which saved me once when I stepped on the speaker cord and unknowingly popped it loose from the cab - which would have shot the tubes otherwise. I could probably have the caps/filters changed for bass on the lead channel but it's not somethng I've looked into.

    It may or may not be something practical for a micro setup but I wanted to experiment with some kind of tube setup that was versatile tonewise via tube swapping. The micro idea was more just an expansion in trying to make some practical use of the experimentation. Plus it seems all the pieces were available to bring it about.

    Even at that, it wouldn't be close to as compact of a setup as a Clarus with a compact cab but it would still be quite practical to transport. And a Clarus is not tube. If nothing else, the micro setep would probably be great for a recording studio setup.
     
  16. Rvl

    Rvl

    Dec 23, 2003
    Aomori Japan
    Yeah.....Fab Amp is incredible on the price $330 for 8watt bass chassis and then the option of head $50 or cab/spkr $100

    My total will be $530 including an upgraded Weber spkr , Red Marshall Tolex cabinet and the bass preamp

    If I choose to go with the KT88 I'll have to upgrade the transformers and pay extra

    The guy from Fab Amp dropped by here earlier and was very interested in your experimenting
    He is a guitar player but plays bass too and really likes Jack Bruce
     
  17. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Yeh I was initially looking into modifying my own from some clean basic 6L6 chassis or the like. But such a chassis is going to run you better than $100 and I had no clue about what I was doing so pretty pointless and I didn't want to dink with something beyond swapping tubes, I wanted to play bass. I could have bought an old Bassman for less than the Univalve but it's not setup to experiment with, the Uni is.

    FWIW: I paid $620 shipped used for the Uni and that's average at this time. Saw one went for $450 before I started looking. Often you can get a hundred bucks and more worth of tubes with one. And they'll take 88's, 66's, 6L6's, EL 34's, or whatever. And of course you can get a Bassman for $400 and under. But neither would be custom made though.

    Tell that dude from Fab to get on-line and jump in. It's free advertising (just joking Paul).
     
  18. Rvl

    Rvl

    Dec 23, 2003
    Aomori Japan
    I was planning on buying an old amp and modding it for bass
    I definitely dont have the ability
    But mostly guitar amps are designed to push the pre-amps to distortion and I'm trying to push the power amp to distortion
     
  19. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Distortion is not something I've messed with or looked into consciously but I've got the distinct impression in my search in talking to different amp manufacturers and repairman that distortion is not something you want in bass. There is a natural clean tube sound that is fat, warm, and kind of swelling that is appealing - and it's a kind of a mild distortion but it's not a low frequency Les Paul/Marshall crunch/chunk distortion.

    That swell vary's with how hard you drive the tubes. Generally optimally from about a volume level of about 3 to 7 on a tube amp. Less and it's wimpy and more and it literally distorts - and not in an appealing way. One of the reasons I wanted a smaller amp was so I could hit that optimal range and therefore get optimal tone at low volume levels. I didn't want to have to go deaf to get tone and if I wanted it at two in the morning, I wanted to be able to get it. You get an amp that's too small and you have to drive it into distortion to get audible volume for bass.

    The guy at Fab could no doubt do a better job of explaining it than I can but I haven't talked to anybody that recommended distortion for bass and from what I've heard from the THD, I wouldn't recommend it from what little I know either. There may be some mix that would work well for bass distortion with tubes but I haven't gone there yet. But I should have kept that speaker I got that had the fuzz distortion and not sent it back cause the one that he shipped that works as it's supposed to sucks. And it may be the speaker is the better approach to go for the distortion route. I don't know.
     
  20. Rvl

    Rvl

    Dec 23, 2003
    Aomori Japan
    I like the sound of a SVT pushing at the edge just before breaking up but the volume required is not reasonable
    I like clean and fat most of all
    I'll try this mini tube amp on stage and run it in tandem with my regular rig
    1)mic the Fox tube amp
    2)DI to mix the signal