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Marshall AVT20 conversion to bass amp

Discussion in 'Amps and Cabs [BG]' started by LeDuck, Jun 2, 2012.

  1. LeDuck


    May 26, 2012
    I wanted a small bass amp that sounded like a vintage tube bass amp without the size, cost, heat or tube issues.

    I selected the Marshall AVT20 as the reviews were very positive, they are very easily acquired at a low price on the used market, and it uses 10" speakers of which there are many to choose from in the Eminence bass guitar and pro sound lines.

    A classic 60's/70's Marshall bass amp was a guitar head with Marshall G12H(55) speakers in a 4x12 cab. Jimi Hendrix used the G12H(55) speaker cabs in conjunction with G12M 4x12 cabs. A G12H and a G12H(55) are the same except for resonance frequency, in this case 55 Hz. HiWatt was also conspicuous in using similar heads for bass and guitar and changing the Fane speakers in the 4x12 cabs. Note Pete Townshend and John Entwistle and their HiWatt rigs of the 70's. The Partridge output transformers in HiWatt DR103 amps were of audiophile quality and made them very suitable for bass guitar (which is why audiophiles bought old HiWatt DR103 heads on the used market, removed the output transformers, installed something cheaper and inferior, and resold them on the used market....BEWARE of an old DR103 at a to-good-to-be-true price). The solid state output stage of the Marshall AVT20 gives extended bass and tight control of a speaker while the tube preamp stage gets good tube overdrive tone, so it's a natural for a guitar to bass amp conversion.

    I installed a Philips JAN 12AT7A tube in place of the Marshall supplied Ei ECC 83 to drop the gain from 100 to 70 in order to have slower breakup and wider control range in the first gain stage. The Ei ECC 83 is a killer tube for lead guitar due to its high gain, quietness, and the fact it is a Telefunken ECC 83 copy.....no need to say more if you are an tube audiophile or lead guitarist. BUT, I think the Ei ECC 83 has too much gain for my Spector ReBop with active tone controls and high output DR Sunbeam strings and high output EMG HZ passive pickups.

    The stock Celestion G10-30AVT is a piece of crap for bass guitar as it falls flat on its face at 80 Hz, and I have the proper test equipment (I'm a degreed biomedical engineering techie) to prove these points. It has a lot of suckout in the midrange for a scooped sound that guitarists like, but bassists have little need of. It farts when you try to get deep bass from it at an appreciable level. It will self destruct if you keep shoving it, so I pulled it out and intend to give it away to a friend's son who is interested in taking up guitar.

    First speaker in was an Eminence B102 of 92 dB efficiency and excellent extension low and high with very linear response. The quality of sound was excellent with the flattest response of the three speakers I have used so far. The fly in the ointment is the low efficiency and the low 20-watt power of the AVT20 .These are solid state watts, not tube watts, equal to about 10 tube watts for comparison. I just had to have more volume, and the quality and range of tones I was getting from the AVT20 started me on a search for a more efficient speaker.

    Searching the Eminence spec sheets for a day lead me to conclude that the Eminence Beta 10A was my best choice at my $100 or less price point. If I need to spend more than $100, then a bottle of Talisker and a bottle of Laphroaig for mixing my secret blended Scotch recipe into cheap grain Scotch gets me the best tone.....for a few weeks at least. :)

    The Eminence Beta 10A in the standard pro sound line seemed the best choice. It is 95-97 dB efficient, depending on how you look at it. Would make the AVT20 sound like you had at least doubled the power relative to the B102. Resonance freq was 53 Hz, close to a Celestion G12H(55) from days past. 2" voice coil, bigass magnet, good response curve, good Xmax, and rated for bass guitar, keyboards, PA use. A giant killer at $70 delivered. Ordered one from Musician's Friend.

    The Beta 10A arrived and I was impressed with the build quality. It has a vented pole piece/voice coil with protective screen. No squawker cone like on the B102, but with a presence peak at 3kHz in spite of that. But off axis, I expected the presence peak to be much diminished, and you generally are off axis from a combo amp speaker. I was expecting a flatter power response due to the presence peak, as it makes up for the narrowing of the "audio beam" at higher frequencies. This keeps overall total power into the room more constant, in spite of the peak on axis. Doing a LIAR test (listening in another room.... if you don't know audiophile speak) while I made the dog his dinner in the kitchen tended to bear that out. The B102 is 200watt rated, but the 250 watt rating (coupled with increased efficiency) of the Beta 10A means it can play about 3-5 dB louder if you have the power. This looks (and turned out to sound) like a killer bass speaker for the price.

    The Beta 10A was installed and secured with blue loc-tite and it was time for listening and testing. Early testing tended to support Eminence's published response curve. Quick bass guitar test proved it to be louder in bass response and much better sounding than the G10-30AVT. More loudness than the B102, if not better sounding. But then again, the B102 is used by audiophiles for making single driver wideband loudspeakers, and not likely to be bettered in sound quality in a lower priced speaker.

    I needed a mono recording to test wideband sound quality, so out came the Beach Boys "Sounds of Summer" album. A CD player is a "flat" input, so it gave me a "flat" setting of my AVT20 tone controls. They are Bass 8, Mid 5, and Treble 0. Remember the Beta 10A has a presence peak, so treble will need a big rolloff. I prefer a bit of bass/midbass boost. This is "right" to my ears.

    The first pass of the album definitely improved the sound, so I listened to the album again, and it improved some more, and I listened a third time. Yes, the Beta 10A is a good PA speaker for sure. It should be a good partner with the B102 in a 2x10 or 4x10 cab. Don't Worry Baby, everything will be all right. Yes....I Can Hear Music. Good Vibrations are going on here. It sounds good....In MY Room. We'll have FUN, FUN, FUN....till the neighbors call the cops. :)

    There were some mods done to the cab to improve it for bass. I'm not a real fan of ported cabinets. My Polk HT bass-reflex speakers have open-cell foam resistive flow plugs shoved into their ports (just make your own and experiment), and by my sound level meter and my ears, it flattened the in-room response and gives a slower rolloff in the deep bass. The U2 and Santana concerts sounded better after that.

    Resistive flow ports (aperiodic vents, also known as variovent) get rid of the unloading of bass drivers below port cutoff frequency and protect against overexcursion as is the case with bass reflex cabinets. It also gets rid of the high impedance at box resonance in totally sealed cabinets and without the need for as much acoustic fiberfill. It very closely approximates a transmission line speaker without the size and complex internal baffles and precise acoustic stuffing issues. It gives a more flatly resistive impedance curve that permits more speaker coil current near resonance and thus better driver control and a "tighter" sound. Should sound more "live" than a tightly stuffed acoustic suspension cabinet. A famous audiophile loudspeaker of the T-line design is the Vandersteen Model 1, if you get a chance to listen to a pair. They are killer for the price. Listen to something else around the $1200/pair price and then you decide. A famous loudspeaker of the aperiodic vent design is the old Dynaco A25 which sold about a million copies in the 70's and 80's. Known for monster bass extension in a bookshelf size cabinet and 10" woofer. Bass guitar cabs are living a few decades behind the times relative to audiophile loudspeakers in sophistication. The Nestorovic bass alignment in the Vandersteen Model 2 and Model 3 is another killer bass system (by Mile Nestorovic, past chief engineer at McIntosh Laboratory), but don't expect it to show up in bass guitar cabs anytime soon.

    Anyway, Madisound or any good audiophile loudspeaker supplier can sell you a Scan-Speak or Dynaudio variovent. Shoving the proper density and thickness of foam in a bass-reflex cabinet can do a similar trick. You can staple 1/4" hardware screen over a cabinet cutout of the proper size, stuff it with fiberglass, then staple another piece of hardware screen on the other side of the cabinet wall to hold the fiberglass sheet in place. That is basically what a Scan-Speak vent is, but uses molded plastic instead of hardware screen. The Dynaco A25 used stapled hardware screen, so look it up on the internet to see what I mean. It's a classic and sure to be talked about in vintage audiophile forums. And it could make a good practice bass cab if you stumble across a pair at a garage sale....but the efficiency will probably even be lower than a B102, so expect a speaker changeout.

    An aperiodic bass enclosure is usually filled about 2/3 full of acoustic fiberfill with a clear pathway to the variovent. Fill rates are 1# to 4# per cubic foot, and that's a matter of personal taste. For bass guitar cabs something closer to the lower figure will probably be to most folks taste.

    Anyway, it's time to plug in the Stingray and ReBop and abuse the AVT20 and Beta 10A for a while before I give it a final response curve test. It's looking good so far. If you want something different, you just have to do it yourself sometimes. Not available in stores.
  2. Wow! A post that size needs a decent reply. So.....
    Interesting :)
  3. Sinomis666


    Apr 16, 2011
    Yeah maybe I'm amazed at how long it would of taken you to write 'this'! Good & interesting stuff
  4. LeDuck


    May 26, 2012
    Well, the torture testing last night revealed a serious deficiency in the AVT20 for continued high level sound at high frequency.....as smoke started to come out of the front phone jacks. BUMMER!!!

    Near the end of the high frequency test range, the problem appeared. Removing the board revealed a resistor R53 had burned and that it was connected to a series capacitor. I suspect it is part of a Zobel network to control HF impedance, and guitars aren't supposed to produce about 4 watts of power up near 20 kHz. Marshall should hire me for torture testing :)

    Being an English product, and the English seeming to be masochistic in their affairs with cars and rifles (one English Sunbeam 1500 (Plymouth Cricket on this side of the Big Pond) car was enough for me forever, and my BSA air rifle bolt likes to sometimes blow open in my face, exposing me to 3000 PSI air, my eye saved by the scope mount more than once), perhaps caused by long periods of dreary weather and lots of time to dink with things, the English seemingly like to design niggling future problems into their gear. I cringed every time I discovered the hospital had purchased some medical gear made in England. But, some biomed techs consider it job security these days and I have known some to recommend products for purchase with guaranteed high maintenance requirements. The Marshall AVT range is going to need some preventative measures taken up front to prevent future problems, and I spent this morning replacing R53 and taking care of a few of the Brit designer loose ends.

    I suggest you pull the board of any AVT amp you buy and give it a going over as well.

    The 10 ohm 1/4 watt carbon reistor R53 was replaced with a 10 ohm 2 watt metal film power resistor and it was stood up well off the board away from the capacitor it scorched a bit.

    When disconnecting the fast-on leads, one of the posts connected to the speaker leads pulled right out of the board. There are several of these posts on the board, and some of them looked to have cold solder joints. Cold solder joints can give you no end of agony in the future. I have spent days on such problems as they age and become intermittent.

    The reason these posts have cold solder joints is because the whole cheap phenolic resin board goes through a wave soldering machine and the big heavy top posts are not heated enough by the tiny pin ends sticking down into the solder, hence the term cold solder joint. The English worker that would have once turned the board over and inspected it and hand soldered any such posts into place properly is retired and on pension and current English economic conditions demand the fastest production methods possible. In short, it ain't a Traynor with a glass epoxy board. It's made out of particle board where the Traynor uses Baltic birch plywood, etc. But, the engineering is good, it's just that the bean counters have went a little too far by overeconomizing in places that you will need to straighten out. This is not unique to Marshall, and this unit if far from the worst I have seen.

    I flowed 94/6 silver solder (Radio Shack) into the suspected post pads and built them up a bit. 94/6 is much stronger than 60/40 solder, but needs a higher temperature, so be VERY careful. It's so easy to lift a pad on these single sided phenolic resin boards. I'm used to working on double sided, plated through, glass epoxy boards as found on higher end electronics. You have to press UP on the posts with long nose pliers while the board rests against the post because they will drop out as soon as the solder melts. IF the post is not FULLY down on the top side of the board, you can pop off the pad on the other side of the board while attaching the fast-on connectors later. Make SURE the posts are fully down against their shoulder stop.

    Now REALLY, who at Marshall thought up that slotted metal standoff post that the board pops down on. There are three of them in question here. The Marshall design guy managed to make them less secure than even cheaper plastic standoffs, and of couse the tension was loose and could easily be lifted up and down and the board could rattle against the post and make a louder sound than any plastic post could ever accomplish. Dremel moto tool to the rescue.....I cut off the slotted top ends and drilled through the rest of the mount and threaded them 4-40 and stuck a 4-40 x 7/8" through from the bottom and ran down a 4-40 nut on the top side to firmly lock it in place from ever loosening. The board had the mount holes enlarged a bit and a small rubber grommet fitted in each hole. With the board in place, and another 4-40 nut snugged finger tight against the grommet and secured with blue Loc-Tite, I now have board mounts that help take care of resonances in the board instead of causing them.

    Do yourself a favor and drill through the mounts from the bottom. The middle mount is blocked from above by the upper front lip of the panel.

    Around 70-100Hz Hz a loud buzz sets in at high volume, and three different speakers and three different tubes did not change it a bit. Therefore, it is a cabinet buzz. It seems to be the Marshall grill cloth flapping against the grill cloth board, as it makes a sound when you remove the grill and tap the grill cloth from behind with your fingers. The big air movement of bass reproduction is going to cause a problem here, and there are several ways to solve it: Secure the grill cloth firmly down against the board. Put some foam strip material between the board and grill. Put a metal grill in place of grill cloth, as many bass cabs have. The fact that the grill cloth visibly moves while playing low bass means that it is making its own sound, beside the buzz as it hits against the backboard.

    Also, the grill board should be secured with screws instead of four flimsy velcro patches.

    The AVT20 conversion saga continues.......

  5. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Wow, that's a lot of turd polishing! I threw my AVT20's chassis into the trash as that's where I felt it did the world the most good. I did keep the cab, it's loaded with an acetone-modded Jensen C10Q RI and is used with a home-made Champ build.

    Good luck with your project. Sounds fun.
  6. craig.p


    Sep 28, 2008
    New Hampshire
    Back in my computer manufacturing days, we averted cold solder joints on large solderables by preheating the stuffed board. But you had to be careful not to exceed component temp ratings. It was a delicate balancing act. Also back then we were allowed to use solder that actually flowed well and cooled well.

    You think those single-sideds are bad? Man, those are nothing. How 'bout a six-layer board where the pad-to-pad connections are made by embedded copper wiring? Those were part of our kit for close-coupling minicomputers. Now there's something to test your rework skills.

    I think your 4 watts of power near 20 KHz test was valid. It was a fuzz box simulation.

    I hereby nominate LeDuck for TalkBass's 2012 "Man Most Likely To Blow Himself Up" Award.

    It's also a pleasure to read posts written by someone who really knows how to sling English. Heigh ho!
  7. LeDuck


    May 26, 2012
    craig.p, I have already been hit by 340VDC while working on an old oscilloscope. It kind of got my attention....but not quite blown yup yet.

    I spent the afternoon testing my "overdesigned but underbuilt" AVT20. It's not a piece of junk, it's just not the type of gear that people who can't work on their own gear should use unless you have a warranty. The very low purchase price on the used market allows you to modify it until you like it. If you botch the job, its not like losing an Aguilar or something like that. You should have heard my youngest brother's old Crate amp. I WISH he would have at least upgraded it to "polished turd" status, but he liked it because it sounded like crap.

    I have a couple of new 15W output transformers And 4 matched Ei EL84 tubes (and Ei tubes are good and have gotten rare). if the AVT20 chassis totally blows. A blown power tranny is about the only lethal failure the AVT20 can have, as everythiung else is cheap to replace if you want to work on it youyrself. Would NEVER pay anybody to work on such a cheap amp. I would probably do a twin channel amp and use my Sansamp RBI preamps to drive it. I have a pair of Heath W5M monoblocks with KT66 tubes that I am rebuilding this summer. Their Peerless output trannies are some of the best that was built during their day, and they have choke filtered power supplies. Very solid sounding little amps and will make good practice amp heads. At about 40 watts output, they will drive a lead guitar cab very well.

    But on to the AVT20 update. Yes, I tested the AVT20 with and without the grill in place. The mids and high frequency response seemed a bit better with the grill in place, as measured by sound level meter in many different locations. I had plenty of short stainless steel wood screws so I just screwed down the grill cloth all around the cutout. Yes, it sort of reminds me of a leather saddlebag on a Harley with all those bright screws against that black grillcloth. That took care of a lot of that 70-100 Hz buzz. I think the cloth has a bit of buzz itself internally, as it is a very coarse weave. Now any fret buzz present seems to dominate, so no need to go further at thius point.

    Up in the living room, so the neighbors could also enjoy my testing this evening, I got a -12 dB reading at 40 Hz. The room is carpeted with lots of drapes to suck up some reflections. Since the response was 0 dB at 80 Hz, and a sealed cab drops off at 12 dB per octave under ideal conditions, this sounds like the expected performance of the speaker. Output was useful down to 63 Hz at about -3 or 4 dB. At these low frequencies, there is no rolling bass waveform, but instead the driver is mainly pressurizing and depressurizing the room and I know from experience there are some major standing waves that are problematic even with my Vandersteen speakers in this room. The AVT20 bass control is useless in this regard as it is broad bandwidth and I need something like 1/3 octave to tame the room peak.

    With bass guitar that is full of harmonics, the sound is very full and rich. After a midbass and high bass hump, you get a very flat midrange and then you get a rising presence region from 2kHz to 4kHz, with extension out to about 5 or 6 kHz depending on your angle to the cabinet. You might say there is a very broad midrange suckout, but nothing like the Celestion speaker that it replaced, and I can get about any tone that I want. It also means you have no need of a trashy piezo horn tweeter with this speaker. Plenty of brightness if you want it.

    It doesn't play all that loud, but a practice amp doesn't have to play very loud. According to my sound level meter, if I played as loud as it will go all the time, I would have hearing damage. You should only do about 2 hours at 95 dB average SPL every day, and it will do that and stay fairly clean. With the grill off, I was surprised how much cone movement I was getting from just 20 watts.

    If I install a cooling fan and bump the fuse up a notch, I think I could drive another speaker in a remote cab and get smoother bass response in the room. I have my eye on an Eden 1x12 cab at the local music store. The cab would also go well with one of my Heath W5M monoblocks. My monoblocks were well stored and look almost new. Power trannies are new Hammonds, and that was the only thing that routinely blew up as they were potted and overheated if left on overnight, as I did one time and woke up to a blown W5M. You can buy a used AVT20 for what I paid for each power tranny or that matched quad of Sovtek KT66 tubes. That is like three AVT20's to screw around with while I still haven't got the W5M's up and running again.
  8. Pic?
  9. LeDuck


    May 26, 2012
    It looks like any other Marshall AVT20, and photos of those are all over the place. The mods I have done should apply to any of the Valvestate series as they are all related from what I have read.

    Pics of the Eminence Beta 10A are on the Eminence website, with response curve and all pertinent data.

    The internal box volume of the AVT20 is suitable for the Beta 10A with no further modifications. The Beta 10A is well suited for sealed or ported enclosures. There is this wonderful stuff at hardware stores called sewer drain pipe that low cost vents can be made from. You can spend 10X as much at Madisound and get plastic pipe with flared mouths. Chrome exhaust tips at your truck accessory store sticking up from the sides of the cab would look cool, in an Eric the Viking sort of way, and could also tune the cab for bass reflex.

    Besides installing a Beta 10A in the cab, stuffing in some acoustic fiberfill is the next cost effective measure. It will bring down the impedance peak at resonance.

    R53 is clearly labeled on the circuit board, and it is 10 ohms if you find an old amp that has a burned R53 resistor.

    I dug out my old Shure microphone, and this rig makes a fairly good portable PA rig. The AVT20 runs very well on a quasi sine wave power inverter, and should only pull about 50 watts at full rumble. That means about a 5A draw on a 12V battery, good for an all day jam on a trolling motor battery.

    Neighbors don't like my audio gear very much, and since my playing sucks........ Later on, I will haul the amp out to the boonies so I can get some freefield measurements without room bass boom. This means sitting on the tailgate of my F150 facing to the rear, or on a tall stool if I want to get fancy.

    I forgot to mention that I painted the rear cab panel, as it comes with no tolex and only the bare minimum of paint to get it looking black (never forget....it's overdesigned but underbuilt). A very hard look will scar the paint down to bare particle board. It soaks up paint like a sponge, so I went through almost a full can of black spray paint to get a smooth sealed surface that is very scratch resistant. I then installed a "The Who" decal on the back panel to suggest the amp has been abused by its owners (I am not the first), to validate the dents on the front panel. Your portable amp's apparent value should appear inversely proportional to its true worth. I would not want certain people thinking I had some really good stuff back home.

    If I can find a small booster amp of around 100W, I might try installing it on the back chassis panel. It would make this little combo really honk.

    I have some sorbothane sheeting, and I was thinking of putting some around the speaker baffle cutout and then screwing the grill board to the cab, compressing the sorbothane. This would deaden the front baffle board even more. I put sorbothane patches between my wooden equipment rack and my sheet glass turntable platform. Since glass and wood have different resonances, this allowed them to use each other for a resonance sink. VPI uses sorbothane pucks instead of springs and damper foam to support the plinth on my turntable. Good stuff to experiment with; can solve some real resonance issues. Try making some big washers out of sorbothane sheets and install them between metal grills and bass cabs. Should work much better than rubber washers.
  10. LeDuck


    May 26, 2012
    I did a near field test on the Beta 10A speaker with the sound level meter a few inches in front of the grill cloth and the meter at a 45 Degree angle to the cab and the meter pointing at the center of the woofer cone.

    I got a response curve on the speaker very close to what Eminence shows on their website. The AVT20 control settings to achieve this are Bass 5, Middle 8, and Treble 2. I also did left and right curves at 30 degrees offset. From 80 Hz to 1600 Hz the speaker is about as flat in response as an audiophile speaker. Then there is a presence peak of about +6 dB around 3000 Hz and the output crosses the 0 dB line around 4000 Hz and falls off very rapidly after 6300 HZ.

    The second harmonics of the low notes on the E and A strings are strongly reproduced under actual playing conditions, and since the higher harmonics are equal in strength, the fundamental is recreated in your mind and this gives a very full and warm apparent response. The second harmonic of the open B string on a B-bass at around 63 Hz averages about -6 dB and this amp is not suitable for a B-bass because of that. It should work OK on a 4-string bass that is detuned to D. The E string is very full and warm all the way to the open E if you are generating plenty of second and upper harmonics, if you pick halfway between the fret and bridge to produce mainly fundamental tone, then the note will sound a bit weak until you come up past the 5th fret or so. But when slapping, popping and picking closer to the bridge that is not the case. Since this combo amp has only broadband tone controls and no narrow band graphic equalizer, the dropoff below 80 Hz is desireable, otherwise room boom would be a big issue and rolling down the bass would kill your midbass as well as the room boom. It's a fairly equalized speaker for real world conditions in smaller living spaces where you are likely to practice.

    Remember that I am running a JAN Philips 12AT7A tube instead of a 12AX7/ECC 83 tube. It should have a more extended high frequency as it has a lower plate impedance, which probably explains why the Treble had to be set at 2. If running a 12AX7/ECC 83, expect to use a higher treble setting.

    My spare B102 will probably end on getting installed in a powered extension cab for the AVT20. Later this summer I should have a couple of rebuilt 40W tube power amps setting around doing nothing.

    Last night I did a test on a Marshall AVT50 with the stock Celestion speaker. The AVT20 with Beta 10A was clearly the better bass amp. The AVT50 speaker begins to fall off rapidly below 125 Hz. On human voice, the Celestion G12 AVT has a hollow sound, probably caused by the scooped response. The distortion channel on the AVT20 sounds more natural than the distortion channel on the AVT50. I don't think I would want to upgrade the AVT50 with a bass guitar speaker unless I wanted a cleaner sound with a regular lead guitar. The DI output of the AVT20 allows it to be used as a gig amp through a decent PA system or straight into a recording console, and it's plenty loud enough for practice. With my Shure microphone, my modified AVT20 can be used as a small portable PA system. Plus or minus 3 dB from 80 Hz to 2000 Hz, which is very flat response for a high efficiency speaker and covers the human voice range.
  11. LeDuck


    May 26, 2012
    Over the weekend I upgraded a Marshall AVT50 with a much better Celestion G12H-80 speaker that was made for modeling amps. That speaker gave such great bass response in an open cab Traynor YCV40 that I thought it might be good for bass in a closed cab Marshall combo amp.

    One thing I noticed in testing is that the dirty channel on the AVT20 has much more extended bass response than the high gain channels on the YCV40 and AVT50. The reason for that seems to be because it is the clean channel of the AVT50 with mid control added. An extra stage of gain is added to go high gain on the AVT20, where you go to a separate channel that is voiced diferently on the AVT50 and YCV40.

    This makes the AVT20 better for hi-gain bass, as you have an extra mid level control and that gain switch, which the clean channels lack on the YCV40 and AVT50. If you hit the OD gain switch on the AVT50, you are switched to the lead channel which tends to suck for bass as the deep bass is rolled off and can't be regained with the bass control.

    The AVT50 works with my ReBop on the clean channel, as the ReBop bass control is centered at 40 Hz and improves the deep bass without affecting high bass or midrange as the amp control does. The AVT50 doesn't work as well with my OLP MM2 as the passive tone controls can't boost bass in the lowest octave only. You can get a brighter punchy aggressive midrange sound, but not a deep classic bass sound like the ReBop is able to get. This difference is not so evident on the AVT20 as the Beta 10A speaker is much more extended in the deep bass.

    The AVT50 with its greater power and 99dB efficient G12H speaker plays much louder, but do you need 110dB in a practice bass amp? The 95-100dB range of the AVT20 is plenty for me. When I now push the AVT50 to clipping, it is too loud for comfort in the living room. With a lead guitar, the AVT50 will now take your head off in the house, similar to a YCV40. Since the original G12-50 AVT speaker was rated at 4 ohms, and the new G12H-80 is 8 ohms, I can now drive an extension cab with the AVT50 if I want. In spite of the slight power loss by going to an 8-ohm speaker, the AVT50 still plays louder as the speaker efficiency more than makes up the difference, and there is no farting when you push the power to max, so you can use all the power available up to amp clipping.

    I'll probably sell the AVT50, because the way I have it rigged now it really roars for lead guitar, especially for heavier styles of music. However, it is useful as a bass practice amp if I don't get a reasonable offer. What it does do is make me appreciate the little AVT20 even more as they way I have it rigged makes it a superior bass practice amp.
  12. Phil, I do believe that you have altogether too much time on your hands!!! :D

    I too have projects sitting on the a shelf in the workshop awaiting my attention. I first have to find the workshop floor!..........

  13. LeDuck


    May 26, 2012

    I actually have more projects than time. The current project is rebuilding a pair of 11-year-old VMPS Ribbon Monitor 1 loudspeakers that are flat to 32 Hz frequency response. 2x8" graphite coned, phase plugged woofers driving a slot loaded passive radiator. The midrange and tweeter are true push-pull ribbon drivers with no panel tension that therefore have no panel resonance and have no need of transformer coupling. Quasi-Second-Order crossovers, Soundcoat damped cabinets, yadda, yadda, yadda.

    No bass cab I know of is engineered so well (even ACME does not use push-pull ribbon drivers) or has wood veneer that looks like nice furniture. I believe I will play bass through these speakers and give up the notion of buying bass cabs that are a few decades behind the times relative to audiophile loudspeaker design and have nowhere near the sound fidelity. These speakers cost me $1600 as a kit that I assembled myself back in 2001.

    The air that the mids and tweeters push is heavier than the panels themselves, and thus they are instantly damped by air and cannot ring and have superior transient response that no conventional cone or horn bass cab drivers can match. The midragne panel covers 166 Hz to 5000 Hz, and can go higher but the tweeter takes over at 5 kHz and goes up to 30 kHz. Crossover is first order at crossover points, so these speakers can reproduce a square wave.

    The 89 dB sensitivity is of no concern as they will be driven by a B&K ST202+ that has 300WPC at 4 ohms driven by two Sansamp RBI preamps and they will be my "house rig". The pair will play at 115 dB in the house before clipping of the amp. If anything, I will have to roll off the bass as VMPS is famous for their deep and tight bass. One of their famous subwoofers outfitted with a mid-tweeter in the cab (or a bass cab plopped on top of the subwoofer) would make a killer bass cab......down to around 20 Hz. A B-bass with octave pedal would rival a pipe organ for low frequency extension. Brian Cheney is like Beethoven in that he believes that bass is the basis of western music. The big full-range VMPS speakers weigh in at around 200# or so and every review I ever read mentions the AWESOME bass extension and power. My 32 Hz speakers are just babies in the VMPS lineup at around 60# weight.

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