Upgrading a VOX AD50VT Valvetronix

Discussion in 'Amps & Cabs' started by Dorian, May 8, 2005.

  1. Dorian

    Dorian Active Member

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    I really enjoy this site. I thank so many of you for giving such useful, creative, quirky and friendly advice and comments on so many topics about the Gibson SG. I have been visiting everythingSG for some time to learn facts and tricks about the SG. Thanks in part to your hints and ideas, my faded special has become a real dream to play. (Of course the ebony fretboard, the upper fret access, and the light mahogany construction help too!) To date I have not had anything particularly useful to contribute to this bulletin board, but I hope this post might be helpful, or at least interesting. :?

    The VOX AD50VT is a wonderful amp. It is especially good if you are exploring different styles and tones because serious simulations of amp models can be driven hard at various volume levels. When experimenting with a new style, such as bossa-nova through a dual rectifier, it is not always appropriate to run the amp at full volume, so the level control on the amp is really handy. (Actually, I decided that this style sounded best at zero volume, but perhaps one of you could make it work. :roll: )

    After playing the amp for about 2 months, I decided it had two weaknesses that I wanted to fix. Both were anticipated, owing to the low cost of the amp ($360 new, including shipping). First, the speaker was lacking in bass, and I thought the speaker sounded a little bit muddy. The speaker in this amp was made for VOX by Celestion, but it is not a top-of-the-line Celestion product. Its frame is thin, and its magnet structure just looks small. Replacing the speaker was job number 1.
    [​IMG]

    Job number 2 was quieting the internal fan. I have never felt any warm air anywhere, so my first thoughts were just to just slow it down, or maybe replace it if this looked easy. The stock fan is low-capacity, but cheap and noisy. Actually, the fan noise did not bother me much when playing, even in the ambient sustain between the notes. When I put down the guitar to read, however, the fan became an annoyance after my hearing recovered.

    I spent some time looking over the products offered by Celestion and Eminence. Harmony Central has a number of useful reviews of different speaker drivers (located under manufacturers of guitar amplifiers). Many reviews were pretty touchy-feeley, but the overall consensus was that there is a lot to gain by replacing the stock speakers in cheap or mid-priced amplifiers with new products in the $50-100 range. There is also an online Tonequest report about Eminence that will bias you towards their speakers over Celestion:
    http://www.eminence.com/eminence/pages/products02/pdf/tonequest.pdf
    Selecting a speaker from written descriptions of its sound was frustrating, sort of like ordering from a menu written in Chinese. I had an email exchange with Dave at Avatar speakers, who suggested that I get a relatively uncolored speaker for this modeling amp. His first suggestion, which I took, was the Eminence Tonker. It cost $69 from Avatar. I thought the name "The Tonker" sounded really dumb, but I learned from wikipedia.com that Tonker is an obscure nickname for a character in the old Doctor Who science fiction TV series. (Will Eminence someday offer a heavy metal speaker called "The Dalek"?) Incidentally, the Eminence "Swamp Thang", which has an even dumber name, might be a good choice too, especially if you like bass.

    Before installing thenew speaker, I wanted to break it in. Speaker breakin is probably a real effect, but it is not entirely clear to me what it does. Exercising the cone and suspension probably causes mechanical softening of these paper components, at least this seems reasonable to me. A common claim is that the bass frequencies are lowered after breakin, and this would be consistent with such a softening. Finding hard evidence for this is not easy, at least not on the internet. There are a lot of different ideas about what to expect after breakin, but I haven't been able to find speaker response curves measured before and after, for example.

    I figured that a good breakin would be to drive the speaker cone at large excursions for an hour or so. This takes a lot of power at higher frequencies, so I made the easy choice of 60 Hz. In the garage I plugged the speaker into a borrowed 5 amp variac. I left the speaker face up in its cardboard box, in part to keep it clean. My wife had a social engagement that night, so I was able to run it at 16 V (something like 32 W) for an hour, followed by 20 V (40 W) for half an hour. It was loud. I ended up closing the garage door. Don't try this inside your home. Listening to a continuous 60Hz tone for an hour or two at high volumes is not recommended.
    [​IMG]

    The cabinet of the AD50VT was built better than I expected. Okay, the walls, front and back are pressboard, but they are mostly 5/8 inch with some 1/2 inch, and there are a couple of reinforcing ribs on the front. (I am thinking about adding some reinforcement to the sealed back.) The electronics chassis fits nicely across the pressboard cabinet of the AD50VT, and adds a little stiffness. The cabinet tolerances are quite good, so the back fits tightly between the walls. It fits so tightly that it was necessary to loosen the screws to the electronics chassis to remove the back. The screws for the back go into strips of plywood that give much better bite than pressboard, and can take some real force. I liked that.

    Comparing the Eminence Tonker to the VOX Celestion was interesting. Tapping the cone, the free air resonance of the Tonker was actually a little bit higher than the Celestion, by a bit less than a half-step, I think. Looking into the cones, the two didn't appear very different. The appearance from the back is markedly different. The Tonker was a bit deeper, but the main difference was weight. The Tonker weighed twice as much as the Celestion -- the Celestion weighs 6.6 lbs, and the Tonker is 13 lbs. Both frame and magnet were much more substantial on the Tonker.
    [​IMG]

    The VOX Celestion has a slightly larger mounting bolt circle than the Tonker. The four mounting screws for the loudspeaker are tightly captive in the pressboard frame of the AD50VT. They just barely accommodated the mounting holes of the Tonker. Fortunately, they did fit without filing, drilling, or thread damage, and re-assembly was easy. It was not really necessary to remove the grille because the speaker screws were tightly captive. I took off the metal grille anyway, because I was wondering why it didn't buzz. VOX seems to have done a nice trick with some rubber pads for grille mounting. They work well in suppressing buzz, even after I re-installed the grille myself.
    [​IMG]

    I was curious about the valve reactor circuit, but I couldn't tell much about how it worked by looking at the analog circuit board in a few minutes. It all seems to be done with conventional components somehow, maybe with capacitor tricks, but there was no reactor to be found. The 12AX7 valve is an Electro Harmonix. It could be replaced easily by opening the back of the cabinet. I wonder what to expect with replacing it with another 12AX7, or perhaps a lower gain twin triode in the same family :?: ) This would be a real guess for me, since I don't know how the biasing and plate loading circuit works in the VOX/Korg circuit, and I am sure that the engineers designed the circuitry around the characteristics of the 12AX7.
    [​IMG]

    The fan looked like an afterthought in the engineering design. It blows on the wrong side of the heat sink, and is mounted with standoff screws that seem to require a lot of hand assembly. I cut the wires to it and checked that it ran off 12 V DC, as expected. Its mounting centers are 50 mm apart. I rummaged around for a suitable resistor, and found one that measured 28 ohms. When I put it in series with the fan, the voltage dropped from 10.6 to 8.6 V. The fan noise was diminished considerably, so I decided to leave in the resistor, at least for now. Unfortunately, replacing the fan looks like a pain. Doing it right would require pulling the amplifier analog board from the chassis, and there is some odd routing of wires around the frame of the fan. I think it might also be necessary to break some of the thermal compound to the heat sink and redo it. A better solution might be to clip the power leads to the stock fan and leave it in place. A better fan could be mounted nearby.

    While this was all exposed, I put in some thicker wire for the speaker leads (but the stock ones are pretty short, so I doubt this makes much difference.)
    [​IMG]

    The first few hours of playing the renewed AD50VT were revealing. First, I expected an increased speaker efficiency and a louder sound for the same settings. This was not so obvious, if true at all. The Tonker was just about the same volume as the Celestion it replaced, but this is hard to tell without more care than I gave to it. Second, I expected more bass, owing to the speaker response curves that were available for Eminence and Celestion sites. This was true. The Eminence had more bass, and it was tighter. I am pleased. The biggest change, however, was articulation at any volume level. The Tonker is much more clear. The differences between amp models are now much more obvious than before. I did not expect such a difference in clarity between the two speakers. In the first minutes of playing, I worried that perhaps a muddier speaker was selected by VOX because the electronics would make annoying sounds, perhaps from the digital stages. If this is so, I still haven't noticed it. The 11 amplifier personalities of the AD50VT are now much more distinct, and I can hear effects of my fingering much more clearly than before. Somewhat to my surprise, I now think that the improvement in articulation is more important than the better bass response. I didn't think articulation was such a problem with the original Celestion, but now that I have the Eminence installed, I can't imagine going back. I gave the VOX/Celestion to a college student, who was delighted to get it.

    Oh yes, the fan is quieter too. I recently found a better one to install sometime. It is bigger and slower, but it really quiet. This is a project for another day, probably after I start getting more serious about recording with this amp.
    [​IMG]
     
  2. Voxman

    Voxman Moderator Staff Member

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    Very nice read, and as discussed before in another thread the quality of the tube also does amazing things to the sound........thanks for the upgrade hints....... :D
     
  3. CharlieB

    CharlieB Active Member

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    The Vox Celestion appears to be a V12-60 the same as I've used in the Deluxe 5e3.

    If you want to sell it... let me know!
     
  4. TNT

    TNT Active Member

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    An amazing and helpful first thread Dorian! Welcome to Everything SG!
     
  5. CharlieB

    CharlieB Active Member

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    Jeeze yes... darn didn't notice.

    THAT WAS ONE HELL-OF-A FIRST POST.

    WELCOME!

    Now for something completely different.

    I'd like to give a caution here. Dorian is obviously fairly well versed in things electronic, and took some limitations on his endeavors. FOR THOSE NEWBIES.... hooking up your speaker to a variac is not really a good practice. Dorians speaker was rated for 150w RMS, and he took it to 50w (more or less) at 60cycles (hertz for your modern types). There are some problems associated with that, which I'll attempt to outline briefly.

    1. The whole idea of the variac leaves much room for problems, not only overvoltage, but also the frequency - which is set to 60 (or 50 if your so located) cycles. You can slip, and poof your speaker, let out the blue smoke, etc... Its a far better practice to run a signal generator into your amp, and let the amp drive the speaker.

    2. Sixty cycles is damned low frequency for most guitar speakers. There are some problems when you run a speaker just off its resonant frequency at decent power. The cone/surround/spider/coil/dustcover want to move at the resonant frequency, but you're just off that when you run at 60cycles. Most speakers are resonant at anyplace from 70 to 100 cycles. If the difference in resonant to line frequency is small, you can get some weird things happening with the cone, especially at moderate to higher power.

    3. The power rating of Dorians speaker was taken at 1000cycles, the standard for this type of measurement and rating. He DID wisely limit his power to about 1/3 of the 1kc rating. The actual 60cycle rating can only be determined by empirical (destructive) testing. That doesn't sit too well with me! I don't wanna be a tester! (at least with my speaker).

    4. You dont need to run 50w to break in a 150w speaker. In fact, one or two watts is ample, but the time takes longer. Case in point, I'd always break in the Eminence 10ALK (now the Legent 102) at about 90cycles at maybe 2 watts. The amp I used was only 12 watts (Princeton), and it was barely crankin'. Used to leave those overnite on a carpeted floor, covered with a blanket. It was still loud! I cannot think of 20v running into a speaker at 60cycles... the volume aaayeeeee!

    5. If you have a bass, the best way to break in a speaker is to just play bass thru it, at moderate volume that does not give much (or any) distortion. If the low E string notes flab out, just avoid them. Running the speaker with percussive signals (ie, bass) and at varying frequency (different notes) is a good thing, and doesn't take much volume, is safer, and is enjoyable!

    Also here is the curve for the Tonker

    [​IMG]

    And the curve for the V12-60

    [​IMG]

    If that Vox/Celestion is the V12-60 (and it appears to be in the images... I noticed it before I read the text!) it ought to have a real good low end. They resonate at 75cycles, very low for a 12. Its 3db under the Eminence Tonker overall, so the Tonker ought to be just noticably louder than the Vox/Celestion. The clarity comes from that nice peak at the very top of the Tonker curve, with the notch at 1500cycles. This is not unlike say... the way a 57 classic appears to have great clarity, because its mids have a bit of a notch (well not the EXACT same... but similar idea).

    I'm wondering what the breakin would do to the Vox/Celestion!! Maybe run at only say... 4vac/60cycles for a while....<smiles>
     
  6. TNT

    TNT Active Member

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    I have questions. If a speaker is not broken in, what will happen?What are the symptoms of this happening? Could this be why the Jensen P8R was cutting out when turning the amp up on my new kit amp?There was no sound but the pilot lamp was on. How long at regular play will it take to break the speaker in?Do they (and if so) how do they break the speaker in at the factory?
     
  7. CharlieB

    CharlieB Active Member

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    Speakers not broken in will have poor low end response until they loosen up, thats all.

    If your speaker was cutting out, then you got other problems!
     
  8. Dorian

    Dorian Active Member

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    Yes, Charlie B makes an important point. Be careful with the variac! Please notice the digital voltmeter in the photo of the speaker in my garage. I watched the voltmeter VERY CAREFULLY as I cranked it up.

    The other thing I watched VERY CAREFULLY was the cone throw, or how far the speaker cone was vibrating in and out. Cone throw is a cool thing to watch. You can see a blur of the speaker cone at the outer part of its suspension. You want to keep it reasonable. I am sorry that I cannot give you a precise measure of how far it was moving, but it was a few millimeters - less than a centimeter anyhow. Larger cone throw can cause the voice coil to collide with something in the magnet structure, either directly, or perhaps by wobbling somehow. Collisions smash the copper windings in the voice coil. This is a disaster.

    The other good point, perhaps a bit obscure, that Charlie B makes is that the speaker was driven below resonance, whereas the normal operation of a guitar speaker is above resonance. Operating below resonance is odd for a speaker, but it is okay for breakin if you keep the cone throw under control. Although potentially dangerous, for breakin I think it might even be okay to run the speaker on resonance, because speakers are rather damped, unlike tuning forks. Damping varies from speaker to speaker, so again you want to watch for excessive cone throw. Remember the Tacoma Narrows Bridge, which collapsed as it resonated under wind loading. (If you haven't seen this movie, it is really cool, although only vaguely related to this forum.)
    http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacoma.html

    A good thing for me about breaking in the speaker at the low frequency of 60 Hz was that my wife owns a pair of Bose noise cancelling headphones. (Actually, I bought them for her last Christmas, geek that I am.) These are effective in cancelling out periodic sounds, such as in airplanes and cars. They were also fairly effective in cancelling out the 60 Hz sound of the speakers. With them, I was comfortable walking into the garage and checking out the cone throw and the voltage from the variac. I suppose that earplugs would be even more effective, but it was fun trying out the Bose for this purpose.

    Finally, a point that I made to the lucky beneficiary of my old VOX/Celestion speaker was to be careful with screwdrivers around loudspeakers. Screwdrivers tend to be pulled directly into the magnet. Unfortunately, this often means that they puncture the paper cone on their way to the magnet. This is a quick disaster, and can happen more easily than you might think.
     
  9. Six String

    Six String Moderator Staff Member

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    Welcome to ETSG Dorian! :D :D :D
     
  10. CharlieB

    CharlieB Active Member

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    Bose headphones... THIS IS NOT FAIR!!!!
     
  11. Dorian

    Dorian Active Member

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    Thank you all for the warm welcome. I had a sense that this was a friendly place. Thanks for the proof.

    I agree, CharlieB. It is unfair. I was only able to use the headphones when my wife was out of the house. She likes them a lot.

    Voxman: I am really unclear about how amp performance would change with a different 12AX7 in a Valvetronix amp. I am sure you are right that there would be changes, but what would they be? Would the high gain amp models be even more touchy? Do you think that the VOX/Korg engineers optimized the sound of the AC30 amp model around this Electro Harmonix tube? I just don't know how to assess the effect of a tube swap on a Valvetronix amp. This seems like a fun thing to do, and I don't see how it would do any harm. I hope to get around to this sometime.
     
  12. skidshark

    skidshark Active Member

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    welcome dorian! i broke in my ad30vt the old fashioned way....trying to keep up with my buds 100watter! i had probably 10 hrs on it playing normally, then jammed with it wide open for several hours. when i fired it up the next day (when my ears had stopped ringing) i noticed the difference!!

    there seems to be lots of info on 12 inch speakers, but not as much on 10 inchers. can anyone point me in the right direction to find the best 10 incher for my vox?
     
  13. paradox

    paradox Active Member

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    I may be way off here skidz, but what about a Jensen Mod 10-50. The mod series are supposed to be pretty transparent speakers.
    I think that is what you would want in a modeling type amplifier. They do have it in a 35watt version (Mod 10-35), but since the goal with the
    modeling amp is to not color the sound with the speaker, I would try the 50watt version for the price.

    Here's a link for it at tubedepot.com ($28.95 + $3.95 shipping):

    http://www.tubedepot.com/sp-jen-mod10-50.html
     
  14. paradox

    paradox Active Member

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    there go my manners again :oops: , welcome dorian & nice post!
     
  15. Dorian

    Dorian Active Member

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    I am sorry about your AD30VT, Skidshark.
    If you are sure that the speaker is the problem, I would suggest that you contact Dave at Avatar speakers. He runs a really low overhead business on the internet at
    http://www.avatarspeakers.com/
    and he has the lowest prices that I was able to find on Eminence speakers, anyhow. Tell him that you have a modeling amp, and he can make a decent suggestion. There aren't as many choices for 10 inch speakers as 12 inch, but there are enough. Also, check the recent thread here:
    http://everythingsg.net/forum/viewtopic.php?t=2406
    You may well like your AD30VT even more with a better speaker in it. Furthermore, if you get a speaker that is a a bit more efficient, you might be able to be able to compete with your bud's 100 watt amp (but then he might have efficient speakers too!).

    Thanks for the welcome, paradox.
     
  16. skidshark

    skidshark Active Member

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    hey dorian! i'm not actually complaining about the amp...just the incessant quest here...!!!

    i normally either p.a. it, or use my 100w 2-12in. SS crate amp to push more air, but nothing wrong with making the amp the best it can be stand-alone right??
     
  17. Dorian

    Dorian Active Member

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    Absolutely, Skidshark, and it may send your quest on a new path. :?
     
  18. lagger

    lagger Member

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    Great post Dorian... welcome and thanks !
     
  19. Voxman

    Voxman Moderator Staff Member

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    Dorian....just put in a a Mullard ecc83 first then let me know what you think.....I can send you one if you send it back, to see! :wink:
     
  20. Dorian

    Dorian Active Member

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    Voxman: This is an offer that is hard to refuse, and I may well take you up on it later. For now I need a little more time to get used to this change of speaker. I am still tweaking pickup heights and pole piece screws on my faded special to accommodate it. Also, I am sure that I have more than one old RCA or GE 12AX7 tube lying around somewhere, but I haven't found it yet. The 12AX7 was important to me a long. long time ago when I bought cheap vacuum tube stereo equipment because I couldn't afford solid state amplifiers. (Funny how these things can turn around.) One of my old friends, who later became my brother-in-law, was a more avid collector. I want to see if he still has a Telefunken, Amperex, or Mullard tube. I suspect he does, and I have challenged him to find it. We both had high opinions of the 12AX7 in our younger days, and we pulled them out of stuff that we later junked. It all seems kind of sad now. We sent some good output transformers into landfill. :oops:
     

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