Upgrading a VOX AD50VT Valvetronix

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

  1. TNT

    TNT Active Member

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    Here's my feeble speaker upgrade. It didn't cost much, only the cost of the speaker.Aound $20.00 for the speaker and I bought the '86 amp for about $36.00 from a Pawn shop about nine years ago. I felt it was needed because the amp actually sounded better through headphones than through the speaker. ;D It actually has a good clean sound and spring reverb. The gain channel is nothing to write home about - about like a SS Marshall. I had to disassemble the cab and take the chassis out to make the change, but it needed some cleaning anyway. The speaker has MUCH better Bass response and a tighter sound overall. It'll get me by until I get something better.I oughta try a back panel!

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  2. skidshark

    skidshark Active Member

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    think it would sound any better if the speaker weren't upside down? :o ;D
     
  3. TNT

    TNT Active Member

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    I always look at it from the top! ;) :)
     
  4. Voxman

    Voxman Moderator Staff Member

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    It's  outta phase that way .... I guess T gave up [​IMG]  one of his secrets there! [​IMG]


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  5. RightTurn

    RightTurn Member

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    Dorian-

    I got the Vintage 30 because that's what the guitar shop by me had- i dont know too much about Eminance.  The speaker sounds best when driven hard and your right it is better for lead than the stock speaker also the effects come through clearer.  I was looking for an Iommi sound but ended up with System of a Down...which is cool too.

    To me theres too much bass with the back on so I will probably go for some kind of partial back.  For the money this amp is real good either way, i like the "power level" selector on the back. 

    Yes I lowered my SG's pickups 3 turns, also replaced tuners with Grover's that are packaged by Gibson which i was told come stock on LP's- hang on for the pics  :).

    TNT-

    Your speaker IS upside down. ;D
     
  6. TNT

    TNT Active Member

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    A speaker doesn't know which way is up. The speaker is not upside down, the sticker is. ;D
     
  7. skidshark

    skidshark Active Member

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    don't feel bad T......i might have put it in backwards!! :lol:
     
  8. Dorian

    Dorian Active Member

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    RightTurn -- Thanks for your comments. Here are a few interesting facts that may have some use. The response curves for my Eminence Tonker and your Celestion Vintage 30 are similar, but they show some differences. The Tonker response seems more intense in the bass and treble, but it also passes more electronic noise and has a more irregular response curve. I took the liberty of using Photoshop to merge the Celestion and Eminence curves to simplify comparison, but I am not confident that the measurements by two different companies are really comparable:

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    The Celestion web site had a FAQ on cabinets. Interestingly, it referred directly to your own Vintage 30 loudspeaker:

    "What is the difference between open and closed-back cabinets?

    The closed-back type of cabinet has much more bass presence and a tighter feel. The sound is more directional, however, and can feel slower or less free and open. For this reason I would usually choose a large magnet speaker (eg Vintage 30) in a closed back cabinet as it is less bass-heavy. The effect of the large magnet outweighs the effect of the cabinet.

    I have heard people say that the Vintage 30 ONLY works in open cabs, and others say it ONLY works in closed so ultimately it's all down to personal taste, your tone and style, and how you set your amp controls."

    [hr]
    For a longer read, the Celestion site has a few comments on speaker cabinet design. Not surprisingly, it lists three contributions to tone: 1) instrument 2) amp 3) speaker, changing the tilt of the discussion from the first two.  (Again a matter of opinion.)  There are other biases in the text below, but most of it seems solid to me. I am still thinking about stiffening up the cabinet in my AD50VT, and these Celestion comments have pushed me a bit closer to action...

    "GUIDELINES FOR CABINET DESIGN
    *A unique area of loudspeaker design

    A loudspeaker is usually a transducer, designed to faithfully reproduce the acoustics of the signal presented to it. An electric guitar speaker however is a creative part of the music, contributing its own character and tonality. The ‘instrument’ an electric guitar player uses is really a combination of guitar, amplifier and speaker. All three parts are vitally important to a good sound.

    As guitar speakers are different, so their cabinets are different to hi-fi or PA cabinets. Deep, thunderous bass is not required (the low E of a standard-tuned lead guitar is 82Hz). High frequency reproduction is a positive disadvantage, allowing unpleasant harmonics and electronic noise to be heard. Distortion-free sound would be a disaster.

    *Two elements, a drive and a box

    The box design is acoustically less critical than that for hi-fi or PA systems, but proper construction is essential. The cabinets should be solidly built to ensure no joint vibration (unpleasant buzzing), and be of adequate strength to withstand hard use. Remember that guitar speakers are quite heavy and amplifiers that sit on top of guitar cabs are even heavier. Internal bracing is generally not required, but battens inside the joints are good if your woodwork skills do not extend to complex corner joints, and a central bracing post can be advantageous in a 4x12.

    Most quality cabs use 15-18mm plywood for the main cabinet, with MDF for the baffle (the part where the speaker(s) are mounted), but they can be constructed of any material. Many budget cabs are made of chipboard (cheap, but poor in terms of strength, ruggedness and sound) or MDF (easy to machine, but heavy and dead). Maple, mahogany and walnut are often used for high quality cabinets.

    The important characteristics of the cabinet material are strength, sound and ease of use. Lively resonant materials, such as plywood or real woods, vibrate in sympathy with the speaker and enhance the sound, but should be at least 13mm / 0.5" thick, or they will unduly colour the sound. Most woods or wood composites will be strong enough at this thickness. When considering price, you should also consider cabinet finish. Cabinets can be painted or stained, or covered with a vinyl or carpet finish. Real woods finished with a stain can be very exclusive and expensive looking!

    Open back or sealed boxes should be used. Open back gives a looser low end with less depth, and 'figure 8' directivity (sound field looks like an 8 when looked down on from above the cab). Sealed boxes give tight, deeper low end but are more directional, giving less spread of sound. Vented / tuned / ported boxes are not recommended for lead guitar, as they can damage guitar speakers. The box size is not critical. The baffle size is more important in open back boxes (larger = more low/mid presence), and for closed back boxes larger volume means deeper but looser bass. Do not use internal acoustic wadding, it is inappropriate for guitar cabs, reducing sparkle and life.

    Mount the speaker securely using bolts into T-nuts, not self tapping screws. Do not overtighten so the housing rim bends. Ensure the speaker is protected from the front, as the cone is easily damaged. The speaker can be front or rear mounted.
    In Summary
    • Plywood or real wood construction is preferable
    • Strong, rigid construction means no buzzes or rattles
    • Size is not critical
    • Ensure the speaker is adequately mounted and protected
    • Avoid air leaks if using sealed box construction

    *CAUTION

    We do not advise mixing different impedances of driver within the same cabinet. This can lead to uneven power sharing between speakers, causing one speaker to be overdriven and damaged, while the other is underdriven.

    *Thiele-small parameters

    Thiele Small parameters are useful for controlling the low frequency response of sealed or ported cabinet systems by changing the cabinet internal volume, and port dimensions. However they are of severely limited use when designing a guitar speaker cabinet.
    • Electric guitar speakers do not reproduce 'low' frequencies (the low E string of a lead guitar has a fundamental of 82Hz) and so the frequencies at which Thiele Small parameters have significance are mostly below the operating range.
    • Also, the parameters are measured at very small signal levels. Guitar speakers become non linear at very low levels compared to other types of speaker, greatly reducing the significance of Thiele Small parameters in actual speaker use. Using the Thiele Small parameters of a typical guitar speaker, you will find that halving or doubling the cabinet size makes minimal difference to the response.
    • They have no relevance to open back cabinets.
    • Guitar speakers are not recommended for use in ported cabinets (as the increase in cone excursion below the tuning frequency can cause the thin paper edge of the cone to tear).

    The cabinet size, shape and construction are of far higher significance than the internal volume. Cabinet design using Thiele Small parameters ignores these most fundamental aspects. Important factors include the material you make the cabinet from, the panel sizes and shapes, how they are joined, how the cabinet is finished, the mounting of the speaker, etc. These, not Thiele Small parameters are the critical factors in the design and ultimately the sound of a guitar speaker cabinet."
     
  9. RightTurn

    RightTurn Member

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    Good info Dorian. 
    I was thinking that the cabinet is too shallow for this speaker, the pic in your original post looks like you reinforced the box or yours is diff than mine.

    What do you think the ultimate cabinet (wood, dimensions etc.) would be for the Vox hardware and these speakers.  I MIGHT give it a shot as ultimately I want to build (with help from a friend) an all tube amp.
     
  10. Dorian

    Dorian Active Member

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    RightTurn -- Good observation. I had to go back and check this. The reinforcement strips on the sides of my front panel came from the factory. The serial number on my AD50VT is 004666. If yours is newer, maybe these strips were deleted for cost savings. If yours is older, maybe they were added for sonic reasons?

    Most of the arguments I have heard about the size of speaker boxes pertain to hi-fi speakers, where the box sizes tend to get really big for the lowest frequencies. The first issue is that bass frequencies have such long wavelengths that the sound from the back of the speaker will cancel out the sound from the front. One solution is to kill off the sound coming out the back by dissipating it in the box. The second issue that comes with this approach is that if the box is too small, the speaker has to compress a stiff spring of air inside the box. (I once had an Altec Lansing 15 inch speaker with a free-air resonance of 18 Hz and a soft compliance. I made a thick door for a walk-in closet, and mounted the driver in the door.)

    Guitar speakers really are a different story, since the lowest frequency is above 80 Hz. Even little bookshelf speakers can get these bass frequencies with no problem. So I think the Celestion advice is right -- the volume of the box is not really so important. Sorry, I really don't understand what they mean by the size and shape of the box being more important than the volume of the box. They might mean that the dispersion of the sound changes a lot with the choice of open back, closed back, or size of the front panel (i.e., you can get sweet spots and dead spots at different places around different speakers).
     
  11. TNT

    TNT Active Member

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    ooooh, serial # ends in 666. :o Does it sound better playing Death Metal? :?
     
  12. Dorian

    Dorian Active Member

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    TNT -- thanks for the observation.  I guess I never realized what I had let into the house. :evil:

    Actually, yes, it does sound pretty good with the Dual Rectifier model and scooped midrange. I found a book by Troy Stetina on metal lead technique, and some of it is pretty easy to pick up. In the past I was not a big fan of metal, but you could say that some of the AD50VT amp models are, well, inspiring. To me anyhow. The metal style seems a good way to clear out the house. :angel:
     
  13. Dorian

    Dorian Active Member

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    The advice from the Celestion web site on building speaker boxes finally got me motivated to improve the stiffness of the cabinet of the AD50VT. The cabinet is made of particle board. The sides and front are reasonably thick, but the back is rather thin. Sorry, I forgot to measure it, but at most it is 1/2 inch.

    [​IMG]

    I found a handy piece of wood that was about 1 x 1.5 inches cross section, and made a "T" out of it. To help suppress resonances, I thought it would be a better to position it asymmetrically. The shape I made is therefore a bit odd, as you can see. It is held to the back by 7 wood screws, but it is also glued for added rigidity. It spans between existing wood screws to ensure stiffness.

    [​IMG]

    After stiffening the back, my first impression of the amp was a boost in bass. The bass is definitely louder -- I had to reduce the bass on my presets. It might be a bit tighter too.

    This was a really low-cost modification, and pretty easy too. I need a bit more time to play with it, but so far I am confident that it was worth the effort. If I had done this first, I wonder if I would have replaced the speaker itself?  My first reason for replacing it was to get a bit more bass. But the articulation of the Tonker is really nice, and I'm glad I have it now.
     
  14. RightTurn

    RightTurn Member

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    ha ha ha 666- it does lean towards the metal side. 

    my # is 002425.  after playing for awhile i see it is better to leave the back on.  that eminence looks cool in there- nice pictures.

    you could say VOX knew what they were doing when they chose the speaker.

    i may buid a box to match my SG...ive got nothing better to do- :)
     
  15. Nikola Rdkvc

    Nikola Rdkvc New Member

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    Well guys.. Measure voltage over output amplifier...my VOX is 42 volts and for LM 3886 recomended voltage is 35 volts. Rewinding mains transformer solved oscilatin problems at all...seems to me that is some xhinese joke...
     
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  16. Biddlin

    Biddlin Well-Known Member

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    @Nikola Rdkvc - Welcome, on your first post you are the winner of 2019's greatest necropost. @Raiyn should be alerted for further salutations.
    Thanks for joining our little kaffee klatch. We love pictures, especially those of us who are sub-literate, so show us your gear.
     
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  17. Clifdawg

    Clifdawg Well-Known Member

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    Holy necropost, Batman! I've gotten my driver's license, graduated from high school, graduated with a BS from college, started my career, got married, bought a house, recieved a promotion, and had a child since the last post in this thread. :shock:

    We're all kidding around and having some fun. Welcome to the forum! Stick around, we're nice folk around here. Remember to bring up tonewoods, bevels, and neck dive as often as possible :rofl:

    (Just kidding, don't do that) ;)
     
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