Upgrading a VOX AD50VT Valvetronix

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

  1. Voxman

    Voxman Moderator Staff Member

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    I didn't want to be a wiseinheimer but that tube functions as a power tube which is part of it's design and that new technology there using hence the change from a Chinese Blackcat ,{sorry :lol: } to one with a better plate chemistry! :lol:
     
  2. vic108

    vic108 Well-Known Member

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    Great thread Dorian.

    Alot of the mods were done by some brave souls earlier on.
    Spunks, SGM, SB, etc... either changed tubes, added a speaker out,
    or swapped out the speaker for another. Soon, I hope to swap out the
    stock speaker for a Celestian Tube 10 and the tube for one made by
    Mesa Boogie. Then, if I get brave, and can find the right part, I'll add
    the speaker out...Thanks for a great read and WELCOME!
     
  3. skidshark

    skidshark Active Member

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    :shock: (vic STILL hasn't found his screwdriver!!

    you're as bad as me with all these projects going vic!!
     
  4. Dorian

    Dorian Active Member

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    Voxman: At last I found the dusty storage box that includes a small stash of 12AX7 and 12AT7 tubes. One of them is a Telefunken 12AX7 with long, smooth plates. :D (Maybe I wasn't such a dumb kid as I thought last night.) Its getter deposit still looks shiny, although I am sure it has been at least 20 years (dare I say 30?) since it was dropped in the box. Yes! I was very pleased with this find. Right now I have raised the heater voltage up to 9.47 DC, and I see a nice glow. I plan to spend the next day conditioning the heater, and then move on to biasing the plate and grid. Obviously I am being very cautious to avoid damaging it, but who knows? :?:

    vic108: Thanks for the hint. I did a search for <AD30VT> on this site, and found some of the previous discussions you mentioned. Interesting indeed. I don't claim to be first here, but I am still enthusiastic, at least while the Telefunken still glows.
     
  5. Six String

    Six String Moderator Staff Member

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    Isn't it best to get your sound stand alone and then PA it.....? That would seem to me to really leave you flat if you rely on going thru the PA all the time. It should give you more variety in your sound... otherwise you're talking about a sound tech controlling your sound.
     
  6. Voxman

    Voxman Moderator Staff Member

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    There very robust tubes not at all like modern issue........and stop foolin' around and put that tone bottle where it belongs :lol: it will be OK as is! :wink:
     
  7. Dorian

    Dorian Active Member

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    Okay, Voxman. I did ramp up the heater with care, but it wasn't so easy to find connectors for a high voltage supply to condition the grid and plate. I just plugged the Telefunken 12AX7 into the AD50VT as you suggested. It worked fine, as you predicted. Here is a comparison of the old and new:
    [​IMG]

    The picture shows the light-colored horizontal mica insulators at the tops and bottoms of the plates. The plates extend between these insulators. The bottom insulators are at about the same height in the two tubes, but it is clear that the plates in the Telefunken at left are a lot taller than the plates in the Electo Harmonix tube that came with the AD50VT. (Funny how the Electro Harmonix tube is fatter, though.) Another thing I noticed, but is hard to photograph, is that these mica insulators were not square with the walls of the glass tube bottle in the Electo Harmonix. They are obviously tilted and skewed in the Electro Harmonix, but the Telefunken mica insulators are a lot more square. I wonder if the two triodes in the Electro Harmonix have the same electronic characteristics? Perhaps the "Made in Western Germany" attention to detail for the Telefunken lets the class AB amplifier models in the AD50VT swing more symmetrically between the positive and negative voltages :?:

    There are obvious differences in the tones of the AD50VT with the Telefunken and Electro Harmonix tubes. Some amp models differ more than others for these these two tubes. I need some time, maybe even weeks, to sort this out before I can write it down. (The AD50VT has 11 amp models, and it might be possible to get similar results for both two tubes in different regimes of gain, for example.) I have been thinking about how to do a proper test. My first plan is to compare the amp with the back off, so I can quickly change the tube and not lose my sense of the tone by sweating the screwdriver, etc. I may post some quick results in the next few days for the different presets that come with the amp, but I am off to France in about a week.

    In the meantime, I couldn't resist taking a photo of the Amperex 12AT7 tube that I also found in the old box. There is something charming about their logo of a vacuum tube blowing a trumpet, even though it is not an SG:
    [​IMG]
    :roll:
     
  8. CharlieB

    CharlieB Active Member

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    **** Perhaps the "Made in Western Germany" attention to detail for the Telefunken lets the class AB amplifier models in the AD50VT swing more symmetrically between the positive and negative voltages Question ****

    Negative voltage on a 12AX7 in the Vox? Interesting....
     
  9. Dorian

    Dorian Active Member

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    CharlieB: Sorry to be unclear. I meant the positive and negative voltages at the speaker output of the AD50VT. I was guessing that visible mechanical differences in the two triodes could affect the symmetry of this waveform in AB operation. I'll try to do some recordings today to check a few things out. If ambitious I might connect a scope, but in the past I have not seen too much this way.
     
  10. Voxman

    Voxman Moderator Staff Member

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    The plate chemistry is crap on the new ones....... The dissipation sucks on almost all of the new ones, not all mind you, such that it almost goes against the definition of the tube there trying to produce, I suggested to a big tube manufacturer that they should change the #'s as the tube there producing is ANOTHER tube based on the characteristics......and he wrote off my argument in the discussion as "Moot".... :shock: Do both New and NOS have the same emission? Strong words I know, :oops: , BUT my ears don't lie....ever! And when one grew up and came of age, during the golden years of tubes, lived thru and used them during their almost demise, { one manufacturer- SOVTEK-! :shock: } and their rebirth again :lol: it's hard to to accept the new tube argument!
    Sorry to [Ed Romain] rant......... :roll: I have to go back Sunday nite to the loony bin........till next weekend!
     
  11. CharlieB

    CharlieB Active Member

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    Dorian no... it wont effect the A/B symmetry.

    With almost certainty, the 12AX7 is before any "phase inverter" section for the power amp. The phase inverter is probably something like a TLO72, but it can be done with a single opamp as well with some gain sacrifice.

    The question I've got - hehehehe - is what happens if you leave the tube out all together!

    I've seen "effects" that had a tube, but if you take it out, you still pretty much get the same effect! That is, the tube was there to sort of buffer the signal a bit, not in the conventional sense, but to act like a pull down (or pushup) for the signal voltage.

    Let me explain....

    Imagine the signal from a solid state device... say our tried and true 741 family opamp. Now imagine this signal doing its thing, getting some gain on several stages... and instead of being amplified by the triode in the 12AX7, lets let the 12AX7 act like a voltage controlled shunt to ground (or some other voltage). As the signal goes high, the tube conducts, but we're not taking output off the plate (with IE drop across plate load). Instead, we're letting the conduction lower the signal voltage... or at least reference it to another voltage. This can be compression, or excitement, or mixing of two signals.

    So, it would be interesting to see what happens with no tube installed.

    Finally... I've also seen designs that put the signal on the FILAMENT!!! Yes, riding on the DC of the filament was an AC signal.... strangeness. You remove the tube, no sound. BUT, you replace the filament with a resistor, and the sound is back... generally very close to the same as the tube provided. In this case, conduction of the tube varied the filament voltage and had some weird effect on the upper harmonics.

    So... again... I wonder how Vox is really using that tube.....
     
  12. E.Buzz

    E.Buzz Member

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    Alright! Another faded guy! :D

    ...Welcome to the jungle! :twisted:
     
  13. CharlieB

    CharlieB Active Member

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    I used to be a faded guy.... then Mr. Flitz came a knockin'.....
     
  14. Voxman

    Voxman Moderator Staff Member

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    CeeB some effects companies put a tube in there design in that manner to just say it's a "tube powered" unit! Nice trick....... :wink: :cry:
     
  15. Voxman

    Voxman Moderator Staff Member

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    From the horses mouth, they said as a power tube.........
     
  16. CharlieB

    CharlieB Active Member

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    Well the one I remember... I cant remember! It was a blue box with yellow letters.... It just worked the same with or without the tube. They do all sorts of nasty marketing to get that tube in there.

    Thats why I wanna see what happens to the Vox with no tube!
     
  17. Voxman

    Voxman Moderator Staff Member

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    I remember one like that...uh real tube? Can't recall!....time to go out for a couple Stout's and a large prime rib.....! :lol:
     
  18. Dorian

    Dorian Active Member

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    CharlieB: Are you suggesting that there are unscrupulous engineers who would design products to cheat unsuspecting musicians out of their hard-earned money? I am shocked. Shocked. :roll:

    I spent part of the day swapping tubes in the Vox AD50VT, and recording different sounds. I'll have more to say about this later, I hope, but I am pretty sure that the tube plays a real role in the signal path in the AD50VT. The Vox amp with the Telefunken 12AX7 had a measurably lower output than with the Electro Harmonix 12AX7. The sounds were more "pure" and less "muddy and buzzy" with the Telefunken, so it is a keeper. I did not think to try your test of running the amp without the tube. Until I read your post, I was optimistic the tube is part of the amplifier chain, as implied with pride in the Vox literature, and is not just a parallel shunt of some sort. (Now I have sealed it all up and may not get around to this for a while.) I would be really disappointed if the amp still worked when the tube was pulled, of course, but I can't disprove your assertion yet.

    Sorry, I don't know enough about the Vox/Korg circuit to comment on your ideas about why the AB amplifier models could have symmetrical waveforms, even if the two triodes in the 12AX7 are not matched.
     
  19. Dorian

    Dorian Active Member

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    Voxman: I have not followed vacuum tube development since I was a radio amateur in the dim dark past. I have to say, though, that I was disappointed with the mechanical construction of the Russian Electro Harmonix 12AX7. The skewed shape is something I can see, and my eyes don't lie. It looks like the electrical assembly was stuffed forcibly into the glass bottle, and it was rather distorted in shape when it went in. My ears hear differences too.
     
  20. Dorian

    Dorian Active Member

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    E.Buzz: Yes, I love the faded SG. I wanted to buy an SG Standard about 25 years ago, but it was just too expensive for me at $660 as I remember. I ended up with a used Rickenbacker that cost $225. The Ric is a well-built instrument, but the neck is too narrow for my hands. I spend decades playing rhythm guitar. Amazingly, the faded special cost me $575 a couple of years ago, It has set me free, musically speaking. Also, it looks a bit beat up with its rough finish and inevitable dents. Just like me. :oops:
     

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