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VOX V15 Resurrection - help needed!

Discussion in 'Amp Central Station' started by bcguitars, Nov 19, 2014.

  1. bcguitars

    bcguitars TDPRI Member

    50
    Jul 23, 2014
    scotland
    Hi there,

    I've dragged this out of the attic where it's been sitting (broken) for over 20 years as it was under threat of being chucked out :eek:

    I've replaced fuses and cable and hey presto it now turns on, which is more than it did last time I saw it!

    All I'm getting is a buzz however, no audible response when touching the end of a plugged in cable. So I took it apart, literally brushed out the cobwebs and a couple of bugs and have identified a cracked valve which is going to need replacing.

    There are five in all - 3 short and 2 longer ones.

    Of the short, there are 2 x noname 12AX7 and 1 x Fender 12AT7 (busted one)

    I can't make out the model number on the larger valves.

    Question: Can anyone tell me what valves I should be putting in here? And also, is this likely to be the reason for the buzzing?

    Many thanks!
    B
     

    Attached Files:


  2. deadbeat son

    deadbeat son Tele-Holic

    882
    Apr 5, 2010
    Evergreen, CO
    3 x 12AX7 and 2 x EL84 are what you need. This could very well be the source of the problems, but like anything in life there are no guarantees. Good luck with the resurrection!
     

  3. ArcticWhite

    ArcticWhite Tele-Meister

    265
    Apr 14, 2009
    Portland Oregon
    That cracked tube is a problem obviously. But don't replace the others! They are probably fine. In fact, vintage tubes are better than new.

    You also need to clean all your potentiometers with deoxit. Do a forum search. Or just Google it.


    But it's likely you have some bad capacitors too. Electrolytic caps simply dry out inside, and they create internal shorts.

    People always just want to change the tubes, because they're easy to unplug. But tubes can last practically forever. Caps have limited lifespan, whether or not they are even used.

    You, or someone else is probably going to have to fire up the soldering iron and replace some caps. The big silver cap can next to the tubes, and all of the electrolytics. The ones that look like cardboard tubes. If the amp sounds scratchy and wimpy after you put the new tube in it, and cleaned your pots, that will be the capacitors.

    I'm no expert. But I've recapped four tube amps, largely using advice I've gotten in this forum. All four amps are healthy now, but they were dead when I started.
     

  4. deadbeat son

    deadbeat son Tele-Holic

    882
    Apr 5, 2010
    Evergreen, CO
    Some good advice here, but I assumed this amp was NOT vintage, but rather a '90s AC15, which wouldn't need everything you're suggesting and wouldn't have vintage tubes. The lack of a tube rectifier is what led me to believe this.

    OP - how old is this amp?
     

  5. bcguitars

    bcguitars TDPRI Member

    50
    Jul 23, 2014
    scotland
    Thanks for the advice! I'm not scared of soldering so no worries there. I've ordered a 12AX7 tube from a russian dude on the 'bay who apparently sells new old stock from the old USSR :) I'm a bit cynical but he has good feedback so who knows ..

    The amp is a V15 not an AC15 hence my posting - there's plenty of stuff on AC15s but next to nothing on mine. It is indeed a model from '89 / '90 though.
     

  6. deadbeat son

    deadbeat son Tele-Holic

    882
    Apr 5, 2010
    Evergreen, CO
    Oh man, I'm sorry! I totally misread your post as AC15, even though it clearly says V15. The 12AT7 was correct for that amp. Sorry if you ordered the new AX7 due to my misrecommendation.
     

  7. bcguitars

    bcguitars TDPRI Member

    50
    Jul 23, 2014
    scotland
    Ah, no worries, they won't break the bank! Is there a major difference between the AT7 and the AX7? I'm almost sure that the one's in there are AX7s but I'm not home right now to check ..

    I'd also be interested in how you know which valves they take - do you have a manual somewhere?? :)
     

  8. MadJack

    MadJack Friend of Leo's

    The Vox Showroom has a small paige about the Rose Morris Vox V15, produced from 1981 to 1983. An interesting little read on this amp.

    Are you interested in restoring the amp to stock, or making a good sounding vintage Vox amp?
     

  9. deadbeat son

    deadbeat son Tele-Holic

    882
    Apr 5, 2010
    Evergreen, CO
    According to your original post, you had 2 12AX7 and 1 12AT7. I found a schematic for the V15 online, and it confirmed that layout. Further, I found a site that had a little information on the V15, but I can't find it now. That site also listed the tube layout as 2 ECC83, 1 ECC81, and 2 EL84. An ECC83 is an alternate nomenclature for 12AX7, and ECC81 is an alternate nomenclature for a 12AT7.

    To answer your other question, a 12AT7 has about 60% of the gain of a 12AX7, so a 12AX7 has 167% of the gain of a 12AT7. The 12AT7 is used in the phase inverter in your amp; I'm not sure what the result of swapping it with a 12AX7 would be.
     

  10. MadJack

    MadJack Friend of Leo's

    While the gain factor is greater on the 12AX7, the max plate dissipation is greater on the 12AT7. This is primarily due to the greater MA output per triode, nearly 10x the output. This is used to create a stronger, cleaner signal to the output tubes. the Phase Inverter is the best place to use a 12AT7, there and the reverb driver, where applicable.
     

  11. DaveKS

    DaveKS Friend of Leo's

    Oct 21, 2013
    KS
    Those 2 12ax7 on left are RFT, if they're not bad/shot I wouldn't toss them, excellent tubes.

    Swapping in a 12ax7 in the PI slot will likely produce a lot of noise, more power amp distortion and it will probably be short lived, 12ax7 aren't design to handle current that socket designed for a high current driver like 12at7 supplies.


    That cracked 12at7 probably was only problem way back then, but after all these years of not being powered up/disuse cap and other components are sacked as well.

    Also socket/tube pin cleaning with Deoxit or the like will be needed, as well as all the pots will probably need cleaned, flushed and relubed.
     

  12. bcguitars

    bcguitars TDPRI Member

    50
    Jul 23, 2014
    scotland
    Thanks for all the replies! Daveks - I've ordered an AT7 and will be testing the components as best I can and replacing where necessary.

    MadJack - I'd prefer a good sounding VOX vintage amp, not worried about restoring to stock. Not being an electronics engineer, I don't know where to start but more than willing to learn if pointed in the right direction! I've read that taking the negative feedback loop out of the circuit is a worthwhile mod - do you have any thoughts on that?

    Thanks
    B
     

  13. MadJack

    MadJack Friend of Leo's

    I found the schematic here. There is a wire going to the output side of the output transformer. That is the negative feedback lead. This will not be either of the wires leading to the speakers. I don't have an actual amp in front of me, to give a more thorough description, but this should be the NFB. Disconnecting this should bring back much of the missing Chime.

    Next, would be the speakers. I've read mixed reviews of the Stock Fane Speakers. I haven't heard them, so I can tell if they have that Vox Jangle. If not, look into the Weber AlNiCo Blue Pups or Eminence Red Fang 10". Both have the AlNiCo Blue type voicing in a 10" speaker.

    Being a 31+ year old amp, new caps will most likely be needed to develop a lively tone.
     

  14. bcguitars

    bcguitars TDPRI Member

    50
    Jul 23, 2014
    scotland
    OK. 12AT7 replaced and now I have a lovely (loud) resonant buzz, tuned strangely enough to "G" !!! The tube us not "lit up" as such but there appears to be a good amount of heat emanating from it. The tone pots change the resonant frequency so I guess that's a good thing :)

    If you plug a guitar in you can vaguely hear it behind the buzz but there's not much there. I've used a cheap multimeter (no capacitance measurement) just across the ends - some of them read consistently like those two huge black 47 microfarad ones at the end of the board and with others you can see the resistance slowly increasing which to me is a bit weird but then I'm not an electronics engineer :)

    Will I need a capacitance meter at some point and do you have any recommendations? Should I literally go round the board and replace every cap? I'd quite like to pinpoint the source of the loud buzz, eliminate that and take it from there but maybe that's not feasible..

    MadJack - is the brown wire I'm holding in the photo the one you are referring to for the feedback loop?

    Thanks!
    B
     

  15. MadJack

    MadJack Friend of Leo's

    I'm not sure if that's the correct wire or not. I would have to know where it comes from. If you look in the lower right hand side of the schematic you'll see the 820Ω resistor, That is the NFB feed. The one end is at the Transformer and it should go to the 820Ω resistor, then feed to a point between the 100nF cap and the two resisters, 22kΩ and 100kΩ. You going to have to trace the leads,as seeing it through those pictures is really difficult.

    [​IMG]
     

  16. bcguitars

    bcguitars TDPRI Member

    50
    Jul 23, 2014
    scotland
    Thanks MadJack, will do!

    Do you by any chance have any tips about the best way to find the source of the buzzing?

    Many thanks
    B
     

  17. Pepi

    Pepi Tele-Afflicted

    Aug 8, 2004
    Indiana
    You need to send it to me for a Black Friday present :)
     

  18. IamJon

    IamJon NEW MEMBER!

    1
    Feb 9, 2015
    Illinois
    I have a V15 and would be more than happy to help you. It would be quite an undertaking to replace all of the capacitors. Let's first figure out what to do with the brown wire. If the nfb tap is out of phase from how it is supposed to be hooked up you will have made an oscillator.
     

  19. ScottJPatrick

    ScottJPatrick Tele-Afflicted

    May 12, 2011
    Stirling, Scotland.
    Did you ever get the amp working properly? Don't know exactly where you are but https://www.facebook.com/TimeTravelAudio in Edinburgh are worth contacting if you're still having problems, good guys and not expensive.
     

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