VHT 12/20 mods

Discussion in 'Shock Brother's DIY Amps' started by jsoucy, Apr 3, 2014.

  1. mcabe

    mcabe Tele-Meister

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    I installed the master volume last night. It wasn't working well at first, because I had wired pin 1 to the solder lug that R28 was connected to. When I turned the master volume down, there was still a lot of volume coming out of the amp and the useful range of the master volume control was really small (the volume was full on with master volume above 2 or 3). I tried swapping pins 2 and 3, but that didn't help. Then I realized that there was a 56K resistor between the solder lug and ground, doh! Flipped pins 2 and 3 back to the original configuration, wired pin 1 directly to ground, and now everything is working well.

    I need to play around with it a little more, but my first impression is that the master volume is a nice improvement. There is a big difference between tweaking the volume at the send, return, or master volume stages. I need to play around with it a lot more to see what sort of settings work best.
     
  2. mcabe

    mcabe Tele-Meister

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    I noticed that my amp was missing resistor R66 (the 470K grid stopper resistor before the input to the phase inverter). I emailed VHT about it, and turns out they updated the design. See their reply below. I've been pretty impressed with VHT customer support btw.

     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That 470k grid stopper was put there to keep the cathodyne phase inverter from being over driven into into nasty double frequency distortion. That usually only happens when boost pedals and other effects push a big signal into the amp. With a master volume you can dial down the input into the phase inverter to eliminate double frequency distortion.
     
  4. mcabe

    mcabe Tele-Meister

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    After studying the inside of the 12/20 for a while, I decided on a much less ambitious project. Instead of trying to add additional gain stages and tone stacks (and make the VHT something it is not), I decided to remove the one knob tone control and add a full TMB tone stack. The stock VHT tone control uses two 470pf caps to boost or attenuate the high frequencies. The advantage of this set up is that the tone control still works with the mid boost engaged (the mid boost essentially bypasses the tone stack). I don't use the mid boost, so a more traditional tone stack makes more sense for me.

    I was able to free up space on the front panel by moving the send and return controls to the rear panel like in the 12/20 RT. There was enough slack in the existing wires, so I was able to move the pots without touching the soldering iron. Just drilled 2 holes in the rear panel and moved the pots down.
     

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    Last edited: Jun 15, 2015
  5. mcabe

    mcabe Tele-Meister

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    Here is a view from the inside of the amp. Controls on the stock amp are: input, volume, tone, send, return. On my amp the controls are: input, treble, bass, volume, pre-phase inverter master volume.

    I removed the mid boost, and put the mid control on the back panel where the foot switch used to be. I also added a switchable 120pf bright cap to the volume pot.
     

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  6. mcabe

    mcabe Tele-Meister

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    The stock tone stack with fixed resistors is located on the right most portion of the eyelet board. The components were attached with huge gobs of solder, so it was a little annoying to remove. Luckily the eyelets are pretty durable and can handle a lot of heat/abuse. I cut out all of the existing components and added new tone stack capacitors and a slope resistor. There is plenty of space on the eyelet board for large orange drops and/or Mallory 150s.
     

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  7. mcabe

    mcabe Tele-Meister

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    I initially had a 500pf bright cap on the master volume, but the amp was much too bright. I found myself turning the treble all the way down and maxing out the texture control with my telecaster. After cutting out the bright cap things were much improved. I also replaced the chromeback speaker with a cannabis rex which really helped to tame some of the highs.

    While I had the amp apart, I added some copper shielding. I wouldn't call the 12/20 quiet, but the noise levels are perfectly acceptable for my purposes.

    All in all, I am pretty happy with these mods. There are a lot of different tones available in this amp (not all of them pleasing), so I wouldn't recommend this mod for everyone. But if you don't like the mid boost/one knob tone control or just like tweaking, then its a relatively easy change.

    Now I just need to find a good/cheap way to relabel all the controls....
     

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  8. zeoy

    zeoy Tele-Meister

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    I have the 12/20 RT version and besides some tube swaps my amp is stock*.

    The only mod I'd like to do is to make the "mid boost/tone stack lift" variable. In other words, having a potentiometer which when on 0 you 'd get no mid boost at all and on 10 you 'd get the full effect as it is right now.
    I was looking at the schematic a few months ago and I remember that I came to the conclusion that you can sacrifice the FX loop send pot, move a cable or two and that's all.
    I'm not experienced at all with serious mods but I have some physics background so I can (kind of) understand a few things when looking at a schematic.
    It'd be great if someone could check if what I describe here is feasible.


    EDIT: the schematic is shown on page 10 here
    http://www.vhtamp.com/images/Manuals/vht special 1220rt manual.pdf
    and I'm talking about putting VR3-100K (send level) in the place of R10-68K (boost circuit )
    --------------------------------------------------------------
    * mostly Tung Sols in the preamp, a Sylvania 6K6 instead of 6V6 for the reverb driver and the stock 6V6s. Sounds great and I get compliments all the time about my tone. Sounded phenomenal with some Winged C 6L6s but they became microphonic after a few months.
     
    Last edited: Jun 15, 2015
  9. mcabe

    mcabe Tele-Meister

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    In theory this should be a straight forward change. Just replace the 68K resistor R10 with a 100K pot. The one challenge is that the VHT 12/20 RT has a lot of the components wired directly to the various pots, switches and jacks, and they are not labeled. You would have to open up the amp and poke around a little to try and find R10.

    Once you find R10, cut out the resistor, solder 2 wires from where the resistor used to be to the 100K pot and you're done. Given that the FX loop send jack has lots of components soldered to it, I wouldn't remove it though. Are you opposed to drilling a new hole in your amp?
     
  10. mcabe

    mcabe Tele-Meister

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    FWIW, on my amp R10 was soldered directly to the push-pull volume pot, but the send and receive jacks had a bunch of components attached to them. You might be able to push the send jack into the amp and out of the way, but removing it all together would be challenging.
     
  11. zeoy

    zeoy Tele-Meister

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    Thanks mcabe! It sounds more like a project for my amp technician rather than me. I use this amp as my main amp in most gigs and rehearsals (and of course at home) and I don't want to mess it up.

    Irrespectively of who's gonna do the mod (or if I ever gonna bother:D), I'm probably missing something from what you said here:
    "You might be able to push the send jack into the amp and out of the way, but removing it all together would be challenging."

    You mean the send pot right? Why would I want to remove it? I don't mind my new "mid/boost control" being on the back of my amp (the FX send pot is on the back in the RT model).
    I don't care about FX send levels. It'd be OK if it could be set at 100% after the mod.

    Now that I think of it again, it'd be probably easier to drill a new hole and use a new pot as a mid/boost control and leave the FX send alone.

    When I 'll have some free time, I'll remove the chassis, clean the pots and I'll see what's going on there.

    Thanks again for your help.;)
     
  12. mcabe

    mcabe Tele-Meister

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    Whoops. My bad. Removing the send pot is what I meant. I think drilling a new hole is the easiest option anyway. There is room on the back panel under the volume control, which is close to where R10 resides anyway. Good luck on the mod!
     
  13. Chevy

    Chevy TDPRI Member

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    Anyone swap out the coupling caps with Orange Drops or Mallorys?

    My 12/20 Special amp to me is not good sounding at all when stock. With tubes upgraded to JJ 6L6 and USA-made old stock 12AX7s, it sounds fair but not yet good to my ears. Using a Celestion G12-65 speaker. I haven't had this amp on the bench yet to see what all is inside, but I assume the coupling caps are cheapies. Has anyone swapped them out with Orange Drops or Mallory 150's to see if that changes things significantly? Wondering if the output tranny is any good.
    Also find myself reaching for a mids tone control at rehearsal... anyone have any good non-destructive ideas for that?
     
  14. mcabe

    mcabe Tele-Meister

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    You are correct that the VHT doesn't use premium components. It uses those dark red radial capacitors as coupling caps and in the tone stack. I think they are polyester. I doubt if switching caps for same values will have a dramatic effect though.

    The VHT actually has a Fender style TMB tone stack with fixed resistors instead of potentiometers that you can tweak. You could try replacing resistors to adjust the settings to something more to your liking. I don't have the schematic in front of me, but I think the default settings are equivalent to having all the controls around 1:00.

    I added a full tone stack to my amp. If you want an easily reversible mod, you could remove the mid boost foot-switch jack, and put a pot in there instead. You could then remove the fixed mid resistor and run leads to your mid pot. Alternatively, just find fixed resistor values that sound good with your set up.
     
  15. Chevy

    Chevy TDPRI Member

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    Hum

    Right, good ideas... some day when I've got time I'd like to put in some known-good caps, maybe even some carbon comp resistors just to see if it would sweeten up. I imagine the output tranny is mediocre as well... but is it worth the time and effort? Dunno.
    One other thing... I found the amp had a lot of inherent hum compared to my other amps, so it's hard to use for studio purposes. Anyone else have this same experience?
     
  16. mcabe

    mcabe Tele-Meister

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    The transformers are custom built in the VHT factory. Not sure about the quality, but they are pretty big and heavy. I doubt the output transformer is a major weakness in this amp.

    There is a bit of hum in my amp, but nothing too crazy. I'd bet that lead dress is the culprit. A lot of bells and whistles in this hand wired amp, so there are lots of components and wires in close proximity. You could try moving wires around to see if your hum goes away.

    Out of curiosity, what settings are you using on the amp?
     
  17. Chevy

    Chevy TDPRI Member

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    When I get it on the bench I'll check out the lead dress, tone stack, etc.
    Settings are (from memory..., amp not here) pentode, max power, 6L6, 16 ohms, Deep at around 1:00 setting, tone around 2:00, volume 2:00 or so, send/return both up full.
     
  18. mcabe

    mcabe Tele-Meister

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    You could try backing off the send and return a little. I keep mine around 3:00. This is a non-R/T version?
     
  19. Chevy

    Chevy TDPRI Member

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    I still haven't got this part tested/figured out, but VHT tech support suggested using the amp with both send/return up full if I wasn't using the effects loop. Yes, non-R/T.
     
  20. mcabe

    mcabe Tele-Meister

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    In theory up full is the default setting and closest to not have the effects loop at all. I have found that send and return definitely effects the tone of the amp. Could help buzz too.
     
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