VHT 12/20 mods

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

  1. jsoucy

    jsoucy TDPRI Member

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    Haven't seen a mod forum for this amp. The special 6 and ultra forums have useful information. But this amp is its own beast.

    Got mine about a week ago. So far, I have changed the power tubes to el34s and also the preamps, tung Sol in v1 and eh's in the other slots.

    Changing the preamp tubes made a huge difference. Prior, the guitar was just way too boomy live. This was on my les Paul. Tele was unusable with the amp. Too bassy. Way too piercing. No mids at all. Now things are much smoother. Not so boomy. Actually have to increase the depth when I use the les Paul.

    What changes have other people made?
     
  2. telecat33511

    telecat33511 Tele-Holic

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    One of the first things I did was run pink noise from my iPad for 2 hour clips at full volume to break in the speaker, and it also tested it electrically to see if something would blow - so far it has not
    2 hour sessions in a garage in the summer in FL , for about a week or more !

    Second I went right for JJ6V6 power-wise and a 1112ax7 for V1-v3
    there is no phase inverter - it is a cathode-follower circuit ( FWIW )

    Third this was each week or 2 I went from 6v6 to E34L to 6L6 and back
    Currently have 6L6 in hi power mode

    Been torturing both the E34L and 6L6 with volume on 3/4 and watts set to room ambiance and band levels

    I have been gigging with this in church, in bars, in rehearsal for all kinds of music
    So far it has held up brilliantly

    No electronical mods to speak of yet
    and I have it for about a year now
    I bought a cover for it at custom amp covers

    http://www.amplifiercovers.com/

    Sorry for the ramble-rant
     
  3. jsoucy

    jsoucy TDPRI Member

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    Love it. I spent a ton of time trying g to get mine to live el34s. They sound great but three different pairs ended up giving me humming and buzzing issues after some play time. Say a show or two. But 6l6? No problem.
     
  4. cowboy rob61

    cowboy rob61 Tele-Meister

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    I'm wondering about speakers. I've done the usual trawling of forums and reviews looking for a good choice of American style speakers.

    I've got a pretty good idea now what's out there but I don't know whether to go with something straightforward bright American or something smoother. I don't really know where the chromeback fits in, in the world of speakers, so I don't want to go with something brighter and end up cracking the windows.

    In a broader sense with speakers, would you go darker speaker and crank the treble if you need it, or brighter speaker and turn the tone down and/or roll back guitar tone?

    Sorry, I seem to have a habit of writing really confusing posts.............
     
  5. cowboy rob61

    cowboy rob61 Tele-Meister

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    Which speakers have people tried, and how do they compare to the chromeback?
     
  6. mcabe

    mcabe Tele-Meister

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    The Special 12/20 is a great sounding clean amp/pedal platform, but I miss being able to overdrive the preamp tubes. Even with the volume dimed, the preamp stays pretty clean with my telecaster.

    I just finished modding my Blues Jr. and am considering adding a switchable marshall style preamp to my VHT Special 12/20. The design that I have mocked up uses a DPDT switch to switch between the stock tone circuit and a JTM45 insipred preamp with an additional gain stage. The VHT 12/20 RT has a fourth 12AX7, so the power transformer should be able to handle the extra load. I gather that modding the 12/20 is not nearly as popular as modding the Special 6, but has anyone tried something similar?

    The Special 6 mod thread is just awesome, so I'll post my experiences with the 12/20 here in case anyone is interested in mods for the "big brother."
     
  7. mcabe

    mcabe Tele-Meister

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    Here is the schematic for the mod I am planning. Planning on mounting a dpdt switch, and pots for the volume and tone stack on the back of the amp. I should be able to PTP wire the mods and leave the components on the eyelet board unchanged.

    Think this will work?
     

    Attached Files:

  8. 410DeVille

    410DeVille Tele-Meister

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    I put a WGS ET90 in mine and love it. Better definition and it just sounds bigger.

    [​IMG]
     
  9. mcabe

    mcabe Tele-Meister

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    Looks nice! Although it looks like you are missing a few tubes... I have a Private Jack on its way that I am going to try out.
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The 12/20 has a cathodyne phase inverter--pretty much a carbon copy of the 5E3's--it even taps the non-inverted signal from the top of the cathode resistor like the 5E3. The biggest difference that brings the cathodyne phase inverter into the 21st century is the simple addition of a 470k grid stopper to the cathodyne's grid which is one of my suggested mods for improving the overdrive tone of the 5E3 Deluxe.
     
    Last edited: Apr 17, 2015
  11. mcabe

    mcabe Tele-Meister

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    Interesting. This probably helps contribute to the blackface sound of the VHT. The signal to the cathodyne phase inverter is coming off of the plate in the 5E3 but off of the cathode of the prior stage in the VHT. I wonder why VHT designed it that way.
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The two previous gain stages are DC coupled to produce a sweeter distortion. From the ValveWizard's DC Coupled Cathode Follower page:

    [​IMG]
     
  13. mcabe

    mcabe Tele-Meister

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    You would need to turn up the send and return levels in the FX loop to get overdrive in this stage though. Would inserting a "master volume" just before the PI allow for overdrive at low volumes? I was under the impression that the send and return levels made a master volume unecessary, but perhaps not.
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    It would be easy to insert a pre phase inverter master volume by replacing R28 with a 1MA pot with input to pin 3, the wiper connected to the 470k phase inverter grid stopper and pin 1 to ground. The MV would only attenuate the signal though, it won't boost it for more overdrive. You would normally want both send and return FX levels at max if you're not using the FX loop.
     
  15. mcabe

    mcabe Tele-Meister

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    I've been using my send/return as a master volume, but perhaps that is not ideal. By diming the send and return as you suggest, I could push V2A and V3B into overdrive, and then attenuate the signal before it hits the phase inverter and wakes up the neighborhood. I'll try the master volume mod and see how it works.
     
  16. mcabe

    mcabe Tele-Meister

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    Does anyone have a schematic for the VHT Special 44? It looks like a fixed biased, full tone stack version of the 12/20 with a negative feedback loop. Would be interesting to see if there are any good mods based on the 44. VHT publishes schematics for its other amps, but there is nothing on the website for the 44.
     
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Since the cathodyne phase inverter doesn't add gain a pre-phase inverter master volume would be an easy and effective way to go. It's a lot easier to add than a post phase inverter mv.

    Here's a pre pi mv on the 5e3:
    [​IMG]
     
  18. mcabe

    mcabe Tele-Meister

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    Thanks for the useful diagram. I see the 5e3 has a capacitor bleeding some of the high frequencies to ground. Not sure if I'll need that for the VHT. What's the purpose of the 470k grid stopper resistor? Cuts down the signal to prevent saturating the PI or something? Easy enough to try it with and without, and see what works best.

    I should have all the parts I need on the way from AES.
     
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The 12/20 already has the 470k grid stopper so you don't need to add that. The cap on the master volume pot is a brite cap that high freqs around the MV to keep the treble up at low master volume levels.
     
  20. mcabe

    mcabe Tele-Meister

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    I was just looking over the schematic again. The return from the FX loop looks to be engineered so that you cannot overdrive the following gain stage. The return volume pot is 250K, and it is followed by a 3.3Meg resistor (R20) and a 220K resistor (R21) to ground. This makes sense because the amp was designed as a clean pedal platform, and in that context adding overdrive after the FX loop is not desirable.

    I think I'll still add a master volume just before the phase inverter, but adding a separate gain stage to the preamp seems like a better place to generate overdrive. Alternatively, I could tweak the values of the return pot, R20 and R21, but then the time and modulation effects in the FX loop will come before the overdrive which may not be ideal.
     
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