Vht Special 6...not A Typical Vht

Discussion in 'Other Amps' started by myersbw, Apr 15, 2019.

  1. myersbw

    myersbw Well-Known Member

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    I posted this on the TDPRI site, but, since this amp has so many Marshall flavors now, I thought I'd add it here, too. Just picked up a Special 6 (not-Ultra) and finished a few mods so I'll share what I did. After gleaning everyone's take on what to do, I incorporated a few with some tweaks of my own.

    Refer to the VHT Special 6 schematic out there in the wild if you prefer to follow along.

    I latched onto the musicgarage.com's on the Fender Princeton tonestack voicing (no add'l pots except the fat-mid boost pot)...this amp uses a "preset tone stack with resistors and no pots"...then parks a tone circuit right after it. Used quality caps (silver mica, Mojo Dijon, audiophiler and Tropical Fish). This mod increased the static tone stack values. I left the tone circuit as-is and here are the value changes,

    C6 - 220pF silver mica
    C9 - 100nF Mullard Tropical Fish
    C10 - 47nF Mojo Dijon
    R10 - 330K
    R12 - 100K
    R22 - Removed the 68K booster and added a 250K pot (rear mounted)...provides adjustable boost and rolls up to raw mode. This elevates the entire tonestack to being non-existent when the pot value is high enough. The pot added lets me control the boost level/raw effect with the footswitch.

    Also, regarding the tone path...

    C11 - dropped from 500pF to 220pF silver mica (compromise between usual Marshall bright cap reduction to 100pF)
    C14 - 10nF Mojo Dijon
    R16 - replaced with 42K carbon comp.

    Using a Mesa SPax7 (LOW noise 12AX7) for V1, here's the stage setup (Kleuck setup with slight changes)

    R3 - 330K carbon film || 100pF silver mica
    R8 - 1.8K carbon film || C8 - 1uF Audiophiler NP cap
    R4 - 470K carbon film || 500pF
    R17 - 3.3K carbon film || C13 - 10uF Audiophiler NP (translated...BIG honking' cap!!!)

    Power stage!

    R20 - left it and paralleled a 5w 470 ohm ceramic wirewound (it's what i had..measured out to 235 ohms), 20V drop after warmup with Great Brit Mullard EL34 in the socket. B+ at 350 ( - 20 at cathode) to settle it at net 330V with 73mA measured at the plate (bias socket adapter). I left the screen resistor as stock and didn't install a bias switch. (Why not? This is not going back to 6V6 land!)

    I didn't add a choke (yet) so no need to adjust the tonestack. The B+ 330ohm dropping resistor is dissipating about 2 of it's 3 watts, so I'll likely take care of that before much play time in on the circuit.

    I contemplated a dynamic tonestack, but figured the "Princeton stack mod" that shifted the frequency scoop coupled with the stack resistive elevation AND the virtual Mid-boost pot now added, was more than enough tonal options to not overly complicate the amp. It truly worked out that way. My guitar is a PRS with 5708's so the elevated boosts and the EL34 let me dime the amp...park the Special6 at a set default...then actively backoff the volume and tone to clean up.

    It works very well with the flat response speaker from VHT, but the golden nugget is a 12" Eminence CV75 in an open back cab. IN regard to the transformers? They were over-engineered and this amp actually was, indeed, DESIGNED to be modded!!! Grab one if you can. My vintage Great Brit Mullard EL34 is dissipating about 24W on the plate and doing fine. After 40 min of playing...the PT is lukewarm to touch...the OT is very cool...a choke will be my last mod to this as I don't like that B+ dropping resistor to be dissipating 70% of it's rated value.

    The sound? Very good JCM800 crunches when maxed and fluid sparkly, glassy cleans when you roll back the guitar volume. Overall, what a beast of an amp!

     
    Last edited: Apr 15, 2019
  2. jimmyjames

    jimmyjames Well-Known Member

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    I scored one at a hock shop coupla years ago. Stuck out amongst the SS dregs that usually plague these places, with a Tung-Sol KT66 hangin' down. So I hid it, did some jooglin', returned and snapped it up, while their 20% amps discount was still hot :thumb:
     
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  3. myersbw

    myersbw Well-Known Member

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    WOW...some are known to just park the big tubes in, but you can get the most if you reduce the cathode resistor to the ideal bias current for class A operation. I was shocked what a EL34 would give me in this 6V6 amp...all with the transformers running cool to slightly warm to touch. First over engineered amp I've seen besides a couple Bad Cats.

    By the way, a local GC had this on their used list for $159...I called and offered the manager $140 sight unseen and parked an old gift card balance to hold it. Drove up and forked over $90 cash and walked out....$150 total including the tax. Score! :)
     
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  4. jimmyjames

    jimmyjames Well-Known Member

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    Yeah mine seemed to have been modded for harp, has a switchable line out on the back. Tried a 6V6, KT better headroom. Li'l Buddy hempcone doesn't readily break up, got that in a Vox 112NT cab with a 10" baffle. Use a treble booster and a Soul Food to bring the vinegar :)
     
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  5. myersbw

    myersbw Well-Known Member

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    Mine matched up well with an open back 1x12 with an Eminence 12" CV75 in it. The CV75 is a very high sensitivity speaker.
     
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