VHT Special 6 Mod thread

Discussion in 'Shock Brother's DIY Amps' started by TNO, Apr 9, 2010.

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Your knob arrangement looks really clean, nice job.
     
  2. Stratospheric

    Stratospheric TDPRI Member

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    Hi all, I’ve just bought a used S6, and have made it through the first 15 pages of the thread. I was hoping someone can give me some quick hints without reading the other 43. I’m after a warm and early breakup for a mid range jazzy tone, without getting into any high gain. The amp sounds great at around 12 o clock with the boost pulled, but hoping to get the same tone at a more neighbour friendly volume. Really after a julien lage tone for the bedroom. Definately after replacement tube suggestions (pair of tung sols look like the best bet at this stage) but happy to pull out the soldering iron and start modding as well, I’ve got plenty of successful pedal builds under my belt... all advice welcome. Thanks
     
  3. jjlemon

    jjlemon TDPRI Member

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    Some possible ways,

    Add a Master Volume, ie a level control between V1b and V2 power tube.

    Install an L-pad attenuator (resistive is cheaper, but some say they alter the tone too much) can be put into a separate box between amp and speaker or built into amp and switchable (Vox AC4TV had one)
    eg http://www.tedweber.com/webervst/lpad.htm

    Install a variable L-pad volume control, can be fitted into speaker cab or amp.
    eg https://www.esr.co.uk/electronics/loudspeakers10.htm

    Add a Variable voltage HT control or similar (like the watts control in the VHT special 6 ultra) (or VVR) These allow variable control of the amps HT supply either to the whole amp ie pre and power amp, or as in the VHT special 6 ultra , just the power amp.
    If you're more interested in mild/moderate crunch tones then controlling the whole amp might be preferable.
    There's quite a lot of info on VVR and master volumes at Sewatt site
    But I'd imagine you can find a lot of info on Robrob's site
    https://robrobinette.com/Special6UltraMod.htm (Ideas for mods can easily be adapted for the non-ultra amp)
     
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  4. Stratospheric

    Stratospheric TDPRI Member

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    Thanks for the suggestions and links. Will be researching a master volume, sounds like a good place to start. Tung sols will be first and are going in this week.
     
  5. jjlemon

    jjlemon TDPRI Member

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    If warm jazzy tones are what you're after, you could try using different preamp tubes in V1. I liked the 12ay7, it has lower gain, so you'll have more sweep on the volume knob before breakup, but I also found it had a less 'grainy' tone than the 12ax7, smoother I suppose. It's a little like using germanium diodes in an overdrive pedal rather than silicon, there seems to be a slower onset to distortion, and goes into and out of distortion in a smoother way. I used a JJ current production 12ay7, it really does sound good (in a special 6 ultra).

    edit: I meant to say diodes and not transistors.
     
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  6. srolfeca

    srolfeca Tele-Meister

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    12AT’s and AY’s are my go-to for quick plug-in experiments, especially in a time when designers are pushing the gain on so many models.

    I always keep a couple around. Even if they don’t quite hit the nail on the head, they can point you in the right direction with adjusting the gain staging.
     
  7. myersbw

    myersbw TDPRI Member

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    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.

    I latched onto the musicgarage.com's take on the Fender Princeton tonestack voicing (no add'l pots except the fat-mid boost pot). 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 - Remove the 68K booster and added a 250K pot (rear mounted)...provides adjustable boost and rolls up to raw mode.

    Also, regarding the tone path...

    C11 - dropped from 500pF to 220pF silver mica
    C14 - 10nF Mojo Dijon
    R16 - replaced with 42K carbon comp.

    Using a Mesa SPax7 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. 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 13, 2019
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  8. ballynally2

    ballynally2 Tele-Holic

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    are you talking about Rob R 'raw switch' option that bypasses the tonestack? I was wondering how that compared to the amp's boost option with the extra tone control..
     
  9. ballynally2

    ballynally2 Tele-Holic

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    if you don't mind Rob:could you please tell me what differences one can expect between the amp's boost option and your 'raw switch' option?

    And, is it possible to use a pot/variable resistor to control the amount of 'raw/boost?
    Or is lowering R22 a better idea?
     
    Last edited: Jul 3, 2019
  10. ballynally2

    ballynally2 Tele-Holic

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    am i reading you correctly that you now have one normal tonestack and the other w just a treble control on boost? Isnt that what the amp does anyway or did you make some adjustments/resistor/cap value alterations?
     
    Last edited: Jul 3, 2019
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The raw switch will add even more mids for a pretty flat response and add volume too. For many the complete removal of the tone stack with a raw switch is just too much. The Ultra's "boost" is a good compromise.
     
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  12. tombob

    tombob Tele-Holic

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    I put a very simple treble cut type tone, similar to a guitar tone control on the boost channel. I found the boost a good bit brighter than normal so I wanted to be able to take a little edge off the treble. It’s about as simple as it gets, a pot that drains a small amount of the highs to ground.
     
  13. ballynally2

    ballynally2 Tele-Holic

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    how did you solder it?
    Im thinking of putting the pot in the low input slot to replace..
     
  14. myersbw

    myersbw TDPRI Member

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    Peek up at my post on values concerning the 68K tone stack lift resistor. I really love the addition of the 250K pot in place of the resistor. It enables you to preset a boost level to a little or an extreme almost raw level and everything in-between. Then the footswitch takes on the mode of being a variable level boost to get your lead boost just where you like it.
     
  15. ballynally2

    ballynally2 Tele-Holic

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    Before you do mods and cap/ resistor values you need to first figure out what valves you are going to be using.
    Or better said:which components are your starting block?
    To give an example: i was looking at the plate voltages of the preamp section,They seem higher than the typical Fender 100kOhm
    Hi Kleuk.Ive been watching your to and fro w Vintage Amps including the french forum.
    Could you please tell me again which filter caps you put in? I think they're Nichocon or Nichoden.Ive been on the Nichocon website but couldnt find the polarised caps (the ones w + and - on either side.Also, VA has different values.I wonder why..
    Im going w 5751 in v1/2 and jj 6v6, experimenting with plate load and cathode bypass resistor/cap values on the preamp side ..VA but the pre plate voltage v1a back to standard 100kOhm while you cranked it up i think.im going to listen to decide the values, especially w the 5751 and lower gain.
    Also, VA mentioned changing the connector ground of 2 filter caps but the attached picture link no longer works..
     
    Last edited: Aug 3, 2019
  16. ballynally2

    ballynally2 Tele-Holic

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    Hi,
    That link to your picture no longer works.Can you (re) link it again please, and/or tell me how you hooked up the sides of the filter caps?
     
  17. jjlemon

    jjlemon TDPRI Member

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    I think Racing left the forum quite some time ago, or at least hasn't returned.
    I don't have a special 6, so I'll refer you to tombob's post no. 1093 in this thread. Rather conveniently he has labelled the caps with their schematic numbers.

    VHT Special 6 Mod thread PostNo. 1093


    If you pull up the second gutshot photo, notice that C46 and C45 have black wires connecting to the negative side of the bridge, immediately to the left of C42. (I have seen some photos of the special 6 where these wires are connected to the negative side of C42, but the advice remains the same.)
    unhook, desolder, the two wires connected at the bridge -ve or C42 and move them to the right and connect them at the -ve side of C44. I'm fairly sure that is what Racing meant.

    That may seem hardly worth doing, but as racing stated the rule of thumb is not to connect anything between the rectifier and the first smoothing capacitor (C42 in this case)

    On my special 6 ultra, I also moved the speaker ground return wire away from the rectifiers and connected it to the negative side of the smoothing caps for the power amp stage, I think that's C46 on the special 6. Not sure whether it was worth doing or not, but it is mentioned in Merlin's (the velvewizard) pdf handout on bus grounding.
    http://www.valvewizard.co.uk/Grounding.html
     
  18. KP Porter

    KP Porter TDPRI Member

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    Hello folks, a new fresh first time poster here.

    I wanted to do this mod to my Special 6, using 6CA7:

    https://robrobinette.com/Special6UltraMod.htm#Kleuck_Switched_Output_Tube_Bias

    To avoid changing several things at once, I decided first to try simply replace 6V6 with 6CA7, leaving the bias resistor and speaker connection as is. Before doing that, still having 6V6 inside, I measured the voltages. B+1 was 349V, B+2 342V. I'm a bit lazy about drawing load lines by hand, so I used a couple of online load line calculators, making sure the both gave similar results for EL34/6CA7 @ 350V plate/340V screen 2:

    Bias voltage - Plate current - Bias resistor
    20V - 107mA - 188 Ohm
    21V - 96mA - 219 Ohm
    22V - 86mA - 255 Ohm
    23V - 76mA - 301 Ohm
    24V - 69mA - 349 Ohm
    25V - 61mA - 410 Ohm
    26V - 54mA - 482 Ohm
    27V - 48mA - 568 Ohm

    So, given that max. plate dissipation current @ 350V for 6CA7 is 71mA, it should be pretty safe to run it with 470 Ohm cathode resistor (approx. 55mA of current), thought I, and switched the amp on.

    Soon after it warmed up, I started hearing loud squeal/roar and immediately switched it off.

    Plugging the 6V6 back returned the device to normal operation.

    6CA7 was a new one from JJ which I bought specifically for this mod. Does the above mean I missed something and accidentally killed it, or could it be faulty straight from the factory?

    As a follow-up question, based on the above numbers, running the power section with the recommended 255 Ohms in the cathode would push 6CA7 good 15mA above the dissipation curve (and even for 6L6, with its higher dissipation limit, it would go a tad above!). What I am missing...? Screen grid current maybe, which would increase the bias voltage, decreasing plate current??

    Thanks in advance!
     
  19. TequilaCaster

    TequilaCaster Tele-Holic

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    I've had many a new power tubes roar and squawk on first powerup. They almost always quiet down in a minute or so. I assume it's just random gas in the tube being consumed by the getter. Your calculations seem sound.
     
  20. KP Porter

    KP Porter TDPRI Member

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    Thanks for the suggestions, @TequilaCaster! As a status report, no, it didn't quiet down. Or, actually, in pentode mode, it did, meaning sometimes it doesn't produce any sound at all (but sometimes still squeals). At the same time I've noticed in triode mode it almost always works (but also sometimes squeals). I will send the tube back and hope the shop agrees it is faulty and sends a replacement. Not really good since I've planned to use it as a power boost for upcoming gigs this weekend, but that's life. Thanks again!
     
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