VHT Special 6 Project

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Optical
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Re: VHT Special 6 Project

Post by Optical »

The lower the volume, the more the bright cap c11 is going to be creating top end in the tone. The tone control is a low pass filter, so that is designed to suppress high end/treble.

Your settings of 4 and 6 don't suggest they were doing much to control treble which begs the question, did you try a higher on the volume control and adjusting the tone control?

^edit yeah as above

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Re: VHT Special 6 Project

Post by Slowy »

Optical wrote: Sat Jan 08, 2022 10:33 am
......did you try a higher on the volume control and adjusting the tone control?
I did. With bandmates screwing up their faces and muttering things like 'Jaysus' and 'Fuck' I didn't stay up there very long. :rofl: I couldn't find a setting which produced a fat tone that didn't also tear our heads off.

There are some low power settings that give a fine, acoustic like tone but it's too thin and small to work with the band.
"We were playing so loud that just one note on the guitar was sufficient".
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Re: VHT Special 6 Project

Post by RectifiedAmps »

You could just clip out C11 and try that since the tone control isn’t helping much. You could also change C14 to a 22nf cap. It might fatten things up too much (farting out) but it would give you an idea of how far you can push things.

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Re: VHT Special 6 Project

Post by Slowy »

RectifiedAmps wrote: Sat Jan 08, 2022 7:30 pm You could just clip out C11 and try that since the tone control isn’t helping much. You could also change C14 to a 22nf cap. It might fatten things up too much (farting out) but it would give you an idea of how far you can push things.
Thank you!
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Re: VHT Special 6 Project

Post by Jay »

Slowy wrote: Tue Jan 04, 2022 10:13 am Putting this here to keep it all in one place:

Thing about the Special 6: It actually has bass, middle and treble controls. Except instead of pots it has fixed resistors inside on its circuit board. They are: R10, R12, R13 and R15 with R9 setting the slope. It's actually related to the Fender Champion 600 circuit that has the same thing, a tone stack with no controls. I should have called the popular "lift R19" Champion 600 mod "The Much Mod" 'cuz I'm pretty sure I originated it. The boost switch on the Special 6 does the same thing as the Champion R19 lift. Unlike the Champion 600 the Special 6 has a tweed style tone control in series with its internal tone stack. Yup... It has two tone controls in series. Like I said, there are bass, middle and treble controls in there with no knobs.

It's relatively easy to juggle nickel and dime resistors substituting for R10, R12, R13 and R15 rather than buying expensive tubes. Unlike blowing a lotta money on tubes it actually works!

Functionally it's a '60s Fender tone stack.

R10 and R12 function as a bass control. The sum of R10 and R12 should add up to approximately 250k. R13 (33k) functions as a treble control. The treble pot in a Fender tone stack is wired to function as a variable resistor. Fender uses a 250k pot so values from zero to 250k set your treble response.

R15 is a 6.8k resistor, same as the 6.8k tail in larger Fenders. Substitute 15k if you want more mids.

Or rig a couple 250k pots in there so you have real bass and treble controls.


For some real fun you could change C6 to 500p / 400v, C9 to .022uf / 400v, C10 to .022uf / 400v and R9 to 56k. That's a Marshall stack.
My few cents... The above is almost correct but I believe he has got the treble and bass resistors wrong. IMO R10 and R12 are for treble control and R13 is bass control. Apart from that it is typical Fender stuff and a version of this tone stack is also used in the Jansen 4/12 Special.

Since it is typical Fender and you need to find out how to remove the over representation of presence, this is what I'd do:

Remove/disconnect C11 and C12 thereby freeing up the treble pot to be used instead of R10 and R12.
Disconnect the top of the volume pot and connect to the middle lug of the 'relocated' tone pot.
Put a temporary new bass pot in of 250KOhm to determine what you like... then measure and replace with fixed value. Alternatively, add the bass pot to the panel or back if there is space.

Re C14 suggestions - test different values after you have played with the 'new' tonestack.
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Re: VHT Special 6 Project

Post by rickenbackerkid »

What an odd schematic! I'd lean towards what NippleWrestler suggested, and trialing various caps across the tone control.

I'd be interested to hear what happens if you bypass the overly complex tone circuit and try it just with the tone control. And finally you could potential do some sort of VOX Cut Control, which is a nice tool to trim down excess treble in the power amp stage.

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Re: VHT Special 6 Project

Post by Jay »

rickenbackerkid wrote: Sat Jan 15, 2022 6:54 pm What an odd schematic! I'd lean towards what NippleWrestler suggested, and trialing various caps across the tone control.

I'd be interested to hear what happens if you bypass the overly complex tone circuit and try it just with the tone control. And finally you could potential do some sort of VOX Cut Control, which is a nice tool to trim down excess treble in the power amp stage.
As Slowy posted in Italics before, there are two tone controls: one with set value resistors and one with a pot. IMO, the 'overly complex' tone circuit should be changed back to pots and the current tone control should be abandoned.

Reason for this is that the fixed value resistor tone circuit comes first and determines what the pot tone control can play with... which is a diminished frequency range...
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Re: VHT Special 6 Project

Post by rickenbackerkid »

Yeah, I'd remove the fixed TMB completely, and see what a simple Fender Tween tone control does.

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Re: VHT Special 6 Project

Post by RectifiedAmps »

rickenbackerkid wrote: Mon Jan 17, 2022 12:23 pm Yeah, I'd remove the fixed TMB completely, and see what a simple Fender Tween tone control does.
I like this option too. Faster to implement and easier to reverse if it doesn’t work out. A TMB tone stack on a low-wattage single-ended amp seems like overkill for a simple circuit.

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Re: VHT Special 6 Project

Post by Mattallica »

I too reckon remove the fixed TMB. I would replace it with a mid filter (fixed framus mid control) that uses the existing "boost" set up. That will give you a faux clean/dirty switch. Clean = mid scoop, dirty = full mids.
You could also rewire the vol and tone to make a sizzle control as per Rob Robinette.
If you only use one input you could replace the other input with another pot. Make the mid control variable or add something else.

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Re: VHT Special 6 Project

Post by Jay »

Right, I think all of the above is probably too much for Slowy... :lol:

So a few pictures may help clarify things. I have used Duncan's tonestack calculator for a Jansen 4/12 just because Slowy has one. And also created two pics of the VHT with pots and pots values set such that it resembles the VHT tonestack. The latter was done for two different C3 capacitor values: the 47nF which is standard in the VHT and then one with a 4.7nF value which is what the Jansen has.

Perhaps this may explain Slowy's 'presence' dilemma? Note that I have assumed that R22 is shorted out!

Edit - wait wrong pics... correcting now
Edit - fixed

Original 47nF
VHT Tonestack C3=47nF Capture .JPG
VHT Tonestack C3=47nF Capture .JPG (101.75 KiB) Viewed 146 times

4.7 nF
VHT Tonestack C3=4.7nF Capture.JPG
VHT Tonestack C3=4.7nF Capture.JPG (101.79 KiB) Viewed 146 times

Jansen
Jansen 4-12 Tonestack Capture.JPG
Jansen 4-12 Tonestack Capture.JPG (101.42 KiB) Viewed 146 times
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Re: VHT Special 6 Project

Post by Slowy »

" I think all of the above is probably too much for Slowy..."

It's always a comfort to be understood by one's mates! :rofl:

Thank you Jay and thank you everyone who has contributed so far. I really am going to attack this thing with a soldering iron (under strict supervision) as soon as my current period of intense work commitment is over.

Looking at those graphs, the first one, the 47nf looks like what my ears are hearing. What it doesn't represent is the intensity with which the sound cuts through a band mix. Normally, this is considered desirable but this amp proves it's possible to have too much of a good thing.

Actually, this might be the perfect amp for your Norlin era mud pie Lester. :think:
"We were playing so loud that just one note on the guitar was sufficient".
Ritchie Blackmore.

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Re: VHT Special 6 Project

Post by robthemac »

Slowy wrote: Tue Jan 18, 2022 5:18 am Looking at those graphs, the first one, the 47nf looks like what my ears are hearing. What it doesn't represent is the intensity with which the sound cuts through a band mix. Normally, this is considered desirable but this amp proves it's possible to have too much of a good thing.

Actually, this might be the perfect amp for your Norlin era mud pie Lester. :think:
It's quite enthusiastic with a Tele, isn't it?

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Re: VHT Special 6 Project

Post by NippleWrestler »

Alternatively you can change the mid resistor to something like 82k for a more flat or even response, it'll lessen the scoop. I wonder if it'll be too fat sounding by changing that cap to a 4n7. If you have sockets though you can chop and change to your heart's desire.

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Re: VHT Special 6 Project

Post by calling card »

I wonder if a variac might work, make it sag.

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