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I have downloaded the Duncan Tone Stack Calculator, but I am unsure as to how I work out the 'Zsrc' (input impedance?) for my particular amp. It is a Vox Night Train, but I have modified it slightly. Here is a schematic of the pre amp section, and I have added in my modification in red. Hope someone can help me out. Thanks in advance!

A 12AX7 preamp stage with a 220K Plate resistor and a 1.5K Cathode resistor (bypassed by a capacitor) has an output impedence of approx 48 KOhms

For about 90 % of those tone stacks the input Z is almost irrelevant,,,, what follows the tone stack is SUPER Critical if you want any decent tone shaping to happen.. High Z after tone stack or it dies in the ass.
I notice the circuit you post has the old trick of lifting the tone stack for more gain,,, Not the best idea I ever heard but some folks like it.
It relies heavily on the basic system tone and on some circuits than can be a pain.
You may need to fine tune C28 to find a good balance between the 2 positions.
Phil.

Thanks to both you guys for the quick reply.

ik6gpy wrote:A 12AX7 preamp stage with a 220K Plate resistor and a 1.5K Cathode resistor (bypassed by a capacitor) has an output impedence of approx 48 KOhms

I changed the plate resistors to 100k. How do you work out the impedance?

phatt wrote:For about 90 % of those tone stacks the input Z is almost irrelevant,,,, what follows the tone stack is SUPER Critical if you want any decent tone shaping to happen.. High Z after tone stack or it dies in the ass.
I notice the circuit you post has the old trick of lifting the tone stack for more gain,,, Not the best idea I ever heard but some folks like it.
It relies heavily on the basic system tone and on some circuits than can be a pain.
You may need to fine tune C28 to find a good balance between the 2 positions.
Phil.

Thanks for that info, Phil. What I am going to attempt to achieve is to shape the tone stack to something a bit more Marshall like, ideally Superlead-esque. But I would first like to be able to understand what is going on at the moment. The tone stack lift switch is labelled Bright on one side, and Thick on the other - Thick is where it is bypassed. It adds a boat load of gain, and for me is a pretty useless feature. I modified the amp for more clean headroom, which I have achieved. But now I would like to shape the tone a bit, to suit my style.

As it stands, the tone controls are quite odd. The treble control sits around 7-8 o'clock, mids between 3-5 o'clock, and bass sits around 2 o'clock. This is the closest approximation of a Marshall sound that I can get from this amp. The gain seems to interact with the tone stack, turning up the gain adds in an ear shattering amount of treble.

Where do I start?!

I changed the plate resistors to 100k. How do you work out the impedance?

With a 100K plate resistor the plate's output impedance goes down to 38k-40kOhms

Try R37 at 33k,,, also C29 maybe better left at 300ish pico.
also make C21 much larger for more bottom end,, try 10uF

R24 and R38 seem rather odd? Better minds may have an explanation for those.
Phil.

Cool, gonna give this a try in the next week or so. Just need to build up an order first.

I have done a lot of reading on the Fender style tone stack since I started this thread, and I have found out some interesting stuff. For example, to get a flat setting.. you back off both bass and treble but crank the mids. It's interesting that I was already doing that, except with bass added, to get the best sound.

What would you think to lowering the mid range cap, currently at 22n, to 15n or as low as 10n? I'm just trying to squeeze a little more mid range out of this tone stack. When I plot this in the TSC, setting all controls at noon gives a pretty similar line as a standard Marshall.

Bare in mind that these tone stacks are only a fine control adjustment. The main (or system) tone is the culmination of everything.
So no matter how good the tone section works it won't turn a Vox into a Fender. From the PU all the way to the speaker and beyond. Even the Room you play in also effects the final result that reaches your senses.
Duncans sim is a great little tool but it won't tell you the whole story.
A 10 Volt input signal swing can loose up to 70% of energy just to drive the tone stack so if you are chasing lots of preamp drive it is a handicap.

I believe Brian May simply used the normal input on his vox amps which have no tone stack, just a volume pot and straight to the PI.

That maximized the pre-drive gain and with a simple treble boosting circuit in front He overcame the excess bottom end of the system tone.
Dead simple idea and very Clever, as it negates a whole lot of messing about inside circuits. Heck why do it the hard way.

You could likely do similar things *Inside the Amp* but judging by the amount of failures I read it might be faster and more economical to work with outboard ideas first. If it's all PCB layout inside then you can easy stuff up the whole Amp by swapping out parts in search of mojo. hint.

You said you get way too much treble,, then try darker speakers as most speakers now have (IMO) way too much treble response. They are used simply because they are louder per driven Watt and the makers know the market will always buy the louder Amp. The result is a lot of gear comes out sounding very harsh and thin.

But as you are already inside the Amp go read up on all the places in the signal path that alter the tone shape. Coupling caps between stages, cathode bypass caps,, even oddball output transformers make a big difference to the end result.

If you are chasing the Marshall sound then look at the Fender Pro Junior for clues as it does a fine job of replicating that sound in a small rig.
Later versions altered C1 and messed it up but it's an easy fix.
In general if you want the old sound then go look at the originals for clues.
Most of the greatest guitar sounds came from really bad/basic designed Valve circuits not hifi modern fancy pants circuits that are sold today.
As to changing midrange caps,,, look it's a fender front end and you are trying to get marshall sound??? I wish you luck.
The 2 circuits are wired different and that alone makes a significant impact on system tone
Phil.

Hey Phil, thanks for you reply.

I appreciate what you're saying, even if I do find some of what you said a little patronising. In fairness I know that I won't get a Marshall sound out of this amp. Besides being a chronic tinkerer, I am trying to learn and understand audio electronics - and the best way to learn is by being hands on, right? I have always had gripes with this amp from the beginning (they always sound great when you try them out - then when you start using them with your band etc, the cracks start to show), and even in the last week or so of tinkering with it I have learned and now understand more than I have in the two years I have owned it. That's got to be good?

I have always thought about speakers, and they are on my shopping list. I am aware that playing Strats and Jazzmasters through an already bright signal path, and bright speakers, is going to yield a bright output. I do take care of that somewhat with the pedals that I use. But, like I say.. I am only trying to learn, and in the process, perhaps I can fine tune the amp in a way that will help others who own it.

Hello sickman,
OK don't get all funny,, the net is not a very good way of discussing complex issues and yes it is easy to misunderstand or to accidentally patronize people.
You get over it and move on,, if you don't you can end up loosing sleep over the most trivial issues.

No harm was or ever is intended by Me. I reserve the right to be a complete idiot and miss the point and Also reserve the right to have my own opinion.

I learned a long time ago to simply convert the word *Opinion* to *Option*
It made my life a lot easier.

Any way no harm done I hope,, Yes if you must tinker,, great but I'm just making you aware that it's a PITA working on/fault finding/redesigning circuits in situ,, Far better to whip up a test board circuit and rebuild what is inside the Amp,,, then you easy find all the worms in the design.
Once done THEN you only have to pull the lid on the Amp ONCE.

Sounds like a lot of effort but having messed with circuits for many years now it truly is the faster way to problem solve.
Murphy's Law number 102; A short cut is often the longest possible path to take.
It can take forever and I just don't have enough days left in one lifetime to eff around aimlessly trying to better crap designs.
That is how I do it but your way may well be better for you.

Search for Valve Wizard pages,, tons of great info to be found.
Tone Lizard, if still up and running? Go to the Tone Lounge pages.
And some Aussie stuff here; http://www.ozvalveamps.org/
All Australian Valve Amps but heaps of good advice resides inside if you go deep.
Cheers mate,, Phil.

The "ozvalveamps " link has a virus tells my virusscanner !! So take care.

Hey Phil,

Sorry, I didn't mean to come across like I was being funny with you.. Like you say, the internet isn't the best place for discussions like this!

Well, I've been away a while.. but I did get round to modding the tone stack and I like the results. The Marshall thing was solved too.. I just bought a 1959SLP reissue on eBay, from 1994. It's absolutely majestic! Thanks for your help by the way, I never meant to sound ungrateful. The links you gave were really handy, particularly the Tone Lizard site.

phatt wrote:For about 90 % of those tone stacks the input Z is almost irrelevant,,,, what follows the tone stack is SUPER Critical if you want any decent tone shaping to happen.. High Z after tone stack or it dies in the ass.

Then why does Marshall (and Fender as well, at least in the bassman) put a cathode-follower just before the tonestack ?
I thought that it was precisely to drive the tonestack with a low impedance.

JiM wrote:

phatt wrote:For about 90 % of those tone stacks the input Z is almost irrelevant,,,, what follows the tone stack is SUPER Critical if you want any decent tone shaping to happen.. High Z after tone stack or it dies in the ass.

Then why does Marshall (and Fender as well, at least in the bassman) put a cathode-follower just before the tonestack ?
I thought that it was precisely to drive the tone stack with a low impedance.

The tone stack itself is low impedance. The cathode follower is to prevent the gain stage previous to the tone stack from being loaded down and is borrowed technology from high-fi designs. It isn't necessary for good results. It just sounds different and has its own compression when driven hard. What phatt was saying is that there needs to be a high impedance following the tone stack or the tone stack will get loaded down and fail to work properly. Usually this results in less than reasonably active tone controls. Normally in this type of circuit you see a high value pot or resistor going to another tube grid.

Thanks Groovenut, Sounds like you have a better command of the situation.

(Not sure what you mean by Tone stacks being low Z? I've learned they where rather high )

From the best of my learning;

Yes you can drive the classic Fender tone stack with almost Anything,, Hi or low Z ,, but without a decent hi Z stage *After* then you loose a lot of signal energy as well as the tone controls no longer work with any real definition.
Ever noticed how a lot of Amps have rather vague tone controls?
**A lot of those Marshall ShredMaster type circuits have a hi ish Z tone circuit hanging off the end of the distortion wizz bits.
Well OK works fine if you happen to have a Hi Z Valve input straight after but the circuit will die in the ass if trying to drive other gear which might not happen to have a hi Z input.
One simple buffer stage after Tone would make it a Pro job and guarantee that it works into any load.

I used to think in terms of *Driving* the tone stack until I discovered (by Accident) that you can drive those Hi Z tone circuits *Direct* from a passive PU. Blew me away, how well it worked! Google *PhAbbTone* by me.

Totally *Passive Input* HiWatt styled tone circuit yet seems to work better than most Amp tone controls,, and that includes some Valve Names also.

Without a Hi Z recovery stage you can loose as much as 70% of the signal swing through the tone stack. Well that is what my sims say and my real world testing shows that to be the case also.

FWIW;
I have personally tested many tone circuits and found that the Hiwatt setup has far better mid notching ability than ALL the others.
Interesting to note that some very early tone circuits where passive (No *Active* frontend preamp,, and sounded rather nice).
I have a vague recollection that one model Traynor PA Circuit (I think) ran passive tone circuit. Microphone>Transformer > Hi Z tone circuit > THEN into the first active AX7. A real head scratch ay???

The price you pay is Signal noise ratio as you need a fair bit of gain in recovery but Every second player Who has heard my Tone boxes has asked me to build a PhAbbTone for them.
So go figure?

I've had great success with running semi passive tone circuits **IN FRONT** of a lot of Distortion circuits Not After.
I say,,Fiddle around with some different ideas and see what happens ,, cant' hurt?
Phil.

phatt wrote:Thanks Groovenut, Sounds like you have a better command of the situation.

(Not sure what you mean by Tone stacks being low Z? I've learned they where rather high )

I should have said fairly low impedance

If you look at the standard FMV tone stack, from an AC standpoint, the pots are in parallel with the slope resistor. Which means that the best case scenario is the Fender TS which has about a 36k input impedance. I realize this isn't low impedance unless you are talking about loading on tubes. I apologize for the mislabeling.