Valvecaster with Tone stack

[fair.child]

Hi folks,

I'm getting ready for my next build. Guess what, it's going to be Valvecaster matsumin again with tonestack option. I do have questions regarding the schematic builds, especially combining between baxandall tone stack (bass and treble mod) to valvecaster. I want to know how to do that though is it even possible ? I did some research on tone stack but not sure if it will apply with 12AU7

I need some enlightenment and feedback about this build please

As always any comments, links and suggestion are always highly appreciated

Thanks folks
Andy

[GibsonGM]

Hi Andy, if there's a different schematic out there that shows a tonestack, that you're thinking of, could you post a link?  If that's going around, I must have missed it.  I will try to give you some 'quick and dirty information' about this, although in reality there is more to it.  This is enough to get experimenting, though!  You certainly will not  hurt a thing if you try this out, in fact, I suggest you do!

If you don't have a schematic to post, we'll start at "square one", and assume you want to just add a tone stack to the 'regular' Valvecaster, the easy way.

Enlightenment: 

1)  ok, if you add a tone stack, you WILL lose some volume (level) as the passive R's and C's 'eat up' signal.   Actually, more than a little will be lost.  That's called "insertion loss".    The VC probably has enough gain to drive a stack, just that it will not give as high an output IF you add the stack just before the output. Whether the stack eats up too much level is something to be experimented with - it COULD really stink, esp. at lower voltages.   I would use an FMV stack rather than Baxandall, by the way - better suited for guitar.

2)  One way to avoid this is to put the stack in between the 2 gain stages.  That will let the 1st stage drive the tone stack, and 'recover' lost signal in the 2nd stage (buffering is better, but beyond the scope of a quick reply).  BUT - because you 'ate' some level in the stack, you won't get the overdrive you probably want!

Because of 2, I'd go with 1.  You will keep the amount of overdrive you got in stage 2, just at a lower output.   

BTW, The valve has little to do with whether you can add a tone stack, it is more about how you drive the stack...in fact, if set up correctly, the 12AU7 is a BETTER valve for this than the 12AX7!   Typically, a cathode follower is used to drive such things, though....also called a buffer.   As this is a simple 1 triode gain circuit, our options are limited unless we want to add more stages.

I would just build the VC, and make a tone stack on breadboard - don't just build it without hearing it!...connect the output to tone stack >> amp, and see what you get.  This will work much better if you can increase the VC voltage, try 18V if that is possible.   If the level is too low, you can then move to a bass and treble conotrol....if too low, try just a treble cut, etc.  Or add a recovery stage if you really 'need' the full tone stack, your call.

If you took the time to do the math/simulate the VC in LTSpice, you can figure out the dB gain (I'd do it but no time right now)...and then see what the stack will take up, get my drift?   That is why learning something like LT is so useful!  If you come out with a negative, you know you're losing signal.   This is a great way to learn how to mod things!  Hope this was helpful....

[fair.child]

Wow, thanks GibsonGM

I really like your opinion about PSpice or Multisim for doing circuit analysis. I used to play with those tools everyday but right now I'm kind a busy with work most of the day and week but still I appreciate the idea and will do to try.

1) I still don't understand why I'll lose some volume. I thought if I increase the filter (R-C) capacitance it will just about coloring the tone. I still need further knowledge and information about how to boost up the volume IF that will happen. I plan to add the Bass and Treble stack before the output. I'll do some study about FMV stack, because so far the only tone stack that I experienced just Baxandall while back a go I was building BB preamp clone.

2) Are you referring with buffer with an op amp ? I concern it will reduce the number of oscillation. I'm trying to understand 'why' and 'how' I eat  some level in the stack. I think you still emphasize from the first point about the level

3) How about 12AX7A runs 18VDC +/-, I'm thinking using rectifier diode to boost up the level if it's possible (this is just my other idea). Roughly but draft and soon I'll try to draw the schematic and post it here

[GibsonGM]

If you're working from a design that HAS a tone stack that you're modding, that's one thing. Just adding a TS to a circuit will 'drag it down', though.  The impedances of the C's are low, so that loads the previous stage.   There is an impedance mis-match at work....you want to drive a high input impedance with a low output impedance, and this is actually trying to make a low out drive a low in!   You CAN increase C in an existing design, and it's not going to be very noticeable...more gain (higher voltage....) will help you drive a small tone stack ok, I believe, as this isn't a 'critical application'.

It's not that important right now, but in many, many amps, they feed a tone stack with a TUBE buffer.  It's a cathode follower...takes the output from the cathode rather than the plate.  In that config, you get cathode feedback that tends to hold the output steady...it acts like current amplifier.   So, when your tone stack draws current, it can source it without being dragged down.   Lots to read up on that later

If you wanted, you COULD drive a TS with an opamp or something, but that sort of negates the whole 'pure tube' experience, ha ha....

The idea of running a 12AX7 at 18V is great!  Post your schematic. Don't forget you have to hold your heaters at whatever you're running them at now, though....there are tone stacks optimized for a 12AX7, and if you post I'll look at your values....

[GibsonGM]

There is an impedance mis-match at work....you want to drive a high input impedance with a low output impedance, and this is actually trying to make a low out drive a low in!

Sorry, mis-type!!!     What happens with tone stacks just added in to an existing circuit is that you have a relatively high output impedance from the gain stage you want to drive it with...say, 40K, a typical value, and that is fairly high.     You have a LOW input impedance to the tone stack.   This causes the TS to load the previous stage.   So you'd be driving a low input Z with a high output Z, not a great thing to do.  The tone stack sucks up signal like a sponge, and it likes high frequencies most. Blah.

A plumbing analogy:  You can picture this as a very small pipe (high output impedance) trying to source water to a larger pipe (low input impedance)...the pressure drops.  If it's the other way around, you get a pressure increase, which is desirable.  Like a valve half closed, trying to fill a pipe after it, but it cannot SOURCE the current to do so.

Often you will see a cathode follower (buffer) used to drive a tone stack.  This presents a lower output impedance so that the tone stack can draw the current it needs without appreciably loading the previous stage.  Rather than increasing the level of the signal, the buffer allows the circuit it is driving to take current while leaving the signal alone (poor explanation, but may be enough to understand what's going on).   

There ARE tone control and tone stack variants that have been "optimized" to work ok with the high output impedance of a gain stage.

Don't worry so much about supply voltage, buffers, etc. right now...go ahead and try some tone controls, have fun, and make some observations.  If you need to jack the level up after, we'll talk then  

[Ben N]

Just look at the schematic of a Tweed Bassman to see how a tube buffer ("cathode follower"--takes the output off the cathode rather than the plate) is used to drive a tone stack after a pair of tube gain stages. There is more in Kevin O'Connor's "The Ultimate Tone", if you can get your hands on a copy. Alternatively, you can do it in solid state. Take a look at "Mosfet Follies" at Geofex.com for an example of that (also in the world of amps). Another alternative is a split plate load, in which you trade some voltage gain in your second stage for a bit more current to drive the tone stack better.

[GibsonGM]

That's a great way to get there, Ben, but the trouble is that the valvecaster is only 1 triode.  So you either lose a gain stage to get the CF, or have to add in another tube...if so, you'd want another triode IMO, add another gain stage and a CF for the TS on the output. Now you're pushing your power supply too much, so you revisit that and...

If you're gonna do that, you might as well get a nice power supply and take it up to 300V, since you'll be making an actual tube preamp!!

I'm willing to bet the VC will drive some form of TS, maybe not an FMV but a bass & treble...I'll have to sim this and see what goes on.  As a PEDAL, it doesn't need to put out a ton of level, after all.   

Good call on using the triode for overdrive followed by something (Opamp, FET...) to drive the TS - seems to have a lot of potential and wouldn't be 'over the top' for a new builder... 

[Ben N]

Ahh, my bad for not looking at the VC schematic. For some reason I was assuming that it was two triodes in series (like a typical tube preamp, as you say).

[GibsonGM]

Wouldn't THAT be nice??    I don't know that it would sound too good at the low voltages, tho!

I seem to come up with a 14dB gain for the VC, the way it is set up now, at 9V.   That doesn't give you much room for a "Tone Stack".   The avg. seems to lose between 10 and 20 dB, if Duncan's Tone Stack Calculator (another free one, Google it) is correct.    So you'd lose the nice drive into your amp after the VC.  Your output would be near unity, and maybe (ya think?) lacking in ways.

My sense is that, other than playing with the tone control already there, you'd have to recover after adding a tone stack.  A basic FET gain stage would do this pretty nicely, and would sound good, I bet!    It would be as simple as omit the existing TS, add an actual STACK, and feed its output to the input of a FET stage.      Just a couple of resistors and a FET needed.....