Advice on Low Wattage, Single-Ended Amp Schematic

[abunch1]

Hello all,

I've recently gotten into guitar amp building and design, and would welcome any advice or suggestions for this latest attempt at a schematic.  It's a simple low wattage, single-ended amp.  Thanks in advance.

-Andrew

[tubenit]

Unless you have a new idea regarding potentiometer's use ................ I think they are probably drawn incorrectly?

Typically,  the volume will follow the tone stack ......... but you could do it as drawn with tone stack following volume. It's just more unusual.

I am not familiar with that type of tone stack & I am not sure if it will work or not.  However, if you've used it or are familiar with it as a tone stack, by all means .....go for it.

with respect, Tubenit

[abunch1]

Thanks for the response.  I mislabeled the pots, so I'll change it in my next go around.  As for the tone stack, it is (to my very limited knowledge) a new design that I would like to try out.  The theory in mind is that the current splits between the low, mid, and high elements, where the pots on the treble and mid are there to decrease potential / current flow and the pot for the bass changes the cutoff frequency for the low pass filter.  The Treble is a high pass filter, and the Mid element is a high pass filter followed by a low pass filter.  Do the output stage and power levels look reasonable?  Thanks again.

-Andrew

[sluckey]

You must put a coupling cap between the plate of the first tube and the gain pot. And you cannot wire the gain pot with the wiper connected to ground. There's no way for signal to pass from the left end to the right end of the gain pot. It will be killed by the ground on the wiper. Same with the treble and mid pot. Turn either to the left end and you completely ground (kill) the signal.

Same applies to the volume pot.

[abunch1]

Thanks for the help, Steve.  How does it check out now?

-Andrew

[sluckey]

It's functional. I don't give the tone stack much hope. Looks more like a huge signal attenuator. Let us know how it sounds if you build it.

I'd add a 10K-22K resistor in the B+ line just to the right of that 8µF cap.

[DummyLoad]

sim indicates that the bass control is useless. mid and treble may not work as well as you'd like. for what you are trying to achieve, a james stack may be a better choice.

respectfully,

--DL

[thelonious]

Hey Andrew,

If you already have the parts, you might as well try building it so you can play around with the part values & circuit and hear/experience what sluckey, tubenit, and DL are talking about. Then try building a couple other tone stacks so you can experiment with them, too. Merlin's book, Designing Tube Preamps for Guitar and Bass, has been an invaluable resource for me as I started designing. It's worth checking out and has an entire chapter dedicated to tone stacks.

Sluckey mentioned attenuation - the values you've chosen for some of the resistors to ground in the stack (such as the 7.5k in the treble control) might give you the frequency corner you're looking for, but they will also act as voltage dividers, shunting some of the signal to ground. The arrangement you've got will attenuate the signal quite a lot. If you've got a few preamp tubes for driving and recovery, a high loss stack might work, but for a simple amp with just one preamp tube, you don't have much leeway for signal loss like that.

If you want to design your own tone stacks, you should definitely consider checking out a sim software like the one DL used to simulate your stack design. It's much easier to change component values and designs with a couple clicks of the mouse than it is to solder and desolder a bunch of stuff. :)

Looking forward to seeing what you come up with.

[abunch1]

Thanks for the comments everyone.  It looks like I'll be going back to the drawing board for a bit and run some simulations.  I'm not sure if this is possible using a single 12ax7, but my goal for the tone stack would be that the Input Signal is split into 3 roughly equal and components (Bass, Mid, Treble) that independently process the signal in a unique way.  In mathy terms, given an Input Signal, I_In, undergoing signal processing by elements f_Bass(w), f_Treble(w), f(w)_Mid: 
I_Out = 1/3 * I_In * (f_Bass(w) + f_Treble(w) +  f_Mid(w))

Am I overlooking something?  Are tone stacks necessarily multiplicative?  ( I_Out = f₁f₂f₃I_In ).   I'll try digging for more ideas based on other tone stack designs.  Thanks again for helping a newbie to these forums and amp design.

-Andrew

[thelonious]

I'm interested to see what some of the more experienced/knowledgeable guys have to say about this, but I don't think it's possible to have a tone "stack", in the traditional sense of the word, while completely preventing interactivity between the controls. It ends up being a complex system rather than the additive one you described. In order for the controls to be independent, they would need to be isolated.

You could do this by placing only one control after each preamp stage, as in triode > bass > triode > mid > triode > treble > triode > EL84, but you would have to add a tube. You could make part or all of the stack active, rather than passive, but you'd have to add a tube for that, too. You could put one of the controls in a local negative feedback loop to isolate it. Or you could combine these strategies. But any way you look at it, you will probably have to add a tube.

It would be an interesting experiment, but whether the sonic benefit would be worth the cost/effort or not... I don't know. It certainly wouldn't fit with your desire to keep the amp simple!

[abunch1]

I think this should work now.  Only capacitors and high value resistors connect to ground to help with attenuation, and  I dropped the Gain Pot as well.  Do I need to avoid using so many caps connected to ground?  The sims I was running on CircuitLab show that the pots are affecting the signal properly, and I like that the lows are emphasized.  I'm still not sure if the signal is attenuated too much.  Thanks again for your comments all.

-Andrew

[DummyLoad]

andrew, running a sim. on your tone stack reveals that it has very limited range - on the order of +/- .2dBV for the treble. i did not check the mid and bass. 

www.circuitlab.com

is where i sim. your ckt. if you wish, sign up and edit the ckt. below.

https://www.circuitlab.com/circuit/p8ffuu/a-bunch-ts/

i made the ckt. public so you can edit it and run a sim. with the parameters i used.

you can only sweep one parameter at this time. pick a pot and sweep the range of the K value from 0 to 1 in .1 increments.

a james stack for reference...

https://www.circuitlab.com/circuit/qbt774/james-stack/

--DL