BK Butler Tube Overdrive. Tone section. More bass and less highs

[nonost]

 Hi. I've recently build another BK Butler Overdrive and I was wondering how to tune the tone stack section.

I've build a few of them and they always have too many highs & mids and lack a bit of bass. I set the treble & mid pots at 9:00, and the bass pot at 15:00. With this set up I get an EQ pretty close to the one in bypass. But the pots don't have an useful range... From noon and beyond I get a really trebly and full of mids signal. I would like a more broad range of use, with pots sitting near noon instead of being close to one end or another.



I have swapped the 22k resistor to GND in the tone section for a 10kb pot for a Mid control. And I've removed the 47nF to GND also in the tone section.

Regarding the HI pot, I'm using a 100kA pot instead of 500kA and removed the 220k across lugs 1&2. You can see it in the picture attached.

Any ideas about what can I do to "center" the EQ?

Cheers!

[antonis]

Any ideas about what can I do to "center" the EQ?

Delete the other 47nF cap, replace 22k resistor with a 25k pot, place a 22nF (or so) between 25k pot wiper and 22k/100nF joint and voila your MID control..

[nonost]

Ey antonis, thanks for your help.

Would this Mid control change the behavior of the Lo and Hi pots?

With my current Mid pot, I have to set the Mid & Hi pretty low. It gets really ice picking with them at noon. And my Lo pot, otherwise, needs to be cranked...

[Fender3D]

100k out level pot is what really loads your lows in this pseudo tonestack... you may try and use 1M pot, but it will be still affected by the following circuit's input impedance.
An output buffer would be really useful

[nonost]

Hi Fender3D. I've tried it in my pedal chain (there are a couple buffered bypass pedals) without too much different. Didn't tried with another Vol pot value, though.

I don't know if the changes Antonis talked about are for a better Mid pot control or a solution for the whole tone section...

Thanks!

[antonis]

I don't know if the changes Antonis talked about are for a better Mid pot control or a solution for the whole tone section...

The later..

[amptramp]

Looking at the signal chain before the tone stack, there is a lot of attenuation for lows.

The input to the first op amp turns over at 4.8 Hz, not a problem.

The turnover between the op amp stages is 10.6 Hz, not a problem.

Going into the first tube grid, the 10 nF and two series 10K resistors form a highpass with a -3 db frequency of 796 Hz.

Going from first tube to second tube, the 47 nF and 32 K resistors form a highpass with a -3 db frequency of 106 Hz.

All of this happens before you get to the tone stack.  The 796 Hz highpass is the obvious choice for changes - either raise the capacitor from 10 nF or the resistors from 10K and you will definitely get more bass.  The other values can be changed, but they are not as critical.  By the time the signal hits the tone stack, it is already boosting the highs and manipulation of tone stack values is not going to do much to recover the bass.

[anotherjim]

I agree with Ron's analysis. The feed to the first grid is super bass light.

[nonost]

Thanks Ron, I will try a 100nF before the first grid. BTW, how's the math for the opamp done? I mean, how do you get the 4.8hz cut-off?
I'm missing something there.

I'm taking a look at the info posted by antonis. Looks pretty useful. I will definitely do the changes needed there too.

[amptramp]

Thanks Ron, I will try a 100nF before the first grid. BTW, how's the math for the opamp done? I mean, how do you get the 4.8hz cut-off?
I'm missing something there.

I'm taking a look at the info posted by antonis. Looks pretty useful. I will definitely do the changes needed there too.

I use the simple method of calculating -3db cutoff points.  Go to this site:

http://www.muzique.com/lab/main.htm

and go down the list until you get to AMZ Online Calculators and pick R C Filter Cutoff

http://www.muzique.com/schem/filter.htm

Just insert the values and it does all the donkey work of determining the frequency.  The owner of that site is a member here and his site has a lot of information of use to all of us.  In the case of the input RC turnover use 33 nF and 1010000 ohms, the sum of the 10 K and 1 meg resistors at the input.

[nonost]

Ohh, I see. Thanks Ron!

Yeah, the highpass filter before the first triode looks weird. I'm trying another cap there.

BTW, can I use a 10uf instead of the two 22uf in series? Between opamps. And remove the 1M resistor to GND. I'm not using the buffered bypass.

Cheers!

[antonis]

can I use a 10uf instead of the two 22uf in series? Between opamps. And remove the 1M resistor to GND. I'm not using the buffered bypass.

Yes..
And replace the 2X680R resistors with a single 1k2 one..
You can even lower 10μF cap vaue for a more effective HPF..
(10μF/1k2 corner frequency is about 13Hz..)

[anotherjim]

I think I read somewhere that low HT "starved plate" tubes have reduced grid resistance. This has 24v so does it count as starved? Will it affect calcs?
Seems a great idea to hang tube stages between +/- power rails. Many SS amps have +/-15v so 30v for a tube or two.

[nonost]

Thanks Antonis. I thought about it as well, but was not quite sure.

I don't understand the RC filters with opamps... Why the resistor + cap (in series) between opamps is a filter? I mean, none of the components are tied to GND. I don't get it.

Related with the latter, before the first grid there's a 10k + 10nf + 10k tied to V--. Is it yet a RC filter? It's not tied to GND but V--

A the input of the circuit there's a filter 10k + 33nf + 1M to GND. This is the kind of scenario that I understand 

So yeah, a bit lost I am

[amptramp]

Thanks Antonis. I thought about it as well, but was not quite sure.

I don't understand the RC filters with opamps... Why the resistor + cap (in series) between opamps is a filter? I mean, none of the components are tied to GND. I don't get it.

Related with the latter, before the first grid there's a 10k + 10nf + 10k tied to V--. Is it yet a RC filter? It's not tied to GND but V--

A the input of the circuit there's a filter 10k + 33nf + 1M to GND. This is the kind of scenario that I understand 

So yeah, a bit lost I am

If you ground the non-inverting input of an op amp, the inverting input becomes a virtual ground because the op amp will provide an output that forces the inverting input to be close to the non-inverting input.  This remains true until the op amp output is forced to go too close to the rails, then the op amp clips, but the signal levels should be kept below this.

V- is a DC level that always remains a certain fixed voltage from ground.  Since the capacitor blocks DC, the AC signal appears to go through the 10K + 33 nF + 1 Meg because in the midband, the capacitor disappears and you just get the 100/101 voltage divider.  The DC levels in the circuit can also be considered to be virtual grounds for AC.

[antonis]

I don't understand the RC filters with opamps... Why the resistor + cap (in series) between opamps is a filter? I mean, none of the components are tied to GND.

Although it isn't correct (from math analysis point of view), it might be better to memorize the below filters:



Left op-amp (buffer) output is considered GND due to negligible output impedance..
The lower the C1 capacitive reactance (XC1) the higher the IC2 gain..
(low R1+C1 equivalent resistance..)

Right op-amp (IC2) inverting input is considered GND for reasons pointed out by Ron above..
The higher) the C2 capacitive reactance (XC2) the higher the IC2 gain..
(high C2//R2 equivalent resistance..)

P.S.
Strictly speaking, both R1/C1 and R2/C2 are HPFs but the later acts in a "reverse" mode due to its placement inside the NFB loop..

edit: modified scheme for filters GND distinction,,