Joe Hart's "Bat Boost" Schematic for the 666 Pedal Competition

[Joe Hart]

Okay, after building the pedal for the competition, I (like always) have tinkered with the circuit I designed and came up with this schematic. It's different from the competition entry, but I think it's an improvement. Also, I am not including the tone stack section from the pedal version.

I started with a simple single transistor booster, but then expanded/modified it from there. And I used a lot of different component values because I used different transistors from my competition entry. So I guess this would be Mark II.

Anyway, does this look like anything? Did I simply reinvent the wheel?
-Joe Hart

[tubelectron]

Hi Joe Hart,

I loved the decorum of your Bat Boost as seen in the pictures section - very kind (an not diabolically awful) for me.

Your circuit is interesting : I wonder if it produces distortion - As I am more specialized in electron tube amplification, I am unable to confirm if it is another wheel or not, but there is one peculiarity as the 2nd Q seems not to be biased - Is it possible to know which Q's you have used with success in your circuit : type ? measured Hfe ? I'd be interested to make a trial.

Thanks & A+!

[Joe Hart]

Hi Joe Hart,

I loved the decorum of your Bat Boost as seen in the pictures section - very kind (an not diabolically awful) for me.

Your circuit is interesting : I wonder if it produces distortion - As I am more specialized in electron tube amplification, I am unable to confirm if it is another wheel or not, but there is one peculiarity as the 2nd Q seems not to be biased - Is it possible to know which Q's you have used with success in your circuit : type ? measured Hfe ? I'd be interested to make a trial.

Thanks & A+!

Thanks for the kind words! It does produce some distortion. I was mostly testing it with an already dirty amp, and that tends to mask some things (but highlights others!), but I tried it out with a clean amp and it seems to be a bit more of a bass boost and has a but of overdrive to it. Much less than a Tube Screamer, but there's still some dirt to it. But when I turn my amp's distortion up, the pedal rocks!

Anyway, I do not know what transistors I am using because I am using "recycled" ones, so all they have are part numbers, but the first one is about 200 hFE and the second one is lower, about 140 hFE. Even more peculiarities! I know in Fuzz Faces, the second transistor has higher gain. And in Tonebenders, the first two are about the same and the third one is higher.

I don't know enough about electronics theory, but maybe the 10K resistor is kind of biasing them both? Or maybe I've piggybacked them in some strange way, so they are acting more like one extended transistor rather than two separate gain stages?

Now, you have me very curious as to what I've done. You clearly have more knowledge about this stuff than I do. Hmm...?
-Joe Hart

[~arph]

I simulated it in spice.

It cuts some bass as expected. The interesting thing is indeed the second transistor. It looks like it is not in the audio path, as you are taking the output directly from the collector of Q1 through the 22nF cap. So indeed when I simulate it with the second transistor stage removed the results are exactly the same.
Also it does produce distortion as Q1 is misbiased. It clips the top of the wave. I'd say it's a lucky experiment

EDIT: for spelling

[Joe Hart]

Ahhh. Crud! I just tried it without the second transistor and it does sound the same. So I basically invented a poorly built (misbiased) LPB-1. How did you know it was misbiased? I thought different transistors had different bias points, so how can you tell just on paper?

Also, it sounds great with the guitar volume full up, but if I cut it down even to "9" the sound thins out tremendously. What can I do to change this? I've tried different value input and output caps, but can't get rid of this problem. I do have a treble bleed circuit on my guitar's volume pot, but the volume works great with everything else (different pedals, different amps, any combination of them). Any ideas?
-Joe Hart

[Brymus]

Hey Joe I too love your Bat Boost very original and clean build.
I remember your thread asking what to do with that high gain Ge.
Have you tried clippers to ground on the output ?
I have had really good results doing this with a Rangemaster variant.
And your Ge has way more gain than the ones I use.
Dont despair ,if it sounds good then its done right !
You could wire it to get more gain from Q2 and try the clippers.
I still have a hard time understanding transiter biasing and have to refer to text to keep things straight.
But basically the way you set the Q point dictates how much voltage swing you have on your output.
So if you set the Q point high then you have a large swing on the bottom (doesnt clip) but very little room to swing on the top (likely to clip the top part of the wave form)
Thats why for the greatest voltage swing(unclipped) you set the Q point in the middle.
this depends on how much current gain you want,ect,ect
A good way to think about it is your bias voltage (half Vref equals even swing) like on an op amp.
You would use two equal resistors to set your bias V like a BJT buffer.
I hope that helps.

Again nice build,I was very impressed.The glow in the dark sponge paint was genius !

[tubelectron]

Ah, Joe - The Spice Simulation done by ~arph confirms that the 2nd Q was misbiased (I would say non-biased, because there is no DC connection to the base, but I may be wrong here, as I am a "tube specialist"), but I did not guess it was rather useless... That's the merits of the simulator.

As you where speaking about Tone-Benders, a quick shot of my Double Tone-Bender, for the occasion :



A+!

[Joe Hart]

Dont despair ,if it sounds good then its done right !

I still have a hard time understanding transiter biasing and have to refer to text to keep things straight.
But basically the way you set the Q point dictates how much voltage swing you have on your output.
So if you set the Q point high then you have a large swing on the bottom (doesnt clip) but very little room to swing on the top (likely to clip the top part of the wave form)
Thats why for the greatest voltage swing(unclipped) you set the Q point in the middle.
this depends on how much current gain you want,ect,ect
A good way to think about it is your bias voltage (half Vref equals even swing) like on an op amp.
You would use two equal resistors to set your bias V like a BJT buffer.
I hope that helps.

Again nice build,I was very impressed.The glow in the dark sponge paint was genius !

Thanks for the compliments with the build. It was my first hand painted box (I've only used "full face" stickers before -- or nothing at all!) and I like how it came out. Thanks.

Anyway, about the biasing, so if I made the 10K and the 1M resistors the same (say both 10K), then it would be more biased? Then I could adjust the resistor going to the collector to fine tune it? Or would I be better off making them both 1M? I don't know if changing the original 1M would throw more things off by keeping the 100K to ground (which would change the relationship between those two resistors at the base of Q1? Thank you for the explanation!!
-Joe Hart

[Brymus]

Well thats where it gets tricky,you have to keep in account your input Z as well.
Using 10K's with out bootstrapping would make for heavy loading on your guitar.
using 1Ms might have other effects as well,aside from raising your input Z.
And I am not advanced enough to guide you through what needs to be done.
But some else on the forum can,there are several members who can help if they are so inclined.

Good luck with the contest !
You get my vote for most improved builder.