EHX Screaming Bird, why it sucks (not)

[Steben]

Hi guys,


I bought a screaming bird for cheap. It is a classic treble booster with a silicon transistor.
I've read a lot of things about it, going from the "fail of the century" to "useful".
So what is so bad about it?
Essentially, it is what all classic TB are: a non-full-bandwith BJT common emitter stage.



It does not feature a bypass cap at the emitter, but a single 390ohm resistor.
The BJT has more gain though than a classic germanium one, so it sure boosts.
The input and output caps are the real culprit here.
I simulated in a simple calculation a classic strat single coil with resistance and inductance at the input and followed all impedances.
Apart from boost at the output and some clipping quality the real difference between treble boosters is made with the caps. Simply because the guitar properties trump the transistor ones.
A rangemaster has 4.7nF at the input, the SB has 2nF. The peak of the boost is around 1.2kHz instead of 2.5 of the SB (with strat). An octave lower. Yes that is audible.
The output cap is not great either, having a low roll off point at +/- 800Hz with the 100k pot. The output alone renders tube screamer honk...

In summary: swapping the caps to 4.7nF at the input and to 22nF at the output is on the table.

[antonis]

It does not feature a bypass cap at the emitter, but a single 390ohm resistor.
Apart from boost at the output and some clipping quality the real difference between treble boosters is made with the caps.

Caps can't do anything by their own..
(just try to alter bias cofiguration resistors values while maintaining same caps values..)

[ElectricDruid]

How does the response change if you simulate a humbucker pickup instead? Does the response change if you change the guitar's volume or tone pot positions?

Some people think that this sort of thing is good because it makes the pedal "reactive", but I'm not amongst them. I'd prefer for a pedal to work the same and sound the same no matter which guitar (or even non-guitar, synth, electric piano, whatever) I plug into it. Fuzzfaces and Rangemasters and stuff are just hopeless from that point of view.

[amz-fx]

I drew that schematic in early 1995 from my hand-drawn original, back when computers were using Windows v3.11, but the actual tracing of the circuit is from many years before that (probably 17 or 18 years). The software to do the drawing was a DOS program because Windows didn't have any drawing software at the time... this was before the Internet became a common thing. I posted it in a Compuserve forum that had a section on guitars, amps, and pedals. Later it was one of the first schematics on my web site that launched in August 1995 about a week or two before Windows 95 was publicly released.

The Screaming Bird was a very small box that had no stomp switch, and the Screaming Tree was the same circuit in a stompbox format. I still have that pedal in a box in my storage building. I am from the "never sell a pedal" club. 

It was too thin sounding for my rig, and I don't remember ever using it at a gig.

Best regards, Jack

[Steben]

How does the response change if you simulate a humbucker pickup instead? Does the response change if you change the guitar's volume or tone pot positions?

Some people think that this sort of thing is good because it makes the pedal "reactive", but I'm not amongst them. I'd prefer for a pedal to work the same and sound the same no matter which guitar (or even non-guitar, synth, electric piano, whatever) I plug into it. Fuzzfaces and Rangemasters and stuff are just hopeless from that point of view.

Pickups: yes that changes a lot. Changing to R and L of Typical humbuckers drops the peak far below 1k.
I for one do like these low Z stages. Volume roll off etc.

[Steben]

I drew that schematic in early 1995 from my hand-drawn original, back when computers were using Windows v3.11, but the actual tracing of the circuit is from many years before that (probably 17 or 18 years). The software to do the drawing was a DOS program because Windows didn't have any drawing software at the time... this was before the Internet became a common thing. I posted it in a Compuserve forum that had a section on guitars, amps, and pedals. Later it was one of the first schematics on my web site that launched in August 1995 about a week or two before Windows 95 was publicly released.

The Screaming Bird was a very small box that had no stomp switch, and the Screaming Tree was the same circuit in a stompbox format. I still have that pedal in a box in my storage building. I am from the "never sell a pedal" club. 

It was too thin sounding for my rig, and I don't remember ever using it at a gig.

Best regards, Jack

I love stories. Thx.
It IS a bit thin. I am planning the cap swap.

[Steben]

This is a simplified calculation without imaginary components.
Upper half is SB values
Lower half is rangemaster values
R1 and R2 in the sheets are bias R's of the booster, not the R's in the drawing.

[Steben]

Changed the caps as said.
Beautiful.

[amz-fx]

Excellent. Thanks for the info.

Best regards, Jack

[Steben]

Needless to say any LPB-1 can have the same swaps.
Or at least the input cap...

[amptramp]

Going to a larger input cap should reduce the noise since below the turnover point, the transistor and its bias resistors are isolated from the input whereas above the turnover point, the output resistance of the guitar is in parallel with the input resistance of the booster.  Putting these resistances in parallel creates a lower resistance with lower thermal noise.  The output cap shouldn't matter as much since the resistances are lower.

[pinkjimiphoton]

yep, changing cap values is hugely under rated sometimes.
the only real dif between fuzz and treb boost is the size of the input caps sometimes.
if ya want a more overdrivey kind of sound on this circuit, try a 10n input cap and a 3.3n or 4.7n output cap... more bass = more grit,
but the output cap tames it back down a bit.

i LOVE stupid simple transistor circuits. so much tone people never bother to look for in them.

[Steben]

yep, changing cap values is hugely under rated sometimes.
the only real dif between fuzz and treb boost is the size of the input caps sometimes.
if ya want a more overdrivey kind of sound on this circuit, try a 10n input cap and a 3.3n or 4.7n output cap... more bass = more grit,
but the output cap tames it back down a bit.

i LOVE stupid simple transistor circuits. so much tone people never bother to look for in them.

One of the first designs I ever built myself was a Rangemaster followed by a second transistor stage, directly coupled.
A high gain treble boosted fuzzdrive was the result.
https://www.angelfire.com/droid/burvenich/fat_germs.htm
Silicon transistors are very likely to be just as interesting here.