Dumble BBC-1 Schematic+Layout

[UltraHookedOnPhonix]

After spending a couple of hours looking at two poor-quality gut shots, I was determined to find out what the Dumble BBC-1 really is and I’ve concluded that it’s basically the FET input of an ODS in a non-bypassable stompbox format (big surprise eh?)

Now I need your help! The circuitry from the input of the effect to the Gate of the JFET is pretty murky in the pictures and I’ve listed a couple of options in the schematic below. What do you guys see? The “Depth” control would to me either suggest a variable impedance circuit, variable input gain or a variable high pass filter. I can’t really make it out…

Also, parts with a (?) after them are components with unknown values. However, since the values I’ve figured out correspond with that of a standard FET input on an ODS, I think it’s safe to assume that there are no big surprises elsewhere except for the input circuit.

I’ve found that the BBC-1 is really close to the Barcus-Berry Hot-Dot Box both layout wise and circuit-topology wise. Wonder which came first?

[drz400]

Is the drawing accurate?
Do you have the pictures larger?

I'm sure not understanding the need for the high gate resitance, this always puzzled me on the original FET input

[UltraHookedOnPhonix]

Is the drawing accurate?

To the best of my ability. It was drawn looking at the two pictures you see. I need help checking it, feel free!

Do you have the pictures larger?

No. These were taken from an Ebay auction long ago.

I'm sure not understanding the need for the high gate resitance, this always puzzled me on the original FET input

Yeah, that's something that's always struck me as a bit strange also. The Barcus Berry preamp doesn't have it though. With the supposed 100pF capacitor bypassing it, it creates a high pass filter at around 1.6kHz.

[pureoldsound]

there is a bit bigger pic here

http://dam.10.forumer.com/a/sonic-titan ... st508.html

[drz400]

Is the drawing accurate?

To the best of my ability. It was drawn looking at the two pictures you see. I need help checking it, feel free!

Do you have the pictures larger?

No. These were taken from an Ebay auction long ago.

I'm sure not understanding the need for the high gate resitance, this always puzzled me on the original FET input

Yeah, that's something that's always struck me as a bit strange also. The Barcus Berry preamp doesn't have it though. With the supposed 100pF capacitor bypassing it, it creates a high pass filter at around 1.6kHz.

Yes but it really shouldn't matter with such a high input impedance of the FET. A small gate resistor is OK for static protection.

[pureoldsound]

This might help...I found it on the net....

[UltraHookedOnPhonix]

there is a bit bigger pic here

Nice, a better quality pic. With this pic I can confirm the existace of the 3.3M resistor which is actually a 3.5M, 10% resistor. Cool! Also, I can see that the bypass cap on the source resistor is clearly marked "10/12". 10uF, 12V.

This might help...I found it on the net....

Yeah, That's the FET input of an ODS that I was referring too. I took it for granted it was common knowledge. It's been in the files section of this forum since the stone age.

Yes but it really shouldn't matter with such a high input impedance of the FET. A small gate resistor is OK for static protection.

If you want the same frequency response with a 1K gate resistor, a .1uF cap could be used.

[FUCHSAUDIO]

How about "BBC" actually means "Barcus Berry Clone"....lol. Maybe he's laughing up his sleeve as we write.

The high input impedance is required for the old piezo transducers in the 70's, which required a very high input impedance to work full range. The idea was to present a very high input impedance and some gain as required.
Without it, a 1-M input impedance would have no low end, and a low signal.

Obviously, something else HAD was inspired by along the way.

[UltraHookedOnPhonix]

How about "BBC" actually means "Barcus Berry Clone"....lol. Maybe he's laughing up his sleeve as we write.

Bingo! Sounds about right haha!!!

The high input impedance is required for the old piezo transducers in the 70's, which required a very high input impedance to work full range. The idea was to present a very high input impedance and some gain as required.
Without it, a 1-M input impedance would have no low end, and a low signal.

I don't know my semiconductor history to well but I suppose that HAD didn't have access to MOSFETs at that time. He could with ease have set up a MOSFET stage with a 10M input impedance. Say...that would be kind of cool to try. Everyone, gut your Zvex SHOs and transplant them into your ODS builds...NOW!!!

[FUCHSAUDIO]

http://www.muzique.com/misc/mosfet11.gif

http://www.muzique.com/misc/mosfet12.gif

Small signal Mosfets go back to the 1967 and 1968 or so...

The ten-meg is only for the transducers benefit. A "regular" guitar pickup seeing that impedance wouldn't likely sound as good as it should. A few places (I think actual Dumble literature) stated this input was for a piezo transducer, so it all makes sense why this style of (or particular) circuit may have been chosen. Those types of transducers (the Barcus Berry and ATD devices) were fairly new at the time.

[UltraHookedOnPhonix]

Speaking of Jack Orman, I really like his MOSFET boost in front of practically any amp. A really nice booster. It might be worthwhile subbing this ckt for the FET one?

[FUCHSAUDIO]

That's a tolerable boost. Just add a 1-M to ground on the input (pre coupling cap), and your guitar should be happy. That has a 10-M or so input impedance as-shown, just like the Dumble Fet circuit.

[Bernardduur]

I think the posted schematic is quite on the spot. The 3m3 to ground both biases the FET as makes the piezo work best. The potmeter as a variable resistor is a great way to make the overal gain clean up; this mod is often used in the Fuzz Face:

from the technology of Fuzz Faces:
If you put a resistor in series with the guitar pickup, it raises the apparent source impedance of the pickup, making it look more like a current source (albeit a tiny one) and less like a voltage source. A signal from a current source lets the input of the effect seek it's own voltage level, and merely supplies a varying current. This can be much more linear than a voltage source drive. As a result, the variable resistor allow you to radically clean up the distortion that the FF produces, producing subtle shades of softer distortion.

The 10 uF was already posted above.

The value of the output potmeter can be guessed as 10k. The depth potmeter can vary from 50k to 1M

Link to the BB Hot Box discussed at Freestompboxes.org

[jelle]

Rogier,

Thanks for chiming in!

jelle