Bazz Fuss Transistor Comparisons? Help Needed

[Bunkey]

Hello all,

This is my first post in the forum so please bare with my verbose writing style, I get to the point eventually!

I'm prototyping a pedal based on the V1 Bazz Fuss arrangement with the single transistor, 100k resistor, C-B diode, 100/470nF in/out caps & a pot on the output, plus a few personal tweaks.
The circuit I'm working with is fully functional but I built it from parts I pulled from an old VCR so I've used a 2sD471 transistor ('K' 200-400 hFE) instead of the generally accepted types.
Datasheet: https://smarttronik.com/index.php?controller=attachment&id_attachment=368

I'm not familiar with fuzz pedals like many of you are but I do love the cascading wall of sound present in records from the likes of Sleep, Kyuss and the RHCP 'Body of Water' B-Side - to my ear it's beginning to tick the right boxes and I'm now in a position where I wish to replicate the circuit..

I've come across some very helpful posts on the site from Mac and others explaining how the circuit works and to my understanding the diode-induced biasing and clipping is responsible for most of the character. Given this, is the transistor used affecting the character much at all? I'm unsure if I can confidently order a lot of d471-type transistors or if I'm missing out on better sounding options that I'm not aware of?

I don't have a great deal of money to experiment so I'm hoping somebody with a bit more experience has some input on this?


Many thanks
Ben.

[duck_arse]

Bunkey, welcome.

even hFE ~400 will have you missing some of the sustain available. get yerself a small signal darlington, like the MPSA13 - OR - for extra points, look up the darlington configuration and then lash two of your D471's together as a discrete darlington. there are literally hundreds of type numbers that will do for your darlington in this circuit.

also - I see an interesting build method there, can we have a clearer picture of it, please?

[Bunkey]

also - I see an interesting build method there, can we have a clearer picture of it, please?

Ah thank you!
I'll order a job lot of the D471's and give your suggestion a try. Though I have to say I'm getting a lot more sustain through the amp's gain channel even with the single transistor!

Since you were so kind as to assist:




This was merely a product of using what I have available.

The 'board' is a piece of high density foam I believe is commonly used in sign-making - I'd used this previously to blank the neck pickup cavity on my guitar hence the drilling - it does not like heat (but fortunately it would eventually melt as opposed to combust!).
I poked holes through it for the component legs then connected everything together on the opposite side using bits of bridging wire taken from the VCR pcb - The resistor legs were trimmed too short to poke through sufficiently so I pushed the bridging wires up through the board and hung the resistor between them on top like a washing line 

The whole ordeal was somewhat fiddely and the solder I had was much too much for the task but it got the job done!


Thanks again.

[Bunkey]

You'll have to excuse me for blowing the dust off this thread but, having tried a few different types of diodes, I see what you mean now about the sustain.

Some diode/single transistor combinations sound very gated even with a ~650mV forward voltage. Based on snippets of info picked up here and there, I think its mostly to do with the transistor biasing (I'm no expert).

If any future peoples stumble across this post:
I had very good results in this regard with the first diode I tried which was a small Zener diode (forward voltage of 850mV) in parallel with a 100pF capacitor; maybe something to try if you want more sustain from the single transistor circuit. Through the amp's gain channel it had a modern, smooth & compressed high gain sort of sound and held single notes for long enough to put a smile on my face at least 

Ben.

[anotherjim]

It's a fun little circuit you can do a lot with, although it can be too fizzy at the top end to my ears.

Adding a capacitor across the collector-base biasing diode makes a huge difference because of the Millar effect which magnifies the capacitance in line with the gain. The effect is less treble which can help a lot. Even the different capacitance of diode types, although small, can be heard. If you use a high gain Darlington, try a 10M resistor instead of a diode. It's not really a Bazz Fuss then, but a very easy way to get that old overdriven BJT pre-amp sound that some of us pine after.

On Darlingtons, the MPSA14 is I think higher gain and according to the manufacturer preferred for new designs over the 13 and the cost's about the same anyway. My stock are 14's anyway so I don't know any better.
If you think the gain could be too high, you can always reduce it by adding an emitter resistor up to 1K. You could make that variable but it will crackle when moved as the DC conditions change.

[Bunkey]

Thanks for your addition Jim!

I had a switchable 3k2 resistor on the emitter with the zener/cap combination for this reason.

Though I use a single active EMG 81 with an 18v supply so the input is very hot!

In practice the audible difference was definition and the ability to do 'hard stops' for rythm playing and chugging palm mutes - with the resistor.

Switching it off unleashed all manner of hell. Its noisy, unrestrained and amazing for crazy solos - Like weilding a fire breathing dragon.
It screams like a demon.
And other such unholy imagery.

[bool]

mpsa13 will sound a little smoother here (than mpsa14); you can also add a resistor (say a 10k trimpot..) in series with the diode. you can also add a resistor in series with the input..

[Bunkey]

mpsa13 will sound a little smoother here (than mpsa14); you can also add a resistor (say a 10k trimpot..) in series with the diode. you can also add a resistor in series with the input..

What effect does adding a resistor in series with the diode have, Bool?

I believe the original design had the pot on the input (before it was moved to the output as a volume control) - which would be useful in attenuating the signal from hot pickups.

[antonis]

What effect does adding a resistor in series with the diode have?

It "adds" a bit of extra voltage drop in series with diode's forward voltage drop..
(so, Collector sits a bit higher than with single diode in NFB loop..)

[Bunkey]

What effect does adding a resistor in series with the diode have?

It "adds" a bit of extra voltage drop in series with diode's forward voltage drop..
(so, Collector sits a bit higher than with single diode in NFB loop..)

Ah thank you!

I have a diode combination I really like the character of but it doesn't seem to bias the transistor quite right, sounding very raw and gated - Maybe this is the ticket.

I'd welcome any further comments on this issue.

Thanks again.

[anotherjim]

I think the collector voltage should give a clue as to how clean it might be. The closer it is to half supply voltage then the cleaner it gets and the distortion becomes more symmetrical. Stock, with just a single feedback diode, you might find the collector voltage is quite low. Increasing diode forward voltage or adding in resistance should tend to raise the collector voltage.

A series resistor in the input can...
Make it work better if a buffered pedal or preceding stage has to go in front.
Lower treble by forming an RC low-pass with the Millar cap.
Reduce the drive by forming a voltage divider with the input impedance.
It can go either side of the input capacitor, so it's easy to try adding the resistor to one that's already built. It can be a pot wired as a variable resistor. A 47k or 50k pot might be a good one to experiment with.