Author Topic: Resistors for Mangler  (Read 1201 times)

Bluestrat

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Resistors for Mangler
« on: October 06, 2011, 10:50:57 AM »
Hi, I'm getting ready to build a MB "Mangler" and will be using an external bias knob instead of the trim pot. I'm using a matched set of PNP trans from Small Bear. These come with recommended resistors to achieve the correct bias. My question is: If I'm able to bias on the fly with the bias knob should I use their recommended values or is that not necessary given that the bias can be adjusted. Thanks in advance!

sgmezei

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Re: Resistors for Mangler
« Reply #1 on: October 06, 2011, 12:36:11 PM »
Well if you have an external bias knob you will not need to use the resistors. The resistors are to properly bias the transistor to 4.5v I believe. Most projects around here use a trimpot to dial this in. I have read that having the pot on the outside is not always beneficial because a large amount of rotation is "useless" sound wise.

I have wondered about this though and hopefully some others will comment. Maybe they could also comment on the power starve idea of under or overpowering transistors as well.

Bluestrat

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Re: Resistors for Mangler
« Reply #2 on: October 06, 2011, 01:47:53 PM »
Well if you have an external bias knob you will not need to use the resistors. The resistors are to properly bias the transistor to 4.5v I believe.

Thanks. That's my take as well. I'm installing the exterior bias partly for looks and partly as an experiment. This is my first Fuzz Face build so I'm still learning.

mgwhit

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Re: Resistors for Mangler
« Reply #3 on: October 06, 2011, 07:06:07 PM »
Well if you have an external bias knob you will not need to use the resistors. The resistors are to properly bias the transistor to 4.5v I believe. Most projects around here use a trimpot to dial this in. I have read that having the pot on the outside is not always beneficial because a large amount of rotation is "useless" sound wise.

I have wondered about this though and hopefully some others will comment. Maybe they could also comment on the power starve idea of under or overpowering transistors as well.

I recommend using a 10K trimmer and a B5K external pot in series.  The original Fuzz Face used an 8.2K resistor, so this total of 15K (10K+5K) will get you pretty close to the value you need with median settings (5K + 2.5K = 7.5K).  The best way to set this up is to set the external pot at half-way (~2.5K), then bias to 4.5v (at room temperature) with the trimmer.  After you box it up you can go 2.5K in either direction with the external Bias knob.

The goal really isn't to produce usable values over most of the pot's sweep (although there is certainly some value in that) as much as it is to give yourself enough variance to be able to dial in an appropriate bias in exceptional environmental (i.e. non-room-temperature) conditions.

If SmallBear recommends a value significantly higher or lower value than 8.2K for your transistor pair, feel free to adjust the trimmer or external pot values appropriately.