What is the purpose of this diode?

[slowpogo]

This is a Tone Bender schematic, from the DFX Screaming Life (JHS Bunrunner clone). What is the purpose of the circled 1N270 diode?

[antonis]

IMHO, D5 prevents issues with elevated temperature, avoiding biasing problems..
(same "leaking" current with Q3 hence bias compensation..)

[slowpogo]

I see. Is the forward voltage rating crucial or would any Ge diode work there? I've got a bunch of russian diodes (D18 I think), would be great if I could use one there.

[Rob Strand]

Antonis could be right as well but it's not uncommon to add diodes like this to prevent blocking distortion which can sound bad.   First notice the diode polarity is opposite to the transistors "diode" base-emitter junction.  If you don't have the diode the 220nF cap charges up in one direction and that will affect the transistor biasing.  With the diode present the 220nF cap will charge in both directions.

At the end of the day you add stuff like this to change the sound, whatever sounds best.

[isobar]

Hi,

I think it is there to hold the voltage at a particular level. The usual voltage drop for a silicon diode is about 0.6 -0.7 volts.

Chris

[isobar]

And I think I should add that this will help bias the transistor to the desired level.

Chris.

[FSFX]

Hi,

I think it is there to hold the voltage at a particular level. The usual voltage drop for a silicon diode is about 0.6 -0.7 volts.

Chris

That may hold true if it was forward biased, however the diode is reverse biased in this circuit and as they are PNP germanium transistors and positive ground then what you say won't be what the diode is doing there. Another thing, the 1N270 is a germanium diode, not a silicon diode.

[PRR]

What is the purpose of the circled 1N270 diode?

Re-draw it for function. Ignore ignorable tidbits.

I choose to re-re-draw it 'plus-up' because I grew-up with tubes, ignored the PNP era, and was forced to understand the NPN era.

The diode goes one way from common. The transistor B-E junction goes the other way from common. This would be a symmetrical soft clipper, except the BIAS resistor offsets it. (There is also NFB through the bias resistor but my 2-cent opinion is "not much".)


There are similar plans to "cancel Base leakage" but as-said, the diode isn't set up for that.

[FSFX]

As the 1N270 is a germanium diode then it will have some reverse leakage current that will be temperature dependant.
This, together with the feedback resistor does affect the bias of that stage compared to if no diode was there.
However, at any one temperature, the bias can be adjusted to compensate for the diode being there or not. Altering the bias resistor value also affects the feedback and so has a small effect on the gain of the stage too.
Whether or not the 1N270 diode actually matches the leakage characteristics of the germanium transistors and adequately stabilises the bias over a range of temperatures is rather doubtful.