Electra Distortion transistor bias

[m7b52000]

I have breadboarded the Electra using the schematic and component values from Beavis Audio. Modelling the circuit in CircuitLab and directly measuring the voltage at the 2N3904 collector both give around 7V. This seems high. CircuitLab suggests that I would need a 50K resistor at R2 to achieve 4.5V at the collector. Am I misunderstanding something?

[Rob Strand]

I have breadboarded the Electra using the schematic and component values from Beavis Audio. Modelling the circuit in CircuitLab and directly measuring the voltage at the 2N3904 collector both give around 7V. This seems high. CircuitLab suggests that I would need a 50K resistor at R2 to achieve 4.5V at the collector. Am I misunderstanding something?

That schematic has been distorted by the internet.  There's quite a few different schematics around for that pedal all with different values.  Some have a 2N3904 and R2 as 47k.    There's also some with different values for the other resistors.   You might even find circuits with 2N5088 transistors which have higher gains and will require different resistors.    Different transistors will affect the sound regardless of the resistors as it can affect the input impedance of the circuit and hence affect the pickup loading.

As for what was the original I haven't got my stash of stuff with me but I know this problem has come up before and you might be able to search the posts on this forum.  I think there was a thread not long ago.

More generally, while I don't think it's the case here, you can bias transistors between  4.5V and say 7V.  The tone can change a bit.


EDIT:           [*** As it turns out this is *not* the original it has been edited and the 3M3 is probably wrong. 
                            See reply #9 below.]

Here's one of the earliest schematics that was around over 20 years ago,



An even though it was the original it could still be wrong!!!

[Buran1997]

You could find the correct value by trial and error. Try replacing R2 with a trimpot, then adjust it until the collector is sitting at 4.5V. Measure the resistance across the trimpot and then pick the resistor that most closely matches that value.

Also, when breadboarding, if you're running your circuit off a 9V battery then you should measure the voltage across the battery terminals in case it's run down, then bias to half of that voltage, e.g. if the battery gives 8V then you bias to 4V, if it gives 6.5V then you bias to 3.25V, etc.

[Rob Strand]

Well, I'm a little confused now.  There are is a distortion module and an overdrive module for the Electra guitars.

See here for some pics,
http://www.rivercityamps.com/electra/wire.php
http://www.rivercityamps.com/electra/modules.php

The MPC 5 Overdrive which has no diodes and one transistor.
It seems to use 4k7, 2M2, 470R and also maybe 120k in series with the input.
Transistor type unknown.

The MPC 2 Dynamic Fuzz which has three transistors and what looks like 2x silicon diodes.
I can't make out the schematic but it looks like it has 47k's on the board.
The underside pic of the board is wrong, it's the MPC 5 pic not the MPC 2.

So there's no single transistor module with 2x diodes.

So what is the "Electra distortion" schematic as we know it?

Another guitar?  A modded MPC 5?  A part of the MPC 2 schematic?

The text on the old schematic says "Power Overdrive Module" with the exception of the diodes.
So it must be a *modded* version of the MPC 5.   The input series resistor has been removed.

Still hanging is the transistor type.
-------------------------
EDIT:  I fixed a few things in this post.

[j_flanders]

So there's no single transistor module with 2x diodes.

This post seems to imply there actually was such a thing:

These values come from an actual Electra distortion module that came in my LP style Electra from early mid 70's
4.7k instead of 47k
470R instead of 680R
1N260 & 1N4001 for diodes
Socket some medium gain transistors, remember these they were imported from japan.
Mine had an unmarked transistor with a yellow dot.
You really have to own one of these guitars to get the real effect,  I think this circuit was really taylored around their special design pickups. Super Magnaflux I believe,known for their extended sustain.

https://www.diystompboxes.com/smfforum/index.php?topic=110302.msg1011376#msg1011376


Overall great thread which explains the components values btw.
https://www.diystompboxes.com/smfforum/index.php?topic=110302

[Rob Strand]

This post seems to imply there actually was such a thing:
..
Overall great thread which explains the components values btw.

Good stuff.  It's another one of those simple circuits with a twisted history!

(PS I edited my post because I missed one point.)

[Rob Strand]

The thread mentions an unmarked transistor with a yellow dot.  For US transistors the yellow color could mean a gain of hFE = 225 (nominal).   The Electra transistors are Japanese so it could mean an entirely different thing.   Maybe there's a Toshiba transistor datasheet with the color codes on it - can't find one yet.

[Rob Strand]

Just for completeness, I found another crack.

The "original/old" schematic shows 3M3 but pic of the MPC 5 PCB module sure looks like 2M2.

In the thread brianq doesn't give a correction for the 2M2, however the schematic in that thread the schematic has a 2M2 so maybe that's why he didn't mention it.

With hFe=225 (assumed, not confirmed), Rc = 4.7k, Re = 470, Rbc = 2.2M I estimate a collector voltage of about 6.4V which is entirely believable.

[Rob Strand]

Examples of hFE Classification Codes from Toshiba

The Color Coded hFE classifications don't correspond
to specific hFE ranges, they just are broad classifications.
The ordering is the same as the resistor codes.


R ~ 80, O ~ 135, Y ~ 220, G ~ 300, BL ~ 450


Color      hFE range   Source

Red (R)      55  to 110   Toshiba 2SC5200

Orange (O)   70 to 140   Toshiba 2SC2229
Orange (O)   80 to 160   Toshiba 2SC5200
Orange (O)   100 to 200   Toshiba 2SC2236

Yellow (Y)   120 to 240   Toshiba 2SC2229
Yellow (Y)   120 to 240   Toshiba 2SC2705 / 2SA1145 ;unverified
Yellow (Y)   160 to 320   Toshiba 2SC2236

Green  (GR)   200 to 400   Toshiba 2SC732TM

Blue   (BL)   350 to 600   Toshiba 2SC732TM

[Rob Strand]

Well I've discovered the "original" schematic in reply #2 has actually been edited.   You can see the part values have a different font.

Here's the original schematic.  Notice the font.

*** Note:  This schematic has a bug. The collector-base resistor is marked 2.3M.
     Based on the pcb pics, I'm pretty sure this should be 2.2M (2M2).





Click to get larger image:

[Rob Strand]

To put this thing to bed.

Electra Distortion - Bias Test
Rob S, 25 March 2020

Measured voltages with real devices.

Parts:  RC = 4k7, RBC = 2M2, RE = 470R
2N3904:   lower gain than usual
BC237:      relatively high gain

     2N3904   BC237
hFE     110      360
Vcc      9.26V      9.26V
Vc     7.64V      5.66V
Vb     0.796V     0.979V
Vbe     0.636V      0.628V
Ve     0.164V      0.359V

Vb, Vbe:  measured with 10M meter impedance
(hFE calculated from Vc, Vbe with test circuit:  RE shorted, RBC connected  Vcc)

Midway between these measurements is hFE = 235 and Vc = 6.7V.

[m7b52000]

Thanks Rob (and others)
The short summary of all of this is that the "standard" component values do produce a high-ish collector bias voltage. Increasing the value of the 4K7 will lower the collector voltage toward the "ideal" 4.5v. This may or may not sound "better"

Is that a fair assessment?

Thanks again

[Rob Strand]

The short summary of all of this is that the "standard" component values do produce a high-ish collector bias voltage. Increasing the value of the 4K7 will lower the collector voltage toward the "ideal" 4.5v. This may or may not sound "better"

Is that a fair assessment?

Yes, exactly.   The 4.5V value isn't always ideal.

I was just trying to get to the bottom of what the thing really was.   That's always a good starting point. (and original circuits can often differ from Vc=4.5V).

There's nothing stopping you playing around with the values to get the sound you like.  That could be a collector voltage of anything from 2V to 8V according to taste!

Notice also that even if you stick with the original values the final behaviour can vary due to the transistor gain.

If we assume that the original unit was around Vc = 6.7V, then it is possible to adjust Rc (4k7) or Rbc (2M2) to get 6.7V (or other) to compensate for the transistor differences.   This is a very common method to kick circuits into line, pretty much along the lines of Buran1997's post.

[rankot]

Maybe off topic, but I was thinking today how to use some of low gain Ge PNP transistors I have - they have hfe from 20-50, so they're not too useful for Fuzz Face or similar circuits. So Electra came in mind, but I didn't want to use positive ground, so it can be daisy chained with other pedals. Came to this by trial and error. So, before I start breadboarding, what do you guys think, is this reasonable circuit or not?

[Rob Strand]

Came to this by trial and error. So, before I start breadboarding, what do you guys think, is this reasonable circuit or not?

It's reasonable but there's a few gotchas.

Low gain transistors inevitably make the input impedance of the stage low which can muddy the signal.  One way out is use large emitter resistors.  Your large emitter resistor helps biasing but because it is bypassed with a cap the loading effect is still there.  You can trade off between gain and loading by adding a non-bypassed emitter resistor.  Another way is to add an input buffer.

The biasing set-up depends strongly on the transistor leakage.  The simulation model has a fixed leakage by perhaps not representative of all devices.

[rankot]

Thanks Rob, glad to see you're here again!

In that case, will try with a buffer, why not! I have noticed that R6 is the most important for proper biasing, but will try to mess with other resistors, too. I was just curious if this setup has any obvious mistakes before I start building it.

[iainpunk]

i'd use the low gain Ge's for the buffer as well. it gives some mojo points at the cost of input impedance. a more conventional modern Si transistor as an input buffer is better, higher input impedance, but less mojo

cheers