How to increase gain (not volume) in an Electra pedal

[jfrabat]

Hi, guys.  I am building an offshoot of an Electra pedal with tone controls.  Here is what I am making:



Some clarifications:

• In/Out 10nF filters are on purpuse (started at 68nF, but went down and down on the breadboard until I hit 10nF and I liked the sound)
• Parallel caps are because that is what I have in stock (did not have the required one, but had the pair)
• OpAmp is actually a TL072 (did not have that on my Eagle, so used the OP249 for the drawing)
• BAT41's are used instead of BAT42, and 1N34A is in place instead of 1N4148 (again, used what I had in my Eagle library)

Anyway, what I am looking for is more gain from the pedal.  Volume level is sufficient, but I want to increase gain.  I tried 2N5088, 2N5089, 2N2222 and 2N3904, and I liked the gain on the 5089 the best, but I want more. 

I also tried another Electra before and after the current one, but I do not like the sound.  Is there a better way?

[PRR]

> Volume level is sufficient, but I want to increase gain.

"Volume level" is mostly the diodes (and any gain after them).

Sounds like you want more gain in T2.

There's only so much gain you can get from one transistor. In this case, reducing the 10k pot reduces the input impedance and loads-down the pickup.

Since you let an opamp in already, you may as well change T2 stage to an opamp stage.

[Rob Strand]

It's very hard to increase the gain without changing something.   Higher supply voltage is one way but it may need adjustments.

So here's one idea which might help give a bit more gain.   I don't know if it's enough for what you want.  The idea is the highest gain of the Electra is the ratio of the 2.2M resistor and the guitar impedance.  The idea is to increase the 2.2M to 10M to increase the gain.   The 2x 1.2M keeps the biasing the same. 

You will need to *decrease* the input cap to get the *same* sound.

I can't guarantee it will work you will have to try it.   I've tried to minimize the effect on the tone but you will need to test that too.    For example when you crank the gain the cut-off becomes higher.  This mod preserves that behaviour.



The problem is a bit like trying to squeeze more juice from a dry lemon. Other changes would be more drastic and really require redeveloping the whole circuit.

You might be able to increase your R5 value to 220k and R6 to 1MEG to squeeze a little more.

[antonis]

Additional to all above said, it might worth try to ground T2 gain pot wiper via a capacitor (10 - 47μF, say..).

It doesn't actually raise T2 gain rather than it "smooths" it relatively to Collector bias..
(there shouldn't be any mis-bias according to gain pot setting anymore..)

In any case, you can't get more gain that that of full gain pot setting (which, in theory, is total Collector load X Collector current divided by 0.025..)

What Rob said about raising 56k & 300k resistors should work a bit (or more than a bit..) 'cause 56k is effectivelly set in parallel with 68k Collector resistor resulting in about 30k Collector load..

[jfrabat]

Ok, I will try the R5 and R6 changes, along with the 10~47uF cap first.  If that does not work, I will go for Rob's other suggestion.

[jfrabat]

Ok, I will try the R5 and R6 changes, along with the 10~47uF cap first.  If that does not work, I will go for Rob's other suggestion.

That worked!  Thanks!

[jfrabat]

I am pissed off...  Designed the boards on Eagle, waited for them to arrive, used all the parts from the breadboard (which was working FINE!) and now the pedal, after soldering everything up, is not working!  I am just ranting.  I know what I have to do (go through it with the audio probe, look for shorts, bad solder joints, etc.) but it just pi$$es me off when this happens (which is more often than I would like to admit!).  Anyway, I just hope it is one of the above and not a missing trace (which has also happened to me!).

[Rob Strand]

Designed the boards on Eagle

If you are using parts from Eagle that you haven't used before sometimes you can get problems with pins being reversed.

[jfrabat]

Designed the boards on Eagle

If you are using parts from Eagle that you haven't used before sometimes you can get problems with pins being reversed.

Well, got the audio probe out today, and this is the weird thing; if I connect the tone generator to the pedal, the signal goes through (I can even set it up for parity volume no problem), but I try it with the guitar, no sound on the amp!  Granted, I do not hear any kind of gain on the tone generator, but at least the signal should get through, right?  I mean if it works with the audio probe (using the collar of the out jack as GND and the metal on the jack that touches the tip).  I don't get it!

[Rob Strand]

Well, got the audio probe out today, and this is the weird thing; if I connect the tone generator to the pedal, the signal goes through (I can even set it up for parity volume no problem), but I try it with the guitar, no sound on the amp!  Granted, I do not hear any kind of gain on the tone generator, but at least the signal should get through, right?  I mean if it works with the audio probe (using the collar of the out jack as GND and the metal on the jack that touches the tip).  I don't get it!

One difference could be the guitar is DC coupled but the tone generator is AC coupled.  So when you plug in a guitar it stuffs-up the biasing on the transistor but with the tone generator it doesn't care..  That would point to C4 shorted or inserted the wrong way around.

You could try measuring the voltages around the transistor with the tone generator (perhaps set to a low level) then with the guitar.   If you see large differences in the voltages it's another sign there's a problem around C4.

[jfrabat]

You could try measuring the voltages around the transistor with the tone generator (perhaps set to a low level) then with the guitar.   If you see large differences in the voltages it's another sign there's a problem around C4.

I will give that a shot, thanks!

[jfrabat]

One difference could be the guitar is DC coupled but the tone generator is AC coupled.  So when you plug in a guitar it stuffs-up the biasing on the transistor but with the tone generator it doesn't care..  That would point to C4 shorted or inserted the wrong way around.

You could try measuring the voltages around the transistor with the tone generator (perhaps set to a low level) then with the guitar.   If you see large differences in the voltages it's another sign there's a problem around C4.

Rob, I am going to take a look into this tonight, but thinking about it, I am thinking that the tone generator (taken from my tablet out the headphone jack) should be DC current, right?  If so, there should not be any difference with the guitar, right? 

[jfrabat]

Did not have time for measurements, but I did connect the pedal to my looper, and I can hear the signal going out VERY FAINTLY.

[anotherjim]

A stereo headphone jack is almost certainly going to be AC coupled. Although I believe there are some new-fangled class-D'ish headphone driver chips for low voltage portable devices that can DC couple.

Have a look at Bazz Fuss circuits - more gain with a single stage using a Darlington transistor. They don't have to use the odd diode biasing so the single R4 feedback bias can work. This usually calls for an MPSA13 Darlington, but you can Frankenstein one from a pair of 2N5088/9's to try it. R4 can probably be 10M with a Darlington.
http://www.home-wrecker.com/bazz.html
Actually, you may like the effect of the diode bias.