GE Giant Bias / Mods

[anthony]

Hi everbody

While tinkering with my loved GE Giant I got a two questions...

1.) I've set the bias by ear before... for some reason I came to measure it and I think it is way off. Is it supposed to bias up to 4.5v on the emitter? Then I guess I need a bigger trimpot :D

2.) The pnp/positiv ground think confuses me when thinking about adding a bypass cap for gain increase (just for testing...). Is it correct that the polarized cap needs to be placed between the collector and ground with the positiv side on ground?

Thanks for any clarification!

Regards,
anthony

[Beaker]

"1.) I've set the bias by ear before... for some reason I came to measure it and I think it is way off. Is it supposed to bias up to 4.5v on the emitter? Then I guess I need a bigger trimpot :D"

So what reading are you getting? The often stated 4.5V for a 9V supply is more important  for JFET circuits. Biasing by ear is the correct way to do this, as the optimum bias point for each circuit you build will be different, due to differences in actual component values and transistor gain.

Bias by ear, and ignore the multimeter reading!


2).Which version have you built - PNP or NPN? (I'm presuming PNP, but don't want to give a wrong answer and confuse you further).

You can use non polarised caps if you have them (if the values you want to try are 10uF or below) and save yourself the headache.

[anthony]

Hi,

1.) I don´t know the exact values right now... was like 2v, 0,5v 0,5v or something like this... can't check exactly until saturday... if it is supposed to be close to 4.5 the 2v is way off.. but it is definitely sounding good... not fizzy and not gating...

2.) Basically this: http://1.bp.blogspot.com/-fx4MxeX-UIs/VoVscaY3g-I/AAAAAAAAAII/iEHSDmxeCcY/s1600/Germanium%2BGiant%2BPNP%2BV3.png ... so it's the PNP with positive ground

Edit: given the link above, I think the bypass cap could be placed from M/G to M/F where M/G is the positiv side...

[induction]

anthony wrote

Hi everbody

While tinkering with my loved GE Giant I got a two questions...

1.) I've set the bias by ear before... for some reason I came to measure it and I think it is way off. Is it supposed to bias up to 4.5v on the emitter? Then I guess I need a bigger trimpot :D

2.) The pnp/positiv ground think confuses me when thinking about adding a bypass cap for gain increase (just for testing...). Is it correct that the polarized cap needs to be placed between the collector and ground with the positiv side on ground?

Thanks for any clarification!

Regards,
anthony

I've seen NPN and PNP versions of this circuit, so it would be good if you posted a link to exactly what you built (layout and schematic if possible), and tell us what transistors you used. Most of the reports I've seen have been for the NPN circuit, which is negative ground, but the Greer Green Giant thread includes a few PNP layouts for the Ge giant.

The reason this is important is that your posts indicates that you are confused about either terminology or circuit topology, but I can't tell which, so it's hard to clarify things for you. But I'll explain a few things in order to help us figure out where you're getting confused.

The Ge giant is a variation on the common emitter amplifier. Whether it's PNP or NPN, the output comes from the collector, and it is usually the collector (not the emitter) where we set the bias in these kinds of circuits. The bypass cap parallels the emitter resistor, so it goes from the emitter to one of the power rails, but which rail depends on exactly what you built. There are a few different common ways to connect the power rails on PNP circuits, so before we talk about how to orient a polarized emitter bypass cap we have to know how you put the circuit together.

[anthony]

Hi,

Sry... I really mixed up emitter and collector in the initial post...

The version i´ve build is http://1.bp.blogspot.com/-fx4MxeX-UIs/VoVscaY3g-I/AAAAAAAAAII/iEHSDmxeCcY/s1600/Germanium%2BGiant%2BPNP%2BV3.png  with the power inverter on a separate daughterboard.

Based on this link I would guess that the bypass cap could be placed from M/G to M/F where M/G is the positiv side...

I measured the 1.9v/2v between collector and ground (not emitter)...

thanx,
Anthony

[anthony]

ah.. and the transistor is an AC126

[induction]

Sorry, I guess I missed the posts after your initial one.

anthony wrote

Based on this link I would guess that the bypass cap could be placed from M/G to M/F where M/G is the positiv side...

That is correct.

As for the biasing, you can use your ear. There is really no 'correct' setting in a DIY circuit, just preferences. Off-center biasing can increase the asymmetry of the transistor clipping (or might not), or can create gating and sputtering (or might not), any of which you may or may not want.

Nerd corner:
Be aware that the bias trimmer will also change the gain in this circuit, so it can be difficult to isolate the sonic effect of biasing and gain. If you install the emitter bypass cap that effect will be mitigated at frequencies that depend on the value of the bypass cap. A big cap (e.g. 10u) will essentially give you maximum gain at all guitar frequencies, at which point the hfe of the transistor will matter more, which means that the effect of swapping transistors may be more apparent. In either case, the bypass cap will add more transistor clipping in addition to the diode clipping that comes afterward.

[anthony]

That sounds good :D At least my understanding with the bypass cap was right :D Lately I really liked values like 47uF for this (e.g. like in the woodrow). I will socked this and give it a try...

Regarding the bias:the reason I´m asking is because I get the ~2v at the max setting of the bias pot... will it be safe to use a bigger pot (10k oder 20k) and try to reach a higher voltage like 4.5 or even 5v? I´m not the expert in studying the transistor data sheets and would definitely look for the wrong values... I don´t want to risk damaging the transistor...

[induction]

anthony wrote

Regarding the bias:the reason I´m asking is because I get the ~2v at the max setting of the bias pot... will it be safe to use a bigger pot (10k oder 20k) and try to reach a higher voltage like 4.5 or even 5v? I´m not the expert in studying the transistor data sheets and would definitely look for the wrong values... I don´t want to risk damaging the transistor...

Totally safe. Try it out, see how you like it. Do you have a breadboard? They make this sort of thing much, much easier.

[anthony]

induction wrote

Totally safe. Try it out, see how you like it. Do you have a breadboard? They make this sort of thing much, much easier.

I´ve git a tiny breadboard... but the GE Giant is already boxed and played for hours.. lately I just wanted to squeeze a little bit more dirt out of it.. and while thinking about the bypass cap I started measuring to clarify whats happing in this positiv ground circuit... well and then there was this "bias value" :D but it is easy to replace the trim with a "bigger" one... transistor and diodes have sockets already... :D

I guess I´ll take my soldering iron, replace the trimmer and add one more pair of sockets for a potential bypass cap... than i´ll check the new bias range... when the gain is enough... fine.. if not... happy cap testing :D

[induction]

That should work.

If you wanted variable gain, you could set up the bypass cap fuzz-face style:



Ignore the values and look at the path from Q2 emitter to ground.

To implement this:
Lose the double link under the trimmer, so the jumper goes from row D to G directly, then put your bypass cap between the wiper and ground. When you change the setting, it leaves the DC bias alone and only adjusts the AC gain. You can leave it as a trimmer, or replace the trimmer with an external pot if you don't mind drilling another hole.

The is a separate issue from the biasing, though. You'd have to sort that out separately.

Just an idea.

[Beaker]

This is such a simple circuit that you can't really mess it up, so experiment away.

There is a limit though to how much extra gain you can squeeze out of the transistor.

An easier way to get a crunchier, gainier sound, along with a fair bit more volume, would simply be to replace the diodes with silicon ones. You could add them to the pedal on a switch to get Ge/Si diode options.

It is still a great sounding pedal with silicon diodes.

[anthony]

Hello all,

I´ve done some testing... when biasing to 4v+ the transistor gets totally unstable. On every signal it is only shortly spitting and then gating... i´ve gone back to a 10k trim which gets me up to 2.8v which represents my usable range and adjusted it completely by ear again (did´t even want to measure it after that :D)

Finally i´ve tested with the bypass cap... works like described. More gain now. I´ve ended up with a 10uf. Now my GE Giant is even more awesome! It sounds huuuuuge and massive! Lots of Bass (both input und output caps 100nf) without getting to muddy, smooth cream GE style dirt - did I mention the low thumb already? :D Its crazy good now! Well, it was really good before, but now it kills!

Thanks for all the help!

Regards,
Anthony

[Beaker]

That's great to hear that it works ok. Would you mind sharing with us how you have fitted the bypass cap?

[rocket88]

[Beaker]

Cheers Zach, I will have to try that out ASAP.

[rocket88]

i really would want to put it on a pot though, but i'm worried doing so would also mess up the transistor bias. can't wait to hear how it works out beaker.

[Silver Blues]

Could put it on a switch for a gain boost,

[anthony]

Hi there,

Zach, that´s not exactly how i´ve done this... I´ve put the cap in parallel to the bias trimmer and it always connects the emitter to ground. Your layout puts the cap in series with the bias trimmer... I doubt it would work this way...

You could place the cap in from F/F to F/E (negative side on F/E for this NPN Version)

You could also add a ~1K gain pot before the cap... as this is only an AC path it should not affect the DC Bias.

This is how the schematic of my current version looks like (converted to NPN):



Regards,
Anthony

[rocket88]

no worries buddy. i must have miss understood the description. made the correction. to the layout above