Germanium Orpheum - best place for a bias pot?

[Snufkinoob]

Not 100% where to put a bias pot here. Any ideas?

I'm guessing the second 47k would need to be a lower value, and then a bias pot put after that cross-roads (1m-.005uf) 

[John Lyons]

the .005 and 22k is a high pass filter.
The 47K / .1 / 47K is a combination bias and low pass filter.
Basically the 47ks add up to 94K which is the bias for Q2 along with
the 1M across the base and collector. I'd put in a 100K pot in place
of the 47ks and take out the .1 temporarily to see what you can get.
It would be smart to put in 1k or so before the 100k pot so there is always
some resistance in line so you don't fry the pot (been there...  )
See what you get and then you can put some fixed resistance back in
and find the pot value you want to go with...and put a cap in between the
resistance to knock out some high end... (where the .1 is now).
Clear as mud?

[PRR]

Modify the 10Meg and 1Meg to taste, 4:1 up or down.

No, this is not easy with trimmers; put studs where the resistors go and tack-solder various resistors.

Some Ge transistors will "work" without these resistors (because they leak so much).

[bool]

I would proceed as follows:

1) replace the upper 47k resistor with a 100k trimmer (or a pot)

...this will both alter the bias and the "filtering" that the RC network with the 0.1u cap provides.

I see this as the most "harmless" mod possible with this circuit.

[Snufkinoob]

the .005 and 22k is a high pass filter.
The 47K / .1 / 47K is a combination bias and low pass filter.
Basically the 47ks add up to 94K which is the bias for Q2 along with
the 1M across the base and collector. I'd put in a 100K pot in place
of the 47ks and take out the .1 temporarily to see what you can get.
It would be smart to put in 1k or so before the 100k pot so there is always
some resistance in line so you don't fry the pot (been there...  )
See what you get and then you can put some fixed resistance back in
and find the pot value you want to go with...and put a cap in between the
resistance to knock out some high end... (where the .1 is now).
Clear as mud?

Hi, John,

Putting a small value resistor (I was going to go with 470R) then a 100k pot was my next guess, so if you're suggesting that at least I know I'm starting to get my head around things. 

I was thinking of just using a pot just as a useful feature should it need re-biasing due to heat etc, and see if it has any useful side effects. I've been building Ge Fuzz Faces and like that the bias pot I put on (just a 10k in place of the 8k2) can create spluttery, kind of gated sounds in addition to having the practical application of biasing things up. At least that was my initial intention. Not sure if I've just made a mod, or whether it is actually going to re-bias it if the transistors start acting all flubby at any point.

Since you mention the option of using a fixed resistor to taste... if I did that, but then still wanted a bias as a practical thing, would it just be a case of going through the motions again? E.g if I put in a 68k resistor purely because I liked the sound best, would I just then need to replace it with a 56k resistor, then a 10k trimmer/pot for biasing that?

Next step is learning more about capacitors.

I see this as the most "harmless" mod possible with this circuit.

Modding might be the next step for me, but I just want to keep things simple for now. Plus any fuzz with more than three knobs (not including volume) just confuses me.

[bool]

You didn't read carefully.

What I suggested was in fact a BIAS mod.

[midwayfair]

1) replace the upper 47k resistor with a 100k trimmer (or a pot)

...this will both alter the bias and the "filtering" that the RC network with the 0.1u cap provides.

This is what I do and it provides a useful range of sounds.

[Snufkinoob]

You didn't read carefully.

What I suggested was in fact a BIAS mod.

But you eluded to it being the simplest, which you're right, it is. I meant I might be trying more elaborate modifications that are purely to get more sounds out of it using component swaps, additions of toggles, extra pots etc, at a later date.

Good, succinct method you suggested though, thanks. I'll probably give that a try first.

[LucifersTrip]

I was thinking of just using a pot just as a useful feature should it need re-biasing due to heat etc, and see if it has any useful side effects. I've been building Ge Fuzz Faces....

You really don't have to worry about the biasing much at all. The main reason you want a pot in there is for tonal variety and the simplest way is just a 50K pot subbed for the 47K on Q2 collector. You should be able to get a useable variety of buzz/fuzz thru the entire sweep.

If you want to get a little more extravagant, you can wire it like the buzz control below. The UMI sounds very similar to the Orpheum with an added control.

Oh yeah...and you definitely don't need more than about 60-80 hfe for either transistor. Believe it or not, I've built more ge Orpheums that any other fuzz (since it's still my favorite) and a good combo is 80 hfe for Q1 and 60 hfe for Q2.

[John Lyons]

Since you mention the option of using a fixed resistor to taste... if I did that, but then still wanted a bias as a practical thing,
would it just be a case of going through the motions again? E.g if I put in a 68k resistor purely because I liked the sound best,
would I just then need to replace it with a 56k resistor, then a 10k trimmer/pot for biasing that?

Yes, just find out what max resistance sounds good and use a combination
of fixed resistor(s) and a pot to get the range of sounds you want

[Snufkinoob]

Thanks for the replies.

Oh yeah...and you definitely don't need more than about 60-80 hfe for either transistor. Believe it or not, I've built more ge Orpheums that any other fuzz (since it's still my favorite) and a good combo is 80 hfe for Q1 and 60 hfe for Q2.

With my first attempt I've used 146 & 147 hfe and it sounds fine. Has a nice and smooth, bright tone to it. Will try other hfe's at some point perhaps.