Low frequency transition, low gain silicons

[mac]

IMHO using a low gain silicon to clone  a FF or another vintage germanium-based pedal does not always guarantee success.
I think that the frequency transition Ft is as important as the gain of the transistor.
RG clearly points to this fact in his FF article when he describes the behaviour of AC128 transistors' gain at high frequencies.
Ft can be thought as gain limiter indication at high frequencies. Big Ft are a good thing for telecom, but a bad thing for us.
See http://en.wikipedia.org/wiki/Gain%E2%80%93bandwidth_product

Some germs, ACxxx, 2sb17x, 2sb77, 2sd72k, 2sb56 have FT close to 1-2 Mhz, 2sd352 about 10khz!!!, unlike some silicons like bd139, 2369, 3904, 5088 and others having Ft well above 100 Mhz!!!

I've been trying to find current production low Ft, low gain, TO-92 cheap silicons without much success, except maybe 2sc1213. All others are power devices, like bd233-5-7, 2n3055 or TIP41.

Just a thought.

mac

[John Lyons]

Interesting Mac. Keep up the research. 

[Davelectro]

IMHO using a low gain silicon to clone  a FF or another vintage germanium-based pedal does not always guarantee success.

Hi, mac

What do you mean by success? No harshness? No oscillation?

In my experiencie, high frequencies in low gain silicon FFs can be controlled rather easily. In fact, these guys can get awfully dull if you try to make them more germanium-like.


Now, background noise (particularly with fuzz control on 10) is something else.

P.S.: 2n5786 seems like a good candidate.

[mac]

What do you mean by success? No harshness? No oscillation?

In my experiencie, high frequencies in low gain silicon FFs can be controlled rather easily. In fact, these guys can get awfully dull if you try to make them more germanium-like.

I mean harshness.
IMHO a small roll off cap helps to remove unwanted highs, but it is not like a real low freq germ.
I think the internal germ parasitic capacitance works differently from silicon roll off cap.

P.S.: 2n5786 seems like a good candidate.

mac

[Jaicen_solo]

Not sure about capacitance, I think it's more to do with its ability to amplify low frequencies better than high frequencies. Perhaps it has the effect of compressing the highs?

[mac]

Not sure about capacitance, I think it's more to do with its ability to amplify low frequencies better than high frequencies. Perhaps it has the effect of compressing the highs?

I think that if we plot gain vs frequency, silicons curve is flat and it suddenly goes to 1 near the transition frequency, while germs goes to 1 long before and more linearly. Much like what you said but in another words
Of course I could be so wrong... i'm looking for data on the web.

mac