piggybacking transistors?

[foxfire]

so can you piggyback transistors in just about any type of circuit? is there a better/ worse type of circuit to try it on?

[R.G.]

I came up with piggybacking for one specific purpose - to make a lower gain transistor.

Modern small signal transistors have uniformly high gains. Some pedals work better with a moderate gain transistor. Hence piggybacking to lower the gain.

Which circuits? In general, ones which originally used germanium transistors. These circuits usually work best with a gain around 100. That's hard to find with modern transistors.

[foxfire]

ok cool. i'm thinking about trying this on 2 of dragonfly's fuzzs. his whirlygig(which i have de-whirled), http://www.diystompboxes.com/smfforum/index.php?topic=58782.msg458101#msg458101 and his montezuma, http://www.diystompboxes.com/smfforum/index.php?topic=58872.msg458953#msg458953. i just love them and have had a ton of fun playing around with them.

as far as how to piggyback, from what i've been able to pick up/understand you connect the bases together, the emitters are connected via a resistor in the 1k-10k based on personal taste, and the collector of the piggybacked tranny can be connected or left open? i read the long thread on "piggybacking" but haven't been able to find a layout or schematic to look at.

thanks for the help R.G.

[brett]

Hi
the collector of one of the transistors is always left "open". 
Otherwise, your decsription is correct. The base-emitter junction of the "active" transistor has a "load" in parallel with it, consisting of a similar base-emitter junction plus a 1k to 10k resistor. If you want to reduce the hFE by a small amount (say 20 to 50%), even larger resistors might be used. 

There are examples where a reduction of about 50% would be useful.  These include the many overdrive and fuzz circuits from the 1970s (e.g. the Bosstone and Mossrite).  Those transistors (often the 2N3904 or equivalent) had hFE around 150 to 200, but their modern equivalents seem to be 300 to 500.
cheers

[foxfire]

thanks Brett.

[John Lyons]

The Whirlygig and Montezuma use silicon transistors anyway right. Lowering the gain on these will radically effect them no?
At least on the Whirlygig, the gain of the darlington is what makes the tremolo do it's thing.
Maybe I'm not seeing what you are after here.

john

[foxfire]

i couldn't get the trem of the whirlygig to sound right to me but i liked the fuzz of the pedal so i de-whirled it and dialed in the fuzz. i added led's and ge diode clipping on a switch. basically piggybacking transistors is new to me and since these 2 circuits are rather easy to build and i really like both of them i figured they would be a good place to learn on.

[John Lyons]

Ok, I see. I was thinking you were trying to keep the trem and use the piggybacked transistors.
Search for Miss piggy and your should come up with some info on this as well.

Happy tweaking!

John

[foxfire]

i found it, thanks for the tip.