Piggybacking transistors in fuzzface Update

[brett]

Hi.

It's quite a while since the idea of "piggybacking" transistors was first discussed.  For those that weren't involved, it's a method of reducing the hFE of modern Si transistors to values more like the good old Ge transistors.  This schematic shows it done in the "Miss Piggy", which is a fuzzface with piggybacked transistors.


Because of their construction, you can't measure a meaningful hFE of piggybacked transistors in a DMM.  But I've been doing some measuring of hFE of piggybacked transistors in their positions in a fuzzface.  The results surprised me a fair bit.

For Q1, the PN100 transistor on its own had hFE=350.
With a piggyback transistor, it was 17 ( :shock: ).
Using different resistors between the emitters of the 2 transistors (as shown in the schematic above), the results were
Piggyback + 1k, hFE = 21
Piggyback + 10k, hFE = 45

I used the same procedure for transistors in fuzzface Q2 position (Q3 and Q4 in the schematic above).  The results were
No piggyback, hFE = 295
Piggyback, hFE = 11
Piggy + 2k, hFE = 42
Piggy + 4.7k, hFE = 62
Piggy + 10k, hFE = 114 (Bingo!)

Anyway, I tried out the Miss Piggy with 2 x 10k piggyback resistors in it (for hFE 45 and 114) and it sounded HEAPS better than before.  While doing all this, I could remember someone in the original piggybacking thread saying that they used 2 x 10k resistors (I think it was Javacody).  For some reason I just settled on low value resistors and never tried a sufficiently wide range.

With the improved hFE performance of the piggybacked Si transistors, my next experiment is to add a little capacitance across the base-collector junction for a slightly smoother (more Ge) sound.

Cheers

[javacody]

Wish I could take credit for your sucess, but it wasn't me.  

What we need brett is a mini circuit board for the piggybacked trannies + the resistor (sockets?) or a trim pot. Then this would plug (somehow?) right into an existing socket or could be soldered in place of a single resistor. Wouldn't that be dang cool? You could buy cheap fuzzes on ebay and retrofit them with this little circuit board. Instant tone!

[petemoore]

I'll take  credit for having said that a long time ago, in one of my wandering threads. I don't want any credit for helping develop Miss Piggy, I don't think there's any concious correllation... I certainly didn't do any experimenting or spend any time on it.
 Just one of those slightly juvenile things...two engines in one car type stuff...whod'da thunk there was anything to it...I, at that time thought [or tried to think] it could possibly increase the load a given transistor size could take or increase output or something, I can't explain how Piggy trasnistore work...
 The good news is: they do what no Si has done before!!!! Get house-trained for FF and Rangemaster Type use.

[vdm]

hey guys,

do you think putting a 10k pot with a 1k in series for the Q1 emitter resistor would be worth the trouble, or can you get the same lower gain sounds with just turning the gain knobs?

you could go as far as to hook up a 10k dual pot... but i doubt it'd be worth the trouble and expense...

any thoughts?

thanks,

trent

[brett]

Hi.  Good idea vdm, but the bias changes, especially when changing the resistor between Q3 and Q4.

I kinda think that the old fuzzface gurus were pretty smart.  hFEs of 70 and 120 sound consistently good in a fuzzface.  Similarly, in the Easyface, I set the gain of Q1 to 100 (33k collector resistor, 330 ohm emitter resistor), and find that Q2s with hFEs of between 100 and 200 are mostly good.  But there's a certain sweetness of Q2s around 140 to 160 which lets the Ge tranny shine through.  The consistent thing seems to be to have higher gain in Q2 by about half, and to keep hFE lower in Si than Ge to avoid harshness.

However, there is a related idea that might be interesting - set up a test circuit for pairs of transistors, using a pot to measure and adjust the resistance needed to get the hFE of piggyback pairs down to the magic range (at the Ic found in fuzzfaces).

cheers

[petemoore]

Maybe a very high Hfe Si in the MP/Q1b position with a 12k or 15k even, would do something.
 I only messed with it a few times, and just figured 10k to be the limit.
 I haven't tested any piggybakcs  nor measured their Hfe's, just tweeked by ear and used I think a 2k2 for 'Q's 1' and a 4k7 for 'Q's 2', and settled on that because it worked all around pretty good.
 I'll have to get it out and see what I think these days. I was sort of waiting for more analysis or cause and effect tales to guide the tuning process, as tuning in 2 more transistors in a FF is just that much more to figure out...
 I think it could be almost time for Miss Piggy to have a litter...a bunch of piggybacked FF types.
 I'm presuming this tech works best with bipolars? Si's and GE's? Mosfets don't work because no current flows at its base, are Jfets in the same category?
 Also a little high end rolloff cap, or two very small ones, either at the output or at Q1's C/B connections makes the high end more pallettable to my ears.

[R.G.]

I'm presuming this tech works best with bipolars? Si's and GE's? Mosfets don't work because no current flows at its base, are Jfets in the same category?

You're correct - MOSFETs and JFETs won't have the same effect because their input "eats" substantially none of the input signal. The effective gain reduction happens because the dummy piggyback transistor input eats some of the input to the working device. The variable resistor allows for the amount of eating to be adjusted.

It's funny - when I first proposed a piggyback transistor to lower Si gains, I thought the gain would drop by half. It actually drops by much more, and needs the resistor to get the gain back up.

[RedHouse]

I used the same procedure for transistors in fuzzface Q2 position (Q3 and Q4 in the schematic above). The results were
No piggyback, hFE = 295
Piggyback, hFE = 11
Piggy + 2k, hFE = 42
Piggy + 4.7k, hFE = 62
Piggy + 10k, hFE = 114 (Bingo!)

Brett,

Did both transistors you used in the piggyback unit have the same hFE of 295 or were they different?.

If so... I'm no math whiz but it seems if they were the same, then those figures you posted above indicate a fairly predictable factoring or piggy-back-factor "PBf" could be done to calculate the target "PBhFE" and "PBr".

(new 'nym's: "PBf", "PBhFE" and "PBr")

For the 2k, 4.7k and 10k PBr example values you posted one could extrapolate a "factor" from them:

2k = PBf:7.02
( hFE:295 / PBhFE:42 = 7.02 )

4.7k = PBf:4.75
( hFE:295 / PBhFE:62 = 4.75 )

10k = PBf:2.58
( hFE:295 / PBhFE:114 =2.58 )

so if all holds true, one could rougly estimate PBhFE's by using the inverse, start with the target hfe and multiply by the extrapolation factor:

ie;
Q1 target PBhFE of 80, calculate 80 x PBf 2.58 = hFE 206 w/10k PBrif
Q2 target PBhFE of 120, calculate 120 x PBf 2.58 = hFE 309 w/10k PBr

seems like average pairs of 2N3906's and 2N5087's should get the job done with 10k PBr's, but it all depends on if the individual hFE's being (roughly) the same so as to exclude individual hFE as a variable in this kind of simplification. I wish I had better math skills, I'm sure there is a way to directly calculate the PBr from the target PBhFE as the resistor values seem to be in a kind of log pattern.

[RedHouse]

Sorry typo...

ie;
Q1 target PBhFE of 80, calculate 80 x PBf 2.58 = hFE 206 w/10k PBrif
Q2 target PBhFE of 120, calculate 120 x PBf 2.58 = hFE 309 w/10k PBr

should have been:

ie;
Q1 target PBhFE of 80, calculate 80 x PBf 2.58 = hFE 206 w/10k PBr
Q2 target PBhFE of 120, calculate 120 x PBf 2.58 = hFE 309 w/10k PBr

[MR COFFEE]

Redhouse,
Your Math skills seem pretty good to me!

[petemoore]

RG Wrote:
 It's funny - when I first proposed a piggyback transistor to lower Si gains, I thought the gain would drop by half. It actually drops by much more, and needs the resistor to get the gain back up.  
 ...A good thing, it turns out, for us FF piggyback tuners...IMO.

[brett]

seems like average pairs of 2N3906's and 2N5087's should get the job done with 10k PBr's, but it all depends on if the individual hFE's being (roughly) the same so as to exclude individual hFE as a variable in this kind of simplification. I wish I had better math skills, I'm sure there is a way to directly calculate the PBr from the target PBhFE as the resistor values seem to be in a kind of log pattern.

As I understand it (ie NOT that well), with similar transistors in similar circuits, the piggyback effect is similar.  But in different circuits the effect is different enough to only semi-predictable.  Going back to basics tells us why.  The piggyback junction "sucks" some current out of the base, effectively wasting it.  This lowers the hFE (Icollector/Ibase).  But the amount "wasted" doesn't vary as much as the amount not wasted (ie double the amount not wasted and the amount wasted increases, but doesn't double).  At low base current, 90% might be wasted, while at high base current, only 50% might be wasted.  So the piggyback effect is strongest at low Ib (and low Ic).  This is apparent in the fuzzface, where a piggyback (+10k)in the Q1 (Ic = 0.2mA) lowers hFE by 70 to 80%, whereas in the Q2 position (Ic=2mA) it lowers hFE by 40 to 60%.

I haven't really looked at it, but I suspect that you can get a really big piggyback effect in high gain transistors (MPSA18, 2N5089).  But you wouldn't usually start with those trannies anyway if you want low hFE.

In power transistors there is already a fair bit of piggyback type current wastage happening, just because of their rugged construction.  Hence medium power devices have medium hFE and high power devices have low hFE.  No need for piggybacking there - most of the hFEs are too low.

cheers

[rodriki1]

Thank you all for sharing the idea of PIGGYBACKING.

It is simply one of the best ideas i got about guitar circuits.

Before PIGGYBACKING simple BC548 sounded TRASH.
After piggybacking i can do those simple cheap transistor
sound sweet. It is unbelievable.

THANK YOUUUUU ALL.