Equal HFE for Fuzz Face transistors

[Sparky]

Does anybody still prefer / find value in  / tolerate  a  FF circuit with the Q2 transistor that has the same HFE as Q1 ?

[Gus]

a link that might give you a hint about your question
https://www.diystompboxes.com/smfforum/index.php?topic=114153.msg1058953#msg1058953

LT spice transistor models often a close to real life builds in fuzzes
So download LT spice and have fun

[Sparky]

a link that might give you a hint about your question
https://www.diystompboxes.com/smfforum/index.php?topic=114153.msg1058953#msg1058953

LT spice transistor models often a close to real life builds in fuzzes
So download LT spice and have fun

Thanx Gus...much appreciiated.

Owing to all the variations in the old [60's and 70's]  silicon Fuzz Face transistor HFE's  I'm pretty sure they didn't take the time to match Q1 and Q2 .    Although much more stable than the germanium FF, there would be differences in the sound of the silicon versions with mismatched transistors.   What about Q1 with an HFE of  550 and Q2 of  400  ?    Would that yield a desirable sound for the user or end up in the trash can?    Personal taste in fuzz sounds really became fine-tuned  later as people became aware of that pesky Q2 voltage. 

[Electric Warrior]

Owing to all the variations in the old [60's and 70's]  silicon Fuzz Face transistor HFE's  I'm pretty sure they didn't take the time to match Q1 and Q2 .

Dennis Cornell claims he auditioned transistors for their sonic properties when he worked for Arbiter.

Although much more stable than the germanium FF, there would be differences in the sound of the silicon versions with mismatched transistors.   What about Q1 with an HFE of  550 and Q2 of  400  ?    Would that yield a desirable sound for the user or end up in the trash can?    Personal taste in fuzz sounds really became fine-tuned  later as people became aware of that pesky Q2 voltage.

That pesky Q2 voltage depends mostly on Q1's hfe. It still varies a lot – because of the fuzz pot's tolerance (or wear). IME the latter plays bigger role than matching or mismatching transistors.

[Sparky]

Owing to all the variations in the old [60's and 70's]  silicon Fuzz Face transistor HFE's  I'm pretty sure they didn't take the time to match Q1 and Q2 .

Dennis Cornell claims he auditioned transistors for their sonic properties when he worked for Arbiter.

Although much more stable than the germanium FF, there would be differences in the sound of the silicon versions with mismatched transistors.   What about Q1 with an HFE of  550 and Q2 of  400  ?    Would that yield a desirable sound for the user or end up in the trash can?    Personal taste in fuzz sounds really became fine-tuned  later as people became aware of that pesky Q2 voltage.

That pesky Q2 voltage depends mostly on Q1's hfe. It still varies a lot – because of the fuzz pot's tolerance (or wear). IME the latter plays bigger role than matching or mismatching transistors.

Hhhmm...   I wonder if Cornell actually measured HFE or just slapped the transistors into a circuit.   I noticed he was also said to have "designed the Fuzz Face"  [??!]     That's a whole other topic, though.
Yeah, good point about the fuzz pot.

[PRR]

> Q2 transistor that has the same HFE as Q1

They run at very different currents and bias different way. I don't know what "match" is in this case.

As EW says, drop in Q1 Rb (Q1 hFE) and value of fuzz pot (minus wiper) dominate the total circuit bias. hFE of Q2 matters very little.

[Sparky]

 So, the "Q2 must be higher HFE than Q1 thingy "  was not all it's made out to be.

[Rob Strand]

So, the "Q2 must be higher HFE than Q1 thingy "  was not all it's made out to be.

As PRR said, the biasing of the circuit is very insensitive to the hFE of Q2.
If you have very low Q2 gains the overall gain of the pedal is reduced.

Q1 can affect the biasing a lot more as Q1's base current must pass through
the 100k base resistor.   Any voltage drop across the 100k gets amplified by
a factor of 5 on the collector of Q2.

[Sparky]

So, the "Q2 must be higher HFE than Q1 thingy "  was not all it's made out to be.

As PRR said, the biasing of the circuit is very insensitive to the hFE of Q2.
If you have very low Q2 gains the overall gain of the pedal is reduced.

Q1 can affect the biasing a lot more as Q1's base current must pass through
the 100k base resistor.   Any voltage drop across the 100k gets amplified by
a factor of 5 on the collector of Q2.

Understood. Adjusting the resistor on the collector of Q2 is too simplistic.    The Vox Tonebenders used a 47K instead of 100K base resistor......that circuit has less gain than the stock Fuzz Face.    Just playing around with that one resistor affects the distortion / gain of the circuit.    I have trim pots on my test FF so with a 100k base resistor I get a Q2 collector reading of 5 v and with a base res. of 47K  [the Tonebender value]  I get a 5.80 v reading.     

[PRR]

> I get a 5.80 v reading.     

Leave the volt meter on when you beat the FF. (Notch the box to get the meter lead out without letting all the buzz in.) Does it stay at 5.8V? If beaten in anger, Q2 C (all DC voltages) will change. This is why the FF is not a simple clipper. And why static bench readings are not insightful.

[Sparky]

HaH!  Yeah, I've done that before.... I'll try it again in awhile.   One of my guitar repair services is making Fender-style pickups [Strat & Telecasters].    When you beat on the magnets with a screw driver as the volt-ohm meter is reading the ohms the reading will jump around.

[Sparky]

> I get a 5.80 v reading.     

Leave the volt meter on when you beat the FF. (Notch the box to get the meter lead out without letting all the buzz in.) Does it stay at 5.8V? If beaten in anger, Q2 C (all DC voltages) will change. This is why the FF is not a simple clipper. And why static bench readings are not insightful.

Yup, it jumped around .

[Lino22]

> I get a 5.80 v reading.     

Leave the volt meter on when you beat the FF. (Notch the box to get the meter lead out without letting all the buzz in.) Does it stay at 5.8V? If beaten in anger, Q2 C (all DC voltages) will change. This is why the FF is not a simple clipper. And why static bench readings are not insightful.

Does the DC change happen even if it is powered from a power adapter? What causes the changes?
Thank you

[Sparky]

I use 9 volt batteries so I couldn't comment on an adapter.

[Lino22]

Yeah, i guess the more exhausted the battery is, the more the DC changes. A solid power adapter should keep the DC steady.

[PRR]

I don't mean power supply change. You bias the amplifier to a desired DC bias point. Beat it. It will clip. It may clip different top and bottom. The DC bias changes.

[Lino22]

And what causes the bias point change?

[PRR]

It may clip different top and bottom.

[Lino22]

oh indeed, thank you.