OK, so I've gone a bit "Fuzz" crazy lately and in doing so, have built around 15 different circuits, both Ge PNP/NPN and SI versions.
Most of these circuits involve a 1k or 2k "Fuzz" pot and well, I dont have any so have been subbing 4k7 pots with 4k7 resistors across them.
This gets me close, but after say three quaters of the pots value I'm well
above where I should be, value-wise ! in particular where a 1k is required.
Many of these circuits have a Q2 or Q3 bias trimmer which can get altered by this "larger" pot value in the circuit.
I think its an advantage !
What i mean is that at the "top end" of a slightly over value fuzz pot I'm
getting a sort of "mis-biased" wild splatty area, and below all the nice correct "bias" fuzz tones are there :D
Sorry if this is "obvious" but its just something I've noticed and I quite
like the effect .... !
Does anyone think that I'm "missing" something by not using the correct
lower value pots in these circuits ?
Cheers,
Marty. 8)
depends on which circuits...
assuming your talking about a FF or FF derivative.
the gain pot value will effect the bias. Using too high a value pot will make it bias 'differently', I'd run that pot down...
Using a variety of resistances across your 4k7 gain pot wafer...you say you have another pot?...clip that on there, turn the gain pot all the way up...then, tune the gain pot's value with the 'extra' gain pot [another pot, wired the same but 'doublepotted'] set max gain for the circuit [Joe Davisson's FF...uh whatzit called...calculator might be of help here to decide pot values for Si and Ge FF's]...anyway set the max gain so the pot is as low a value [or wherever the heck you want] ...that way the FF can be biased 'better'...I was using larger than necessary FF gainpots, someone pointed out to me that this will influence bias...I started 'tuning' the gainpots resistance per circuitboard...things seemed to 'clear up' a bit as a result.
Anyway I turn the 5k gain pot all the way up, and have a *5k pot, [2k'd probly do] wiper and an outside lug, wired across the gain pot...then I turn the gain down with the *5k pot 'till I can 'just stand it' [thinking where I preset this will be enough for SC's etc.]....take off the *5k pot, measure, replace it with a fixed resistor of ~that value.
The Easy Way...turn the gain all way up...grab a 1k or 1k5 or 2k2 or...resistor, "C shape the leads, touch them across the gainpot, pick one.
stick the pot on your breadboard, with your DMM probes on the outer lugs. Set the DMM to resistnce. Put various resistors in parallel with the pot until you see a value on the DMM reasonably close to 1K.
I'm beginning to think getting that gain pot as the right value may have some benefits.
Analog Alchemy's FF Calc page leads me to believe this, and, tuning the size of the gain pot does also. The ones I was using 5k GP's on got beat by the ones I started gain set high using larger than necessary [say 5k or 2k2] gainpot, then taming it down with 'pot taming' resistors...
Maybe I was reading between the FF Calc pages lines, but IIRC it suggested a different sized gain pot for Ge's and Si's.
I think I'm getting a touch more FF mileage this way, but I can't verify that with any technical reasons as to exactly why, figureing it has to do with the symmetry/assymetry of the clipping or swing range decrease of the FB amp.
I can see why they are "called for" in various FF circuits and I know that, for instance, you CAN play with gain/vol pots on other op-amp/diode clippers with some very useful results..... I just made a "Grace" overdrive with a 2k gain pot and a 50k vol pot, with very nice results.
What I'm saying is that I LIKE that top end of the pot mis-biased area
and if you use a 2k log where a 1k log is recommended, you DO have the 1k range probably up to 2/3's of its travel anyway, so your "true" sound
is there....
Interesting stuff...... :D
Marty.
I only did sketchy research on this.
The only results are from two different FF's so it's not a 'lab test' by any stretch.
I have noticed though, when using 5k or even 10k [to start with in the absence of say a 2k2 pot] that it does change the sound...exactly where in what circuit I Don't Know...so I devised pretty easy to do methods for setting max gain by pot size...a couple test clips and a 5k pot...maybe next time I compare big pot turned down some to smaller pot turned all the way up.