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finishing up a PUFF, and I replaced the 20k trim with a 10k B external bias control and 4x1k resisters in series. The entire sweep of that pot sounds like it’s gated fuzz territory, and I’m not sure if I should add or take away resistors, or if it’s not quite wired correctly. I wired #2 pad to #2 terminal on pot bit wired with 4x1k resistors, and likewise 3 pad to terminal 3 on pot. Any insights to make that sweep more useable would be appreciated. I’d like to go from gated sputtery under biased stuff all the way to over-biased cleaner stuff.
If it’s gated throughout the entire pot range that would suggest too much resistance ie the transistor can’t reach correct bias voltage
Leave it as it is and check Q2 collector voltage with the pot fully CCW and CW to make sure that’s the only problem
I would just simply wire the pot to TR1 lugs 2 and 3 with no series resistance you shouldn’t have to place a jumper between lugs 2 and 1 if you’re only using lugs 2 and 3
A wee point to remember with 2 and 3 being used resistance will decrease as you turn it CW so obviously fully CW should allow more voltage through
So if you want it to go gated as you turn the pot up use lugs 1 and 2 to increase resistance CW
I have built 2 Ge FF’s in the past (Not PUFF’s but NPN Ge FF’s) that used 25K trimpots for the biasing. I replaced the trimmers with 25K pots for external bias controls. The 25K value worked fine in this circuit with no problems like you have experienced. Maybe think about switching to a 25K linear pot for external bias control without the added resistors. As mentioned above wire the pot to the bias pot lugs 2 and 3. You won’t have to place a jumper between lugs 2 and 1 if you’re only using lugs 2 and 3.
Ended up using a 4.7k resistor and the 10kb pot, and it has a really usable sweep and sounds great.
When biased high, it’s a great clean boost, and can get into the gnarly gated sounds when voltage is bled off.
I have another kit, and may experiment with less resistance (maybe down to 1k with the 10k pot)..
one thing that puzzled me, is that the DMM never quite read what it was supposed to at the collector of Q2. I’m assuming operator error because it sounded great once all the kinks were worked out. What was I supposed to use for the multi meter settings, and where was I supposed to put the probes?
The DMM should be set (at least on mine) to 20V DC and just put the negative probe/lead into the output jack sleeve.
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