I'm looking for R.G.s take on the RCA Hobby Circuit #43

[johngreene]

Hey Marty, I think everyone else lost interest in this thread, so for you I'm going to mention a little "secret".

Go back a few posts where I posted the scan of my copy of this circuit. Now grab that scroll bar at the bottom and scroll over to the right page. There is an interesting looking 1-transistor compressor circuit. I'm going to try that too. I tried that one about 30 yrs ago and wasn't very impressed - but I'll readily admit it was probably %^&*pit error. This book came out in 1974 and I got it when I was in high school... There's a few other interesting circuits in it too, ones that I haven't seen around here yet.

Doug

I've simmed the compressor and it looks like it should work. It appears that it is using the doide on the output (cathode to ground) as a pin diode attenuator. The diode connected to the resistors before is a half-wave rectifier and as it builds up voltage, it starts to forward bias the diode and therefore shunting more signal to ground. With the 50uF cap to ground it has a looooong time constant so it comes on really slow, like .5 seconds. But lowering the value of that cap makes it faster. I redrew the schematic so it's a little easier to see things.



--john

[Doug_H]

Thanks, John.  I have not played with the compressor yet- neither with a sim or the breadboard. I want to try it next. IIRC, the complaint I had about it when I built it in high school was that the delay was way too long to make it usable. Sounds like it will be easy to remedy. Now I just have to find a 15v battery...

One more quick note about the fuzz: It seems like the main contributor to the compression is the biasing of the follower. With the 3.3k resistor the emitter is at around 3.5v. Bigger resistors will stiffen it up, if you want that. A smaller series input resistor produces a hotter signal which exaggerates the compression more. So you can tune your preference of gain/compression with the input and emitter resistors.

Doug

[Doug_H]

Hey guys, if you really want to have some fun with this RCA fuzz circuit, sub a BS170 mosfet for Q3. You might want to increase R10 (47k) to 100k as well, to keep it smooth.

Holy screamin' rock&roll city, Batman!

Doug

[col]

Is that compessor schematic run by 15v or 1.5v. I'm afraid my screen isn't quite good enough to tell!

Col

[MartyMart]

Now I just have to find a 15v battery...
Doug

There you go !

MM.

[Doug_H]

It should work at 9v with the R2 & R3 (base bias resistors in my scan) set to 33k and 8.2k respectively. Have not tried it yet but that should work.

Doug

[MartyMart]

One more quick note about the fuzz: It seems like the main contributor to the compression is the biasing of the follower. With the 3.3k resistor the emitter is at around 3.5v. Bigger resistors will stiffen it up, if you want that. A smaller series input resistor produces a hotter signal which exaggerates the compression more. So you can tune your preference of gain/compression with the input and emitter resistors.
Doug

Just an update, I like the input resistor at 33k - 47k, you still get a little compression
at max fuzz, but can dial it back enough and I quite like the effect !
Emitter reistor is a 3k9 now too.

MM.

[Doug_H]

With the mosfet in the 3rd stage I ended up liking the 100k input resistor and a 100k in place of the 47k between stage 2 & 3. Up to 75% gain it stays pretty stiff and gets a considerable amt of breakup/gain. Higher than that it starts compressing, so you can sort of get the best of both worlds. I still like the softer 2n2222 output stg for more of a "vintage" overdrive sound too. Like you I prefer 47k for both of the series resistors in that case. It compresses a little there too and that adds to the "vintage-y" quality. With the 3rd stg and resistor tweaks, you could easily make 2 fairly different sounding pedals out of the same basic circuit.

Doug

[WGTP]

Correct me if I'm wrong, but looks like some of he resent darlinton stuff might work well with this.   

[Doug_H]

Correct me if I'm wrong, but looks like some of he resent darlinton stuff might work well with this.   

Give it a try!

Doug

[armillary]

I'm working on building the number 43 Fuzz Box circuit from the RCA HM-91 Hobby Circuits Manual. I used DIYLC to map the circuit to vero. I plan to omit the DPDT and use a 3PDT with wiring board for convenience.

While searching for more info I found this thread from 2005. 

I recently got some original SK3020 and SK3005 transistors from ebay and am posting their readings from a DCA55 in case they're of use to anyone, since there was some discussion about appropriate substitutes in this thread. 

SK3020
I measured several of these. They were all about the same. One had current gain of 38.
NPN Silicon Transistor
Current gain = 47
Test current = 2.50mA
Base-Emitter V = .70V
Test current = 4.70mA 
Leakage current = .00mA

SK3005
I measured two of these, in a cool room with minimal handling. They were pretty different.
PNP Germanium Transistor
Current gain = 166
Test current = 2.50mA
Base-Emitter V = .35V
Test current = 5.09mA 
Leakage current = .06mA

PNP Germanium Transistor
Current gain = 332
Test current = 2.50mA
Base-Emitter V = .39V
Test current = 5.05mA 
Leakage current = .12mA