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

[jmusser]

I thought I'd go on a fishing trip here, to see if R.G. was around. Mainly, because I have a sneaking suspicion that he has probably had some dealings with this circuit at one time or another. R.G., I don't know if you read about my build and bastardizing of this circuit in the "60s Style Fuzz box Build" post or not. I don't know what I'm looking at as far as the design goes. I believe it should work as written, and I CAN get to work partially. I just feel that it's a circuit worth pursuing. It may have been so specifically geared to the parameters of the SK transistors, that using anything else would be a biasing nightmare.Thanks

[R.G.]

I'll take a look. Where's the schemo?

[Stevo]

http://www.muzique.com/schem/sixties.gif

Here is the scheme ..I am wondering why Jack has not pursued this..It would be great to build a different fuzz as the fuzz face has been around and the tone can get old if you play it to much..So if this is a workable circuit or maybe something is wrong somewhere..I can not get it to work..Looks really cool as if it would have a real distinct tone..I asked Jack many years ago why he posts a circuit that does not work but no reply

[R.G.]

The thing is hypersensitive to the bias on the second transistor, the PNP. Diddle with the 100K to ground from its base until you get a voltage between 1 and 7V on the PNP collector, to taste.

[jmusser]

Thanks R.G.! The reason I never included the schematic link, is because I had it in the build post, and I was hoping you'd meander over there to read what I had come up with. I actually made the thing work (and not sound too bad either BTW), by turning the 2nd transistor around so that the collector was towards the 9V rail, and then, putting an MPSA13 in Q3 position. I did eventually try it as intended, and got a very low, minute, output that sounded sort of like a slightly fuzzed tone Bender. I had the voltages over on the othere post as to what the transistors had measured also. Anyway, thanks for the tip. I will give it a try this evening with a 100K pot, and see what I come up with. I believe it will sound at least decent.

[Stevo]

 

I have all but given up on this years ago...Found a circuit in my stuff with NTE103 for 1 and 3 trans and an NTE158 for 2 trans....  All germanium .....Flipped around the transistor ,the second one so it was right, and put a 470k on the base to ground on the second instead of the 100k to add more voltage WHAMMO it wails now...Not a real fuzz but more like a nice germanium distortion with the warmness of germanium transistors...Thanks RG how can you do that ..Really thanks a bunch..I think Jmusser needs a 470k also I hope he reads this and finds a trim 100k will not do it...

[jmusser]

Hey, I couldn't wait either. As soon as I got off here, I put it back to a 3904,3906, 3904 in there, cut the 100K out, and stuck in a 1 meg panel mount pot. She is wailin! I subbed Q2  from the 3906, 2 an ECG102A germ. It worked fine too, but needed less collector voltage. It seems like around 5. 4 for the 3906, and 2.3 or so, for the 102A. thanks R.G.! I knew all I had to do was go to the master. I'm not worthy! I'm not worthy! I will play with it some more after i get home from my meeting.

[Stevo]

Let us know what the resistor you ended up using is?  My collector voltage for Q2 is a little more than 7 volts but sounds really wailin to me...I havent played it with my tube amp yet got to use the headphones its a school night for the kids.....I ended up with a 470k I guess that is right for different transistors that you can use....

[jmusser]

Well, I played with the #43 circuit last night, and settled on the 2N3904 for Q1, ECG102 (no A suffix) for Q2, and another 2N3904 for Q3. I subbed out several different transistors in different spots, but this group sounded the best. I would believe that this would be a fairly good approximation of what the original circuit would have sounded like, since the SK3005 in Q2 actually cross referenced to an NTE100 gemanium, while the other SK3020 crossed referenced to NTE123 silicon. I tried a lot hotter germanium in Q2 position. It was a Russian GT308B. It sounded good, and headed the fuzz more towards Tone Bender intensity, but, I already have that sound. I had various higher gain Si trannys in Q3 position, but they wrecked the tone for me. I know were trying to build fuzzy sounding circuits, but the higher gain Q3 takes all the focus out of the tone. It sort of like when you overdrive a mic at a fast food place, by talking too loud and close to it. I found this also to be true with RDVs Si Tone Bender MkII, when scaling it back from a 2N5089, to a 2N5088. With the ECG102 in the Q2 position you get the germ tone, and it takes the harshness out that you get with the 2N3906 in there. This circuit needs that, because it pretty trebly. This thing is loud! I am showing around 220K for what used to be the 100K base resistor for Q2. I believe this resistor alway ought to be a a 300 to 500K trimmer pot, so you can dial in which ever transistor sounds good to you. I would NOT put a fixed resistor in there. For one thing, there are different tembres you get by varying that bias voltage on that transistor, and what I come up using this specific transistor may not be what works best or sounds best to you. Plus, you have a lot of choices for that position, and a lot of tembres to be gleaned form a variable resistor. We've already had a fixed value resistor in the base position, and look where it got us! Switched to the neck position, and the tone controls rolled off, it has an excellent thick fuzz, and in the bridge position (Fat Strat), it has a real nice cutting overdrive no matter where the tone knobs are set. Maybe it don't take to much to thrill me, but I am thrilled with this circuit for several reasons. Number one, I looked through the archives, and unless I didn't do the correct search wording for this circuit, no one has had it work up until now. Number two, I believe I'm in the ball park of the tone that the initial design was supposed to give us, and number three, I believe a very nice sounding circuit has been reserected from the dead! I want to thank R.G. again for heading us in the right direction and solving the mystery of this circuit. This circuit is the first one I have ever seen, that can work with Q2 turned upside down! That's amazing to me. As you may have read in the previous thread, I had this thing grinding into a 70s Classic Rock tone, by turning the collector towards the 9 Volt rail, and using a MPSA13 darlington in the Q3 position. This needed no changes at all to the circuit resistors or caps. I used a 2N3906 initially for Q2, but settled on a lower gain 2N2907. It's a good sounding circuit either way. Unbelievable!

[MartyMart]

Hmmm, still not sure that I have this right ?
I had a 500k trimmer from Q2 base to ground and it wouldn't give me any joy !
Could have been bad .... so tried a 470k r
Q1 is 5088  Q3 is 5088 and Q2 is an AC128  ... WHICH LOOKS REVERSED  to me ?
I have a ton of boost and a touch of ge warmth, hardly any "fuzz" to speak of
Well, its working but is not any kind of "Fuzz" that I'm used to  !!

Marty.

[MartyMart]

OK, update """""
Jeff's idea of the 1M pot is probably the way to go.
I just tried my fixed 470k with a bunch of "old" AC128's
( the fat ones with no tabs and a flat top )
Several were coming up sounding great !!
I settled on one with hfe of 126, which has a nice
tone and is breaking up well, kind of an "expensive overdrive" tone !
Great for "pop" and "blues" work.
This is "loud  clear and fat" sounding 
It's now the correct way round in Q2   !!!

Marty.

[jmusser]

Hey Marty, glad it worked out for you. It has a real nice tone to it, that I thought was all it's own. It was weird about the pots burning up. I believe that's the same circuit that I burned my thumb and finger on pulling the transistor out of the socket. I believe having Q2 turned around like that gives it some interesting sound qualities, and also some weird bias problems too. It was definitely a first for me to smoke a pot on a fuzz circuit. 

[Stevo]

The thing about Q2 it is PNP needs a positive ground!!!! The reason why it is situated like it is, is to have a positive ground...The resistor on the bottom is the bias for the collector connected to negative...The base is also used to bias this transistor...The Q1 is just an emitter follower (no gain there)...It is all done in the last two transistors for gain and fuzz I THINK THIS IS RIGHT.....We had a problem biasing Q2 which was our gain-fuzz that was missing!!!

[Doug_H]

This cracks me up. I was just working on this one:



I didn't know it was the "60's fuzz/RCA" circuit.

I couldn't get it to work until I did some 2nd/3rd stg tweaking with emitter resistors and etc. It was a "nice overdrive" sound, no great shakes IMO but decent.

I'll try it again with the trimmer.

Synchronicity...

Doug

[Doug_H]

I just ran a sim of this and I believe the key is to get around 1v on the Q2 collector. This sets up Q3 such that its collector voltage is around 4.5v. The verbiage in the book I got it from (in the scan) mentions that Q2 is "biased near cutoff" so the 1v value seems consistent. (Or would that be near saturation? I can't remember and the pnp thing has me all flipped upside down and inside out.)  In the sim, using a 2n2904 for Q2 and a 2n2222 for Q3, R8 needed to be around 180k for those values.

The trimmer is a good idea, but be careful, you may want to leave a limiting resistor in series. I burned up an AC128 playing with the base bias on this circuit.

I'm going to try this one on the breadboard again.

Doug

[MartyMart]

The trimmer is a good idea, but be careful, you may want to leave a limiting resistor in series. I burned up an AC128 playing with the base bias on this circuit.
I'm going to try this one on the breadboard again.
Doug

Yup, burned up an OC71/electro cap and a pot on this so far   !!

"Fire .... da da daah .... I take you to burn.... da da daah.... bap bap"

Marty. !!

[Doug_H]

I just breadboarded this again and got it working. I used a pair of metal 2n2222's and an AC128 for the pnp. It took a 1M trimmer along with a 220k fixed resistor to dial in the bias (in place of the 100k) ( ). As the sim predicted, around 1v on the Q2 collector gives around 4.5 v on the 3rd stg collector.

It sounds pretty nice! Makes a *great* booster for the amp too. I hate this word but it is very transparent. It has a lot of headroom and cleans up well. One thing I like is the slight compression, kind of like a saggy amp. It gently clamps down on chords and then releases- a nice feel and fun to play with.

I would suggest experimenting with the 100k series input resistor for different sounds. Eliminating it makes it much more aggressive and adds some "rawness" to the sound. I kind of like a 10k in there- 100k can be a little too "soft" sounding- although it makes a nice light overdrive. The 10k keeps it smooth but you get more feedback and other fun stuff.

Doug

[MartyMart]

EUREKA !!
Thanks for the tips Doug
15k input resistor and a 1M from Q2 base, Q1/Q3 2n2369's
Q2 AC128.
Nice Fat "compressed" aggresive fuzz !!

PHEW  !!!!

Marty.

[Doug_H]

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

[MartyMart]

I dont miss things like that Doug .... 
"clocked" it a few days ago ......

Ta,
Marty.