Op-Amp "stuttering" !! LM741 ??

[MartyMart]

This is wierd .... I'm designing a "cut down" Rat  around an LM741/TL071
using minimum parts and 1N4001 clipping diodes to 4.5v
It works fine, up to about 1/3 rotation of the drive pot (250k) then for a
very short extra turn I'm getting "square wave tremolo" - which is quite
cool actually, though NOT what I want.... then a 1mm turn more and the
signal "dies" being replaced by a regular "thump...thump"

Pinout :
Pins 1 & 8 not connected
Pins 2 & 6 feedback loop with 250pf cap and 250k pot plus 300ohm/2.2uf
to ground off pin 2.
Pin 3 input via 1M to ground 0.1uf cap 470k to 4.5v and 1k2 to pin 3
Pin 4 grounded
Pin 5 not connected
Pin 7 to 9v  ( 8.5 )
Pin 6 output continues through 1k resistor to 2X 1N4001's to the 4.5v line ( 2x10k's and 10uf VB )
a 3n3 cap to ground, then 2.2uf to output jack .......   phew !

All I can think thats missing is a 4.7uf/2,2uf after the feedback loop before the diode pair ... !

any ideas as to the cause of the "st-st-st-stuttering"  ??

Cheers,
Marty.

[MartyMart]

all those views and nothing .......  :shock:  !!

OK, seem to have solved it, but that posses another question,
(as normal !)

I "futzzed around" ( technical term for "adjusting" ) for a while, adding
the 4.7uf coupling cap to the diodes, changed the 470k from 4.5v to a
1M ( as in the "multi-rat" ) and also a 30pf comp cap across pins 1-8 ..
though I KNOW that the 741/TL071 are "internally compensated" - I was
clutching at that particular "straw" !!
..... NONE of that worked, nor did it have any bearing on the stutter/osc
whatever you want to call it .....
NE5534ap pushed the onset of the "stutter" further up the gain pot ......

...... EUREKA...... changed the gain pot to a 100k .... FIXED  !!  

So, question .... how the heck does the "Multi-Rat" work with a 250k pot?
or ....... doesn't it ?
Anyone built one and have oscillation problems ... ?
and why does a 250k pot cause such "devastation" in this circuit but not
in similar "op-amp" OD's / Distortions .......  TS-9/SD-1 etc etc   ?
Is it due to connecting my diodes to 4.5v instead of "ground"    !!!!

BTW - It's now a fine sounding Dist/Fuzz without the "tone pot" and hence
no need for the transistor at the end for volume recovery  

Cheers,
Marty.

[hank reynolds 3rd]

Hi Marty,
I made the multirat i think...a switch for 'vintage' and turbo???..

Built it on perf and it works fine...except for the centre area on the gain pot....it oscillates like f**k!!!!
I think the 100p cap in the gain section needs raising (i read it somewhere in a thread...),...but apart from that it sounds pretty hot!!!

Ta
Sam

[hank reynolds 3rd]

oh yeah...I just used a 100k pot too, as i saw other rat schems using this,..and it still oscillates!!! so i'm guessing the 250k (220k???) would send it nuts!!

And the opamp is an lm308, so i'm guessing its something to do with the compensation cap...maybe it works differently to the 301...

[Paul Perry (Frostwave)]

Hart to say! but, I think somethingis oscillating at an ultrasonic (prob RF?) frequency, and this is being rectified by the electronics somewhere & shifting bias so it goes on & off.
Try shielded input and output. Try caps (.1 ceramic and 100uF electro) across power input.

[WGTP]

I'm using a 250K pot on my Rat clone and haven't had any problems with oscilation, although after 1/2 way it really starts sounding like the end is near...   :evil:

Also, the JFet is a buffer, IIRC and not a gain stage, to isolate the tone control from the output.  I think making it a gain stage is a cool idea though.  Just a suggestion, but a Vulcan (?Jfet) stage would be worth trying.  

[MartyMart]

Hart to say! but, I think somethingis oscillating at an ultrasonic (prob RF?) frequency, and this is being rectified by the electronics somewhere & shifting bias so it goes on & off.
Try shielded input and output. Try caps (.1 ceramic and 100uF electro) across power input.

Thanks guy's, at the moment power rail has a 1N4001/47uf across it then
VB from 2x10k's with a 10uf to ground off that too ....
perhaps i could "squeeze" in a 100uf then then a  .1 ..... !!

It's sounding pretty "FAB" with the 100k anyway now  
WGTP, yeah the end is near at about 3/4's of the 100k .. !!!!  :twisted:

Marty.

[EdJ]

Hi,coincidently i had the same effect in an MXR DISTORTION+.
Haven`t had the time to look at it again.
Don`t suppose they put a few shy 741`s on the market did they?
Greetings,Ed

[MartyMart]

Hi,coincidently i had the same effect in an MXR DISTORTION+.
Haven`t had the time to look at it again.
Don`t suppose they put a few shy 741`s on the market did they?
Greetings,Ed

Hmm, I did think of that for a moment, but it was the same with a TL071
and an NE5534ap ..... !!

Marty.

[R.G.]

Likely intermittent ultrasonic oscillation that happens as the gain goes up. You have an exceptionally high gain available the way that is set up, and any quirk of wire placement could make it sing.

[EdJ]

That is very likely happening with mine;i was going to try out a few ideas and inserted a few wires that are going to a breadboard.
Haven`t got the time right now but i will route the wires better and see if that will cure.
The effect is actually kind of cool really :lol:
Greetings,Ed

[GuitarLord5000]

I know that I'm raising this thread from the dead, but I'm having a similar issue.  I purchased one of the newer Proco Rat pedals a few years ago.  Swapped out the opamp for the lm308.  Replaced the 30 pf compensation cap with a mylar, and switched to a green LED (because it looks cooler.  sue me).  During all of this, I ended up damaging the 100 pf cap across the drive pot, so I drilled out the pcb and placed a much larger (physically) 100 pf cap in its place.  Everything seemed to work fine for a while.  Then, it started to stutter.  I thought that maybe one of the solder joints had started to ground out against the chassis, or something similar.  But that's not happening.  While I was troubleshooting, I turned the drive down from full on to about halfway.  As I was turning the drive knob down, the THUMP....THUMP.... started going faster thump thump thump thump.  A little less drive and it started going thumpthumpthumpthumpthumpthump.  Turned it down just a tad more and suddenly I had sound!  This one has me shaking my head.  My first thought is that somehow the opamp got screwed up.  I have a socket, and I intend to check that first.  Just wondering if anyone has any other ideas.

[PRR]

I did not know you could get 30pFd in a Mylar.

If a 30nFd slipped in, the ultrasonic compensation would transcribe to the thump zone.

[GuitarLord5000]

I did not know you could get 30pFd in a Mylar.

If a 30nFd slipped in, the ultrasonic compensation would transcribe to the thump zone.

You're right.  Sorry.  Meant to say 30pf Silver Mica.

[PRR]

Re-read R.G.'s old post.

[GuitarLord5000]

I always pay attention when RG 'speaks'.  Lol.  That is one super smart dude.  I just find it hard to believe that there is a quirk in the wiring all of a sudden in a factory made pedal that worked previously (the wiring harness on the new proco rats are pretty damn good.  my minimal tinkering hasn't changed the layout of the wiring at all).

I changed out the opamp several months ago.  It worked flawlessly after the opamp change.  My recent changes were the 33 pf silver mica cap, the green LED, and the 100pf replacement cap.  The problems started after those changes.  Last night, I swapped out the opamp for another 308 that I have handy and it still stutters.  I'm convinced that the opamp is fine.  I'm convinced that the wiring is fine.  I'm beginning to wonder if the 33pf cap is bad, causing problems with compensation.  Or if something is wrong with the 100 pf cap.

[PRR]

I would not change a Ceramic cap under 1,000pFd (1nFd). The small ones are as perfect as caps get. (Yes, large ceramics are made of different stuff and do have problems.)

We used to think that Mica was great stuff. In the 1930s it was the bee's knees. In WWII the good stuff ran low and we were testing the chum for quality. Demand for Mica is down in the 1950s and I would think the mica in old Fenders is good stuff. However recently I have been seeing too many tales of mica gone bad. Not so I know what to avoid. But maybe you should try other caps, including small Ceramic.

[pinkjimiphoton]

sounds to me like your issue is the way the board is made.
you likely only have connection on one side, but the socket for the 100p cap is probably connected to both sides of the board. by drilling it out and putting a larger one in you may have messed one of these connections up. the 100p is really just a snubber isn't it? i'd have to look at the schematic for it, i just built one of these last nite with an lm301 instead of a 308 (no difference tonally with that, the 308, the 741, the tl061/71 that was noticeable really, so i went with one of the venerable 301's just for mojo factor, them old can ic's just look cool)

so... i'd say check your solder there. you may find ya need to get a little creative to make sure that the cap is fully connected. if one side of the trace is damaged and not making contact, that could definitely make the circuit unstable enough to motorboat.

i learned of this issue fixing a prescription electronics experience pedal that was all buggered up. THAT was a bitch, but it was exhibiting similar behaviors for similar reasons. i had to run jumpers in some cases from the top and bottom side of the board to the component to enable proper connection. but it worked.

+1 on paul's suggestion of ceramic caps. i get 'em in decades at tayda for like a penny a piece.
to my ears, ceramic often sounds every bit as good as more expensive caps, and in many cases better.