Whiny Tubescreamer

[Ortiz]

I've made a Tubescreamer from General Guitar Gadgets and I'm having some trouble.

Whenever the tone pot is turned to full, a high-pitched squeal comes through. A similar squeal is also heard at varying intensities as the drive pot is turned up. So it sounds great as long as both the tone and drive pots are turned down.

I don't even know where to begin as this is my first pedal.

Parts are per the layout except a 47pF cap in place of the 51pF cap, which I was told would be negligible.

Voltages:
IC
1 - 4.22
2 - 4.23
3 - 4.22
4 - 0
5 - 4.22
6 - 4.23
7 - 4.23
8 - 8.54

Q1
E - 2.857
B - 3.364
C - 8.55

Q2
E - 2.723
B - 3.23
C - 8.55

Thanks!

[BrianJ]

Check C5 and R11 again.  Without those two parts, the tone control amplifier would be running "open loop" at high frequencies.  Oscillations would be mandatory.  Is the drive pot noise similar or identical?  It may be a different problem entirely.  Is the circuit in a grounded enclosure?  Sure was an ambitious first pedal choice, but it looks like you jumped most of the obvious hurdles.  Do you have a scope?

Good luck

[Ortiz]

By "check C5 and R11 again", do you mean check if the components are still good? I believe they are.

Actually, I should've said the drive pot affects the frequency of the squeal, not the intensity. So as you turn it, it's as if you're tuning a radio. Actually that's just what it sounds like. The noise on the tone pot is more of a shrill screech. So yes, they sound different.

I don't have or have access to an oscilloscope but I have a mic and this nifty oscilloscope software I found a while back.

Yes, it's in a grounded enclosure, but I haven't heard a difference whether it's in the enclosure or just sitting on my desk.

Thanks for the help!

[BrianJ]

I meant for you to check if they are insalled properly.  Are you bypassing the power supply (specificlly the 4.5v ref) to ground with a cap?

[Ortiz]

I meant for you to check if they are insalled properly.

Yes, I believe they are.

Are you bypassing the power supply (specificlly the 4.5v ref) to ground with a cap?

I'm not sure what you mean by that but I followed the layout exactly. I didn't really look at the schematic. 

[Ortiz]

Hmm. I was just fiddling with the guts of the effect while listening to the squeal, when suddenly it stopped. I froze and realised that indeed it was gone but was unable to get it to go away again. I'll try resoldering all of the off-board connections but it's possible that I touched or moved something else.

[Ortiz]

Okay I've done that and now the squeal from the drive pot is gone, but the squeal from the tone pot is still there. Any ideas?

[Ortiz]

Any ideas? Anything I could try at all?

[Ortiz]

Anything?

[gaussmarkov]

Voltages:
IC
1 - 4.22
2 - 4.23
3 - 4.22
4 - 0
5 - 8.54
6 - 4.23
7 - 4.23
8 - 4.22

hmm.  it looks to me like you might have pin 5 swapped with pin 8.  the 9v supply is supposed to be attached to pin 8.  pin 5 should be at the same reference voltage as 1, 2, 3, 6, and 7.  did you use your own layout or the ggg layout?  on the other hand, you may have the hookup correct but not the labels above.  just to be sure, check the labelling of the ic pins in the faq (8 is across from 1). search for the phrase "Dual Op Amps have this pinout." i hope this helps.

[Ortiz]

Voltages:
IC
1 - 4.22
2 - 4.23
3 - 4.22
4 - 0
5 - 8.54
6 - 4.23
7 - 4.23
8 - 4.22

hmm.  it looks to me like you might have pin 5 swapped with pin 8.  the 9v supply is supposed to be attached to pin 8.  pin 5 should be at the same reference voltage as 1, 2, 3, 6, and 7.  did you use your own layout or the ggg layout?  on the other hand, you may have the hookup correct but not the labels above.  just to be sure, check the labelling of the ic pins in the faq (8 is across from 1). search for the phrase "Dual Op Amps have this pinout." i hope this helps.

Yeah, thanks, I had 5 and 6 labeled wrong, they're correct in the layout. I used the GGG layout.

[Ortiz]

Has anyone successfully built a Tubescreamer from the current GGG layout? I'm starting to think this my be a design flaw.

[PharaohAmps]

When you say that you used the GGG layout, does that mean you built a PCB using Press'N'Peel Blue?  Or did you buy a PCB from JD / GGG?  Or that you used some perfboard from Radio Shack and copied the parts layout?  I've not built a TS with this layout but I know folks that have.  If this is your first pedal, please don't be so quick to assume that you did everything right or that there's a "design flaw."

Let's get some information first:  Voltages are helpful, of course, but if you're building this pedal on perf, then you might get oscillations from anywhere.  What / how are you building it, and are you sure your offboard connections are okay?  Positive?  We'll all try to help as much as we can here.

Matt Farrow

[Ortiz]

I bought a PCB from GGG and used that. Sorry for not making that more clear.

I'm fairly certain the offboard connections are good.

[gaussmarkov]

have you looked for any accidental bridges across traces made by solder?

[Ortiz]

No solder bridges. I'm certain of that.

[gaussmarkov]

when your pedal is squealing, is the squeal sensitive to moving any of the hookup wires?

[Ortiz]

when your pedal is squealing, is the squeal sensitive to moving any of the hookup wires?

Yes, mostly from the wires going to the drive pot. The squeal is also affected by touching the pot itself.

[gaussmarkov]

when your pedal is squealing, is the squeal sensitive to moving any of the hookup wires?

Yes, mostly from the wires going to the drive pot. The squeal is also affected by touching the pot itself.

well, that is mighty suspicious.      i would try to see whether i could remove this effect by replacing the pot with another pot or, if you do not have a spare, replacing the pot with a fixed resistor.  you can use any resistor from 0 (wire) to 500k.  it's like setting the pot at a fixed point.  the more resistance you use the more gain.  too much resistance and you get squealing/oscillation.

i am not expecting this to fix all of your problems, based on what you have described.  but i would try to fix this problem first.

someone else surely has a better grip on these things than i do (and will pipe up).  maybe the best place to start is with the pot replaced by a wire.

[Ortiz]

Hmm. I haven't tried it with a resistor yet, but shorting it out does nothing. However, when I touch the wire coming from R8 (the blueish wire on the layout), the squeal stops and I can turn the other two pots and there is no squeals.