Whiny Tubescreamer

[Ortiz]

I'm afraid I must take back what I said before. The squeal is still there after changing R6. I thought it was gone because I was using the humbucker on my guitar. However, after turning down the volume I realized it was still there. (I think the squeal was gone because of the higher output, not the humbucking.)

As I turn down the volume on my guitar the pitch of the squeal goes down, and stops when the volume is all the way down. So there seems to be a connection between the squeal and the intensity of the input signal.

One thing I never thought to mention but might be useful is that whenever the squeal is present, the signal sounds gated.

[phaeton]

One thing I never thought to mention but might be useful is that whenever the squeal is present, the signal sounds gated.

I can't offer you any help at the moment, but thanks for mentioning that.  I'm battling the same problem in the second transistor stage of my current project (my own design, not TS tho- but i've been silently following this thread) .  According to my measurements the oscillation is screwing up the biasing of the transistor.  I wasn't sure if that was possible or not, but your statement confirms it.

Thanks for sharing, and at least know you're not alone, heh.

Back to the bench!

[gaussmarkov]

ortiz, i had a quick thought so i am dropping it off for you to try.  besides using an audio probe, you could test whether op amp 2 (IC1b) is oscillating by following dave's earlier suggestion: put a cap across R17.  something small, like the 47pF you used for IC1a.  if the squeal stops, then we know that IC1b is oscillating (i think).

[johngreene]

I think that if the guitar volume control is causing it to change then the whole thing may be oscillating. I know you probably have done is several times but I would check the grounds from input to output again. Try a different guitar cable too.

--john

[johngreene]

In the TS9 "R6" goes to ground, and not the Vbias supply.  I've always wondered what the need is to bias here as well as the input signal to this stage.  Surely if the input is sitting idle at half the supply, this pin could be grounded with no affect?  The feedback path in the tone stage goes to ground on the TS808, and not Vbias. 

There really isn't a 'need' because it's connected to Vbias through a capacitor. However it can provide a couple of advantages, no DC differential across the cap so there's never any 'charge-up' or 'settling' to happen. But probably the true reason is just layout related. Vbias was easy to get to than ground at that place on the PCB.

There's also disadvantages to connecting it to Vbias, the big one is that if Vbias is not bypassed well enough you get coupling from one stage to another.

--john

[Ortiz]

ortiz, i had a quick thought so i am dropping it off for you to try.  besides using an audio probe, you could test whether op amp 2 (IC1b) is oscillating by following dave's earlier suggestion: put a cap across R17.  something small, like the 47pF you used for IC1a.  if the squeal stops, then we know that IC1b is oscillating (i think).

Well there's some good news and some bad news.

The good news is the squeal stopped when I put a 0.02uF cap (I tried a 68pF cap, but it just changed the frequency of the squeal) across R17 and the squeal stopped!

The bad news is I still get a squeal when I turn the drive pot up.  This squeal is a little different. It's a little lower pitched and the volume on my guitar doesn't change the pitch, it only stops the squeal when the volume is all the way down.

Edit: Before and after I put the cap in, I tried different cords and there was no effect.

[gaussmarkov]

I think that if the guitar volume control is causing it to change then the whole thing may be oscillating. I know you probably have done is several times but I would check the grounds from input to output again.

so johngreene seems to be right.  (thanks john! )  you clamp down on IC1b and you still have a squeal.  looks like you have to meticulously check your connections, especially to ground. 

[Ortiz]

Argh! I just did all that. I guess I'll look again.   

[gaussmarkov]

o.k.  i have a successful breadboard build of the ts808.  i had to make one sub: a 51K resistor became two resistors in series that total 60K.

here is my breadboard layout and here is my schem for component labels.

my battery was 9.4v.  my voltage readings were

Q1: c 9.4, b 4.25, e 3.81
Q2: c 9.4, b 4.25, e 3.81
IC: 1 4.7, 2 4.72, 3 4.69, 4 0, 5 4.7, 6 4.7, 7 4.7, 8 9.4

so these check out.  no squealing  , which doesn't help you much (yet).  i guess i am going to experiment with creating some squealing.

[Ortiz]

I haven't really had the time to really sit down and look at it, but what I think I'll do is either resolder absolutely everything on the PCB and offboard connections, or I'll buy another PCB and move everything. Although, that won't exactly tell me what's wrong with it. It could possibly be the PCB since I already found one tiny flaw that shorted two paths.

Thanks again for the effort you're putting into helping me. I guess this is a leaning experience for both of us. 

[gaussmarkov]

Thanks again for the effort you're putting into helping me. I guess this is a leaning experience for both of us. 

yep!   and you are most welcome.  i have always fournd the forum to be a generous place.

[yeeshkul]

I have very similar problem: osculation starts when all the pots are all the way up - it can be eliminated by any of the knobs turned down a bit. Also, the volume knob on my guitar makes strange oscillation sounds ... 

[snoof]

Re-check your grounding scheme.  I've had this problem a few times while testing various OD's, and it's always been that I've forgotten to attach ALL the grounds properly.