"Torn speaker" distortion sound on OD...thoughts?

[greiswig]

I originally put this under a different thread, but it seems to be unique enough to merit its own.

Okay, now I'm getting this buzz in my modified D'Lite. But it is in the OD channel. On the clean channel, no matter how loud I get it, no buzz. But on the OD channel, as soon as I turn up the overdrive high enough (9:00 with the trimmer at about 10:00), no matter how soft the volume is through the speaker, I get a sound as though the speaker cone is torn. This is particularly obvious when I play low notes in the low and up a fifth or so from there, but I think it is happening on higher notes, too. It keeps going until the string decays to where it is quiet enough to basically be a clean tone. If I turn down the OD pot far enough to get out of any distortion, and I can get it to go away. When it is happening, turning down the bass pot also gets rid of it for the most part.

This is most noticeable on the neck pickup (I play a Tele and a Strat for the most part, so the pickups aren't very hot), since the bite of the bridge pickup tends to mask that buzz a bit more.

I've changed the .05uF coupling cap between CL and OD to a .01uF, but it is still there. I have 4.7uF cathode bypass caps on both the V2 cathodes. I am using a Bluesmaster PI, and a skyliner stack. I've changed tubes on V2. V2 voltages are in the 198 range. Cathode bypass caps on V2 are both 4.7uF. I still have the 68k grid stopper on V2a.

If I look at a scope while playing (hard to do because the scope won't precisely lock on), it looks to me like there is a spike at the beginning of each of the "knees" that starts the clipped part of the wave. I can't reproduce that waveform with my sine generator until I get an octave or so below the low E.

Any ideas would be appreciated. I'm trying to attach a .wav file that shows it.

[heisthl]

You have tried this with a different speaker right? If so, next you've measured for leaky caps in the od section right? And lastly you're sure both grids in the OD(V2) have a ground reference?

[Bob-I]

A few things to check.

Replace V2, even swap V1 and V2

Check the plate and cathode voltages on V2

It almost sounds like an oscillation. Try looking with your scope at the output of the OD section with no input. Adjust for frequencies well up above the norm, 6kHZ and up. You may find something appearing well above audio that's screwing with the low end.

[greiswig]

You know, the only reason that I have any confidence that this amp will end up the way I want is because of this remarkable group of people. Thank you! I owe several people on this forum the beverage of their choice.

Heistl, Bob, I did try two different speakers. Same effect. I also swapped tubes. No change. The capacitors in that section seem to check fine; they read close to the actual value using my Fluke multimeter.

Heistl, I'm not sure what you mean by a ground reference: this section of the circuit is unmodified from the D'Lite.

V2 pin1 reads 192.8, and pin6 reads 193.8. Pin3 reads 1.695, and pin8 reads 1.704.

I have a borrowed Tek 465 scope that I am *trying* to learn how to use. Bob, at your suggestion, I took a look for issues at the output of the OD section when the input is grounded out. There IS something there: if I read my scope correctly, there's a choppy waveform of about (EDIT) 100mV P-P, as measured at the entrance to the bass pot. I think it's right around 120Hz (A#-ish?). It's audible in the speaker.

If I engage the PAB, it all but disappears from the scope, but the distortion is still audible. Turning down the bass pot has nearly the same effect as engaging the PAB. The amplitude of this wave depends on the volume and the overdrive pots. Is that the likely culprit?

[phsyconoodler]

A parasitic oscillation usually takes place in the output section.Try upping the grid stopper resistors from 1.8k to 5k or more.If there is any change you are on the right track.You can use a jumper so you don't have to remove the resistors until you are sure it's there.

[greiswig]

A parasitic oscillation usually takes place in the output section.Try upping the grid stopper resistors from 1.8k to 5k or more.If there is any change you are on the right track.You can use a jumper so you don't have to remove the resistors until you are sure it's there.

Thanks...maybe I'll give that a try. Per a suggestion from Scott, I'd already bumped them up to 3.3k. But if it's in the output section, why would it be measurable at the entrance to the tone stack at 100mV?

[ampdork]

120Hz would mean rectified AC so I would look at the power supply nodes ripple. Check all the grounds as well.
Light dimmers, ceiling fans, fluorescent lights etc will cause 60 cycle hum but I would still eliminate those from the equation. I promise I will get your shields in the mail but borrow one from another amp if you can. Open chassis will often pick up noises but if you have narrowed it to 120Hz I'd say look at the power supply closely.
A few other thoughts that are likely off the mark considering 120Hz..
Using a choke instead of the 330 ohm resistor.
Elevating the heaters, or even by just using 2 100 ohm R's instead of the CT.
Always wondered about the input grounding to the bussbar. Without an isolated jack this could possibly create a ground loop.

Check the grounds around the tone stack and relay, in fact just check all the grounds but in particular there as well. having it go away with PAB on seems to point to a grounding issue. Still though if it's 120Hz your seeing I am pretty sure it can only be power supply and rectification related.
Maybe suppressor caps acorss the diodes like in a real D amps would be worth trying?
Betting a choke too might work well. Again check the ripple on the nodes and even bias supply (though that is half wave rectified so 60Hz)...
Just some thoughts...

[heisthl]

, I'm not sure what you mean by a ground reference: this section of the circuit is unmodified from the D'Lite.

Ground reference is making sure when you measure from the grid to ground you see the correct ohmage. Say you set the input trim at 50%, from the grid of V2A you should see 68k + 50K or 118k to ground. on V2B you should see grid resistor + drive level wiper to ground.

Another stupid question - you do have the 0.1 cap on the second grid of the PI right?

[greiswig]

No such thing as stupid questions, heistl: as it turns out, no...for some reason* when I redid the PI, I put in a .02 there, so there's a pair of those on the PI. What effect might this have? (*EDIT: there is a discrepancy between the schematic and the layout for the original D'Lite. The layout shows a pair of .1uF caps on the PI. I put a pair of .02uF caps in when I changed to the Bluesmaster PI because of this symmetry. However, when I temporarily jumper a .1uF cap in parallel with the second .02uF cap on the PI, it does not change this 120Hz wave sitting on the OD output)

Shad, thanks for chiming in. 120Hz made me think of switching noise, too. I had "upgraded" the diodes in the rectifier to the UF4007's, because I had heard they were quieter. Is there any way to tell if these are the problem without just replacing them all? And why would it appear on the OD channel only?

[heisthl]

.02 is OK too I was just checking that there was some sort of cap there. It was a stupid question in that with no cap on the second grid even the cleans would sound very distorted.

[greiswig]

, I'm not sure what you mean by a ground reference: this section of the circuit is unmodified from the D'Lite.

Ground reference is making sure when you measure from the grid to ground you see the correct ohmage. Say you set the input trim at 50%, from the grid of V2A you should see 68k + 50K or 118k to ground. on V2B you should see grid resistor + drive level wiper to ground[/quote

Gotcha. Okay, I checked, and the resistances to ground appear correct. I guess this isn't going to be trivially easy.

[greiswig]

heistl, is what you hear in my clip anything like what you were able to fix on yeahyeah's HRM amp?

Also wondering if changing to the Precision PS might help with any of this...

I'm still flummoxed.

[ampdork]

I had the exact same thought this morning. I was wondering if there has been any real D amps with the old power supply and larger plate load resistors. FWIW I have no idea why I would wonder that, no real rhyme or reason except that in my first builds I ended up having to beef up the supply to be rid of some ripple. never did get a choke on it to see.
There again I wonder if any real D amps had higher plate loads without a choke? The more I think about some of the issues I have had in the past with the old power supply (yucky tones like the ones you have come across) the more I wonder how much a choke would help.
One other thing I had a thought about was the cathode resistors. Did i read that you may have had a trim pot on some cathodes for experimentation?
If so I am sure you have checked them out but in the past when I did this I notice there were some real ugly tones if the tube wasn't biased properly. Clangy and gated sounding noises very similar to the clip you posted.
Again just thinking aloud and throwing ideas out there...
Also where was that cap network with the foils together? If it's anywhere near the suspect area I would probably look at it closely and maybe try a swap.
IIRC when you suspect a cap and jump it with a known good one it will only correct things if the suspect cap is going open, not if it is leaky. Again IIRC a leaky cap will just keep on leaking. If it is passing DC then it could be throwing the bias of the next stage off, enough to make ugly sounds.
Well I'd better let some more experienced guys chime in before I get looking in all the wrong places.
Ok one more useless idea. bass pot seems to affect the problem...Have you checked for leaky coupling caps?
Good procedure for testing for leaking coupling caps here:
http://www.geofex.com/ampdbug/ampdebug.htm

[dogears]

I have personally seen these.....

I was wondering if there has been any real D amps with the old power supply and larger plate load resistors. yes

There again I wonder if any real D amps had higher plate loads without a choke? yes

[greiswig]

Thanks again for keeping the ideas coming. Okay, I cleared up the "ghost" 120Hz wave I was seeing at that stage without anything plugged into the amp...it was a cap that I'd put in the PAB circuit after the relay to tame some highs when that was engaged. Took that out, and POOF...the wave went away! There is a little, teeny bit of barely measurable noise there now, with an occasional small spike, but nothing else. Good news, eh?

Wrong! The fizz is still there. I reheated all the solder joints, etc. Still there. I've tried moving wires around. Still there. I don't know if this is yet another red herring, but the fizz is worse with the scope probe hooked up to the output of V2.

Oh, and I scoped the HT leads between each of the resistors, too. There is slight ripple on the HT- A and B leads, but every other HT lead is flat.

I'm attaching a shot of a 75Hz wave sampled after V2. I set up the P-P voltage on the signal generator to match as closely as I could the output of my guitar pickup. Volume is set at 12:00, OD trimmer at about 10:00, and the OD knob at 10:00. Is this supposed to be what it looks like?

If I try to capture a waveform on my scope from a plucked string, it's pretty much impossible...but I think I see that the "spikes" at the front of the squared off part of the wave are particularly high and sharp when this fizz is happening. Is that any kind of a clue?