Using Oscilloscope to Track Oscillation

[tubeswell]

Here's what I tried.

-I chopsticked the EQ leads quite a bit. Seems they are the best they will be as is now
-I put various caps (18pf to 250pf) between g-p, k-p and as snubbers across Rp on stages 1, 2 and 3 (currently have a 250p across Rp on stage 2 only but a 18pf across Rp of stage 3 seems to slightly help)
-I have the gain pot rotated 90 deg with the bright a DPDT with 100p and 500p, both caps are super short sitting against the top/bottom of switch with short leads to pot lugs
-Moved the MV to PT grid wires, no change (thinking about shielding these)
-Ran a coil around the 50p treble cap in TS and grounded one end to chassis
-grid leads are all shielded (except Ptube grid runs - these are twisted unshielded leads running from faceplate to back of amp across PS caps)
-Paul Ruby diode mod across 220k Ptube grid leaks (2xel84 amp)

And you tube-swapped?

[surfsup]

A few but all my tubes are non-NOS crapola like JJecc803S, JJecc83S, TungSol, et al.

My main concern is the noise I see on the scope that masks being able to see oscillation, etc.

[overtone]

Might the noisy fuzz be due to digital distortion somewhere inside Reaper, perhaps the plug-in is running too hot at some stage?
If you can borrow an analogue signal generator you could compare.
Best, tony

[vibratoking]

The scope probe itself will pickup noise. One source is the loop that is formed by the probe and the ground wire/clip. Depending of the scope, the input amp has a noise contribution as well. This should be neglible on a good scope.

The fuzz you are seeing is basically the noise floor of your scope system. Notice that the fuzz appears larger on the channel set to 50mV versus the one set at 5V or 2V. The noise is present on both channels, but you are magnifying the display when you set the voltage range to 50mV vs 5V. It just looks bigger. This is all perfectly normal and expected. You can sometimes improve on the noise coupled into the probe by using a shorter ground wire/clip or by a different orientation of the loop. I suspect you will see the fuzz on the other channel if you change the voltage range from 5V to 50mV.

Your first plot shows an oscillation of roughly 20kHz, which is kind of inline with what you describe - high pitch whining. Also, depending on the circuit, attaching a normal scope probe can kill or change the oscillation frequency due to the loading of the probe. Many times a special type of probe such as a FET probe or other low capacitance/high impedance probe is used to investigate oscillations, but they are expensive and you most likely don't own one. Continue with what you have, but understand what can be happening.

Double stops do different things on different amps, guitars and location of the fretboard. The intermodulation distortion takes on a diffferent character depending on the amount of bypassing capacitance at each stage in the power supply. I have experienced big changes in this regard by changing the power supply capacitance, resistance, choke, etc... Sometimes more capacitance helps and sometimes less.

[surfsup]

overtone, Reaper is pretty good at replay of WAV files, hence it is a recording software. I will work on the amp to take more scope shots this weekend of the PS, etc.

Vibrato thanks. Yes I have the free probes. I figured the same about 2 cycles per square at 100us = 1/50us=20kHz

I didn't think this was oscillation because of the fact it didn't happen on a sine wave at all. I jut thought it was a weird frequency/gain issue due to the square wave being all odd order and amplified at different levels/phase shifts throughout the amp.

[surfsup]

Ok an update, the slight oscillation IS still there in the amp...sigh. So I chopsticked for an hour, no discernable difference on wires I can move around. I did redo two plate leads and one PI screen lead to put more of it against the chassis. I think one plate lead made it slightly better. My PS is this (all caps are film):

15u parallel to 15u (30u)->choke (15h)->15u (screen)->3k3/15u (PI)-> 10k/10u-(cold clipper) ->10k/10u (input and 2nd gainstage)

I put the scope on the three PS nodes of the preamp. Paralleling four 33u electros with all existing caps except the 30u resevoir resulted in no difference in oscillation. Putting a scope on it showed no differences in ripple.

[surfsup]

ok another update. I think I found the oscillation source. the treble cap in the tone stack. I am using a polystyrene cap and here is the scope shot with a probe before (blue) and after the cap (yellow). The yellow clearly shows an oscillation going on.

Its getting late so I will swap that cap tomorrow and see what's up. Any commentary from the peanut gallery is appreciated.

[surfsup]

Okay so figuring the oscillation after the cap would be caused more by the lead to the pot, I experimented with the TS leads. I shortened them, lengthened them, along the chassis, away from each other, next to each other, I currently have the treb lead shielded and the other two longer and against the chassis. Seems to be the best. I think the oscillation on the scope is a function of the square wave, because it looks like that at the input as well.

So here's some interesting info. First stage plate normal and with a 500p cap across the load R. the cap solves the 36kHz "oscillation" but when I play the amp there is still a ringing and fizzy dying of the notes that is subtle on double stops. Single strings sound GREAT. I am starting to think this is blocking distortion not oscillation. Why? Because in addition to the TS rewiring, I basically rewired the entire amp around V1/V2 and the PI, chopsticked, did a rain dance for the gods, as well as ran new leads to the Ptube grids and upped the grid stops from 2k2s to 10k3W - absolutely nothing seems to help. I even tacked in 33k grid stop on stage 2, 100k on stage 3 and 100k on the PI! If anyone has a clue to help I'm all ears.

[Jana]

You're all ears? Interesting visual.

When you did the rain dance, did you rotate clockwise or counter clockwise? It matters.

I haven't read the whole thread so maybe my suggestions have already been covered. If so, go back to dancing.

It looks like a "ringing" on the trace.

Have you tried winding a coil of wire around the leads going to the tone controls? Like Fender used to do. Ground that coil on one end.

Have you tried putting a small poly cap, .047 or .1 uf, in parallel with the filter cap for that stage, maybe the stage prior as well.

Do you live close to a radio transmitter tower? A cell phone tower? Maybe put a 47pf cap on the input jack (parallel the 1 meg).

Have you swapped the treble cap?

Did you rub the fur on your lucky rabbit foot?

[surfsup]

Have you tried winding a coil of wire around the leads going to the tone controls? Like Fender used to do. Ground that coil on one end.

I did fo this around the treble cap. About 40 rotations, 3 inches long...

Have you tried putting a small poly cap, .047 or .1 uf, in parallel with the filter cap for that stage, maybe the stage prior as well.

Yep, across the first two caps (input tube and 2nd ax7)

Do you live close to a radio transmitter tower? A cell phone tower? Maybe put a 47pf cap on the input jack (parallel the 1 meg).

Have you swapped the treble cap?

Swapped the treble caps to gain pot and tonestack. Didn't try a cap at input will do that in the am...

Did you rub the fur on your lucky rabbit foot?

Damnit i knew i forgot tto do something! (Thanks for the suggestions!)

[surfsup]

Update...changed the PIcouplers to 10n from 22n. Also paralleled a bunch of resistors on the grid leaks - finally settled on 110k grid leaks (2x220k on each side in parallel). Helps...gets rid of a lot of fizzy stuff that was there in a subtle way. Bass response is still fine with hb and a sc guitar neck/bridge. Sounds good. DropD as well. But there's still some subtle blatting on double stops that just won't go away.

Tried to parallel another 100u on the cathode, didn't help. Might need a 1000u.
Putting a 250p across Ra of stage 1 smooths out the scope waveform but doesn't sound any different.
Putting the 250p across the backside of the pi couplers smooths it out but again doesnt sound much different.
Cap from input to ground does nothing.

Strange observation: when i dime the amp and run into a dummyload, it sounds like there is a mousewheel running in my amp when i hit a power chord. Like loose change sliding around in a metal bowl. Also my screens are glowing, not at idle, just when i start sending lots of signal - will up the screen resistors also, but any idea on the noise?

I'm so close yet so far....

[surfsup]

Update...I swapped the OT for a pacific OT. New OT so I assume this is a good OT, same issue. Stuff I tried today besides that:

-Swapped ALL the tubes. No change.
-Added a 10R to cathode bias (10.2V to 10.8V). No change.
-Changed 10R to 24R in series with power tube cathodes to get the EL84s up to 11.2V bias. No change. Seemed to actually make it worse.

When I roll the treble down at the guitar, I can really hear this buzziness quite well. I am willing to try anything, even niobium titanium nitride coils!!! Bueller?

[surfsup]

Damn I am gettin' no love here lately. I noticed when I bring up the MV, the PI signal into the PA gets larger until about 7, then from there as the MV goes to 10, the PI output starts dropping rapidly. I figure this is an impedance issue, but I get the buzzings even at low MV setting this shouldn't contribute to the problem, would it? Figured I'd ask.

I should add if I change the grid leaks from 220k to 110k R's, the MV@10 gain is the same peak to peak.

[Structo]

I wish I could think of a suggestion for you.

I think it is common for the OT to make some kind of noise when the amp is turned up into a dummy load.

So I take it you have tried smoothing caps all over the place?

Like between preamp, PI plates?

Across Ra of preamp.

You know, Andy La Blanc wrote a piece about fizz and he attributed it to the screen and plate voltage on the power tube.
He suggested a voltage divider to keep the screen voltage below the plates.

[surfsup]

Structo thanks for checking in!

I think it is common for the OT to make some kind of noise when the amp is turned up into a dummy load.

Thanks for confirming. The other OT I have gator clipped in (pac iron) does the same thing so I'm sure the Edcor is fine.

So I take it you have tried smoothing caps all over the place? Like between preamp, PI plates? Across Ra of preamp.

See photo below. I had a 500pf across stage 1 currently moved it to stage 2. 250pf across stage 3. 180 pf on both Ra at PI. Not sure what you mean by between preamp, PI plates. I also had/have smaller film caps across power supply, also a 33u in addition to the 30 u on the resevoir. I have a 1000u on the PT cathode and the 125R bias resistor for the 2x6n1Ps has a 220R5W in series as well. This upped the plate voltage from 325 to about 370 but I tried a 10R, 24R, 100R, 130R and 220R and still have this situation. The oscillation or blattiness moves around the neck depending on the snubbers I use and where.

You know, Andy La Blanc wrote a piece about fizz and he attributed it to the screen and plate voltage on the power tube. He suggested a voltage divider to keep the screen voltage below the plates.

I am actually about to try this. I was plannning to put two resistors from the PI couplers to the MV dual pot instead of twisted wire. Somehow me thinks the 6N1Ps just don't handle huge signal like this. According to my loadline, the 6N1Ps are at a gain of 12. I am sending about 40-50 Vpp into the poweramp, so that is 170-215V swing. But, I still get this problem with the MV at a very low setting of like 1 or 2.

Maybe one of the preamp tubes is getting slammed too hard... and now for your entertainment: