67 Super Reverb ugly distortion

[earache]

Vintagekiki

“Same story turns around over 100 posts, and we still don't know what Fender signal test voltages are.
Question
Set volume and all tone controls on 5, Reverb 0, Bright sw OFF
How many mV test signal 1 kHz on normal and on vibrato channel jacks gives a clear signal on dummy load of 2 Ohm and how many is this voltage value.”

* Will try this in next or later post

“If distortion occurs after some time (when the tubes are warmed up) indicates GZ34 and 6L6. Check it.
Question
Are these tubes currently in the FSR new or "as new" ?”

*The rectifier and both output tubes are new, just out of the box. Have been since early in this saga.

“Distortion at low tones and at higher power usually occurs when poor DC voltage filtration or insufficient PT.
When distortion occurs
- measure (+465 VDC) the DC voltage on the first elco, does voltage drops with increasing power.”

* I measure 446 VDC which drops briefly to 415 VDC when I hit a loud “E” chord with the volume over 5 on the Vibrato channel.

“If the + DC voltage on the first elco drops with increasing power, check the GZ34 and the first elco

- measure (360 VAC) the AC voltage at the anode GZ34, does voltage drops with increasing power.”

* I measured 349 VAC at pin 6 of the 5AR4, which dropped briefly to 342 at the loud “E” strum and distortion. Is that a significant amount?

“If the AC voltage at the anode GZ34 drops with increasing power, check the PT.


Question
- Does distortion occur at low tones and at higher power only occurs on FSR 4x10 cab or on other guitar cab.”

* I have no other 2 ohm cab to test this on, or 4 ohm cab, for that matter. I’ve disconnected each of the 4 speakers individually and played thru the remaining three. The distortion problem persisted when playing thru any combination of 3 of the 4 speakers.

- Whether distortion occurs at low tones and at higher power occurs on both channels or on one channel.

* The distortion occurs on either channel after a few minutes, above “5” on either channel

Since you have working on FSR modes, check that one of the mode elements has not changed its value.

*I’m sorry, I don’t understand this question.

[earache]

Dude

Judging from this legend on the back of my scope, I don’t think I should use it to test the caps



The probe I have is a Tektronix P6117 that says right on it “300V”

[earache]

Dude

I think I figured out another way to use this scope to measure the caps as a DMM.

This portable scope also functions as a DMM and has banana plug inputs for test leads.

With regular test leads connected to the scope and measuring the Amp like this



...I see this waveform if I press “scope” and “auto run”



Here’s the DC reading with the scope as a DMM



Here’s the AC reading



Can I ask what you interpret from these readings? I’m really new to scope usage

Comments on the method of test and any other scope usage hints would be welcome

[The Dude]

Ok. First, I'm not at all familiar with that scope, but if it says, 300V is the limit, then so be it. On to your pictures. I don't see anything that's a real problem there. 9V AC on a push pull tube amp B+ shouldn't be a problem. I think the few millivolts of noise is just that- noise being picked up from something via the unused scope probe or probe input when you put the thing in scope mode without being connected to anything.

[Helmholtz]

...I see this waveform if I press “scope” and “auto run”

The scope display shows a time-base setting of 5µs/div and the signal has a period of 2 divisions. i.e. 10µs. This means a fundamental signal frequency of 100kHz with lots of higher harmonics. To me this looks like noise from a nearby switched power supply.

Are you sure the thing works as a scope when you use the DMM inputs? Please check with the scope's manual.


Your power supply voltage drops (sag) look normal.

[vintagekiki]

* Will try this in next or later post

Not one amplifier does not say that it must have a clean sound at 10.
Usually a fat tone occurs when the volume is more than 5-6. Therefore in the later versions a master volume was introduced.
First let's wait what say ... Will try this in next or later post ...

Newer PUs give a higher level than old (vintage) PUs, therefore the amplifier earlier goes into overdrive.

Are you able to compare your FSR with the same or similar (from your colleague)

[earache]

dude

I haven't completely delved into the scope's manual, but - it creates an output image (as you saw) and not an "error" message on the screen when I push the "scope" button with the clip leads/banana plugs attached ....so... I guess it works that way!

Helmholtz -

Thanks for the interpretation of what I'm seeing, and see above for "if the scope works as a DMM" answer

[Helmholtz]

and not an "error" message on the screen when I push the "scope" button with the clip leads/banana plugs attached

Why an "error" message? I think you're expecting too much. The scope most probably only works correctly if you use the scope input - otherwise the unterminated input collects some noise. Most likely the scope and DMM work independently and can be used in parallel with different input signals. If so this would be a great and very useful feature

There's no excuse for not studying the manual.
(Man, I envy you for this nice tool.)

[earache]

Helmholtz

I came to own this Tek scope when the service office I worked at closed. I was the local service and install guy. The company was based overseas, and they basically abandoned the place and the gear when they closed.

As I stated in an earlier post, I am trying to learn how to use an oscilloscope in general, and this scope in particular for working on guitar amps. I have read thru some of this scope's manual, but not the specifics to all of its features...yet. I wanted to be able to post some results and so just forged ahead with the DMM and clip leads to try and get some data to post here. So, I hit the "scope" button while doing a DMM action, and the scope put up a graphic, and not a "this function not allowed" type of message, so I guess it works that way to some degree. Further research is needed obviously.

As always, thanks for your input and interest!

[Helmholtz]

So, I hit the "scope" button while doing a DMM action, and the scope put up a graphic,

Yes and the scope "graphic" seems not related to the DMM input signal. That's why I think that scope and DMM might work independently and require separate/own input signals, but I might be wrong.
BTW, I never expected any of the many scopes I worked with to tell me what is not allowed - actually none of them did.

[earache]

Helmholtz

I see what you mean about the scope/DMM non-interaction, thanks.

Tonight I went over the output section again.

I cleaned and retensioned the rectifier, output, and PI tube sockets and used a tiny brush and DeOxit to clean the inside of all the pin sockets.

I replaced the Phase Inverter tube with a different old used one that I had...both the recto and the output tubes are brand new, and the problem predates me replacing the tubes.

STILL the problem persists.

I tried some different scope measurements and have posted the results to dropbox as videos.

In all the following videos I'm playing the amp just a little louder than "5" into the Vibrato channel, and hitting a hard E chord.This seems to be the most reliable way to replicate the problem.

If you listen to the audio, you can hear the ugly broken up distortion

Here's the scope measuring across the output, and a separate meter is set to read DC at that same point.

https://www.dropbox.com/s/vs28rexh56..._0160.MOV?dl=0

Same setup, the meter is set to read AC.

https://www.dropbox.com/s/phh4s91a3y..._0161.MOV?dl=0

Here's the scope at pin 1 of the phase inverter

https://www.dropbox.com/s/29d6rku7fj..._0164.MOV?dl=0

Here's the scope at pin 6 of the phase inverter.

https://www.dropbox.com/s/pruw3wo79j..._0165.MOV?dl=0

Comments/interpretations/suggestions welcome...thanks in advance

[Greg_L]

Have you tried injecting a sine wave instead of banging a guitar chord? Might be easier to trace if you're not having to play guitar at the same time.

[earache]

Greg_L

What makes this hard is that the distortion is upon a hard transient, like a heavy strum of a low note on a guitar.

Without hearing it I am having a hard time trying to figure out how to replicate something like that with a sine wave signal...I don't know how to set up so that it would be similar.

I guess I'm gonna have to experiment with that more...maybe input 330hZ (low E) with the signal generator, with the amp connected to a dummy load, and keep cranking it up until it goes weird...
so when it goes bad, I would use the scope to trace back thru the output section until it doesn't look wrong??

Just thinking out loud here, looking for some ideas.

[Helmholtz]

It is more or less impossible to interpret scope or DMM data of an ever-changing guitar signal.

I suggest to follow vintagekiki's instructions using a sine input signal.

Also it is confusing that at one time the ugly distortion seemed gone and then "reappeared" on both channels.

Please mind that it is quite normal for a SR to start distorting above a vol setting of around 4, depending on PUs. And the SR distortion is not one of the nicest.
So a comparison with another amp of same type would be helpful.

[Greg_L]

Greg_L

What makes this hard is that the distortion is upon a hard transient, like a heavy strum of a low note on a guitar.

Without hearing it I am having a hard time trying to figure out how to replicate something like that with a sine wave signal...I don't know how to set up so that it would be similar.

I guess I'm gonna have to experiment with that more...maybe input 330hZ (low E) with the signal generator, with the amp connected to a dummy load, and keep cranking it up until it goes weird...
so when it goes bad, I would use the scope to trace back thru the output section until it doesn't look wrong??

Just thinking out loud here, looking for some ideas.

I'm no expert by any means, so anything I might suggest is just thinking out loud right along with you. But I am a guitar player, and I too tinker with amps, and the distortion you're getting is IMO some nasty shit that isn't part of the usual tube breakup most of us know and love.

As mentioned by Helmholtz, and it's a very valid point, a guitar signal has way too much tonal and harmonic information in it to get a good scope reading. A guitar signal looks like a mess on it's best day, so you're not gonna see anything useful from testing with a guitar.

I suppose a workaround could be, and I've done this myself, connect your scope to your guitar and bash a low E. See how the scope reacts. See how "hot" your pickups send a signal into the scope and make a note of that voltage. Then set up a signal generator (you can download one as an app on your phone) and inject a sine wave from the signal generator into your scope. Set the output and frequency to closely match what you saw with your guitar.

Then, pump that signal into the amp. You should hopefully be getting a constant approximation of the low frequency and amplitude of your low E string and from there you will be free to signal trace from point to point with the scope.

I've done this myself and it was very handy in diagnosing my own HF oscillation problems on a blackface build I did last year. I didn't try to approximate a guitar signal, but just using a signal generator to constantly inject signal so I could probe around was invaluable.

Also, have you checked that all of the coupling caps are blocking DC? It kind of sounds to me like a tube is going haywire when it's being pushed. If DC is getting to the pots and grids that will make stuff go crazy. I had a phase inverter coupling cap leak a little DC on an old Marshall one time and it kind of sounded like that.

Again, I'm certainly no expert. I'm just spitballin here.