67 Super Reverb ugly distortion

[vintagekiki]

Online Tone Generator

https://music-electronics-forum.com/showthread.php?t=49531&p=537538&viewfull=1#post537538

http://onlinetonegenerator.com/
Online Tone Generator

Sound from sound card output jack, connect via the shield cable to the amplifier output stage.
Testing amplifier through preamplifier does not produce the right results, because the signal passes through different filters (tone control) and therefore it is recommended to test alone output stage because it's frequency linear.

Disconnect the .001 capacitor from the 2 x 220k mixer point and connect a test signal via .001 capacitor directly to the output stage.

Instead of a speaker, connect a dummy load, rated resistance and rated power.
In parallel, with dummy load connects oscilloscope and AC voltmeter.

Volume from a tone generator or online Tone Generator set to min. to avoid damage to amp, and gradually increase until output signal clipping occurs on the oscilloscope.
When the first signs of output signal clipping appear, read on the AC ~ voltmeter the voltage value and calculate the power of the amplifiers.

[earache]

Hmmm can’t seem to upload the video from the IPad...

Here’s a Dropbox link to a video I took of me playing guitar into the amp and the scope signals and sound produced

https://www.dropbox.com/sh/16wz6p5je...WJ7QpLF4a?dl=0

[earache]

I am very flattered at all of the replies and interest.

As a hobbyist I have worked on and fixed amps for over ten years. I have often bumped up against the limit of my abilities in cases such as this. I’m really excited to finally make the leap into solving audio issues that require an oscilloscope.

Thanks advance for your patience and help. Tomorrow I leave for California for a few days so this post will take a little rest.
I will be reading and rereading all of the comments and suggestions and will get back at it on Tuesday or Wednesday

[earache]

"First I measured the output of the iPod and tried to set it to 500mA" (mv?)

That is a pretty 'hot' signal level.
I would go with 100mv.

Yessir I meant 500mv...

[Helmholtz]

therefore it is recommended to test alone output stage because it's frequency linear.

Agree that it is generally a good idea to test the output stage separately with a dummy load. Problem is that this requires a signal level at the PI input of more than 3Vpp for a LTPI, which is not easily available to everybody.
In addition I like to use a triangular test signal as this allows to judge linearity/transfer characteristic and makes optimal biasing easy.

I think JMF was proposing a different route using the speakers and a 400mV input signal simulating heavy string attack and judge by amp sound and scope.

My strategy would be to analyze DC circuit voltages (plate, cathode and supply voltages) first.

[Pedro Vecino]

I've seen the video. That kind of sound artifacts (not a distortion in the usual sense) I associate them with a defective output transformer.
The first thing I would do is measure the resistance between pin 3 of each socket and the center tap of the output transformer trying to find some symmetry.
It is not a conclusive test but if you find 50 and 100 Ohms (it is an example), then it would be.
I hope I'm wrong.

[vintagekiki]

I’m really excited to finally make the leap into solving audio issues that require an oscilloscope.

An oscilloscope in servicing audio amplifiers (except Class D) is only needed in 5% of cases.
40-50 years ago, the oscilloscope was mostly a rarity, and the amplifiers was then well repaired.
All you need from the instruments is an AVO meter (20k Ohm or more), tech schematics and solid technical knowledge.
When the voltages at the test points were brought to the prescribed value, the amplifier is 90% or more was repaired.
Tiny details mostly experience brings it in order.

tip

When the amplifier is serviced, the failure is first removed from the output stage and then from the preamplifier.

[vintagekiki]

My strategy would be to analyze DC circuit voltages (plate, cathode and supply voltages) first.

Logically absolutely correct.

[vintagekiki]

How test the output transformer

The first thing I would do is measure the resistance between pin 3 of each socket and the center tap of the output transformer trying to find some symmetry.

The short circuit between the 2 neighbors primary winding will not be noticed by the instrument, but will be reflected in the sound through a drastic drop in power, distortion and lack of bass.

The fastest and the safest way to test the output transformer TR3 is to measure the transmission ratio between the primary and the secondary, with known AC voltage at the secondary (5 ... 10 VAC), does it exist symmetric AC voltage between the 6L6GC anodes relative to the mean terminal of the primary of the output transformer.

[earache]

The short circuit between the 2 neighbors primary winding will not be noticed by the instrument, but will be reflected in the sound through a drastic drop in power, distortion and lack of bass.

The fastest and the safest way to test the output transformer TR3 is to measure the transmission ratio between the primary and the secondary, with known AC voltage at the secondary (5 ... 10 VAC), does it exist symmetric AC voltage between the 6L6GC anodes relative to the mean terminal of the primary of the output transformer.

Vintagekiki please elaborate on how to set up and perform this test. I have a variac

[Leo_Gnardo]

An oscilloscope in servicing audio amplifiers (except Class D) is only needed in 5% of cases.

Oh the bursts of ultrasonic oscillation I've found ... which I never would have seen without a 'scope. Without a scope, maybe I can make an amp sound OK, sort of. With one, I can make it sound fabulous.

However, now that I have - half a dozen (!) - all those extras are no big advantage. Heck, I've just become the owner of a scope museum. My 1960 vintage vacuum tube Hewlett Packard, veteran of the U.S. Air Force, is still the main workhorse despite its modest 200 KHz bandwidth. Hardly an amp passes through here without experiencing its scrutiny.

[vintagekiki]

Vintagekiki please elaborate on how to set up and perform this test. I have a variac

You can use variac only if it is galvanically separated from the mains.
I'm using 6.3V ~ from some mains transformer.


EDIT 191003
Connect the AC voltage source (5 ... 10 VAC) to the secondary OT (speaker jack).
Connect one end of the AC voltmeter to middle of primary OT (+ 465V) and the other end on pin3 (octal sockets 6L6GC), and measure AC voltages.
If both primary voltages on TR3 symmetric OT is correct.

[Chuck H]

Oh the bursts of ultrasonic oscillation I've found ... which I never would have seen without a 'scope. Without a scope, maybe I can make an amp sound OK, sort of. With one, I can make it sound fabulous.

However, now that I have - half a dozen (!) - all those extras are no big advantage. Heck, I've just become the owner of a scope museum. My 1960 vintage vacuum tube Hewlett Packard, veteran of the U.S. Air Force, is still the main workhorse despite its modest 200 KHz bandwidth. Hardly an amp passes through here without experiencing its scrutiny.

And, because I can't give two on the button, another

[vintagekiki]

Oh the bursts of ultrasonic oscillation ... ...

Dear colleague
The amplifiers are failed and had ultrasonic oscillation 40 to 60 years ago.
But 40 - 60 years ago, technicians had the knowledge to eliminate malfunction with a minimum of instruments.

[Leo_Gnardo]

Praise The Scope!

Dear colleague
The amplifiers failed and had ultrasonic oscillation 40 to 60 years ago.
But 40 - 60 years ago, technicians had the knowledge to eliminate malfunction with a minimum of instruments.

With all due respect to you vk, and I really do appreciate your input here plus the scads of schematics you have posted.

I s'pose it depends on the level of perfection one is trying to attain. I swear some of the old Fenders & other amps I work on were entertaining dogs and bats with their ultrasonic bursts from the day they were made, 50 years ago. Coincidentally, about the same time I started working on amps. Early 70's college labs had them, it's there I learned to use them. I also worked for the college, maintaining and repairing lab equipment, including scopes. Once I had the power of the oscilloscope in my hands, starting @ 1976, I could eliminate the imperfections that dogged the users of those amps: "hoarseness," "cruddy tone," "ratty distortion," "signal drops out when I play loud," like that, and have their amps deliver clean clear signal instead. If having parasitic oscillations and all the racket they make is taken as acceptable, iow "that's what we expect the amp to sound like" well maybe that's good enough for those folks that accept those conditions. Which includes exactly none of my customers. Maybe I'm lazy? If I can see the fault on the screen as well as hear it, I can do better at analyzing the problem and effecting a repair. For instance, if it sounds like the speaker is bad, and all the speakers I swap in also sound bad, are all the speakers bad? When I can see that ultrasonic burst riding the waveform, and quench it by applying a small disc cap, it sure saves a lot of time swapping in speakers. Maybe I'm not as clever as those old time techs that had no scope. If that's the case, I guess I'll just have to accept my shortcomings.

BTW I first saw a scope in a TV/radio repair shop about 1962. Of course, I was fascinated. And resolved "I gotta get me one of those gadgets, someday." That 9 year old kid is still inside me now. And totally delighted with being "scope rich." I have occasionally had to apply a field fix, like adjusting bias "by ear" on an amp with awful crossover distortion. But I'm not going to go back to the good old days of doing without a scope on the work bench. I'm already doing enough "flying blind" driving places without a GPS. That's about to change very soon.

OK, back to the Super Reverb. Can't wait to find out what's ailing it. Stay tuned for further adventures!