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Thread: 67 Super Reverb ugly distortion

  1. #106
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by earache View Post
    VK - the Fender schematic you posted is much more recent, do those voltage settings still apply?
    Have time for schemarics, voltages, scope pictures ...
    First needs to be started Vibrato ch step by step.

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  2. #107
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    Quote Originally Posted by earache View Post
    So it looks like I might have a set back here.

    I set up the latest test as Vintagekiki described, by putting in V2 and V4 tubes, and playing thru the "Vibrato" channel with controls at 5, reverb and vibrato off.
    The test lead is still connected from point "X" to the .001 capacitor.

    The setback, is that I'm now getting some of that distortion upon heavy attack of notes.

    Perhaps it's because no one is home right now and I'm hitting the notes harder...

    I switched back to just having V1 in place and connecting the test lead from P6 V1 to the .001 cap and still the distortion is there upon heavy attack, playing thru "Normal" channel.

    i checked the speakers for voice coil rub but it didn't seem like there was any.

    I am out of time for today, but the first thing I will do tomorrow is check the output jack and make sure that the ground connection to the chassis is good.
    https://music-electronics-forum.com/attachment.php?attachmentid=55537&d=1570868287

    If you followed the story neatly you would notice that (red point X) referred to Normal ch.

    We will repeat the test procedure for Vibrato ch.
    Temporarily connect test wire from .001uF with anode V2 (pin 1)
    Turn on the guitar in vibrato ch and try the sound.
    If the sound is a loud temporarily connect test wire to the anode V2 (pin 6) and try with a guitar.
    If the sound is loud and clear and the volume and volume controls works, temporarily connect test wire to the anode V4 (pin 6) and try with a guitar.
    If the sound is loud and clear and tone and volume controls work, check .1uF (pin 6 V4), network resistor 220k (blue point Y)

    If the sound is still ugly:
    - check the anode and cathode voltages on tube V2 and V4
    - cleans from oxide ground connection input jacks on vibrato ch to the chassis
    - check tube sockets V2 and V4 for cold solder joints

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  3. #108
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    “We will repeat the test procedure for Vibrato ch.
    Temporarily connect test wire from .001uF with anode V2 (pin 1)
    Turn on the guitar in vibrato ch and try the sound.”

    The sound was loud, and not controllable with vibrato channel volume.

    “If the sound is a loud temporarily connect test wire to the anode V2 (pin 6) and try with a guitar.”

    The sound was NOT as loud and the volume and tone controls did not work.

    “If the sound is loud and clear and the volume and volume controls works, temporarily connect test wire to the anode V4 (pin 6) and try with a guitar.”

    I tried this test and the volume control and tone controls work. After a couple of minutes of chugging a low “E” chord, the bad distortion happens again upon the attack of the note.

    “If the sound is loud and clear and tone and volume controls work, check .1uF (pin 6 V4), network resistor 220k (blue point Y)”

    I will follow up with this last test in a little while

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  4. #109
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    Quote Originally Posted by earache View Post
    Temporarily connect test wire from .001uF with anode V2 (pin 1)
    Turn on the guitar in vibrato ch and try the sound.”

    The sound was loud, and not controllable with vibrato channel volume.

    OK

    “If the sound is a loud temporarily connect test wire to the anode V2 (pin 6) and try with a guitar.”

    The sound was NOT as loud and the volume and tone controls did not work.
    If volume and tone controls not work at anode V2 (pin 6) check capacitor connection .1uF; .022 uF and 250 pF with tone control pots.
    Check the anode and cathode voltages on tube socket V2.

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  5. #110
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    “...check .1uF (pin 6 V4), network resistor 220k (blue point Y)”

    I lifted one end of the .1 uf cap and the 220K resistor

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    If the sound is still ugly:
    - check the anode and cathode voltages on tube V2 and V4”

    V2 P1 anode. 279
    V2 P6 anode. 282
    V2 P3 cathode. 2.3
    V2 P8 cathode. 2.5

    V4 P1 anode. 283
    V4 P6 anode. 276
    V4 P3 cathode. 2.6
    V4 P8 cathode. 2.6. (soldered to P3)

    I saw your last post and will continue to follow the directions and post results.

    Sometimes I find that posting a picture of the test results is preferable as you can see exactly what the results were and the method of test (if it’s wrong you could comment)

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  6. #111
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    The voltages on V2 and V4 are fine.

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  7. #112
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    “If volume and tone controls not work at anode V2 (pin 6).....
    check capacitor connection .1uF;

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    Looks ok

    .......022 uF

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    Looks ok

    ....and 250 pF with tone control pots”

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    Value out of spec?

    I continued on down the line and found what looks like a few bad caps

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    I unsoldered one leg of each component during test

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  8. #113
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    Quote Originally Posted by earache View Post

    Click image for larger version. 

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    Or don't have good contact with elco, or cathode resistor is defective.

    Click image for larger version. 

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    I unsoldered one leg of each component during test
    Correctly
    Before testing, soldering elco contacts or clean from the oxide.
    When testing cathode elco unsolder cathode resistor from elco

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  9. #114
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    Vintagekiki

    Just as you had thought, the parts that tested bad in the last post tested ok after cleaning the corrosion off of the leads.

    I have resoldered them all back in place.

    Since my last post I have done the following:

    Took off the output jacks, cleaned the chassis at that spot and then reattached the output jacks

    Did the same for both input jacks at the normal and vibrato channels

    Reversed the mod that put reverb and vibrato on both channels

    Touched up the solder on all preamp tube sockets

    I also disconnected one speaker at a time and played thru the amp to see if just one speaker was causing the distortion.
    That didn’t prove anything as the ugly distortion is still there.

    ==============================

    The distortion doesn’t show up right away when you turn on the Amp.

    It happens especially when chugging on low notes, and above “5” on the volume

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  10. #115
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    Quote Originally Posted by earache View Post
    ......The distortion doesn’t show up right away when you turn on the Amp.

    It happens especially when chugging on low notes, and above “5” on the volume
    That is symptomatic of a failing filter cap much of the time. If you have a scope, look for ripple on supply lines. If not, measure AC with your DVM. You could also tack a known good cap with proper voltage rating across each filter and see if the problem goes away.

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    Last edited by The Dude; 10-17-2019 at 03:54 AM.
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  11. #116
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    Dude

    I have a scope I’m just learning to use, can you elaborate on how to look for ripple on the supply?

    Is some AC ripple acceptable? If so, how much?

    I replaced all the filter caps back in 2011. Might it be a bad solder joint there? I used Sprague Blue Atoms back then... but I suppose anything can fail...I’ll check the solder tomorrow and look for your direction on the scope usage...thanks!

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  12. #117
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    First, make sure your scope is rated above B+ DCV. Use a X10 probe. There will be some ripple, but it will be a very small percentage in relation to the DCV. It could be a flaky joint or a bad cap...... or neither- I'm not guaranteeing that's your problem, that's why we test. Even new caps do sometimes go bad, especially if the vendor has had them sitting on a shelf for a while.

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  13. #118
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    Dude

    Noob questions:

    Would I just clip the scope leads over each individual filter cap to test?

    Would I need to input a signal?

    The scope would need to be rated better than the capacitor's rating I imagine, like 500V or better, correct?

    Couldn't I just use my meter to measure for AC across the capacitor?

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    Last edited by earache; 10-18-2019 at 01:35 PM.

  14. #119
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    Same story turns around over 100 posts, and we still don't know what Fender signal test voltages are.

    https://music-electronics-forum.com/showthread.php?t=49793&page=3&p=541209&viewfull=1#post54 1209
    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.

    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" ?

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

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

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  15. #120
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    Quote Originally Posted by earache View Post
    Dude

    Noob questions:

    Would I just clip the scope leads over each individual filter cap to test?

    Would I need to input a signal?

    The scope would need to be rated better than the capacitor's rating I imagine, like 500V or better, correct?

    Couldn't I just use my meter to measure for AC across the capacitor?
    1) Would I just clip the scope leads over each individual filter cap to test? Yes, that will work.
    2) Would I need to input a signal? Nope.
    3) The scope would need to be rated better than the capacitor's rating I imagine, like 500V or better, correct? Basically, yes. Technically, you're only concerned about the actual voltage that is there, but usually caps are rated accordingly.
    4) Couldn't I just use my meter to measure for AC across the capacitor? Yes, that's another way to do it. IMO, it's easier to see it on a scope.

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  16. #121
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    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.

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  17. #122
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    Dude

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

    Click image for larger version. 

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

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  18. #123
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    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

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

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

    Click image for larger version. 

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    Here’s the AC reading

    Click image for larger version. 

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    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

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    Last edited by earache; 10-23-2019 at 02:26 AM.

  19. #124
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    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.

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  20. #125
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    ...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.

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  21. #126
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    Quote Originally Posted by earache View Post
    * 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)

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  22. #127
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    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

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  23. #128
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    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.)

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  24. #129
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    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!

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  25. #130
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    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.

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    Last edited by Helmholtz; 10-24-2019 at 05:05 PM.
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  26. #131
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    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

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  27. #132
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    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.

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  28. #133
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    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.

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  29. #134
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    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.

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    Quote Originally Posted by earache View Post
    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.

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  31. #136
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    Quote Originally Posted by earache View Post
    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.
    Get the story back in the technical flow.
    How many voltage gives a clear signal on dummy load of 2 Ohm

    Unanswered question
    Quote Originally Posted by vintagekiki View Post
    Are you able to compare your FSR with the same or similar (from your colleague)
    Maybe your guitar gives a heavy strum of a low note.

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  32. #137
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    Two weeks the thread is inactive.
    Does this mean that the FSR has been repaired, and run around the stage.

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  33. #138
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    Vintagekiki

    No, the amp isn't working yet. I'm getting together a more credible signal generator and will continue testing once I have it working.

    I've also printed off the entire thread and am going to review every thing that's been suggested to make sure I haven't missed anything.

    Thanks for your continued interest! You and all of the other posters on this thread have taught me a lot!

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  34. #139
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    Easy, easy pace.
    Just so you know the forum has not forgotten you.

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    I'm going to relate an anecdote from years ago.
    I had a silverface Bassman head that worked fine, then seemingly started cutting out momentarily while playing. A bunch of techs took a stab at it, including Gerald Weber, twice. Nobody could fix it.
    Turns out that what was happening, is that I had started to play louder in the band I was in. I was resting my palm on the strings just in front of the bridge. With the pickups I was using, as I picked, I would slightly compress the lower strings sending very low frequency signals to the amp.
    The amp was trying it's best to amplify those 1 to 10 hertz, or whatever, pulses, but just didn't have capability. It would spend all the available power trying to amplify those low transients and the audible signal would cut out. I could even make a tremolo sound by hitting a note or chord, and push my palm into the low strings at intervals.
    Of course, once the problem was identified, it could be properly addressed. The amp worked as originally intended, but I was definitely not using it as Fender intended.
    Some small circuit tweaks and a bass cut cap on the input cured it right up.

    The moral? This problem might be outside "the box". Is there a small strand of steel wool stuck on the reverb tank recovery that only shorts on high volume transients? ... or something like that?

    I just had a guy bring in a Les Paul that had been to a Gibson warranty tech but still had "ground noise issues".
    The "issue" was that he was using coated strings, which had partially worn through, so as he played, he was alternately grounding and ungrounding and getting that little static pop that you hear, a gajillion times as he made and broke, and made contact with every wind along the strings.

    EDIT: I forgot to add the time a Traynor head would make a godawful static on one note in particular. After some sleuthing, and figuring out the amp HAD to be on the speaker cab, so the problem was vibrational, but poking everything inside failed to make any noise, I saw that on the most problematic note, at volume, one ceramic disc cap with slightly long leads, would resonate and physically oscillate quite a bit further than I would have guessed it could, and was shorting on whatever lead was nearby. BzzzzzzTttt!
    A little bend and some goop, good to go.

    Did a voice coil rub get ruled out? Does this amp make the same ugly distortion plugged into a completely different cabinet?

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