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Thread: 67 Super Reverb ugly distortion

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    67 Super Reverb ugly distortion

    Hello all

    I’m looking at a 1967 Super Reverb that, when you play it hard or turn it up, it has ugly distortion.

    It happens on both channels.

    Sending the sound to a different speaker cab has no change.

    I’ve cleaned and retensioned all tube sockets.

    I’ve swapped in known-good tubes for all the preamp tubes, one at a time,no change.

    While turning the Amp on and off during the preamp tube swaps, the rectifier arced and blew the fuse.

    I’ve replaced the rectifier with a brand new Tube Doctor 5AR4.

    Poked around inside the Amp with a chopstick while it was live, no apparent bad solder joints or broken/burnt components.

    I’ve replaced the power tubes with brand new Sovtec 6L6GCs.

    Bias of the new power tubes looks good at 37mA and is even at both tubes.

    The Amp was serviced in 2011. It had the power caps replaced with a full complement of Spragues.

    The bias cap was replaced then as well, and also the resisters on that little board.

    The 100K resistors that tended to make that “sizzley bacon frying” sound were also all replaced at that time.

    All of the other components are original.

    Has anyone else in their experience had this situation?

    I have to think that the problem is closer to the output...I haven’t tested the speaker jack very closely but I have cleaned the contacts on it and all jacks and pots

    The Amp has been actively gigged since it had the service in 2011. It’s my sons amp so I know it was gigged locally and also been in and out of cars and clubs like 20 times a year.

    Click image for larger version. 

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    http://ampwares.com/schematics/super_reverb_aa763.pdf

    Thanks in advance for your thoughts

    Earache

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    Last edited by earache; 09-28-2019 at 02:41 PM.

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    Senior Member Pedro Vecino's Avatar
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    I imagine you have both channels assigned to reverb and vibrato, right? It only has a 220K resistor connected at the PI input.
    Measure resistors around this circuit: 82K / 100K / 2 x 1M/ 470R / 22K / 100R ...
    An inactive triode in the 12AT7 it could also generate that problem. Measuring voltages on the plates could detect a severe imbalance.
    If all is fine you could have a problem in the output transformer.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by earache View Post
    I’m looking at a 1967 Super Reverb that, when you play it hard or turn it up, it has ugly distortion.
    I'm going to ask for clarity on "play it hard". Does this mean something like a hot input and/or heavy attack, but the amp isn't turned up loud? And, are we certain this problem never existed before and is not just being discovered now because of new playing circumstances? I'm wondering about a possible parasitic oscillation that may have always existed.

    Quote Originally Posted by earache View Post
    It happens on both channels.
    So it's past the mix resistors following the preamps. EDIT: Pedro may have noticed a "reverb on both channels" mod. Please report on this. And on that note, can you tell us if any changes were made to the circuits immediately prior to the problem?

    Quote Originally Posted by earache View Post
    I’ve cleaned and retensioned all tube sockets....

    Poked around inside the Amp with a chopstick while it was live, no apparent bad solder joints or broken/burnt components...

    I have cleaned the contacts on it and all jacks and pots...
    This is all well and good. But just to rule out mechanical causes further you should give the live amp a whack to see if it makes any noises.

    Are other performance characteristics unchanged? When not "played hard" the amp sounds no different than it ever has? That is, no additional audio symptoms to report? Volume at setting is the same as before? etc.

    If you have a signal generator, can the ugly distortion be recreated with a steady input signal? If so then a scope might be useful for tracing where in the circuit the anomalous distortion starts.

    Is the distortion a seemingly mono tonal noise (crackle or fizzle) or does it seem to be sympathetic to frequency? An audio clip might be helpful.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Best start with measuring/verifying tube voltages from the schematic and post results.

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    Is the amp fitted with a grounded power cord, that has its earth wire properly bonded to the amp chassis?
    The board ecaps are all original; maybe they all tested good when serviced but I'm rather sceptical of that
    The power tube 1k5 grid stoppers look original, if so they've been cooked and flexed for many decades now (perhaps thereby developing invisible cracks from those stresses); hence my view is that they can't be considered suitable any longer for their mission critical role (of supplying bias to the power tubes).
    The 100 ohm 'longways' resistor at the LTP's 0V return is a popular candidate for developing cracks and bad connections, due to the warping of laughably crap board substrate material used.

    Note that the 'fx on both channels' mod used uses the channel's 2nd stage coupling caps in a reactive (rather than resistive) mixer; that results in a bass boost for the 'normal' channel, bass cut for the 'vib' channel, due to the differing cap values of those caps.

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    Additional notes: after the replacement of the rectifier tube I initially turned the Amp on while connected thru a light bulb limiter. The bulb did not glow wildly at first, and it also dimmed as a minute went by, so I’m hoping no shorts in the power supply.

    There is a 3 prong plug installed, with the ground lug crimped on and attached to one of the transformer bolts with a star washer underneath.

    Chuck H: the onset of the distortion is more with heavy attack. It occurs on both channels, and after turning up past 4 on either volume knob. I have the chassis outside of the Amp presently, and when i bang it on the workbench it doesn’t create the noise.

    I will now turn the Amp on again and take some measurements per the instructions I’ve been given. I will also check to see if the reverb on both channels mod is in place

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    I will now turn the Amp on again and take some measurements per the instructions I’ve been given.
    Just to make sure : Voltage measurements should be taken without the bulb limiter. And please also measure actual line voltage at the PT primary as well as heater voltage.

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    Last edited by Helmholtz; 09-28-2019 at 07:20 PM.
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    All measurements taken with the amp plugged into the wall outlet, not the light bulb limiter

    Plate P1 Plate P6
    V1 279 V 284 V
    V2 274 V 277 V
    V3 437 V 436 V
    V4 282 V 277 V
    V5 *433 V * 386 V
    V6 245 V 242 V

    *V5 P1 calls for 280V but I measure 433V*

    Also V5 P3 wants 2.5V but I see no voltage there
    V5 P8 wants 170V but I see no voltage there either

    Other oddities

    The bias pot calls for -52V but I measure -45.7

    Pedro V - resistor measurements follow. Any resistors not mentioned tested in spec (10%).

    On the layout drawing, above and between V1 and V2, the two 1500 resistors measure 2000 and 1900

    Above V3 the 2200 resistor reads 2600 and the 820 measures 1200

    Above V4, left to right, 2700 reads 3200, 820 reads 1100, 820K reads 175K, 470 K reads 175K, 3M3 measures 3M7

    Above V6, the 82K reads 93K

    Chuck H:
    "Are other performance characteristics unchanged? When not "played hard" the amp sounds no different than it ever has? That is, no additional audio symptoms to report? Volume at setting is the same as before? etc." The ability to produce loud volume seems unaffected. The amp sounds really good at 2 on the volume.

    "If you have a signal generator, can the ugly distortion be recreated with a steady input signal? If so then a scope might be useful for tracing where in the circuit the anomalous distortion starts." I am going to try to get a signal generator going. I will need some direction on where and how to take the scope measurements.

    "Is the distortion a seemingly mono tonal noise (crackle or fizzle) or does it seem to be sympathetic to frequency? An audio clip might be helpful."
    Audio clips coming soon, after I return the chassis to the case.

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  9. #9
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    Well, vibrato not working might not seem like a related symptom. And maybe it's not. But it's still a performance issue that was unreported. I assume the tremolo doesn't work.

    It's odd though that you would have no conduction in either triode of V5 since they're separated circuits. Including their grounds. It's either a remarkable coincidence that both cathode circuits are compromised or that tubes filament is bad.

    We had a case here not long ago where someone BF amp was behaving strange (ugly distortion, etc.) and it turned out that the ground connection for the PI and the second vibrato triode was bad. The amp was still working because the two circuits were still grounded through the NFB circuit and OT secondary.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by earache View Post
    I’m looking at a 1967 Super Reverb that, when you play it hard or turn it up, it has ugly distortion.

    It happens on both channels.


    http://ampwares.com/schematics/super_reverb_aa763.pdf
    Ugly distortion on both channels indicates a fault in the output stage (12AT7 and 6L6GC) and (or) power supply (GZ34 and some elco (20u / 525V).
    To begin, replace the GZ34 and both 6L6GC with new tubes and check the resistors in the g2 output tubes (470 Ohm/ 2W)
    Adjust the bias to -52VDC or 25mA each 6L6GC tube. If you do not get bias -52VDC check (replace) the parts in the bias circuit, (20u/ 70V, diode, 470 Ohm/ 1W)
    Standby switch to on. Measure the voltage before and after GZ34 (360VAC / + 465VDC). If the AC voltage (360VAC) is in the range but the DC voltage (+ 465VDC) is low, replace both elco before the standby switch (20u / 525V), they are most likely to dry or lose value.
    Measure the voltages in voltage point B (+ 460V) and D (+ 410V). If the voltage deviates significantly, check the resistors (1k and 4.7k) in the filter circuit.

    If all of the above is OK, the speakers are OK, but the sound is still ugly distortion on both channels test the output transformer TR3.
    The fastest 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.

    Edit 190929
    Questions
    - When FSR has ugly distortion, do you play it without pedals only guitar (clean) or through some pedals (effects).
    - After the "general" service in 2011, is it the FSR was put in proper condition and how it worked then.
    - When it appeared and what preceded the ugly distortion.

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    Last edited by vintagekiki; 09-29-2019 at 03:11 PM.

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    Save a lot of time and SCOPE IT.

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    V5 P8 wants 170V but I see no voltage there either
    Acc. to schematic V5 P8 should be 6.4V. This refers to a 56k cathode resistor. Later versions with a 100k cathode resistor have a cathode voltage of 12V to 17V.

    170V at P8 seems impossible (would cause the 25V ecap to explode).

    The plate P6 voltage seems normal and indicates that the second tremolo triode is conducting (otherwise plate voltage would be around 460V). But that means that there must be some volts at cathode P8. Could you please repeat this measurement?

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    Actually the schematic (AB763) DOES say 170V. And would seem to be correct from what I've seen in CF type circuits. But it would only go up that high when the tube is conducting and that's dependent on the oscillator. The earlier version schematic (AA763) does say 6.4V and much later schematics (mater volume models) say 12V or 13V. I'm guessing that this a measure taken when the tube is not conducting. And you're right that the 25V cap should pop. I guess I never noticed this anomaly in the circuit before and I must admit that I don't understand it.

    EDIT: The AB568 schematic is illegible and the AA1069 and AA270 schematics say 417V for V5 pin 8 !?! Which is, of course impossible as that's higher than the plate voltage. However, it's probably that the voltages (plate vs. cathode) could have been measured under different circumstances of conduction and some sort of peak was measured? It looks wrong to me.

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    Last edited by Chuck H; 09-29-2019 at 04:21 PM.
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    About measurement

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    When specifying measured values, be based on the original version schematic, not the earlier or later version schematic.
    Specifying various measured values from earlier or later version schematic, lead go to nothing and the thread loses its original meaning.

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    Last edited by vintagekiki; 09-29-2019 at 04:34 PM.

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    Hello all

    I apologize - I think I inadvertently posted the wrong schematic. I linked to and I think have been referring to AA763 when it should have been AB763.

    https://www.thetubestore.com/lib/the...-Schematic.pdf

    I think it’s AB 763 because it has grid stoppers, I haven’t looked too closely at any other details yet.

    Vintagekiki - as I wrote in an earlier post, both power tubes and rectifier are now brand new and all measurements taken above were with the new components in place

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by vintagekiki View Post
    Click image for larger version. 

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    We're all aware of that. But the vibrato function circumstances are not specified and would greatly affect the voltage in question. But thank you for always being vigilant with the information.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by vintagekiki View Post
    Ugly distortion on both channels indicates a fault in the output stage (12AT7 and 6L6GC) and (or) power supply (GZ34 and some elco (20u / 525V).
    To begin, replace the GZ34 and both 6L6GC with new tubes and check the resistors in the g2 output tubes (470 Ohm/ 2W)
    Adjust the bias to -52VDC or 25mA each 6L6GC tube. If you do not get bias -52VDC check (replace) the parts in the bias circuit, (20u/ 70V, diode, 470 Ohm/ 1W)
    Standby switch to on. Measure the voltage before and after GZ34 (360VAC / + 465VDC). If the AC voltage (360VAC) is in the range but the DC voltage (+ 465VDC) is low, replace both elco before the standby switch (20u / 525V), they are most likely to dry or lose value.
    Measure the voltages in voltage point B (+ 460V) and D (+ 410V). If the voltage deviates significantly, check the resistors (1k and 4.7k) in the filter circuit.

    If all of the above is OK, the speakers are OK, but the sound is still ugly distortion on both channels test the output transformer TR3.
    The fastest 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.

    Edit 190929
    Questions
    - When FSR has ugly distortion, do you play it without pedals only guitar (clean) or through some pedals (effects).
    - After the "general" service in 2011, is it the FSR was put in proper condition and how it worked then.
    - When it appeared and what preceded the ugly distortion.
    Perhaps you're just going through the steps as a matter of record, but the OP stated that he has replaced all the tubes in the course of the tests already and that the power supply caps are eight years old and no other filtering symptoms present.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by earache View Post
    Vintagekiki - as I wrote in an earlier post, both power tubes and rectifier are now brand new and all measurements taken above were with the new components in place
    I follow a neat thread, but I don't see any comments on
    Quote Originally Posted by vintagekiki View Post
    Questions
    - When FSR has ugly distortion, do you play it without pedals only guitar (clean) or through some pedals (effects).
    - After the "general" service in 2011, is it the FSR was put in proper condition and how it worked then.
    - When it appeared and what preceded the ugly distortion.

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    I apologize - I think I inadvertently posted the wrong schematic. I linked to and I think have been referring to AA763 when it should have been AB763.

    https://www.thetubestore.com/lib/the...-Schematic.pdf

    I think it’s AB 763 because it has grid stoppers, I haven’t looked too closely at any other details yet.
    This version has the 100k cathode resistor. Voltage at V5 pin 8 should read 17.0V (I checked with other schems.) Actually I see a tiny dot on my print-out.
    Decimal points often got lost in original Fender schematic. 170V across a 25V rated ecap sound scary.

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    Last edited by Helmholtz; 09-30-2019 at 08:33 AM.
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    Let's start again from the beginning.
    Ugly distortion is on both channels. YES

    Since ugly distortion is on both channels, feel free to remove all preamp tubes from the vibrato channel, leave only 12AX7 (normal channel) 12AT7 (phase inverter) 2 x 6L6GC and GZ34 and start to the search for failure from power amplifier and (or) from power supply.
    Since ugly distortion on both channels, what is the status of T3 (output transformer)?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by vintagekiki View Post
    Let's start again from the beginning.
    Ugly distortion is on both channels. YES

    Since ugly distortion is on both channels, feel free to remove all preamp tubes from the vibrato channel, leave only 12AX7 (normal channel) 12AT7 (phase inverter) 2 x 6L6GC and GZ34 and start to the search for failure from power amplifier and (or) from power supply.
    Since ugly distortion on both channels, what is the status of T3 (output transformer)?
    But this doesn't entirely apply if the amp has the reverb on both channels mod since the reverb circuit is only part of the vibrato channel in a stock amp. There was another thread here recently that ran for pages and pages involving an oscillation due to the reverb circuit. In this scenario it might apply and the problem can't, as yet, be isolated to the PI or forward.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    So - I have a short vacation coming up so the posts may become sporadic for a bit...but for now, a couple questions.

    If I am to run a signal they the input jack and scope this amp, to get it to the volume level it needs to make the ugly distortion it will have to be pretty loud. As this is a Super Reverb the speaker ohms rating is 2ohms. Do I need a 2 ohm load specifically or can I get b wit something different?

    Next Noob question - what procedure to follow in testing with the scope? Where to probe, what to look for?

    Are there places to avoid with the scope so I don’t damage it?

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    Quote Originally Posted by vintagekiki View Post
    ...check the resistors in the g2 output tubes (470 Ohm/ 2W) ...
    Just to note that the Fender spec is 470 ohm 1 watt. I've never known resistors of this spec to fail in this application other than as collateral damage from a tube short. My perception is that their rating may have been chosen so that they would probably fuse in the case of the screen grid shorting.
    Original CC resistors here should be replaced as part of normal amp service, replacements should be metal oxide with a flame retardant / proof coating.
    I'm happy to uprate them if that's done in conjunction with proper HT fusing being put in place.

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  24. #24
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by earache View Post
    So - I have a short vacation coming up so the posts may become sporadic for a bit...but for now, a couple questions.

    If I am to run a signal they the input jack and scope this amp, to get it to the volume level it needs to make the ugly distortion it will have to be pretty loud. As this is a Super Reverb the speaker ohms rating is 2ohms. Do I need a 2 ohm load specifically or can I get b wit something different?

    Next Noob question - what procedure to follow in testing with the scope? Where to probe, what to look for?

    Are there places to avoid with the scope so I don’t damage it?
    You'll want a two ohm load. But it's easier to source standard value resistors so I'd say a pair of 4.7ohm resistors in parallel (for a 2.35 ohm load) would be fine. Maybe better than fine really when you consider that an actual speaker is typically much higher than it's rated ohms at all but a moderate swath in the midrange frequencies. Since it's tough to measure loads that small with some meters to confirm accuracy I would just get 1% or 5% tolerance components. The aluminum housed resistors have proven affordable and reliable for me building dummy loads. Here's a link to a pair of 50W, 4.7 ohm, 1% resistors available through Mouser for $3.87 each:

    https://www.mouser.com/ProductDetail...MK9zLoBoBNY%3D

    A pair of these in parallel is good for a 2.35 ohm load at 100W.

    You would start with about 80mV of signal to the amps input. Amp controls set to wherever they are when the problem occurs. Actually, start with the volume low and then bring it up during testing to that place where the problem occurs. Scope the DC decoupled signal at all points in the signal path. That is, those points after coupling capacitors in the signal chain that will have AC signal voltage on them with little or no DC. You don't want to scope points that have HV on them right now. This CAN be done, but there are special considerations and it's not necessary at this time. The problem is likely beyond where the two channels join (which is still in question because of the probable reverb on both channels mod) so the most suspect places would be the mix triode grid, the PI input grid and the power tube control grids.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by earache View Post
    If I am to run a signal they the input jack and scope this amp, to get it to the volume level it needs to make the ugly distortion it will have to be pretty loud.
    Well, yes.
    From your description you have no problem at low levels, only when played loud, so you´ll need to reach exact same volume/level, whatever it is, or you are not reatually testing your problem.

    I suggest you plug your guitar in, set all controls as usual, rise volume until you DO get that "ugly distortion" , replace guitar with oscillator, or play an MP3 sinewave, 440Hz is a fine frequency and watch output.

    Inject a nice strong signal, say 400 or 500mV, to mimic a high output guitar, rise volume from 0 to whatever´s needed to sound ugly, as soon as it starts becoming ugly look at the screen: what has changed?

    Normal is to start with a reasonable sinewave , don´t expect perfection because it´s a gueetar amp, nonlinear by nature, but it should be recognizable.

    Typically both (top-bottom) halves will not be mirror images (tube mismatch) and when louder will show some kink (bias shift) ; that is normal in all amps and by itself part of "tube sound"

    A few examples of what to expect :

    Normal showing some kink. This is NOT crossover distortion, people trying to avoid it overbias tubes to death:




    Tube mismatching (or PI not well balanced), to this degree quite normal.

    When driven hard one tube stays clean, the other shows lack of stamina. So far no big deal.

    Note: these are copypasted images just used as examples, built in comments do not necessarily apply to your problem, if at all.




    Similar.

    This comes from a stereo Hi Fi tube amp (nothing less than a mighty Mc Intosh 250!!!) : there is not such a thing as perfect matching and even if you get it , in general tubes won´t age the same so after shorter or longer time they will diverge.
    To this degree, no big deal, even less on a guitar amp.




    Look at these Trainwreck waveforms, going from clean to heavily overdriven, all afre normal:
    http://home.polstra.com/amps/wreck1/scope/

    Going to do seme useful (as inaid for) work, later will post a gallery of horrors, ugly waveforms.
    You might have one of those, or post your own.

    As this is a Super Reverb the speaker ohms rating is 2ohms. Do I need a 2 ohm load specifically or can I get b wit something different?
    It will be most realistic if you use the actual speakers.
    Yes, it will get noisy ... but not noisier than actual rehearsing or playing live besides a drummer.
    And no need for 2 hour long boring tones, just 1 or 2 minutes will show either normal waveforms or ugly ones.
    Next Noob question - what procedure to follow in testing with the scope? Where to probe, what to look for?
    See above.
    Are there places to avoid with the scope so I don’t damage it?
    In principle, measure at speaker out only, easy 15 V RMS or so.

    Why?: if you can hear it, you must see it too.

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    A pair of these in parallel is good for a 2.35 ohm load at 100W.
    Yes, but requires adequate heatsinking. Without heatsink the HS50 is specified only at max 14W. So without heatsink 5 HS50/ 10 Ohm in parallel would be good for 70W.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Yes, but requires adequate heatsinking. Without heatsink the HS50 is specified only at max 14W. So without heatsink 5 HS 10 Ohm in parallel would be good for 70W.
    Yes! Thank you. Probably best to point that out here. Mine are mounted at some distance apart in a 1/8" aluminum chassis with thermal paste. Heat sinking according to the datasheet is vague but seems to imply an aluminum chassis. Nothing fancy or expensive with fins. I expect a good sized aluminum plate, say, 12"x12" at 1/8" or thicker would do. This is still more economical than buying huge, uber high wattage resistors. I'm using the HS50's and I've abused my switchable load box pretty well. It's held up fine.

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    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by J M Fahey View Post
    In principle, measure at speaker out only, easy 15 V RMS or so.
    Name:  Stop.jpg
Views: 231
Size:  8.1 KB
    15 VRMS
    Overload
    (2 Ohm/2 x 6L6GC)

    Load
    Ohm
    Power
    W
    2
    112.5
    4
    56.25
    8
    28.12

    For FSR/ 50W/ 2 Ohm
    Speaker voltage 10 V RMS

    For FSR/ 70W/ 2 Ohm
    Speaker voltage 11.8 V RMS

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    Supporting Member Jazz P Bass's Avatar
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    What is the chance that one of the four speakers no longer works?

    That would be a game changer.

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    Only then would the user find out what is ugly sound.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by vintagekiki View Post
    Name:  Stop.jpg
Views: 231
Size:  8.1 KB
    15 VRMS
    Overload
    (2 Ohm/2 x 6L6GC)

    Load
    Ohm
    Power
    W
    2
    112.5
    4
    56.25
    8
    28.12

    For FSR/ 50W/ 2 Ohm
    Speaker voltage 10 V RMS

    For FSR/ 70W/ 2 Ohm
    Speaker voltage 11.8 V RMS
    Read my answer in context.
    Are there places to avoid with the scope so I don’t damage it?
    In principle, measure at speaker out only, easy 15 V RMS or so.
    OP is NOT asking me what´s the amplifier power output, what is the overload point, NONE of that.

    He´s worried about damaging his scope, which is reasonable because there are hundreds of volts inside.
    So I tell him not to worry, it´s an easy job, I am close enough by saying "15V or so".

    Obviously I am wasting my time here.

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    JM and all, you’ve made loads of effort to explain and help me figure this out - thank you! Now I’ve got to get the load resistor sorted out and I will proceed from there, most likely after my short vacation, returning Tuesday

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    So I’ve got part of the gear sorted out I think. While waiting for Amazon to send the resistors that you suggested (thanks Chuck H), I found these resistors at my work

    Click image for larger version. 

Name:	D04DCE01-6188-4BB2-85A4-80A5F7328B07.jpeg 
Views:	17 
Size:	2.65 MB 
ID:	55367

    They measure exactly at 2ohms in parallel. They are rated at 50 W each, what is their power rating in parallel?

    I dont have a signal generator, so here’s my signal source - my old iPod...

    Click image for larger version. 

Name:	032578DF-6CEF-4038-8EA3-5E824DEE4937.jpeg 
Views:	11 
Size:	2.00 MB 
ID:	55368

    So now to make a jack that I can solder to the resistor leads...

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    Supporting Member The Dude's Avatar
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    3) 8 ohm 50W resistors in parallel would be a 2.666 ohm load with 150W capability. You'll need one more if you want exacty a 2 ohm load. They are 1% resistors, so I'd believe the stamp over the meter. If you're not concerned about accuracy, they'd probably be close enough. Also, for them to actually be able to safely dissipate full power, you'll need to mount them on some sort of heat sink. Mine are on a big aluminum plate.

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    JM Fahey

    After rereading your excellent post more thoroughly, I will try to measure and post the waveform(s) using the original speakers as you suggested.

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