Strange sine wave

PeaveyToMarshallGuy

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Hello, i'm pretty new to the whole poking arund in a tube amp thing and really new to an oscilloscope. But i bought one for testing my jcm800 2204 -85 since i have some problems and i figured why not.
anyway, came up with this, blue is input and yellow is output. not sure what to make of it.

any ideas?
 

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Pete Farrington

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What does the wave look like at the master volume?
With presence at min, what does the max sine wave output look like, at the onset of power amp clipping?
Consider injecting the sine input at the power amp input (eg as if there was an fx loop), to avoid the preamp confusing things.
 

PeaveyToMarshallGuy

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that was using high input.
here is using low input gain almost to full and master to where it's starting to clip
 

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2L man

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On first photo the drive is too high!

To get an idea what amplifier amplify it is better to use non inductive resistor on output. Loudspeaker impedance react too much to frequency.

Start testing how much clean power you get out using 1kHz. Tune all volumes starting from signal generator, EQ and Presence. Sometimes different stages must be "looked" if they distirtort signal before the power stage. First triode stage does not clip easily so SG output can be relative high but second usially does clip so first volume should be lowish. Soon you know what effect where.

By Clean I mean that sine wave output look good to you. Your second photo sine wave is clean but line is strange thick. Possibly there is high frequency noise? Turn oscilloscope speed knob to see if there come another signal which run on this audio frequency?

Then measure the output. I measure Vpp, divide by 2, divide by 1,41, ^2 and divide by resistor value and it is Power. There are web pages where you add Vpp or Vrms and resistor value and it answers the power.
 

william vogel

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Your waveforms look right in line with what I’ve observed with mine. Yes the first waveform you posted is clipped and has that dip in it but I’ve seen that exact pattern. It also sounds okay when playing the guitar through the cabinet, it’s gainy and harmonic. It looks nothing like a Plexi but it’s not a Plexi. There’s enough preamp gain to get the early stages (v2) to clip and even to get the second stage (10k cathode) to compress. The tonestack is what’s causing the dip in the clipped wave. Adjusting the controls will make it more sinus. Waveforms can be confusing when comparing what you find appealing from playing and what you see on the scope but it is what it is. You also have to remember that the signal generator is outputting one fundamental and the guitar is a fundamental with several harmonics always. The guitar produces much different waveforms than the signal generator. We use the signal generator because it allows us to focus the signal into an easy to view display of what the amplifier is passing through and amplifying.
 

PeaveyToMarshallGuy

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On first photo the drive is too high!

To get an idea what amplifier amplify it is better to use non inductive resistor on output. Loudspeaker impedance react too much to frequency.

Start testing how much clean power you get out using 1kHz. Tune all volumes starting from signal generator, EQ and Presence. Sometimes different stages must be "looked" if they distirtort signal before the power stage. First triode stage does not clip easily so SG output can be relative high but second usially does clip so first volume should be lowish. Soon you know what effect where.

By Clean I mean that sine wave output look good to you. Your second photo sine wave is clean but line is strange thick. Possibly there is high frequency noise? Turn oscilloscope speed knob to see if there come another signal which run on this audio frequency?

Then measure the output. I measure Vpp, divide by 2, divide by 1,41, ^2 and divide by resistor value and it is Power. There are web pages where you add Vpp or Vrms and resistor value and it answers the power.
I have a two notes torpedo 8ohm as loadbox, so i'm not sure about the resistance, since it's induvtive. but i have ordered a 8ohm 100w resistor, it should come later this week.

There is some high frequency noice, when not playing, that's new. Wasn't last time i played it.
 

PeaveyToMarshallGuy

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What is the problem on your 800 amp?
It cuts after playing a while. But i belive that it's the fuse that have gone bad. It's an resetable fuse (Swedish amp)that apparently have some heat protection aswell. And 2ohm resistance when i tested it, so seems a bit high. I ordered some glass fuses so i'll try those and see if it'll work.

I was mostly curious about the sine wave because i thought it looked strange. Looking at videos about checking an amp whit the scope it looked like it should have a smoother wave form at output.
 

PeaveyToMarshallGuy

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Your waveforms look right in line with what I’ve observed with mine. Yes the first waveform you posted is clipped and has that dip in it but I’ve seen that exact pattern. It also sounds okay when playing the guitar through the cabinet, it’s gainy and harmonic. It looks nothing like a Plexi but it’s not a Plexi. There’s enough preamp gain to get the early stages (v2) to clip and even to get the second stage (10k cathode) to compress. The tonestack is what’s causing the dip in the clipped wave. Adjusting the controls will make it more sinus. Waveforms can be confusing when comparing what you find appealing from playing and what you see on the scope but it is what it is. You also have to remember that the signal generator is outputting one fundamental and the guitar is a fundamental with several harmonics always. The guitar produces much different waveforms than the signal generator. We use the signal generator because it allows us to focus the signal into an easy to view display of what the amplifier is passing through and amplifying.
Ok, so i shouldn't worry about something being wrong then? Maybe lower gain from v1?
Since it obviously has enough..
 

william vogel

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Ok, so i shouldn't worry about something being wrong then? Maybe lower gain from v1?
Since it obviously has enough..
What I recommend for testing is to turn the master volume full clockwise and control the input voltage to a low enough level to achieve a clean sinus output waveform with the gain control at about half (12:00 position). Adjust the tone controls. I like to trace the waveform through the amp and adjust the controls to maintain the clean waveform and then adjust and observe the changes to the waveform. I prefer the sound of power tube clipping usually but because this amp allows preamp gain adjustment and the master volume, I run mine with the master at about 2:00 and the gain at about 2:00. I use my guitar volume to adjust but my amp settings are loud.
 

XTRXTR

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As far as the cutting out, once you get your dummy load, get a nice sine wave going and then chopstick around ground points on the board, input jacks etc, also the suspected reset fuse tap around all these areas kind of hard. Looking for intermittent issues. Also its an old amp so tube sockets, input jacks, should be cleaned out.
 

PeaveyToMarshallGuy

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What I recommend for testing is to turn the master volume full clockwise and control the input voltage to a low enough level to achieve a clean sinus output waveform with the gain control at about half (12:00 position). Adjust the tone controls. I like to trace the waveform through the amp and adjust the controls to maintain the clean waveform and then adjust and observe the changes to the waveform. I prefer the sound of power tube clipping usually but because this amp allows preamp gain adjustment and the master volume, I run mine with the master at about 2:00 and the gain at about 2:00. I use my guitar volume to adjust but my amp settings are loud.
That's a good idea, lowering the input signal, i'll try that later today and see if it's possible.
Yeah that sounds loud. Even with the -20db atteunator i think that would be way to loud in the small rehersal room i play in
 

PeaveyToMarshallGuy

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As far as the cutting out, once you get your dummy load, get a nice sine wave going and then chopstick around ground points on the board, input jacks etc, also the suspected reset fuse tap around all these areas kind of hard. Looking for intermittent issues. Also its an old amp so tube sockets, input jacks, should be cleaned out.
I did change the powertube sockets, and put some contact cleaner in the preamp sockets so they should be ok. Also measured resistance to ground all over the place, but i'll try some tapping when i can get a good wave. Got a gig in about 2 months so would be nice to be able to trust the amp by then. And have it sounding good. :)
 

PeaveyToMarshallGuy

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Small update.
The fuse change seems to work, could play for over 1h yesterday.

Lowering gain from v1 did somewhat help getting a clean wave at output. Or could be going into a reststor instead of an reactive load. Or a combination.

Output power seem to be only 38w, gonna do some more testing. Remeasure and recalculate. Should i measure at the resistor or at the output terminals? I don't see that it would matter?
 

PeaveyToMarshallGuy

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Oh yeah, and on v1a there is connection(mm beeps) between heater and pin7 i belive it was, green cable signal in..
Is that normal? It's connected through ground. When i removed ground from transformer it stopped beeping.
 

2L man

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Small update.
The fuse change seems to work, could play for over 1h yesterday.

Lowering gain from v1 did somewhat help getting a clean wave at output. Or could be going into a reststor instead of an reactive load. Or a combination.

Output power seem to be only 38w, gonna do some more testing. Remeasure and recalculate. Should i measure at the resistor or at the output terminals? I don't see that it would matter?
Yes, resistor load should make amp produce more clean power than true reactive load and guitar loudspeaker load because it react only resistively :)

There are quite a many variables which effect the amp output power but if it was good looking 38W sine wave I think it is not far what a pair of "big bottles" can comfortably produce.
 

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