Hey group,
I am using LTSpice to do some circuit mockup/testing and I often run into the issue as shown in the attached image. The input signal to the triode is a perfect sine wave (in this case 440 Hz) but after it passes through the tube, the wave form is shifted more to the negative and also seems to be flattening (clipping) on the top part of the positive swing.
Can anyone explain why this happens and what should be looked at or revised to get a pretty sine wave on the back side of the tube?
Offset Waveform?
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Offset Waveform?
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Stevem
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Re: Offset Waveform?
When a signal goes into a tubes grid and gets pulled off the plate it has reversed phase.
This phase change can be better seen when a circuit and the signal is drawn like this that I posted.
This due to the audio signal having what can be called a positive and negative half to it and the fact that the power supply voltage is only positive.
The offset that you ask about is due to that tube not being biased at it’s needed center point for the signal that your putting into it , or due to its condition in real life as to it not amplifying as much as it when it was fresh/ in good condition.
I don’t think I have done justice to explaining this to you as it very complicated and I have been up since 2 am and it’s now 8 pm, lol!
I hope someone else jumps in for you here and gets your full question answered.
This phase change can be better seen when a circuit and the signal is drawn like this that I posted.
This due to the audio signal having what can be called a positive and negative half to it and the fact that the power supply voltage is only positive.
The offset that you ask about is due to that tube not being biased at it’s needed center point for the signal that your putting into it , or due to its condition in real life as to it not amplifying as much as it when it was fresh/ in good condition.
I don’t think I have done justice to explaining this to you as it very complicated and I have been up since 2 am and it’s now 8 pm, lol!
I hope someone else jumps in for you here and gets your full question answered.
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Last edited by Stevem on Sat Mar 05, 2022 1:15 am, edited 1 time in total.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Stevem
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Re: Offset Waveform?
Sorry that drawing was upside down!
Ok I will try a a better short and sweet explanation for you.
A tube is powered by a positive DC supply voltage that is measured from 0 to whatever the peak voltage is.
A AC sign wave is made up of both a 0 to peak positive voltage and a 0 to peak negative voltage.
The only way a tube can amplify the negative portion of a sign wave is to bias the tube up from idling at zero when no sign is being fed into it.
For instance if a tube needed to cleanly amplifie a 6 volt signwave ( as in no clipping) it would have to be biased up so that it would produce a output at its plate that went from zero to 3 volts positive , back down to zero ( it’s new idling point remember!) and then produce a negative voltage that went from zero up to 3 and then back down to zero .
This then would amplifying the complete 6 volt audio wave that was put in at the grid.
I will not get into the difference between clipping and cut off even though they both look the same on a O scope.
This setting up is what the combination of the plate resistor and cathode resistor does.
This is called class A which is when one gain stage amplifies the full 360 degree audio sign wave.
PS, also note that in a cathode follower type circuit when the signal gets pulled off of the cathode, no phase inversion takes place .
Ok I will try a a better short and sweet explanation for you.
A tube is powered by a positive DC supply voltage that is measured from 0 to whatever the peak voltage is.
A AC sign wave is made up of both a 0 to peak positive voltage and a 0 to peak negative voltage.
The only way a tube can amplify the negative portion of a sign wave is to bias the tube up from idling at zero when no sign is being fed into it.
For instance if a tube needed to cleanly amplifie a 6 volt signwave ( as in no clipping) it would have to be biased up so that it would produce a output at its plate that went from zero to 3 volts positive , back down to zero ( it’s new idling point remember!) and then produce a negative voltage that went from zero up to 3 and then back down to zero .
This then would amplifying the complete 6 volt audio wave that was put in at the grid.
I will not get into the difference between clipping and cut off even though they both look the same on a O scope.
This setting up is what the combination of the plate resistor and cathode resistor does.
This is called class A which is when one gain stage amplifies the full 360 degree audio sign wave.
PS, also note that in a cathode follower type circuit when the signal gets pulled off of the cathode, no phase inversion takes place .
You do not have the required permissions to view the files attached to this post.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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wpaulvogel
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Re: Offset Waveform?
It looks to me like it’s really hot biased with a very small cathode resistor value or it’s grid current biasing with no cathode resistor. The top of the waveform is compressed because of grid current conduction and clamping the input.JJH0906 wrote: ↑Sat Mar 05, 2022 12:16 am Hey group,
I am using LTSpice to do some circuit mockup/testing and I often run into the issue as shown in the attached image. The input signal to the triode is a perfect sine wave (in this case 440 Hz) but after it passes through the tube, the wave form is shifted more to the negative and also seems to be flattening (clipping) on the top part of the positive swing.
Can anyone explain why this happens and what should be looked at or revised to get a pretty sine wave on the back side of the tube?
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Matthews Guitars
- Posts: 155
- Joined: Mon May 24, 2021 1:11 am
Re: Offset Waveform?
The cathode resistor value greatly affects the operating point of the tube.
Vary the cathode resistor from a low value like 750 ohms up to various values as high as 39K.
A very clean value would be 820 ohms. 1.5K and 1.8K are very common standard values. Start increasing that value and you begin to bias the tube toward cold clipping,
and in a Marshall master volume 2203 or 2204 they use a 10K cathode resistor for cold clipping. The Soldano cold clipper value is 39K.
To find the cleanest value, adjust the input level and cathode resistor values to get clipping onset equally at both the positive and negative peaks of the waveform at the same time.
Now back down the drive level.
That is your cleanest setup.
Vary the cathode resistor from a low value like 750 ohms up to various values as high as 39K.
A very clean value would be 820 ohms. 1.5K and 1.8K are very common standard values. Start increasing that value and you begin to bias the tube toward cold clipping,
and in a Marshall master volume 2203 or 2204 they use a 10K cathode resistor for cold clipping. The Soldano cold clipper value is 39K.
To find the cleanest value, adjust the input level and cathode resistor values to get clipping onset equally at both the positive and negative peaks of the waveform at the same time.
Now back down the drive level.
That is your cleanest setup.
Re: Offset Waveform?
Many thanks to all the comments! After hearing this for about the 20th time, I think it is starting to sink in!
Not entirely sure what I did with the SPICE model, if anyone has used it, they understand there are a few quirks with the software you have to watch out for!
I went back through the circuit and got it working "normally" again. I've attached a shot of the signal as it travels through the circuit. A 50 mV signal is applied to the first preamp and then you can see the signal coming out of the first stage (Vpre1) and also leaving the second stage (Vpre3) All pots are set at 50%. The signal looks fairly well balanced (seems to be biased more towards the negative swing) but interestingly, the signal from the second stage seems to have shifted about 45° out of phase from the first stage signal. Is this due to the TMB circuit? This circuit was derived from Merlin Blencowe and it's a pretty standard Fender/Marchall type preamp circuit, other than I added a tone shift pot after the slope resistor.
So, I think the circuit is OK, just not sure why the second stage has shifted over and if that's normal or something else should be looked at?
Not entirely sure what I did with the SPICE model, if anyone has used it, they understand there are a few quirks with the software you have to watch out for!
I went back through the circuit and got it working "normally" again. I've attached a shot of the signal as it travels through the circuit. A 50 mV signal is applied to the first preamp and then you can see the signal coming out of the first stage (Vpre1) and also leaving the second stage (Vpre3) All pots are set at 50%. The signal looks fairly well balanced (seems to be biased more towards the negative swing) but interestingly, the signal from the second stage seems to have shifted about 45° out of phase from the first stage signal. Is this due to the TMB circuit? This circuit was derived from Merlin Blencowe and it's a pretty standard Fender/Marchall type preamp circuit, other than I added a tone shift pot after the slope resistor.
So, I think the circuit is OK, just not sure why the second stage has shifted over and if that's normal or something else should be looked at?
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Stevem
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Re: Offset Waveform?
Also keep in mind that every cap that the audio signal passes thru will introduce a small amount phase shift.
These shifts are additive and when the total amount gets up to close to 180 or more degrees then the amp can become unstable and oscillate .
These shifts are additive and when the total amount gets up to close to 180 or more degrees then the amp can become unstable and oscillate .
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Offset Waveform?
Any deviation from a flat frequency response will also result in deviation in phase response.
Setting tone stack pots to 50% (whatever that means, eg electrically, rotationally?) does not equate to a flat frequency response.
Setting tone stack pots to 50% (whatever that means, eg electrically, rotationally?) does not equate to a flat frequency response.
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