High Voltage Tube Pre-amp build debugging  [SOLVED]

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esist
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Post by esist »

Hello Community,

I am building Tube Preamps in boxes since quite some time now. My current project is a Peavey Classic 30 preamp. The preamp is already functional and sounds quite good, I just have an effect, that the output volume of the Drive channel is lower than from the clean channel.
During my investigations, I found that at input to stage 3 I have an unsymmetrical waveform (I am feeding 440Hz sine way to the input).
440 Hz sine at input to stage 3.jpg
The measurement was done at the red marked box.
Classic 30 schematic.png
At the green box position, the signal is still perfectly symmetrical and has as expected 2x amplitude compared to the red box position.

Any idea where this could come from ?

Thank you all in advance. At the end I am sending you a picture of the housing of this build.
Preamp pic2.jpg
Preamp pic1.jpeg
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Post by phatt »

Well that clipped wave is why it's called Drive channel,, a Normal situation.
If the volume imbalance bugs you then lower the value of R7.
Install a temporary pot at R7,, dial in the desired level then measure the value and replace with the closest value resistor.

VR3 is only 10k,,while VR1 is 250k so yep it's not and ideal design.
Phil.

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Post by esist »

Thanks Phil for your answer. The volume imbalance fix is straight forward, I will go straight ...

About the clipping I am surprised to see asymmetric clipping happening. I expected to see symmetric saturation at the top and bottom side of the waveform.
Also the fact that the waveform at "green" is still symmetric and at "red" it is clipped surprised me. Does clipping/distortion not normally appear first at the plate of an over-driven tube ?
My bad.
Markus
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Post by phatt »

Yes Most assume the magic juice is inside the glass bottle but Valves don't work without the passives. :hmmm:
Those two resistors forming a divider (R5&R11) load down the previous Anode to push through the hi resistance. (Or something like that, Others here might know more about this little quirk)
There was a very good explanation somewhere on the DubDubDub and It's actually got a Term but stuffed if I can remember it now. :scratch:
These are Hi Z circuits and a lot of hidden quirks exist that are Not directly related to Valve mojo.
Phil.

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Post by bmxguitarsbmx »

esist wrote: 30 Jan 2022, 15:10 ...
About the clipping I am surprised to see asymmetric clipping happening. I expected to see symmetric saturation at the top and bottom side of the waveform.
Also the fact that the waveform at "green" is still symmetric and at "red" it is clipped surprised me. Does clipping/distortion not normally appear first at the plate of an over-driven tube ?
My bad.
Markus
When you drive the grid positive, at a certain point electrons will be attracted to the grid from the cathode. It's called typically "grid clipping ". It is an interesting and important part of guitar tone. It can charge up the coupling capacitor and cause bias shifts. If too much grid clipping is happening, it can leading to "blocking distortion". The typical way to limit blocking distortion is to increase the size of the grid stopper resistor (or add one). A certain amount of grid clipping is wanted as it causes slight bias shifts that make the sound more complex. If you are get into spice simulation, you will want to find spice models that include grid current in the model otherwise, your simulations will be way off. Your situation is totally normal and totally surprising! Very cool.

Another way to think about grid clipping is that at a certain positive grid voltage the grid is no longer high impedance and you can imagine it as being a diode with the cathode to ground. In fact, you will see solid-state circuits modeling grid clipping this way. In the case of AMT, it is diode and series resistor connected to ground or a split cathode resistor. Later they modeled grid clipping with a J177 Jfet that turns on according to Drain voltage. Now I'm just rambling. Haha

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Post by esist »

I see. As usual, there is a steep learning curve ahead even getting steeper on the way. What I am taking away is that assuming the grid has "infinite" input Z is good, but does not make the voltage divider behave "linear" ?
Anyway, that thing does what it should, it sounds fat on higher gain settings, maybe a bit dull ....
The Classic 20 pre-amp sounds much better. That extra gain I will get by one of the pedals.
Thank you Phil.
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Post by Manfred »

For a deeper analysis I would like to have the values of B+ and B++.

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Post by bmxguitarsbmx »

esist wrote: 31 Jan 2022, 17:24 I see. As usual, there is a steep learning curve ahead even getting steeper on the way. What I am taking away is that assuming the grid has "infinite" input Z is good, but does not make the voltage divider behave "linear" ?
Anyway, that thing does what it should, it sounds fat on higher gain settings, maybe a bit dull ....
The Classic 20 pre-amp sounds much better. That extra gain I will get by one of the pedals.
Thank you Phil.
Your pedal is awesome and the best way to learn is by discovering the way you did. I love it when those peculiar things are noticed. What the heck causes this? It's so cool. You cant just read the chapter on how a vacuum tube works and then never be surprised by how a tube reacts. It elucidates itself and it can all be a good mystery to unravel.

Basically, The Grid is Ground referenced and the Cathode is kept at ~ +1.5V in a normal sitting state. Electrons are flowing from the Cathode to the more positive Plate. Normal everyday linear operating triode. If you run a hot enough signal into the Grid that part of the waveform goes to a more positive voltage than the Cathode, all of a sudden electrons are not only going to the Plate, but are being attracted to the Grid. Once current flows in the Grid, it is no longer Hi-Impedance. The part of your waveform that is "lopped" off is the part of the waveform that tried to go above the Cathode voltage. The important part is that it sounds sick!

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Post by esist »

bmxguitarsbmx wrote: 31 Jan 2022, 17:16
When you drive the grid positive, at a certain point electrons will be attracted to the grid from the cathode. It's called typically "grid clipping ". It is an interesting and important part of guitar tone. It can charge up the coupling capacitor and cause bias shifts. If too much grid clipping is happening, it can leading to "blocking distortion". The typical way to limit blocking distortion is to increase the size of the grid stopper resistor (or add one). A certain amount of grid clipping is wanted as it causes slight bias shifts that make the sound more complex. If you are get into spice simulation, you will want to find spice models that include grid current in the model otherwise, your simulations will be way off. Your situation is totally normal and totally surprising! Very cool.

Another way to think about grid clipping is that at a certain positive grid voltage the grid is no longer high impedance and you can imagine it as being a diode with the cathode to ground. In fact, you will see solid-state circuits modeling grid clipping this way. In the case of AMT, it is diode and series resistor connected to ground or a split cathode resistor. Later they modeled grid clipping with a J177 Jfet that turns on according to Drain voltage. Now I'm just rambling. Haha
Thank you bmxguitarsbmx,
that makes it much clearer. I met the term "grid clipping" already several times, now I have something real to connect with it. Makes perfectly sense.
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Post by phatt »

bmxguitarsbmx wrote: 31 Jan 2022, 17:16

When you drive the grid positive, at a certain point electrons will be attracted to the grid from the cathode. It's called typically "grid clipping ". It is an interesting and important part of guitar tone. It can charge up the coupling capacitor and cause bias shifts. If too much grid clipping is happening, it can leading to "blocking distortion". The typical way to limit blocking distortion is to increase the size of the grid stopper resistor (or add one). A certain amount of grid clipping is wanted as it causes slight bias shifts that make the sound more complex. If you are get into spice simulation, you will want to find spice models that include grid current in the model otherwise, your simulations will be way off. Your situation is totally normal and totally surprising! Very cool.

Another way to think about grid clipping is that at a certain positive grid voltage the grid is no longer high impedance and you can imagine it as being a diode with the cathode to ground. In fact, you will see solid-state circuits modeling grid clipping this way. In the case of AMT, it is diode and series resistor connected to ground or a split cathode resistor. Later they modeled grid clipping with a J177 Jfet that turns on according to Drain voltage. Now I'm just rambling. Haha
Thanks for the better informed explanation. :thumbsup
I remember reading something similar can happen with BJT's but can't recall where. :scratch:
But yeah Grid clipping is the term :slap:
Phil.

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Post by Manfred »

Manfred wrote: 31 Jan 2022, 23:57 For a deeper analysis I would like to have the values of B+ and B++.
I see the problem is solved and the questions answered.
I would appreciate it very much if I could get the values for B+ and B++, I did not find these values on the internet.
Grid clipping is actually called grid current clipping.
BTW, this cannot be simulated with PSpice as this feature is not supported in the tube models.
I thought it was about a circuit adjustment to make the level balance of the clean and drive signal, I guess I was wrong.

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Post by bmxguitarsbmx »

Great correction Manfred. "Grid current clipping" is a much better term. Thank you for that.

Phatt: Joe Davisson, I believe, talks about this happening with BJT and JFet. Very interesting stuff. I haven't messed with it, but I do think that Grid Current Clipping is a very important thing to get the full effect of the "tube sound", so that may be something to look into with BJT and JFet. The part where the clipping causes bias shifts can really make the distortion more interesting. As the waveform clips off the positive part of the wave, the integration that happens with the previous stages coupling cap will average a more negative voltage, which means less current. But, has that integrating time content of the R network with the C of the coupling cap. Luckily, some spice vacuum tube models DO include grid current, so you can experiment and watch these bias shifts happen.

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Post by esist »

Manfred wrote: 31 Jan 2022, 23:57 I would appreciate it very much if I could get the values for B+ and B++, I did not find these values on the internet.
As I am using a DC/DC booster, I am feeding the same 250V to all B's. If you are looking for the "original" voltage levels I am of no use.
Sorry.
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Post by Manfred »

I found this schematic whose preamp has the same anode and cathode resistors, the voltage values for B++ and B+ are written in it as 275 volts and 260 volts.
Peavey_classic30.pdf
(844.9 KiB) Downloaded 72 times
Here is some more reading stuff concerning grid current clipping and blocking distortion:
Common_Gain_Stage.pdf
(1.46 MiB) Downloaded 69 times
Starts on Page 19.
http://www.r-type.org/articles/art-171.htm
https://www.valvewizard.co.uk/gridstopper.htm
https://www.aikenamps.com/index.php/wha ... distortion
Last edited by Manfred on 04 Feb 2022, 00:23, edited 1 time in total.

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Post by bmxguitarsbmx »

The Common_Gain_Stage pdf looks to be Merlin Blencowe's. He has a great writing style and an amazing ability to describe this stuff. I highly recommend his book on Guitar Preamps.

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Post by Manfred »

bmxguitarsbmx wrote: 03 Feb 2022, 17:30 The Common_Gain_Stage pdf looks to be Merlin Blencowe's. He has a great writing style and an amazing ability to describe this stuff. I highly recommend his book on Guitar Preamps.
Yes, the paper is by Merlin Blencow.
I have the books that were written by him.

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Post by phatt »

esist wrote: 31 Jan 2022, 17:24 I see. As usual, there is a steep learning curve ahead even getting steeper on the way. What I am taking away is that assuming the grid has "infinite" input Z is good, but does not make the voltage divider behave "linear" ?
Anyway, that thing does what it should, it sounds fat on higher gain settings, maybe a bit dull ....
The Classic 20 pre-amp sounds much better. That extra gain I will get by one of the pedals.
Thank you Phil.
BTW if your output is taken directly from the treble wiper then just note that it will limit the efficiency of the tone controls if running a long cable to amp. Also may degrade the signal swing.
A buffer stage will guarantee no loss. :secret:

These are Hi Z tone circuits and suffer from a thing called insertion loss.
i.e. a 1volt input signal can be reduced by as much as 70%. :shock:
There are many common stomp box circuits that run tone stacks with no output buffer but they can degrade the signal a lot.

I found this page which gives an example of interstage loading effects.
https://ultimateelectronicsbook.com/max ... e-loading/

Down the bottom is a simulation of a simple resistive stage verses 2 active follower stages.

I've built many circuits over the years and maximizing signal transfer between stages or in this case stomp box interconnections is key. Also helps keep signal to noise lower.
Phil.

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Post by esist »

phatt wrote: 06 Feb 2022, 14:42 BTW if your output is taken directly from the treble wiper then just note that it will limit the efficiency of the tone controls if running a long cable to amp. Also may degrade the signal swing.
A buffer stage will guarantee no loss. :secret:
Thanks Phil. I am running my pre-amps in the effect loop of my HX Effects, which is buffered at in and out.

Another topic: Do you think a Cathode Follower before the tone stack would increase the bass response ? I found the bass control to be inefficient at the 2nd half of the pot.
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Post by ppluis0 »

esist wrote: 11 Feb 2022, 14:55 I found the bass control to be inefficient at the 2nd half of the pot.
Hi esist,

The treble potentiometer specified in the schematic is a linear taper and the bass must be log taper.
According your observation perhaps you have installed a linear taper pot in both positions ? :roll:

Cheers,
Jose

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Post by bmxguitarsbmx »

esist wrote: 11 Feb 2022, 14:55
Another topic: Do you think a Cathode Follower before the tone stack would increase the bass response ? I found the bass control to be inefficient at the 2nd half of the pot.
No. A cathode follower will decrease bass response. Plate fed tone stacks are known for having a little extra "whomp" because the higher output impedance driving the TS puts a slope on the response that favors the low end response. Also, plate feeding the TS loads down that tube and gives it a lot of extra harmonics and sustain due to load of the TS limiting the headroom. If you buffer this stage with a cathode follower, it will totally change the sound and mostly make it much cleaner. ppluis0 has the good idea with the taper of the pots. Make sure the bass is Audio per and the treble is Linear taper.

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