jelle wrote: ↑Wed May 17, 2023 1:37 am
I am sorry I do not have answers, only questions. Why do different types, brands, and even batches of resistors, or even individual resistors and caps sound different? Why does the guitar sound change when reversing the guitar cable? The longer I am in this field, and its been 25 years, the more questions I have. Its wild. But I learned how to fix them in the amp at hand. The way I understand it is that this is what puts the art into the amps, and how Dumble could take a Fender amp, and without even touching the schematic, make it a phenomenal amp.
+10000000
Lynxtrap wrote: ↑Wed May 17, 2023 3:06 pm
3dB at 10kHz? As a tone shaping option, that's not worth the price of the cap
jelle wrote: ↑Tue May 16, 2023 11:22 pm
If the cathode cap is 'perfect', as well as the leads/solder joints to ground making a perfect zero ohms path...
The imperfection is the ESR of the cathode cap, of course, so yes that creates a small negative feedback effect.
I thought the snubber cap between the cathode and anode can be looked at as being parallel to the internal resistance of the tube. So the cathode bypass cap is, as i look at it, not really relevant for the effect of the snubber cap. The HF frequency's don't have to pass to ground trough the cathode cap. Ofcourse the size and quality of the cathode bypass cap does have an effect on its own on the frequency bandwidth of the amplified signal.
If i am correct the cutoff frequency (-3 db) would be easy to calculate: 1 / 2xπxRixC which in the case of an 12ax7 and 270p cap would be about 9430Hz.
I think in total there are 4 high roll off filters in series that can be added together. The grid resistor and input capacity of both triodes rolling off highs and both snubber caps parallel to the Ri of each triode.
Lynxtrap wrote: ↑Wed May 17, 2023 3:06 pm
3dB at 10kHz? As a tone shaping option, that's not worth the price of the cap
Well, I (and Dumble) seem to disagree with you. When I first started playing with these circuits, I thought the snubbers were for stability or to prevent oscillation. I carefully build the circuits without them and found them stable. Hmmm, so why are they there ?
A buddy of mine (a vacuum tube genius, the late Ted Hammond), showed me how multiband tube equalizers used this method to make bandpass filters. Look at the Blonder-Tonque "Audio Baton", and you will see many circuits, each with roll off on the way in and out (for lows) and these snubbers for high frequencies around the triode sections. https://elektrotanya.com/PREVIEWS/63463 ... .pdf_2.png
Back to the case of the snubbers, don't dismiss out of band artifacts causing problems within the audio bandwidth. There is a definite difference when you take 100, 220, 330, 470 pf caps. Yes, they eventually start encroaching on the high end as the values go up, but even at 10-K I believe they can clean up transients and make an overdrive smoother and more musical.
jelle wrote: ↑Tue May 16, 2023 11:22 pm
If the cathode cap is 'perfect', as well as the leads/solder joints to ground making a perfect zero ohms path...
The imperfection is the ESR of the cathode cap, of course, so yes that creates a small negative feedback effect.
Isn’t it positive feedback?
With the output taken from the anode, I’m thinking of the grid as an inverting input, the cathode as non inverting.
It's really kind of neither. The snubber cap current is phase shifted ~90 degrees from the plate voltage, so the voltage produced on a resistor to ground is crossing zero when the voltage on the cathode peaks. In the simulation it isn't quite zero, just slightly negative, but in any case it's negligible.
martin manning wrote: ↑Thu May 18, 2023 11:23 pm
It's really kind of neither… in any case it's negligible.
Thanks for that perspective, sometimes it’s better to think in terms of current rather than voltage
I’m thinking it would work out pretty much the same if the cathode was unbypassed?
pdf64 wrote: ↑Fri May 19, 2023 10:00 amI’m thinking it would work out pretty much the same if the cathode was unbypassed?
Indeed it does. For the same parameters as described above the gain is reduced ~6 dB as usual, and the clipping is limited to the higher values of input voltage sweep.
Small phase changes in the harmonics stifle the sustain, huge. The highest loudest finger noise is a good place to put the knee of low-pass. The treble response from all the pickups should be similar there. Band-passing is the voice of the amp!
Lynxtrap wrote: ↑Wed May 17, 2023 3:06 pm
3dB at 10kHz? As a tone shaping option, that's not worth the price of the cap
Well, I (and Dumble) seem to disagree with you. When I first started playing with these circuits, I thought the snubbers were for stability or to prevent oscillation. I carefully build the circuits without them and found them stable. Hmmm, so why are they there ?
A buddy of mine (a vacuum tube genius, the late Ted Hammond), showed me how multiband tube equalizers used this method to make bandpass filters. Look at the Blonder-Tonque "Audio Baton", and you will see many circuits, each with roll off on the way in and out (for lows) and these snubbers for high frequencies around the triode sections. https://elektrotanya.com/PREVIEWS/63463 ... .pdf_2.png
Back to the case of the snubbers, don't dismiss out of band artifacts causing problems within the audio bandwidth. There is a definite difference when you take 100, 220, 330, 470 pf caps. Yes, they eventually start encroaching on the high end as the values go up, but even at 10-K I believe they can clean up transients and make an overdrive smoother and more musical.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
martin manning wrote: ↑Thu May 18, 2023 9:40 pm
I wasn't sure if Lynxtrap was joking or not. The traces on the previous page clearly show how the snubber takes the edge off of a 5kHz waveform.
I was half joking. Theoretically, 1 or 2 dB's around 5-7kHz is neglible, but I don't doubt it can be heard in an otherwise finely tuned circuit with a good speaker. I certainly do not doubt your experience in these matters.
I tried 220pF in a build I'm working on (has a thread in the Technical section), it made no difference whatsoever, but I guess 220pF is too small.
"Hey mister, turn it on, turn it up, turn me loose!"
Lynxtrap wrote: ↑Sun May 21, 2023 3:43 pm
Theoretically, 1 or 2 dB's around 5-7kHz is neglible
WhopperPlate wrote: ↑Thu May 18, 2023 1:35 am
One man’s negligible is another man’s imperative.
Absolutely, but nothing is imperative to me if I don't know what it is or what it does. That's why I started this thread, and the answers have been very enlightening.
I actually thought I was replying to Fuchsaudio in my previous post, but @Martin Manning: you showed the facts and that's great!
I was just kind of doubting how audible the effect would be in practice, and that was addressed by Fuchsaudio (who has about 99.999% more experience than me in this matter).
"Hey mister, turn it on, turn it up, turn me loose!"
Lynxtrap wrote: ↑Sun May 21, 2023 6:22 pm
I was just kind of doubting how audible the effect would be in practice, and that was addressed by Fuchsaudio (who has about 99.999% more experience than me in this matter).
You had doubts , others chimed in with their highly experienced perspective (as you noted as well) and I put an exclamation point over it all. What’s confusing ?
