Can we reach a consensus that for regular classic tube guitar amps, with controls set to realistic stage levels, that as the signal level is increased from 0, the first point at which significant waveform distortion (eg clipping) occurs is at the power tube grids?
The above should be readily verified by a basic bench test kit, and act as a common reference point from which the affects of the waveform shaping of earlier stages (eg as the signal level is increased beyond the above point) can be investigated and analysed.
Pete
Elementary questions on gain.
Moderators: pompeiisneaks, Colossal
Re: Elementary questions on gain.
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Re: Elementary questions on gain.
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Last edited by matt h on Fri Mar 27, 2015 3:47 am, edited 1 time in total.
Re: Elementary questions on gain.
It seems that the PI / PA waveforms with negative feedback illustrates the distortion occurring first in the power tubes. The funny looking waveform of the PI is different than the output of the PA or input to the PI because of the errors introduced by the PA are fed back into the PI. If there were not PA distortion all the waveforms would look like the input. This happens well before the PI reaches any limits of its own. Without the NFB, you will see a progression of distortion as the input signal is increased starting with the power tubes. I'm not just talking about flat top clipping but the gradual progression towards that end.
The gain at each stage guarantees that each successive stage is closer to its limits (unless you have a radically exaggerated voltage dropping string between stages or voltage doubler for the plates, like a music man).
The "Gain" that guitar players talk about seems to me to be a combination of compression and distortion that produces a signal of fairly constant volume with varying harmonic content as the input signal diminishes. You can get there in many ways.
Good sounding distortion can happen anywhere along the signal chain. One half of this site is dedicated to amps that focus on pre amp distortion into a clean power amp. While the other to power amp distortion.
The gain at each stage guarantees that each successive stage is closer to its limits (unless you have a radically exaggerated voltage dropping string between stages or voltage doubler for the plates, like a music man).
The "Gain" that guitar players talk about seems to me to be a combination of compression and distortion that produces a signal of fairly constant volume with varying harmonic content as the input signal diminishes. You can get there in many ways.
Good sounding distortion can happen anywhere along the signal chain. One half of this site is dedicated to amps that focus on pre amp distortion into a clean power amp. While the other to power amp distortion.
Re: Elementary questions on gain.
epiphone VJ with reduced interstage attenuation, with a 500mv pk-pk 500 hz i/p, with the amp cranked into an 8ohm dummy load, the increase in current draw from quiescent was 73%
To clarify, current calculated from voltage drops across supply node dropping resistors and cathode resistor. Yes, 73% sounds rather specific, no idea why I quoted that figure, guess it must have been from comparing quiescent voltages to full power voltages, and the need to give a figure representative of my findings. The important thing was that I needed to calculate a suitable power rated dropping resistor for the power supply.I have to be honest, I don't really trust that 73% figure.
Agree that significant increase is due to increased screen current dissipation.
Tell you what, go measure the current, quiescent and cranked on a SE amp of your choice. I'm not being antagonistic, just interested.
Anyway, on this threads topic, I like to hear distortion from all and various stages, and not necessarily just 'clipping' distortion. As far as I can tell, each stage adds it's particular vibe, and the blend is preferential and fun to obtain.
Re: Elementary questions on gain.
matt, thanks for your review; good point, I agree that any global negative feedback loop needs to be opened, otherwise the 'inside loop' waveforms are difficult to interpret.
Also it's good mitigation for blocking distortion to prevent excessive signal swing at the LTP outputs, so yes, it's good if the LTP isn't too far behind the power tube grids..
However, I think a resistive dummy load would be better than a real speaker, due to the following -
1/ a real speaker will get v loud! Especially with continuous signals, it won't be domestic friendly and is risky in regard of hearing damage, unless an isolation box is arranged. My hearing is pretty shot and distorts loud sine waves all on its own, which also sets the tinnitus going.
2/ inductive loads will result in waveforms on the output and power tubes that are more difficult to interpret, for me at least, and will be frequency dependant
3/ different speaker types will have differing inductances and frequency / impedance characteristics, which will also be cab dependant to some degree.
Pete
Also it's good mitigation for blocking distortion to prevent excessive signal swing at the LTP outputs, so yes, it's good if the LTP isn't too far behind the power tube grids..
However, I think a resistive dummy load would be better than a real speaker, due to the following -
1/ a real speaker will get v loud! Especially with continuous signals, it won't be domestic friendly and is risky in regard of hearing damage, unless an isolation box is arranged. My hearing is pretty shot and distorts loud sine waves all on its own, which also sets the tinnitus going.
2/ inductive loads will result in waveforms on the output and power tubes that are more difficult to interpret, for me at least, and will be frequency dependant
3/ different speaker types will have differing inductances and frequency / impedance characteristics, which will also be cab dependant to some degree.
Pete
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
Re: Elementary questions on gain.
Full reactive dummy load to allow silent 'real load' testing.
I've got the gear, but funnily enough I'll be all tested out for a while.
I've got the gear, but funnily enough I'll be all tested out for a while.
Re: Elementary questions on gain.
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