An interesting thing to do is look at an amplifier with a 1khz square wave into a resistive load with different amounts of feedback. An even more exciting thing to do is to connect the same square wave and output the amplifier into a real loudspeaker. Does not need to driven hard and see what a mess the speaker makes with its very inductive and capacative load. Some amplifiers show little difference but others really distort the square wave badly. Probably a real reason some amplifiers sound better than others.
+ElPaso TubeAmps looks like your'e getting the phase shift to the exact point of oscillation. i've seen that on my Electrostatic headphone amplifier which the circuit i found on headwize, it's called the Kevin Gilmore Solid State. I also seen that on my first well built tube amp using 6550. I was getting a very huge overshoot on the 1kHz SQ wave. That is clearly some sort of oscillation as Michael pointed out here. I solved the problem by adding a 100pF cap to the feedback and got rid of the oscillation. The solution for the headphone amp was to add a very small value capacitor between base and collector of the voltage amplifier stage. in fact the capacitance value was so small that a pair of twisted wires was just enough. On the low frequency you could move the roll off frequency of the coupling capacitors to way below 20Hz and see what happens.
It's ok to run the B+ to 530vdc as long as your cathode voltage is in the 125v range or your running more negative bias voltage..I agree that those 6b4gs are incredible sounding in PP.
+Mike Samra Hi Mike - yes, it is hard to describe why I tend to prefer the simple low power tube amps to the McIntosh but maybe our ears actually like the artifacts of an amplifier. That is, of course, within reason as when the THD and noise are below maybe 0.5% and -70 or more dB S/N. I still haven't gotten into the SE amps with no NFB - except for my attempt with the 833A. :-) Someday I will have to pick up a 100 watt SE OPT and try again with the 833A.
+Mike Samra Yes, I agree with 500 volts on the plate being OK, as far as the tube goes, because the cathode is indeed 100 volts above ground. One thing that does start coming into play is the cathode resistor (850 ohms, 10 watt) becomes too hot and burns up as it is dissipating almost 12 watts - I know because it happened to me... :-)
+Nicholas Bantell Thank you, Nicholas. I hope some of these examples help make those that don't work with decibels more comfortable with the idea. From what I have seen on RUclips concerning tutorials on decibels, all I usually see is an academic regurgitation of a classroom lecture that is not very effective. I sure wish I had had someone explain some of the fundamentals to me (with real examples) when I was a college kid back in the '60's instead of banging my head on books for so long to (hopefully) get a fundamental concept so I could move forward. 73 WA4QGA
You commented that one of your amplifiers did odd things at low frequency as the NFB was increased. I would expect that was due to phase shift, probably in the output transformer. As the phase shifts moves more towards the +ve it can make the amplifier unstable at those frequency's or even oscillate. I have also done experiments on amplifiers with and without feedback. Apart from the obverse gain loss in applying NFB. I have to say that (valve) amplifiers tend to sound better with quite low amounts of total feedback ie about 8/10dB However at these low levels, distortion, frequency response and noise is not good. I remember a transistor amplifier circuit from the early 60s, boasting a massive 60dB of NFB now THAT is a lot of gain to loose!!but these early transistor amplifiers needed this amount of feedback to make them sound passable.
Thanks for the video. There is also local NFB in that Dynaco Mk III due to the output stage being Ultralinear. Have you measured the amount of local NFB in the output stage?
Just look at the distortion level at 45W w/o feedback, you got 2.5% and from other videos on your channel this amp clearly does 0.25% at full power. let's round that 19.3dB feedback to 20. 20dB of gain equals 10X gain in terms of voltage. the NFB would then equal to -10X of gain of the amp and look how the numbers add up! you get 10X lower THD with feedback hooked up. same thing with the output impedance, it drops by a factor of 10, so you get a damping factor 10X higher. In fact the 20dB NFB is quite the most acceptable NFB for commercial brands like Mcintosh, Marantz and Dynaco. in solid state amplifiers however, you might see things like 80dB feedback and thats why you see THD numbers like 0.003% but to the ear they sound harsh at high volumes.
6550 tubes, one of my favorite, is now being made again!!! so much for the kt tube series,
AES has at least 5 makers of 6550 tubes.....
An interesting thing to do is look at an amplifier with a 1khz square wave into a resistive load with different amounts of feedback. An even more exciting thing to do is to connect the same square wave and output the amplifier into a real loudspeaker. Does not need to driven hard and see what a mess the speaker makes with its very inductive and capacative load. Some amplifiers show little difference but others really distort the square wave badly. Probably a real reason some amplifiers sound better than others.
+ElPaso TubeAmps looks like your'e getting the phase shift to the exact point of oscillation. i've seen that on my Electrostatic headphone amplifier which the circuit i found on headwize, it's called the Kevin Gilmore Solid State. I also seen that on my first well built tube amp using 6550. I was getting a very huge overshoot on the 1kHz SQ wave. That is clearly some sort of oscillation as Michael pointed out here. I solved the problem by adding a 100pF cap to the feedback and got rid of the oscillation. The solution for the headphone amp was to add a very small value capacitor between base and collector of the voltage amplifier stage. in fact the capacitance value was so small that a pair of twisted wires was just enough. On the low frequency you could move the roll off frequency of the coupling capacitors to way below 20Hz and see what happens.
It's ok to run the B+ to 530vdc as long as your cathode voltage is in the 125v range or your running more negative bias voltage..I agree that those 6b4gs are incredible sounding in PP.
+Mike Samra Hi Mike - yes, it is hard to describe why I tend to prefer the simple low power tube amps to the McIntosh but maybe our ears actually like the artifacts of an amplifier. That is, of course, within reason as when the THD and noise are below maybe 0.5% and -70 or more dB S/N. I still haven't gotten into the SE amps with no NFB - except for my attempt with the 833A. :-) Someday I will have to pick up a 100 watt SE OPT and try again with the 833A.
+Mike Samra Yes, I agree with 500 volts on the plate being OK, as far as the tube goes, because the cathode is indeed 100 volts above ground. One thing that does start coming into play is the cathode resistor (850 ohms, 10 watt) becomes too hot and burns up as it is dissipating almost 12 watts - I know because it happened to me... :-)
Was wondering if you still worked on amps? Thank you for the new video and hope all is well. Thanks. Ron
This great example that is easily explained regarding dB. I sure enjoyed it. 73 KC3FKX.
+Nicholas Bantell Thank you, Nicholas. I hope some of these examples help make those that don't work with decibels more comfortable with the idea. From what I have seen on RUclips concerning tutorials on decibels, all I usually see is an academic regurgitation of a classroom lecture that is not very effective. I sure wish I had had someone explain some of the fundamentals to me (with real examples) when I was a college kid back in the '60's instead of banging my head on books for so long to (hopefully) get a fundamental concept so I could move forward. 73 WA4QGA
You commented that one of your amplifiers did odd things at low frequency as the NFB was increased. I would expect that was due to phase shift, probably in the output transformer. As the phase shifts moves more towards the +ve it can make the amplifier unstable at those frequency's or even oscillate. I have also done experiments on amplifiers with and without feedback. Apart from the obverse gain loss in applying NFB. I have to say that (valve) amplifiers tend to sound better with quite low amounts of total feedback ie about 8/10dB However at these low levels, distortion, frequency response and noise is not good. I remember a transistor amplifier circuit from the early 60s, boasting a massive 60dB of NFB now THAT is a lot of gain to loose!!but these early transistor amplifiers needed this amount of feedback to make them sound passable.
Thanks for the video. There is also local NFB in that Dynaco Mk III due to the output stage being Ultralinear. Have you measured the amount of local NFB in the output stage?
13:28 - What frequency are you measuring "ACR" at?
I can't remember for sure but I was pbly robausing the GR 1650A and its internal oscillator is 1KHz.
Oops, I think I answered the wrong question. I think I got the 2 ohms of ACR by using the ESR meter.
Just look at the distortion level at 45W w/o feedback, you got 2.5% and from other videos on your channel this amp clearly does 0.25% at full power. let's round that 19.3dB feedback to 20. 20dB of gain equals 10X gain in terms of voltage. the NFB would then equal to -10X of gain of the amp and look how the numbers add up! you get 10X lower THD with feedback hooked up. same thing with the output impedance, it drops by a factor of 10, so you get a damping factor 10X higher. In fact the 20dB NFB is quite the most acceptable NFB for commercial brands like Mcintosh, Marantz and Dynaco. in solid state amplifiers however, you might see things like 80dB feedback and thats why you see THD numbers like 0.003% but to the ear they sound harsh at high volumes.