I find that most SMPS supplies give off alot of RF which is captured by the amplifier inputs. it manifests itself as a FM like hiss between stations limiting the s/n to 50db. i am getting 20v p-p from 6922 running on only 45 volts. distortion must be 4 -5 % but the sound is great. I have to make the B+ tripler so I get 140v B+ after regulator.
400 v B+ with a transformer yields a maximum 1600v P-P from end to end. if one tap is pulled to 0 volts the other tap goes to +800v in a tube setup, the lowest voltage is about 50v so the highest tap voltage would be 750v for 1500v p-p across the plates
20:1 transformer
1500 /20 = 75v p-p = 37.5 vp / 1.414 =26.52 v RMS pwr = 26.52^2/8 = 87.9 watts RMS
400 v B+ with a transformer yields a maximum 1600v P-P from end to end. if one tap is pulled to 0 volts the other tap goes to +800v in a tube setup, the lowest voltage is about 50v so the highest tap voltage would be 750v for 1500v p-p across the plates
20:1 transformer
1500 /20 = 75v p-p = 37.5 vp / 1.414 =26.52 v RMS pwr = 26.52^2/8 = 87.9 watts RMS
You are right there is a motorboating like noise and another more strange compound noise in the background but you have to put your ear close to about 1 foot from the speaker to hear it. This is also the reason why i haven't decided if i will use smps all around or i will make a traditional regulated power supply.
Tomorrow i will try some more LC filtering of the power supply rails and see what happens. THE 829B amplifier works very well in terms of distortion clarity and power indeed!
Chris
Tomorrow i will try some more LC filtering of the power supply rails and see what happens. THE 829B amplifier works very well in terms of distortion clarity and power indeed!
Chris
Motorboating is low frequency oscillation, often due to feedback via the PSU - which in some cases could have too high an output impedance at subsonic frequencies.
Squegging is repeated stop/start high frequency oscillation, which could be due to stray capacitance between input and output circuit nodes with the same signal phase. Squegging can sometimes mimic motorboating, so you could have either of these.
Squegging is repeated stop/start high frequency oscillation, which could be due to stray capacitance between input and output circuit nodes with the same signal phase. Squegging can sometimes mimic motorboating, so you could have either of these.
PS… with both easily found with an oscilloscope.
Never underestimate the power of a (these days for a GOOD one) $250.00 device to — for the rest of your life — show you vitally important details about the internal real-time operation of your present and future projects.
Just saying.
“An oscilloscope is worth a thousand wrong turns”…
GoatGuy
Actually what i observe is a 1 to 2 hz oscillation like someone is turning up and down the volume but just a little.
The waveform is clean with no distortion in it. It goes away when i turn up the output power and it does not return again until i power cycle the amplifier.
I also tried the oscilloscope and there is nothing visible but then again i will look in to it again tomorrow when i will try cathode bias instead of fixed bias (just to gain experience).
I do have a spectrum analyzer also so i think i will find the cause but i have not seen a behavior like that again.
Chris
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I agree! Anyway i THINK I found the offending part. As you were all expecting it was the 200v preamplifier-phase splitter smps as it has a 40ms oscillation in it's output. I will replace it with another design and check again. i am thinking to use linear regulation alone or after the smps for the preamplifier only.
Chris
Chris
Hi.
I thought that it might be a good idea to let you know what smps i use now for the preamplifier and the whole amplifier is dead quiet, i use this one DC 12V to 150V-420V DC High Voltage Power PSU Board f Tube amp/ Preamp/ Filament 699911313721 | eBay
For the output power supply i use this one MINI DC-AC Inverter 12V To 18V220V/380V 500W Boost Step UP Power Module New Hot | eBay
Right now i am getting 460v at 15v input (you need to change the input capacitor in order to raise the input voltage) with one of those but at full power the damned thing is cold to the touch! Right now i am listening to good old Creedence Clear Water Revival and i am very happy...
Chris
I thought that it might be a good idea to let you know what smps i use now for the preamplifier and the whole amplifier is dead quiet, i use this one DC 12V to 150V-420V DC High Voltage Power PSU Board f Tube amp/ Preamp/ Filament 699911313721 | eBay
For the output power supply i use this one MINI DC-AC Inverter 12V To 18V220V/380V 500W Boost Step UP Power Module New Hot | eBay
Right now i am getting 460v at 15v input (you need to change the input capacitor in order to raise the input voltage) with one of those but at full power the damned thing is cold to the touch! Right now i am listening to good old Creedence Clear Water Revival and i am very happy...
Chris
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I use that same supply for a 6N13S amp (160V output) and EL34 440V, 14.4V input. Works like a charm!
Glad to see you like the results. I am making an 829B (GU-29) amp myself, but am planning on running it at 440V, 220V screens, 4-5K Ra-a (both sections in parallel, 10R plate resistors for load sharing)
Glad to see you like the results. I am making an 829B (GU-29) amp myself, but am planning on running it at 440V, 220V screens, 4-5K Ra-a (both sections in parallel, 10R plate resistors for load sharing)
What did you use for the plate caps? I got this style: 10 pcs brown bakelite tube anode caps for 829 FU-29 FU-32 FU 829B FU29 832 DIY | eBay
The HV wire I'm using is just test lead wire as far as I can tell. Silicone covered, rated for 200C/3KV/18ga...
The HV wire I'm using is just test lead wire as far as I can tell. Silicone covered, rated for 200C/3KV/18ga...
Global negative feedback opinions?
Ok i decided against global negative feedback. Although the THD measurements are far better (i used only 6db) my ear is complaining about dullness and lack of detail and it is not my opinion only, has anyone else experienced this? So far many want to build it, it sounds that good, my home JVC amplifier has the same symptoms but worse so i suspect that Global NFB must be the cause. Btw the speakers i have are excellent so this is not the reason. Maybe i will fit a switch in the final version which will start in a week or so from now (i need to build a nice wooden enclosure and get some more parts)
Chris
Ok i decided against global negative feedback. Although the THD measurements are far better (i used only 6db) my ear is complaining about dullness and lack of detail and it is not my opinion only, has anyone else experienced this? So far many want to build it, it sounds that good, my home JVC amplifier has the same symptoms but worse so i suspect that Global NFB must be the cause. Btw the speakers i have are excellent so this is not the reason. Maybe i will fit a switch in the final version which will start in a week or so from now (i need to build a nice wooden enclosure and get some more parts)
Chris
'Dullness' is what hi-fi is supposed to do. 'Lack of detail' means less distortion.
Feedback should not be merely dialed-in or dialed-out; it needs to be designed-in or designed-out. To get the real advantage of feedback you need enough feedback, and this usually requires some frequency compensation to keep the loop stable which then means that without feedback the amp has HF droop.
People new to feedback often add a little (6dB is a little) and then decide they don't like it. They prefer the louder slightly more distorted version without feedback. They never try 20dB of feedback, which is where you begin to get the benefit of it.
Feedback should not be merely dialed-in or dialed-out; it needs to be designed-in or designed-out. To get the real advantage of feedback you need enough feedback, and this usually requires some frequency compensation to keep the loop stable which then means that without feedback the amp has HF droop.
People new to feedback often add a little (6dB is a little) and then decide they don't like it. They prefer the louder slightly more distorted version without feedback. They never try 20dB of feedback, which is where you begin to get the benefit of it.
Ok i understand. From measurements the amplifier has a 13db gain at 20Khz (0,8v at 20Hz, 1v at 1Khz, 5v at 20Khz and 3v at 30Khz) so maybe some compensation is needed? I will try to add 20 db or more and see what happens but it is not the louder amplifier that makes the difference in my ears, is this lack of detail or whatever i perceive like that. Thank you very much for the help!
Chris
Chris
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Snubbers are for stopping local parasitic oscillation. To stop a feedback loop from oscillating you need frequency compensation.
Be aware that a louder amplifier will sound better, even if it is actually worse. This is an unavoidable feature of human psychology. If you adjust it to exactly the same (measured) sound level, then the fact that the volume control has to be more advanced with more feedback will still feel like a problem - even for people who understand this.
Be aware that a louder amplifier will sound better, even if it is actually worse. This is an unavoidable feature of human psychology. If you adjust it to exactly the same (measured) sound level, then the fact that the volume control has to be more advanced with more feedback will still feel like a problem - even for people who understand this.
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