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| Voltage sag problems in tube amp (SS rectifier) |
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| floobydust:
Some guitar amps have mushy power on purpose, as part of their overload sound. Choke-input I find regulation is good but very hard on rectifier diodes, they must be ultra-fast recovery such as UF4007 and the big choke needs a snubber across it or you will get kV switching hash with the leakage inductance ringing. Not a problem with most tube rectifiers they don't have trr unless mercury vapor lol I don't know what the circuit looks like now, post #1 has no cathode follower and a capacitance multiplier. OP you can try just add a zener at one of the taps or for the cap multiplier, get some say 91V 5W parts in a string to add regulation. I think PSUD2 is missing audio frequency loads and that the load currents are not static or always in-phase. Y I would use 10-22nF to the O/P tube grids to limit the low frequencies. Just Spice sim the whole amplifier and see what the freq. response is like. You might just run the O/P tubes (and maybe screens) right after the rectifier, leaving the choke only for filtering the small stages. You'd see 15-22uF compared to your 2.2-5uF and large value resistors, so the caps could be larger unless they are film caps you are using. |
| trobbins:
Can you confirm that you have measured the unloaded secondary HT voltage (eg. is it 380-0-380), and the choke series resistance (shown as 105 ohm)? Can you show the simulation results section as well, as that shows other details, and helps interpret your plot (as the plot doesn't show parameters), and allows others to quickly cross-check your sim? How did you identify the sim cap ESR ? Eg. the first filter cap is shown with an ESR of 16 ohm. Can you confirm the recommended anode+screen dissipation of the 6P3S is circa (0.5*480V*70mA)/0.7 = 24W? What is your application for the amp? The initial post just mentioned 'play something through the amp'. Can you post a schematic of the whole amp, along with idle operating voltages? The initial comment of 'flubs out' may be related to distortion from earlier stages. |
| pardo-bsso:
Besides what everyone else has said, can you scope the current on the rail? Perhaps you need to add a bit more of idle load so that the choke current is continuous instead of dropping to zero on every other line cycle. |
| ELS122:
--- Quote from: trobbins on June 05, 2020, 01:17:04 am ---Can you confirm that you have measured the unloaded secondary HT voltage (eg. is it 380-0-380), and the choke series resistance (shown as 105 ohm)? Can you show the simulation results section as well, as that shows other details, and helps interpret your plot (as the plot doesn't show parameters), and allows others to quickly cross-check your sim? How did you identify the sim cap ESR ? Eg. the first filter cap is shown with an ESR of 16 ohm. Can you confirm the recommended anode+screen dissipation of the 6P3S is circa (0.5*480V*70mA)/0.7 = 24W? What is your application for the amp? The initial post just mentioned 'play something through the amp'. Can you post a schematic of the whole amp, along with idle operating voltages? The initial comment of 'flubs out' may be related to distortion from earlier stages. --- End quote --- yes the power transformer unloaded outputs 380V - 0 - 380V RMS AC. and yes the choke DC resistance is 105 ohms, the primary resistance of the power transformer is 11 Ohms and secondary is 100-0-100 Ohms. and the line voltage is 232V RMS AC. 50Hz. the power transformer has some unsused terminals on it, I might measure the voltages on them. I looked at the datasheets of the capacitors. they were meant as 2 47uF caps in series with 1M balancing resistors, and each cap is rated at an ESR of 8Ohms. same deal with the others. I built a push pull EL84 cathode biased amp with another power transformer and it worked really good, non of that terrible sag. it seems that I am just aiming for too much power on that 6P3S amp. the power transformer can't keep up with it. so idk, either I regulate the B+ voltage, or get another power transformer. I could use another power transformer for everything except the plate supply for the output, but that would be not only really cluttered, but even still it would sag about 75V under load. The output is biased at 78% I drew up a load line but forgot to count in the screen voltage :-\ I'm probably just too tired, I'll go sleep and continue this tomorrow anyway here's the load line with 250V screen voltage and also 450V plate voltage which also is too small, it should be 480V both. also I accidently drew a load line at 0V grid curve. somehow trying to re calculate it I came up with over 50 watts output max, idk. but I redrew it again and now it seems alright, appart from the voltages I'll make a shematic of the whole amp if that helps ok nvm, I think I forgot to save the load line. well anyway, the only thing that was changed really was the max output line, which came out at 40W not 26watts. |
| TimFox:
Although PSUD does not have sinusoidal load current, it does have load current steps to check for unwanted ringing in the filter, etc. Fast rectifiers are good. Note that the choke input filter reduces peak rectifier current. For snubbing, I refer the reader to the textbooks. Choke input was almost required with mercury vapor rectifiers, since they were very unhappy with excessive peak currents. |
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