| Electronics > Projects, Designs, and Technical Stuff |
| Voltage sag problems in tube amp (SS rectifier) |
| << < (8/9) > >> |
| trobbins:
Ta. Note that electrolytic ESR specs are a maximum spec value at 20C, and typically will be much lower in value - but no issue. Also the choke inductance will be different as it will have a different ACV and DC current than your test setup. You measured the idle DC current through the choke by adding an extra 100R in series and that indicated 47mA and 150mA at 'full volume'. That aligns with a simpler PSUD2 sim where everything after C2 is replaced by either a 10k load for about 47mA output, or a 2.55k load with 150mA output. Can you describe your intended application, as full volume for a hi-fi amp is the onset of sinewave clipping, whereas a guitarist pretty much wants a square wave output from a sinewave input. Using PSUD2 with a stepped load current change shows no ringing for that setup (with 100R test resistor moved), just some under or over shoot. You should find similar operation when increasing filter caps as long as you don't do anything too drastic (ie. keep output capacitance sufficiently larger than input cap). That should get B+ sag down to circa 40V. Whether you need to reduce sag any more perhaps again depends on your application. For example low distortion doesn't necessarily require zero sag - there are so many other issues that would dominate that aspect. |
| ELS122:
--- Quote from: pardo-bsso on June 05, 2020, 01:32:24 am ---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. --- End quote --- don't have a scope :-[ I put a 100 ohm resistor in series and messured the current tho. 47mA at idle, 485V 150mA at full blast, 385V I either need to somehow regulate the voltage, have a constant current load at idle. adjust for a LOT less output power, or find a new power transformer. --- Quote from: TimFox on June 05, 2020, 03:01:17 am ---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. --- End quote --- no? when I was messing around with diffrent choke values, and diffrent capacitor values sometimes I would get some major ringing when a load was applied. also PSUD2 is really slow when I have really small or big values. like putting another choke with a 100nF capacitor in front, the simulation takes about 180 seconds. but 100nF isn't usually used in the power supply filtering so, I guess it isn't a problem. --- Quote from: trobbins on June 05, 2020, 05:28:18 am ---Ta. Note that electrolytic ESR specs are a maximum spec value at 20C, and typically will be much lower in value - but no issue. Also the choke inductance will be different as it will have a different ACV and DC current than your test setup. You measured the idle DC current through the choke by adding an extra 100R in series and that indicated 47mA and 150mA at 'full volume'. That aligns with a simpler PSUD2 sim where everything after C2 is replaced by either a 10k load for about 47mA output, or a 2.55k load with 150mA output. Can you describe your intended application, as full volume for a hi-fi amp is the onset of sinewave clipping, whereas a guitarist pretty much wants a square wave output from a sinewave input. Using PSUD2 with a stepped load current change shows no ringing for that setup (with 100R test resistor moved), just some under or over shoot. You should find similar operation when increasing filter caps as long as you don't do anything too drastic (ie. keep output capacitance sufficiently larger than input cap). That should get B+ sag down to circa 40V. Whether you need to reduce sag any more perhaps again depends on your application. For example low distortion doesn't necessarily require zero sag - there are so many other issues that would dominate that aspect. --- End quote --- yes I actually calculated the resistor values at first when I didn't know that you can just change it to be a current load not a resistive load. guitar amplifier, 40watts, high gain amp, but there's a design I'm trying to replicate, but change just a tiny bit so I can call it 'my own' ;D the amp I'm trying to replicate is the guitalogist gromms high gain amp conversion. I've drawn a schematic by what you can see in the video. the NFBL switch is a mystery, he claims that it switches between 2 different resistor values. Here's the shematic: https://easyeda.com/evalc2636/guitalogist-high-gain-gromms-amp-conversion the amp he's fixing I'm pretty sure is a Grommes 50PG2, in the video he claims it to be a Grommes 55PG, but the output transformer on it is in a diffrent place. the grid stopper on the first stage seems quite high but in the last shot of the inside of the amplifier, the resistor is still the same 100k. I'm having a hard time deciding how I can get as close to that sound as I can with fixed biased 6P3S's, since I need another 2 coupling capacitors to filter the DC bias voltage. but the big problem is the sag, I guess I could fix that by cathode biasing the output but I like the ease of use with fixed bias and want more power ;D but I guess if my transformer can't keep up, than I have no choice. I have seem to have lost the shematic for my amp, it's pretty much the same, except I've got a 10k resistor with a 470pF capacitor on the 1st stage. an AC cathode follower with a 430ohm biasing resistor. and I have the fixed biased output. I've changed the resistor values on the PI to the same ones as they are in the Grommes amplifier and I still have that massive sag. I'll draw a shematic of the amp and put it in this post. it's not low distortion, it's just that it quickly drops in volume a LOT, and then goes back in about 1.5 seconds. which doesn't sound good, at least for me. and PSUD2 shows this very clearly. here's the shematic of my amp https://easyeda.com/EdLS/ranom |
| ELS122:
ok update, I have a huge power transformer from a tv, I'll use that one. it outputs way less voltage. but if I use a doubler, it sags only about 12V from the doubler circuit while outputting 350V at idle. the source resistance is 7.3 ohms on that transformer as opposed to 130 on my current one, so it will not sag nearly as much :D I also considered removing the transformer entirely but I had the transformer from the TV that I'll use, also in a guitar amplifier even if I have every precaution to not have a chassis connected to the live wire. it still not a great idea. maybe in a radio. but not in a guitar amp. and if I still have a problem with the ~12V sag, I can have another supply for the preamp since it has 2 ""HV"" secondary windings, I could put them in series, I'll just do some testing and figure it out. anyway thank's for all the help :-+ |
| trobbins:
I suggest you confirm correct operation of the PI/feedback cathode circuitry - it presently shows the switched feedback connected to one side of the PI as providing the dc idle bias of the PI, as well as a feedback signal path to one side of the PI. If you don't have the same output stage gain or output transformer turns ratio then you could have an unstable amplifier. If you put a link over the 100 ohm, then that will effectively remove the feedback signal, and allow the level of feedback to be determined and allow you to confirm the DC biasing of the PI stage is ok. Perhaps if you move the screens to a resistor dropped node as per the linked schematic, as well as add say 100 ohm screen stoppers, and use ss rectifier (2x UF4007 in series should provide sufficient PIV margin, and use diodes from the same batch to avoid any need for balancing parts) with those filter cap values and see how it sounds. If you still think sag is an issue, then I think you need to verify that all other aspects of your diy amp are operating correctly, and then experiment with stiffening up the B+ and screen capacitors. Just replacing the PT is imho a knee jerk reaction, as you lose any experience with what the present setup can provide. Also perhaps add a grid stopper to the 4th 12AX7 triode as that could easily be grossly over-driven. The 100k grid-stopper for the 1st stage may be more about rolling off some HF response as the amp could be too bright - but that would depend on if you are using 12AX7 and your guitar. |
| ELS122:
--- Quote from: trobbins on June 05, 2020, 11:19:35 pm ---I suggest you confirm correct operation of the PI/feedback cathode circuitry - it presently shows the switched feedback connected to one side of the PI as providing the dc idle bias of the PI, as well as a feedback signal path to one side of the PI. If you don't have the same output stage gain or output transformer turns ratio then you could have an unstable amplifier. If you put a link over the 100 ohm, then that will effectively remove the feedback signal, and allow the level of feedback to be determined and allow you to confirm the DC biasing of the PI stage is ok. Perhaps if you move the screens to a resistor dropped node as per the linked schematic, as well as add say 100 ohm screen stoppers, and use ss rectifier (2x UF4007 in series should provide sufficient PIV margin, and use diodes from the same batch to avoid any need for balancing parts) with those filter cap values and see how it sounds. If you still think sag is an issue, then I think you need to verify that all other aspects of your diy amp are operating correctly, and then experiment with stiffening up the B+ and screen capacitors. Just replacing the PT is imho a knee jerk reaction, as you lose any experience with what the present setup can provide. Also perhaps add a grid stopper to the 4th 12AX7 triode as that could easily be grossly over-driven. The 100k grid-stopper for the 1st stage may be more about rolling off some HF response as the amp could be too bright - but that would depend on if you are using 12AX7 and your guitar. --- End quote --- yeah, I was just thinking about the feedback circuit. it's also getting the DC bias from it. in the video, he says "you can also put the NFB switch in the middle where the is no negative feedback, but I don't recommend that at all" which seems to imply that he changed the circuit and it doesn't get the DC bias for the PI from the output transformer. well, he could've made it connect to ground in that position but it wouldn't make sense if he 'doesn't recommend it at all'. my idea is that he connected the cathode of that stage to ground trough the stock 1k resistor and then had another resistor connect from the output transformer that he switched. would that work? and what feedback resistor would be adequate? I haven't seen NFBL in paraphase PI amps much. or to be exact I've rarely seen paraphase PI's at all. so I have no idea on how to add negative feedback to it. or does the NFBL signal get 'divided' from the 1k resistor from the OPT and with the 100ohm resistor to ground essentially? so adjusting the 100ohm resistor would change the amount of feedback, and slightly the balance of the outputs? |
| Navigation |
| Message Index |
| Next page |
| Previous page |