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| Voltage sag problems in tube amp (SS rectifier) |
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| ELS122:
--- Quote from: T3sl4co1l on June 03, 2020, 08:44:07 pm ---That's 24:1 for total primary? And, 8 ohm load, so that'll be 4.6kohm a-a. In class AB, each tube sees 2.3kohm for the initial class A range, decreasing to 1.15k as the opposing tube goes fully into cutoff. Seems pretty low, for two tubes and such a stupendously high supply voltage... Yes, a 16 ohm load would probably be better suited, of course that will likely sacrifice some low-frequency response. Unless the transformer is in fact designed for 9.2k to 16 ohms, in which case there you go, your load is wrong and that's that. 6P3S is rated 20W or so, so a 15W idle is 28mA at 530V, or 44mA at 340V. In class AB, the peak current will be maybe 3-4 times higher, or 176mA tops (assuming enough screen voltage to draw that current at Vg1 = 0). 176mA into 1.15 to 2.3k is 200 to 400V. Maybe 300V mean, since we're going between the conditions in class AB. 530V seems to be an incorrect supply voltage. 8.4H and Q 19.1 is very believable for a choke of that type, that's good. 8.4H resonates with 4uF at 27Hz, so it's not a resonant type filter. Likely, if the transformer and choke were original and as intended, it was supposed to be a choke-input filter (which gives 0.9 * Vrms or 340V DC). 8.4H at 50Hz mains, full wave rectified, has a critical load of 7.56kohms, or 45mA. That's one tube at idle, kind of marginal but should be okay. Load shouldn't be much less than the class A setting so this should be okay. The supply voltage will swell very high (530V) until the tubes come up. So yeah, remove the 4uF, and increase the other cap if ripple is a bit irritating. Tim --- End quote --- the 4uF cap, choke, and 20uF cap after the choke all are original to the amp. by what I calculated, a 22uF cap before the choke would be ideal. 24:1 is from one side to the center tap, not from plate to plate. most Russian tube amps I've seen have really high B+ voltages with the 240V mains nowadays. for example, I have a Rigonda 102 that has 400V on the first B+ node. I don't believe I ever said that the B+ is 530V, I said it's 490V or around that. and drops to 380V under max volume. but I swaped to a tube rectifier (5Z4S) and it now only drops to 420V. and also solid state diodes shouldn't drop nearly as much, well afaik they have 0 sag, but counting in the voltage drop on the transformer under load. I would estimate is to drop like 15V at that node but it drops more than 100V so I am pretty sure those solid state diodes are underpowered, dead, or something else. I bypassed the choke but that didn't help the sag. so yeah I'm pretty sure I just need new diodes. |
| TimFox:
--- Quote from: ELS122 on June 03, 2020, 08:17:23 pm ---well, I'm reading about choke filters on valvewizard but I'm dumb at math ;D. what does '[2 pi f]' mean in this equation? 'L = 1 / ( C × [2 pi f]^2 )' --- End quote --- As noted, that is the angular frequency for a sine wave. That equation is for the inductor L that will resonate with that capacitor C at frequency f, not what you want in a rectifier circuit. Note that a choke-input filter for a rectifier is a different concept than a resonant LC circuit. With a true choke-input filter, the choke tries to keep the current through it constant, and the voltage swing at the node between rectifier and choke is rather large, with a reasonable ripple voltage after the choke. Again, I highly recommend PSUD2 software, but you need to look at the textbook treatment of rectifiers and filters. Putting a small capacitor before the choke is a somewhat recent idea; back in the day, only the choke and output filter capacitor were used, sometimes followed by another L-C stage. |
| ELS122:
well again, the 4uF capacitor before the choke was original, it had no standby switch (if maybe it was some weird "pop" elimination cap or something). and all the solder joints were untouched. either someone magically soldered a 4uF cap that also is the same type of capacitor as the other ones were, also was labeled "C39" and also made the solder joints exactly as the other ones look. then it's a clear sign that it was from the factory like that. also, this is not an amp from like the 40s, as I've estimated it is from somewhere around 68-75 I am pretty sure a lot of tube amps by that time used capacitors before chokes. for example, the JCM800 super lead. it was made from 1959 and had a 100uF cap before a 5 henry choke. |
| T3sl4co1l:
Doesn't mean it was designed properly, or at all. Audio is full of "it makes sound, ship it!" Clearly either the supply voltage or the output transformer is grossly mismatched to the tubes in use. It would be marginal to OK for 6L6GCs, but not 6L6GB/6P3Ss. Or at 16 ohm load if the transformer losses and frequency response are adequate. Tim |
| greenpossum:
You say the transformer ran hot when you had a tube rectifier. Maybe this was due to the extra heater draw and the transformer is deliberately underrated because they didn't expect it to be used continuously at full volume hence the droop. Maybe that's just the way it is. |
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