| Electronics > Projects, Designs, and Technical Stuff |
| Overhauling high voltage vacuum tube (valve) power supply |
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| T3sl4co1l:
Yes, ESR in those old caps (the few that survive without horrendous ESR!) was on the order of ohms. Miliohms here or there on a film capacitor will go completely and utterly unnoticed. As I expected, my intuition was in the right ballpark: The cap's Irms = 158mA is quite reasonable. 108mA DC output at 432V, probably a bit over ratings (the choke says 100mA, but I don't know what it's actually expected to run at). As it turns out, the capacitance is too large to fully transition to choke-input mode operation at rated current (that'll take more like 200-300mA). So I'll admit to being wrong about that. Accordingly, the output voltage is more like 1.0 x Vrms at rated current, up to 1.4x at no load (and following some curve inbetween). 134Vp-p is still kind of a lot of ripple voltage, more than an electrolytic would want to deal with. So yeah, film is still a good idea. One of those EPCOS "DC Link" caps would be more than enough Note this simulation includes 200 ohms series resistance (representing the transformer and diodes' series resistance), and ideal capacitors and inductor. 200 ohms is probably being generous (that might be the transformer alone, not even including the 5R4/5V4), and the choke is probably 50 or 100 ohms again; actual output is probably more like 350V at the same load. Tim |
| proschuno:
BTW, the choke is 375 ohms (it says so on the label and it measures that.). So are these components purposely designed, or could I actually tweak this design to improve performance slightly I wonder? This was homebuilt, so I'm sure the guy who built it just threw together what he had on hand, so I could go back and make it better…. Second Question: Would these caps be suitable to replace the oil caps? I hooked one up to a 1khz square source on my oscilloscope in series with a resistor and it measured to be 4 microfarads, so I'll suppose the other is in the same condition (don't know if I stated this before, but noise, ripple, and nothing else like that was ever a problem with this. But in the event I would need to, I'm just curious, could the film caps below (attached photo) be used? |
| T3sl4co1l:
Whatever it is, it has to be rated for 600VDC absolute minimum; I'd probably get 1000V caps myself. Then, if the value is alright, doesn't matter much beyond that. Tim |
| oldway:
Why do you want to replace the oil caps ? If it has no leakage, there is nothing wrong with this caps. It is doing his job and will do it for 50 years more. Never replace what is not really necessary. If ripple is low, you don't have to worry about the caps. You want to make better this power supply. It is a very old design and has a lot of shortcomings. To make it better, you should make a new one. A new design would be a SMPS with power FET, voltage regulation and current limiting, digital readings, ... For a sixties design, I think that is not a very good one: - there would be several voltage ranges with transformer secundary taps comutations for reducing anode power dissipation. - OA2 voltage stabilisators are not that good : noisy, not very stable, often oscillates, ... - 5V4 (more serial resistance) and 6X4 could be replaced with silicon rectifiers. In those times, I used BY100 silicon rectifiers. - I would prefer to use EL34/6CA7 in place of 1619. If it would be a later design (seventies), i would choose to use high power color TV pentode PL/EL519. An example of a far better design: An other one with 7 ranges: |
| dannyf:
For high impedance / low current devices like tubes, high ESR is either irrelevant or desirable. |
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