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| Overhauling high voltage vacuum tube (valve) power supply |
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| dannyf:
Maybe it is a long-term solution and you can start working on smaller boost-up converters first, with the goal to build your own sometime down the road. Getting a transformer for a smps, or winding one, is not that difficult, especially if you don't need lots of power output. Transient response could be an issue, however. If it is used to power tube stuff, they are likely class A so transient response isn't a big deal. |
| calexanian:
The ripple in those input caps is HUGE in those circuits. It you are going film make sure you have a very low esr. Also I do not recomend series. Parallel is a great idea as it lowers esr And distributes heat. Yes...... They do heat up in that kind of service! Those regulated circuits are kind of an animal all their own.. They however can be extremely reliable. |
| T3sl4co1l:
FYI, the resonant frequency of that cap to the choke is 28Hz, so it's not a resonant filter. It's just a really awful C-input filter. This gives terrible supply resistance (VDC = 1.4 * Vrms for light loads, dropping to maybe 1.1x for... 30% loading maybe, down finally to ~0.85x for 60-100% loading), but the low capacitance is required to keep the rectifier happy while running near ratings. Tube diodes cannot withstand large peak currents; typically the absolute maximum cap-input value is 40uF, and that's with derated current capacity. The ripple through the capacitor is miniscule: we're talking 100s of mA here. Okay, fairly large compared to the ratings of an electrolytic that size, but sheesh, you can put whole amperes through a film cap, no biggie. The ripple *voltage* however is fairly large, much more than the 10-20% you'd want on an electrolytic. The voltage swing will be the full peak voltage at heavy loads (where the diode current is continuous, and the filter is acting as a choke-input filter), but less at lighter loads (cap-input filter action). In a regulated supply, I suppose the supply resistance doesn't matter much, so who cares. But that said, I don't know why they didn't just leave it choke-input altogether, why bother with an input cap at all? They certainly could've added a few more henries for the cost of that oil-can cap, probably getting better performance (less ripple, better regulation) in the end. Tim |
| calexanian:
Believe me. Put an ac meter in there and you will see much higher ripple than that! The resonance is supposed to be less than 120hz. Far less. It's not a dc filter in a traditional seance. It's to provide an ac component to the current flowing through the choke to provide proper filtering there. You just have to play with those circuits to understand. Look up the section in the RCA radiotron design handbook under power supplies. It's the best description and explanation of how those circuits work anywhere. You can see how they did it before our modern stuff. I highly recommend that book to everybody. Just the basic circuit theory in there is better than any modern textbook. |
| oldway:
--- Quote from: calexanian on February 16, 2014, 03:42:08 pm ---The ripple in those input caps is HUGE in those circuits. It you are going film make sure you have a very low esr. Also I do not recomend series. Parallel is a great idea as it lowers esr And distributes heat. Yes...... They do heat up in that kind of service! Those regulated circuits are kind of an animal all their own.. They however can be extremely reliable. --- End quote --- Concern with ESR for an old valve/tube equipment looks very funny to me: in this time, whe did'nt even know what ESR was... :palm: In the sixties, i have build a lot of vacuum tube audio amplifiers and repair hundreds of vacuum tube tv's (black and white and colors tv's) and i had never hear somobody speaking about ESR. Later, when I worked with high power fast SCR's inverter, concern was not to exceed RMS current in the electrolytics. It was low frequency ripple (120Hz/180Hz/360Hz) ESR concern was important for SMPS because the high frequency ripple. |
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