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| Overhauling high voltage vacuum tube (valve) power supply |
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| proschuno:
So I"m overhauling a 50 year old home-builtPSU in serious need of refitting. Using two pass tubes of them has a busted filament and the other was replaced about a year ago. Should I worry about getting a matched pair or just get a single tube? Also, the circuit uses two oil filled caps that I would like to replace for good measure (along with all the other ones in the schematic). Seeing that these on ebay are quite expensive along with any other electrolytic alternative, I'm wondering if I couldn't simply tie two 450V caps in series as shown for the +B supply? This would be much cheaper than getting another oil cap or similarly rated electrolytic ($US 80 on Mouser).. I notice one of the oil caps' leads is tied before the choke, so I wonder why so? Proschuno And for the missing piece: The OA2 tubes, along with the 1st and 3rd leads of the pot, are in parallel with the 30MF caps, and the cathode of the error amp tube is connected to the wiper of the pot. And BTW, taking apart the PSU I found the schematic; notice the dateā¦. it's literally as old as my mother lol ) The pass tubes are 1619 (or VT164), the error amp is 6sj7, the rectifier for +B is 5v4g, and the 2nd one is 6x4. |
| SeanB:
Just be careful of those oil filled caps, they likely contain PCB oil, and should only be handled with gloves, and if there is any oil leakage clean it off with solvent and dispose of the rags, gloves and caps as dangerous waste. You could replace them with 4u7 1kV film capacitors with no problems. |
| calexanian:
The 8uf oil cap and that choke form a VERY important network!!!!!!! That is a capacitor input to filter type circuit. Much of the ripple rejection of that supply comes from that combination of a low capacitance and the choke. Otherwise you will have a lot of ripple and the plate voltage will be way too high for the pass tubes to handle. Just bring the supply up slowly on a variac and I bet that oil cap will be fine. Monitor the current draw to the unit and with no tubes apart from the rectifier tube. Watch the voltage on that cap as well. As the filaments heat up in the rectifier it will self soft start so to speak. Depending on age those electrolytics are toast. If the oil cal is bad, replace it with as close to 8uf as you can get. |
| SeanB:
Just looking 4u7 630V caps are hard to get. You can use 2 parallel 2u2 630V film capacitor to replace them, they are $4 each. The electrolytics are a lot cheaper, you can use 33uf 450V Nichicons as replacements for the 47uF ones, they are under $2 each. If you want to use to replace the 4uF oil then use 10uF 450V capacitors with a 100k 1W resistor across each capacitor like the other side. |
| proschuno:
Ok also too, my concern is wattage rating, if it should be one; is this also a reason oil caps were used too? Or can I literally replace them with the polypropylene/ethylene caps that I have, or are we talking some high power film caps? |
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