tube Heating 6.3V  and variac (for a HV PSU) - Page 1
Forum section: electronics / repair

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: April 27, 2015, 06:36:32 pm
content: Dear All,I just got a high voltage regulated power supply of 1971 (1200 V, 10-80 mA). Mostly with tubes.I would like to revive it but with cautious.I have not yet checked anything, but I wonder about the heating voltage requirement of 6.3 V ± 0.6 V of a 8068 tubeif ever I use a variac to start slowly the PSU.As the 6.3V is obtained through a direct transformer from the main, the voltage will be much lower than 6.3 V.Is it still advised to use a variac to reform the capacitors by powering at a much lower voltage than the 220 V for which the PSU is designed ?Thanks for any hint.

author: SeanB
author_rank: Super Contributor
timestamp: April 27, 2015, 06:51:27 pm
content: No problems there, the lower voltage means the heater is not going to emit as much, and will limit the current flowing through the diode. It will still emit even if it is not glowing, but will be enough to charge up the capacitors in the circuit to reform them. Start first though by checking all capacitors are not shorted, leaking electrolyte ( white deposit by the rubber bung) and that the tubes still have a shiny getter inside ( no white deposit on the inside of the glass, silver gray to brown is fine) and the transformer is set to the correct line voltage.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: April 27, 2015, 08:43:16 pm
content: Thanks for this advice.I will look to all the caps before powering anything, as well as on all components and verify connections, but I will only be able to do this on next week end. Meanwhile, it is a useful thinking time to avoid doing anything wrong by excess of precipitation.What would be the recommended voltage to start with the variac for the capacitor reform, owing that the normal voltage of the unit is 220 V.I will post pictures later on, but I can already post the schematics which is the only (and most useful) piece of information I have for this PSU.

author: SeanB
author_rank: Super Contributor
timestamp: April 27, 2015, 09:04:14 pm
content: HV transformer 4564 will not get power till the mains is about 140VAC, because of the current trip relay, so you probably want to make a dim bulb tester to do the initial reform, with a temporary connection across those 2 relay contacts so as to power the HV transformer 4564. I would start with a 40W lamp in the dim bulb, and start at around 50VAC for a half hour or so and then power off and check for warm transformers or capacitors. Then if all is fine go to 100VAC and do the same. Then for the third step remove the temporary connection across the relay and then apply 150VAC and press the HT start button and the relay should turn the transformer on. If no hot other than the valves ( they should all have a faint heater glow at this voltage) then you can go to the full 220VAC and do a check of operation with the lamp still in place. It will probably be glowing dimly at this point.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: April 27, 2015, 09:23:15 pm
content: Quote from: SeanB on April 27, 2015, 09:04:14 pmThanks for this detailed information, but I am missing some background to understand properly. What is a "dim bulb tester", and what should be the proper connection to make ?

author: sync
author_rank: Frequent Contributor
timestamp: April 27, 2015, 10:17:59 pm
content: I only see one electrolytic cap (3300uF 25-30V) in the schematic. The other caps are non polarized. Maybe the high voltage caps are oil paper type. AFAIK they can't/needed to reformed. If you are unlucky they contain PCBs (http://en.wikipedia.org/wiki/Polychlorinated_biphenyl). That stuff is evil.Also there are no bleeder resistors across the high voltage caps. They can hold the charge (voltage) for a lot time (days till months). Even if the power supply is disconnected from mains it is very dangerous.

author: oldway
author_rank: Super Contributor
timestamp: April 28, 2015, 07:58:57 am
content: Be carefull because it is not a voltage regulated HV power supply but a 1200V power supply with ajustable current limiting.Output voltage IS NOT ajustable ! It has a fixed output voltage (with no voltage regulation, only current limiting) of 1200V ...I wonder what such a power supply could serve.It is only extremely dangerous, even deadly.Better stop with this !!!!!!

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2015, 06:40:14 pm
content: Output voltage is adjustable, using the 806 pentode as a triode to regulate the negative rail.The dim bulb tester is a 60W incandescent lamp ( 220VAC tungsten filament one, not a CFL or led lamp, though you can use a halogen lamp here) in series with the mains supply, so that if there is a large current flow for some reason, then the lamp will light to indicate this, and the increased resistance of the hot lamp reduces the chances of damaging other components.

author: oldway
author_rank: Super Contributor
timestamp: April 28, 2015, 08:35:48 pm
content: QuoteI do not agree.Only the cathode current of 806 is regulated, not the output voltage.I started learning electronics with vacuum tubes, I made a lot of audio amplifiers and other vacuum tubes stuffs in the sixties.

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2015, 08:57:22 pm
content: There is a pull up provided by the meter, so the cut off 806 will act as a voltage divider, there is enough negative drive available to bias it to cut off with that as a load.Would not class the output terminals as touch safe under any circumstances, it just will be best to treat them as being always at 1200V, including the negative terminal. But it will provide 50mA at an adjustable voltage from probably 100V to 1200V.

author: sync
author_rank: Frequent Contributor
timestamp: April 28, 2015, 09:04:12 pm
content: Where is the voltage adjustment? I don't find it in the schematic. This looks like an adjustable constant current source for me.

author: oldway
author_rank: Super Contributor
timestamp: April 28, 2015, 09:10:19 pm
content: There is no output voltage regulation, nor voltage adjustement, only a current limitation from 10 to 70mA.I think SeanB does not understand how this schematics works.It's a vacuum tube schematic, perhaps he is too young to know how it works.QuoteAbsolutly wrong...such wrong information is very dangerous because JacquesBBB could think there is a low output voltage and try to measure it with multimeter.But output voltage is always 1200V...!!! This exceed max. input voltage of most of the multimeters and may be lethal.

author: sync
author_rank: Frequent Contributor
timestamp: April 28, 2015, 09:23:18 pm
content: I only know little about tube circuits.I have a question about the schematic. What is the purpose of the left PM07. It's a current sink, right? But what it does in the circuit?

author: oldway
author_rank: Super Contributor
timestamp: April 28, 2015, 09:27:39 pm
content: This is an ajustable current sink intended to make calibration of the current limitation.Pot MP2a (50k) has to be ajusted for current limiting at 10mA when current selector is on position 10 mA.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: April 28, 2015, 09:47:34 pm
content: Thanks to all for your information and your concern.I am aware that this is a PSU with voltage of 1200 V at no load, but with adjustable current from 10 to 70 mA.It was used for some scientific device. Maybe some photomultiplicator tube. I will have to check. I will be very buzy the two next days, but will post some pictures this week end.

author: TimFox
author_rank: Super Contributor
timestamp: April 28, 2015, 09:48:54 pm
content: The schematic seems to be of an adjustable constant-current source with a high compliance voltage.This can be dangerous, since when the load is disconnected, the output will go to 1200 V (or so).My Fluke voltage/current calibrator has an extra control to limit the maximum compliance voltage.(A current-limited source is normally a voltage source with a non-precision adjustment of maximum output current.)

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: April 29, 2015, 07:18:58 am
content: This is a rapid picture of the item ( right now in my trunk) .More to come this week end.

author: oldway
author_rank: Super Contributor
timestamp: April 29, 2015, 07:24:04 am
content: Another safety tip:The insulation between primaries and secondaries and between secondaries and earth should be checked.In the event of insulation failure, very dangerous high AC voltage can exist between the dc output and earth.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 01, 2015, 04:53:10 pm
content: As promised,here are a few pictures of the outside and inside.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 01, 2015, 04:55:14 pm
content: following ...

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 01, 2015, 04:57:24 pm
content: I have checked the caps,they are well within specs. I will probably try to power it without changing any of the caps, as I do not see what could be gained by changing with new ones.Here is an example for an electrolytic cap.

author: SeanB
author_rank: Super Contributor
timestamp: May 01, 2015, 05:16:14 pm
content: Now we have pics.... Looks like it was made for an avionics test bench, probably by SNECMA. Layout, wiring methods, placement and component choices all look like their style of engineering, along with the case and the connectors.The capacitors will likely all be fine, they are all made to milspec standards, and are ultra high reliability, aside from those white Precis foil types, which are known to sometimes fail. I have a few of that same series in a box somewhere. The big foil HV caps will be fine, they are pretty bulletproof. The valves all look OK, though you might find that the 806 tube will glow for a while at startup, it is slightly gassy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2015, 06:03:31 pm
content: Probably need to reform the electrolytics (take them up slowly, watch for heating), but otherwise yeah, looks greatTim

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 01, 2015, 06:05:14 pm
content: Thanks for the comments Sean, Its really nice to have some feedback, as I am really new to this game.Concerning the constructor, its not SNECMA, but Siemel, which was a small company in the south suburb of Paris,specialized in HT and THT power supplies.(seehttp://retro-forum.com/viewtopic.php?p=70259&highlight=) .These were designed for scientific tubes. Maybe to power some hydrogen tube.I will have to wait in order to power it, as I am missing the power plug on the rear panel.As I have another HV PSU with the similar plug, I will try to get one.https://www.eevblog.com/forum/projects/any-reference-for-these-power-plugs/msg636072/#msg636072

author: oldway
author_rank: Super Contributor
timestamp: May 01, 2015, 07:08:24 pm
content: I personally do not see any interest to power on this power supply.Output voltage and current are extremely dangerous.Why take unnecessary risks?As it is designed, it has no possible application in the amateur field and it is a completely outdated design for professional use.It is a museum piece, no need to turn it on.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 01, 2015, 07:38:21 pm
content: Quote from: oldway on May 01, 2015, 07:08:24 pmCuriosity. I want to get acquainted with both high voltage and tube devices. I have also two old klystron HV power supply that are waiting in my basement.QuoteMany things are dangerous. The important is to know where the danger lies and thus use proper attitude.This is why I look for advice on the forum. This PSU was used everyday in a lab, and it is an equipement of good quality. So it is not dangerous by itself, but just if it is not used properly.I am sure that some chinese everyday consumer devices are much more dangerous.QuoteI have at present no other constant current source of such high voltage. I do not have an immediate use of it, but this may come.QuoteA museum piece is even more attractive if it can be powered on. The best would be to be able to build a completesetting using this power supply, which is what I am aiming on the long run.Thanks Oldway for your concern. As you seem to know much more than me on these devices, I will be grateful for the help you could give me.