Selenium rectifier question

A 600V rectifier on a 600 volt supply may not be such a safe thing to do...

You obviously need much more than 600 volt diodes. It is the PIV which is important. What is the open circuit transformer output AC voltage, and what is the circuit configuration used. ? You can thencalculate the needed rating of the diodes and then add a good safety factor.

Peter

So would I, along with 10 watt wirewound resistors with a starting value of say 1k

Arfa

Jeez, how big were the seleniums? The biggest ones I've seen were only a couple inches square.

Your PIV rating for the new diodes is much too low. You should have at least a 1KV diode. Also you don't need that high a current diode.

Chuck P.

I've seen high current ones that were almost six inches on a side.

Agreed. As a proof, I would check the diodes. They should be dead now. If they are not, there's still another fault at work. Maybe the one that caused the death of the selenium rectifier.

Regards, H.

Old battery chargers used selenium rectifiers that there at least 6 inches square/

One note on voltage drop of selenium rectifiers: Rectifiers were constructed by placing a set of plates is series. The size of the plate determined the current rating. A new rectifier had about 1.5 volts drop per plate at rated current. Each plate could withstand about 45 to 50 reverse volts. Most designs pushed the reverse voltage right to the limit. As the device aged, the forward drop would increase, power dissipation would rise, and the stack would eventually fail with a very obnoxious smell.

David

The diodes are probably breaking down. You need to have them rated at

2.8X of the voltage for the peak to peak from the AC. I would use diodes rated to at least 3000 V to 4000 V minimum in this case because the no load voltage is over 900 V. You can use 4 of 600 V in series to get the proper voltage rating. I would also put caps at about 0.005 uF / 5 kV rated across each diode. This would be for HF noise suppression.

You should find out the required current needed for the supply load. The diodes should be rated to at least 3X the required current at minimum. This will allow for the inrush when the power supply is started up. The electrolytic filter caps have to get charged up.

In series with each leg of diodes in series, I would put a 10 ohm 5 Watt resistor in an average tube power amp supply.

If you use 1 Amp rated diodes, you can easily find diodes rated to 1000 Volts. This would allow for 900 Watts maximum load at 1000 Volts. Using

3 in series on each leg of the rectification path would be very adequate for a 1000 Volt no load source.

I would not re-use the present diodes at this time. Most likely they may be damaged even if they read okay.

Before going to all this trouble, make sure you find the cause of the original rectifier failure. It may be age, but a short or something pulling too much current should not be ruled out.

If you want to go more authentic and you have the space, get an 8 pin octal socket, and a 5U4-GT tube. Knock out the holes in the chassis and install the tube rectifier assembly. You will need a high voltage isolated floating 5 Volt 2 Amp supply just for the heater. The heater is tied hot to the cathode of this tube. You can then feed the AC 900 Volts plate to plate, and the rectified plus source would be on the cathode.

--

Jerry G.


"EricM"  wrote in message
news:b8ec8e3c-4c72-4a4e-b892-fb3962620934@f63g2000hsf.googlegroups.com...
I have replaced two old Federal/ITT selenium rectifiers in an
amplifier power supply with new silicon diodes;  the original part
numbers are 103H4AX1 and 104B1AX1.  I can\'t find any reference to
either part on the web - anywhere - ITT can\'t even provide
information.  Since one of the legs of the circuit supplies the plate
voltage (600V) I used 600V 10A diodes (NTE5815HC) to be safe.  Problem
is, there is a time-delay relay that closes to engage the 600V
circuit, and without it attached to the amp; all tube supplies,
biasing voltage, etc. are normal.  If hooked up, once the relay closes
- the main power fuse blows.  I\'m guessing I need a dropping resistor,
but it would be nice to have the original data on the selenium parts
to be able to figure out the value.  The 600V and the -38V biasing
voltage are derived from the same part of the power transformer.  The
-38 side works, but the 600V side reads upwards of 927 volts without a
load.  Using a Variac, the plate voltages were above 600V at about 70
percent.  Is there a way to find the original specs on these selenium
parts?  Any leads would be greatly appreciated!

OK, I must have grotesquely underestimated the values of these, since no literature is available. The two NTE5815HC I replaced for the

104B1AX1 seem to work fine; that part of the supply circuit has only 15 volts max capacitors in it and the output voltages seem to be fine (12.6 for tube filaments which measure around 17 with no load, and around 9.7 under load). The 600V side of things has two Sarkes Tarzian 1N1239s (octal base plug-in center tapped units) in it, along with the 600V replacement NTE5815HC that I swapped for the 103H4AX1. From what you all have suggested, what I think is happening at this point is that the NTE unit *has* failed once power was first applied under load and is now conducting instead of doing what it's supposed to, and this causes the main power fuse on the primary side of the main transformer to sense an overload and blow. So it sounds like if I replace this unit with one of the 3-4 KV diodes and then add a 10 watt dropping resistor to control the higher forward voltage on the silicon unit everything should be OK? Thanks for all the input BTW!

I'm not sure that 10 ohms is going to do too much to limit the inrush current at these sorts of voltages, and will produce little useable running voltage drop to compensate for the higher DC that will be produced by the replacement silicon diodes over the original selenium stack. As far as grafting in a 5U4 goes, I think that I would want to know a bit more about the actual AC supply level, as this device is rated maximum 450-0-450, and if the HT supply was up at over 600v DC off load (ie before the delay circuit had applied the DC to the output stage) by the time that the variac was at 70%, this would indicate that more than this was being produced by the transformer whilst it was off load.

Depending on the voltage rating of the main filter caps, I would feel inclined to use 3 x 1000v PIV 1 or 2 amp diodes in series, as Jerry suggests, with a couple of high voltage 3 watt zeners in inverse series with each rectifier string, before the filter caps. That will ensure that the forward voltage drop of the original seleniums, which will have been taken into consideration by the designer when he specced the caps and the output stage supply, is effectively preserved by the replacement rectifier arrangement. The zener value would be chosen to provide a similar drop to that which the selenium stacks produced

Another possibility might be to use microwave oven diodes, which are also constructed as multi-diode 'stacks', so might better emulate the original selenium stacks for forward voltage drop. They are extremely high voltage working and quite meaty, current-wise. I must add, however, that I have never tried any in this sort of application, so they might have more or less voltage drop than you need.

Jerry's advice to bypass the diodes with suitably rated caps is good, and I would absolutely concur on that point. Also, on not re-using the diodes that you have in there at the moment, and on making sure that there is not some other problem that led to the demise of the seleniums. These were never the most robust devices, if subjected to long term overload ...

Arfa

I would have guessed pushing the current to the limit was more dangerous (at the same power dissipation). The common radio set with up to 100 mA seems to have been a safe haven for seleniums as long as the electrolytic caps in the power supply held or the fuse blew early enough?

Regards. H.

I agree, 100 ohms is probably a more reasonable value. When working on old equipment like this I try to add enough resistance to keep the B+ at least 10% below the rating of the filer caps If the filter caps were rated at 450 I would add enough resistors after replacing selenium diodes to keep the voltage around 410 or so. This is just kind of a best guess way of doing things when a minimum amount of information is available.

Jimmie

First; thanks for all the insight on this. There just wasn't ANY info out there on the original Federal parts at all. I did find a rectifier listing on an old catalog page on a website, but these weren't listed at all. As mentioned, the one side of the circuit works fine. The part of the circuit in question is the 600V plate supply for two 7027A output tubes. I'm assuming since the tube plate current is rated from about 65ma to 450 ma there isn't a whole lot of amperage involved here, so I do agree that the 10 amp jobs I used is probably overkill. From the responses I'm assuming that the voltage rating isn't enough and replacing it with a 1-4kV unit should do the trick, along with a dropping resistor that after doing some circuit analysis seems necessary since there is a 15K 10W resistor in series with this part of the circuit, that is shunted by a relay switch once a time-delay tube relay times out and powers the coil of the switching relay - I think to enable output once the filaments of the preamp and voltage control circuits in the amplifier are heated. It sounds like I need a 1KV or greater diode with anywhere from 1-6 amp rating (I found some that are 1kV 6A in the shop) and just add a dropping resistor to get the voltage down to within spec under load.

I also forgot to mention that the unit in question was originally 6 -

1 inch square pieces in the stack, but only two connections. The other unit that I replaced with the 10A 600V units was four pieces about 1 1/8" (or just slightly larger) square, but had three connections. This particular unit was on the 15V side of the supply (filament supply for several 12 volt filament tubes) and the 600V 10A silicons seem to work fine. The other part of the supply circuit - the 600V plate supply - is where I used one of the 600V 10A jobs to replace the 6-stack 1" square two-lead unit. Smaller but more in the stack must have equaled more PIV handling than the 600V 10A silicon I put where it was (with no dropping resistor). Don't know why it's so hard to find info on these older rectifier units. They hadn't gone bad either, I'm just replacing them to prevent filling the cutting room with toxic stink if they should decide to fail...

I admit I am very confused at this point. The three leaded rectifier was actually two selenium diodes with a common cathode or anode. Your replacement here should work fine but check the filament voltage since it will be higher than before by one or two volts. 12.6 volt tubes will not like 14 volts over a long time period.

There is no way a single 6 plate (stack) selenium rectifier can be a half wave rectifier for a 600 volt supply. Each rectifier plate can withstand only about 50 reverse volts and as others have said, you need at least three times that for a PIV rating.

David

Here's a link to the schematic; after replacing CR1 and CR2A/B with

10 amp 600 V silicon diodes, when the relay closes to enable the 600V plate voltage, the main power fuse F1 blows. I'm not sure I need dropping resistors, because the output voltage on the 12.6 and -38 terminals is very close to what it should be. Is there something I'm missing? Thanks.