On Apr 13, 7:34 pm, EricM wrote:
On Apr 13, 3:48 pm, "Jimmie D" wrote:



"Arfa Daily" wrote in message

...

"Jerry G." wrote in message
news:WoydnU8t2MHTYp3VnZ2dnUVZ_vWtnZ2d@uniservecom munications...
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.

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 originalseleniumstack.

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
replacingseleniumdiodes 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...