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FREDs on PSU

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alexg

alexg

Member
Joined 2005
Tom Bavis said:

When an input choke is used, there will be a snubber cap or an MOV to limit the turn-off spikes. Because there ARE diode failures without them.
Click to expand...

I was revisiting this thread because I smoked another set of diodes.

I want to try the MOV approach now, if my secondary is 230VAC, can I use a 250VAC MOV across the secondary before the diodes?

I will also try snubber (0.1uf with 100ohm resistor across each diode).

Thanks.
 
Ty_Bower

Ty_Bower

Member
Joined 2007
tubelab.com said:
I few users have experienced blown IXYS DSEI12-12 FRED diodes in SimpleSE amps when the standby switch is flipped. One user had a 5AR4 fatally arc over.
Click to expand...

I wonder if people are blowing the rectifier diode when they flip the standby switch closed, or if they are flipping the standby switch open.

I put a standby switch on the center tap of a UF4007 rectified FWCT supply. The PT is an old Allied unit rated for 330-0-330. I'm pulling maybe 150 mA DC at ~400V from the supply. I use the standby to allow the output tubes to warm up for a half minute before I hit the B+. I never ever flip the standby after the amp is on. I've also put a 1 megohm resistor in parallel across the standby switch so the center tap is never truly floating, even while the standby is open. So far, I haven't had a problem with the diodes or the switch in over a year.
 
alexg

alexg

Member
Joined 2005
Ty_Bower said:


I wonder if people are blowing the rectifier diode when they flip the standby switch closed, or if they are flipping the standby switch open.

I put a standby switch on the center tap of a UF4007 rectified FWCT supply. The PT is an old Allied unit rated for 330-0-330. I'm pulling maybe 150 mA DC at ~400V from the supply. I use the standby to allow the output tubes to warm up for a half minute before I hit the B+. I never ever flip the standby after the amp is on. I've also put a 1 megohm resistor in parallel across the standby switch so the center tap is never truly floating, even while the standby is open. So far, I haven't had a problem with the diodes or the switch in over a year.
Click to expand...

Actually, my diodes are blowing when I turn off the amp (I did not use a standby switch).
 
alexg

alexg

Member
Joined 2005
Tom Bavis said:
A 250V MOV across the 230V secondary should do the job. You don't need a large on as typically used for line surge suppression - there is not much that energy stored in the transformer leakage inductance. But it's more than a reverse biased diode junction can take...
Click to expand...

Would this protect the diodes? It was my impression that the energy is coming from the plate chokes downstream from the PSU.

Thanks.
 
alexg

alexg

Member
Joined 2005
Tony said:
Alex,

how about an 1n4007 reversed bias diode accros the plate choke?, this ensures that build up of current at turn-off will be discharged by the 1n4007....i do not know how it will impact the sound, but may be worth a try imho...
Click to expand...

Tony, how would I wire the 1n4007 reversed bias? Would I wire the 1n4007 with orientation opposite the diodes on the bridge rectifier?

Thanks.
 
alexg

alexg

Member
Joined 2005
Tony said:
alex, the banded end of the diode is towards the B+.....

this a trick used in SS amps the protect the CE junction of output stage, may as well work here.....
Click to expand...

If I connect it this way (diode across the choke with the banded end towards the B+/away from the bridge rectifier), I am bypassing the choke, as the diode will have less resistance than the choke. Or did I get it wrong?

Thanks.
 
N

needsp

Member
Joined 2006
Hi All

Has any one tried using the 1N5062 avalanche diodes in a tube heater supply- particualarly in the filament supply of a directly heated triode? Do you think a rectifier with a lower breakdown voltage would be more suitable in this lower voltage application? I'm using 300B with filament supply of 5 V/1.3 A. Just ceramic cap-bypassed schottkys at the moment, with paralleled Cerafine and BG NX caps

Thanks

Paul
 
Jeb-D.

Jeb-D.

Member
Joined 2005
diyaudio.com/forums/swap-meet/157276-garage-sale.html#post2023179
Your probably better off using schottky diodes for filament. Lower voltage drop, quicker + softer recovery. Readily available for low voltage medium current applications.

There are some Avalanche diodes that have a fast, soft recovery. These type of avalanche diodes are typically only available in high voltage/ low current. So good for B+, but not for filaments.

I do not believe 1N5062 has a fast+soft recovery characteristic, though I could be wrong.
 
C

CLS

Member
Joined 2005
Hi there,

Late to this. A stupid question: what factor(s) are we looking at for the performance of an SS diode? (Of course except for the forward current and PIV rating.)

Recovery time? Or capacitance? Or something else?

As I see the specs, recovery times of those 'ultra fast' diodes fall between 40~60nS. nS is very short I know, but is it this we are concerning? How about the 'soft' portion? Where do we see it?
 
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