WAD 300B PSE Cathode Resistors & Caps

[paul(555)]

I've just suffered a reoccurrence of a failure of a cathode resistor and cap. I'm using Vishay thick film RCH 25w resistors, as per Max from many years ago, and black gates.

Is this a regular failure on these amps or should I look for something that is causing the problem?

Any recommendations for replacement parts now that black gates are no longer available gratefully received. I used Elna Silmic last time.

[Richard]

Hi Paul, they do run quite hard at 3 - 4W so mount them in free air if poss.

Personally I'd take the chance to re-evaluate any tweak components by rebuilding with high quaility commercial ones and seeing how it sounds.

I'd try RS for 199-5690 Welwyn wirewound resistors and 226-7047 Rifa caps.

[paul(555)]

Thanks Richard. I've done some poking about today and the failure is limited to two resistors and one cap between the two amps. I'll follow your advice to get things going again.

[John Caswell]

Hi Paul,
I cannot believe that these have failed catastrophically.
I assume that they are bolted to the chassis with heatsink compound.
I have identical types in my 300Bpp and they are well within their 25Watt rating.
Further information required please.

John

[bob orbell]

Originally Posted by Richard

Hi Paul, they do run quite hard at 3 - 4W so mount them in free air if poss.

Personally I'd take the chance to re-evaluate any tweak components by rebuilding with high quaility commercial ones and seeing how it sounds.

I'd try RS for 199-5690 Welwyn wirewound resistors and 226-7047 Rifa caps.

Just to say I used these capacitors in my WA88, ( big fan of Evox Rifa ) but 3 failed short curcuit in as many months, went back to cheap one's, bingo.

[John Caswell]

Hi all,
The usual reason for elco capacitors going s/c is being subjected to over volts, and in a cathode bias situation this usually means that the cathode voltage has increased due to excess current through the valve.
This is generally due to leaky coupling capacitors to g1, the valve developing a g2-g1 s/c, a g1-cathode s/c or a heater-cathode s/c.
I am surprised that the Evox Rifa caps failed as in my (very limited:-) ) experience they have been extremely reliable. Further investigation is required I feel.

John

[paul(555)]

John,

I fear the failure is due to shabby assembly, no heat sink compound and part of the resistor over sailing the slots in the top of the chassis. I'll do better this time!

[John Caswell]

Hi Paul,
Thought it might be something akin to that.
Never mind.

John

[paul(555)]

Final score three cooked resistors and one failed black gate. Now all back together with all resistors properly fixed to the chassis this time via some newly drilled holes and with heat sink compound. Sounding good with my recently acquired Bastanis speakers.

Paul.

[Cyprus Martin]

Hi,
I've had three Black Gates fail over the past few years in my 300B PSE, one went open circuit and caused my mains tranny to burn out. I used Silmic ones afterwards.