300B PSE Problems

[paul(555)]

Back from holiday and chilling out after the drive home from the airport with some music, when after one side of vinyl one of my monoblocs emitted a rapid machine gun type noise through the speaker.
After urgently switching off I then carefully switched it back on while keeping a hand on the power switch. After a few seconds I saw a flash at the top of the 300B valve accompanied with a crack from the speaker. I didn’t give any chance for further fireworks and switched off immediately.
I run my amps with just one 300B per side, not needing any more power for my very efficient speakers.
Before I just risk trying another valve, does anyone have any idea what the fault may be or where I should check?

[John Caswell]

Hi Paul,
Not sure why you are running your PSE with just one 300B as it would be working into an incorrect anode load.
Any way, the first thing I would check is the coupling capacitor to the grid of the 300B to see if it is leaky/short circuit. Second check the resistance from the heater pins to chassis earth. That should the same as the "cathode" resistor. If not I would suggest either the "cathode bias" resistor has changed value or the "cathode bias" capacitor is leaky/short circuit. I appreciate that the 300B does not have a "cathode" as such, but the heater does the same thing.
Failing that it sounds just like catastrophic failure of the 300B.

John

[paul(555)]

Many thanks John. Hopefully I’ll have time to investigate on Monday.

[paul(555)]

After digging out my stash of old 300Bs and checking a couple in the working amp I simply plugged one of these into the problem amp. Result, it’s been working happily for the last hour. So clearly the problem was a very sudden and catastrophic failure of the original valve. Time for some new valves I think.

[paul(555)]

Hmm, not so fast. After noticing the replacement valve had a red glow spot I tried a couple more valves, all with the same result.
So I finally did what I should have done straight away and followed John’s advice. Measurements show R14 and C6 have both failed. I’m now off to check my box of spares.

[paul(555)]

Finally removed R14 and C6 today. R14 actually checked out OK but C6 was short circuit. That makes three of the original four Blackgates in C5 and C6 positions in these amps which have now failed over the last 12 years, taking a 300B with them each time. Given that record I guess I should replace the remaining one in the other amp.
I’ve quickly replaced this one with a standard cap from my box of spares and all is working well.

[John Caswell]

Hi Paul,
It may actually be the other way around, that the 300B goes faulty raising the "cathode" volts above what the capacitor can stand and bingo s/c capacitor.
Of course you don't suspect a faulty capacitor, plug in a new valve and trash that with excessive current. Sods law rules Either way a new adequately rated capacitor will help and check the resistor for cracked insulation as well showing that it has overheated
Believe me we all learnt the hard way - I sure did as an apprentice in the dim and distant past with many a clip around the ear and a b********g to boot

John

[paul(555)]

Hi John,

Thanks for the wise words. My training is in civil engineering not electrical so I’m learning from a very low base. I did, however, have the sense to use a new resistor, on a just in case basis, as I had a spare thick film resitor at hand.

Paul.