WAD 300B PP Problem - help please (high volume rustling sound)

[tomtidswell]

Dear all,

I built a PP clone 3 years ago which has given sterling service, but has now developed problems.

The initial problem was a popping, quickly followed by a burst capacitor (either C6 or C7 - rated at 100microF at 200V), presumably either due to a capacitor fault or voltage overload exceeding the 95 V specified. The GZ37 glowed too brightly and I switched off just before the capacitor burst (with smoke).

I took apart the 300B, and decided to rebuild the tag strip boards on the amplifier side (having checked that the voltages from the mains transformer were appropriate and the solid state rectifier delivering power to the capacitors C9/10 which had about 600V across them. The reason for the rebuild was an intermittent hum (worse on tapping the chassis) that had developed in the last year and this was found to be a loose connection to the interstage transformer, but on the other channel to the side that the C6 or 7 capacitor blew.

After the rebuild the amp started up well, and played music for 20 minutes on both channels, then developed a loud rustling noise (sounded like near DC switched on and off randomly through the speakers at near full volume) and I have switched off. No excess glowing valves this time, and no capacitor smoke.

The problem is, I don't know where to restart the fault finding and daren't switch back on until I know. I wonder if it could be one of the 6072 or 5687 valves (which appear normal) - if they throw random voltage swings to the interstage then this could change the voltages across the C6/7 caps causing the initial fault and capacitor blowing, but does seem to be an intermittent fault as they were passing through music for 20 minutes (after the rebuild) before tonight's fault.

Any help would be appreciated. Best wishes - Tom