[GrimJosef]

I have a stereo amp in for service. The design is simple. Each channel has a small-signal gain stage using a 6C45P triode followed by a single 300B triode as the output stage. There are three HV supplies - an HT one for the 300Bs, an HT one for the 6C45Ps and a third one (-100V) for biasing the 300B control grids.

I first saw this amp (it belonged to a friend) 6 years ago when I traced the circuit and took some voltage and audio measurements. My friend has since sold it on and its current owner has now brought it back saying it's developed a hum and is now running physically hot. In particular the smoothing choke in the 300B HT supply becomes too hot to touch and the GZ37 rectifier there is red-plating.

Attached is a circuit of part of the small-signal HT supply and all of the 300B bias supply. I suspect the amp may be some kind of prototype. The chassis and wiring are rather untidy and circuit modifications have clearly been made. For example, the EZ80 in the bias supply has been bypassed by a BY127. Some components are also being over-run e.g. the reservoir capacitor in the bias supply, which is rated at 450V but is running at 560V !

The bias supply had in fact failed, which would account for the over-running of the components in the 300B HT supply. Specifically the two resistors circled in red were open-circuit. When I first saw this amp, 6 years ago, these resistors were 'cement-block' ones (perhaps 3W ?) and showed clear signs of overheating (labelling all but burnt away, values drifted high). I replaced them with a pair of Welwyn W22 vitreous enamelled ones (nominal 7W). The second picture shows them now. One, at least, has run so hot that the vitreous enamel on it has melted and run into the gap between them, 'glueing' them together ! This must have happened before their open-circuit failure.

I've modelled the bias supply using PSU Designer II by Duncans Amps and it shows, once things have reached the steady-state, the pair of resistors passing a train of 50Hz current pulses with a foot-to-foot duration of ~3.2ms and a peak current of 92mA (so a peak voltage of 77V or so). Even if those were square pulses the average power dissipated in the resistors would only be 1.1W or so. In practice their truncated sine nature means the average power should be well under a watt. I have fitted replacements and sure enough they are running cool to the touch. All of the other bias supply components have survived whatever it was that blew the resistors (despite some of them being routinely pushed beyond their ratings), and the fault must have persisted for long enough to melt the vitreous glaze.

For completeness I should add that the HT winding on the mains transformer delivers 455V-0V-455V off load and each half of it has a resistance of 52 ohms.

I confess I can't think of a recoverable fault that could have caused the resistors to overheat. But I have seen it happen twice now. I would be really grateful for any thoughts ...

Cheers,

GJ