WAD 300B PSE Problem

[paul(555)]

Where to start looking?

One of my monoblocks has a fault which appears after about one hour of use when the signal collapses to a muffled and distorted low level. So far I've swopped over the power valves and the rectifier valves with no change. I'm just about to swop the smaller valves but very much doubt that will reveal anything.

Has anyone else experienced this problem or has any guidance on where to start looking?

Thanks,
Paul.

PS I run battery bias on V1 and with just a single 300B

[bob orbell]

Hi Paul, have you or can you check the HT voltage, before and during this problem? it may be a capacitor in the power supply going part short circuit. If you have douts about checking this kind of voltage, have someone more qualifide to do it for you,be carfull, it can KILL!. BOB

[paul(555)]

Thanks Bob, I'll get the meter and rubber gloves out and check the next time I have it running.

[paul(555)]

Finally found the time to get the meter on this.
The base readings are: Pnt A 473v, Pnt B 414v, Pnt C 195v, Pnt D -195v and Pnt E -42.5v. So points A & B are reading over 50v high.
Now to get it working and measure when the fault reoccurs.

[Richard]

Hi Paul,
Expect the HT at points A and B to be high as it is unregulated and under low load as you are only running 1 instead of 2 300Bs.
Your problem sounds more like a dry joint, bad connection, or component that fails with heat. Difficult when it's intermittent

[paul(555)]

Thanks Richard. Yes it's proving to be elusive, I ran it for four hours yesterday with no sign of the problem. I've got some time today so I'll try again.

[Richard]

To confirm you could compare voltages and various resistance readings between the good amp and the problem amp