B&H 179

[angelodp]

Helping a friend who has a great old B&H 179 in remarkable condition... given its from the 40's. We pulled the amp out of the projector and put it on the bulb limiter and slowly brough it up on variac.... all good, but quite a lot of hum. It occurred to me that the projector itself has a steel box in the cavity where the amp nestles into. I though that this may well be acting as a faraday cage. Given that there is a tube that has an external grid connection on the top of the tube, its an antennae outside of the projector. Sure enough it worked back in the projector, and very quiet. The amp is back at friends house and he reports that after ten minutes of playing at low volume the amp goes to a fizzy low volume tone. After a rest it then plays again. I am gonna get it back to test tubes and see about bias level. It does not have a three prong plug yet and the power caps are in a steel box of sorts and likely not doing well. The innards of these amps are crammed with components, but the fact that it does play clean albeit intermittently gives me hope.

Any guidance gladly accepted.

A

[Stevem]

Change all the power supply filters and the dropping resistors between the nod's.

Change all the plate load resistors.

Change all the cathode bypass caps.

Check all solder connections.

The fizzing out thing is many times a bad tube socket, a tube socket is good only when it takes some effort to remove the tube.