Guys:

I have a Hampton cathedral (TRF - one of those companies that came and went I think).The audio goes up and down - starting ok when cold then disappearing when hot. I put my hand on the bulb of the type 45 audio output and it is burning up. Any thoughts? Incorrect screen voltage perhaps? The tube tests good in the tester.

Lou

45 tubes run pretty hot anyway. Not sure that the "touch method" is a good test for detecting a problem. If the plate glows dull red with the set in a darkened room, then there's a problem. Otherwise, its probably OK.

The 45 tube is a triode, so it doesn't have a screen. It's possible that the DC grid bias is changing as the set heats up. I don't have a schematic for this set but there should be a couple of resistors hanging off the power transformer high voltage winding's center tap that set this grid bias voltage. If the bias voltage drifts too far from the set point it will drive the tube either "on" or "off", causing it to either conduct excessive current (glowing red plate), or shutting it off entirely. Either condition will tend to make it insensitive to the AC signal on the grid. Recommend that you unplug the 45 tube if the plate is glowing a dull red (it can be damaged by excessive current), clip a voltmeter to the 45 socket's grid pin, and take measurements as the set warms up from dead cold. If the grid voltage changes dramatically you probably have a thermal problem with the resistor divider that sets the bias voltage to the tube. It is far more likely for one of the resistors to open up than to decrease in resistance, so one fadeout scenario is that one of the divider resistors is opening up, driving the 45 tube into nonconduction.

However, if the 1st audio amp is RC coupled to the 45 tube grid, another possiblity is that the coupling cap has shorted and is dumping B+ from the driving tube's plate DC into the 45's grid circuit. (Obviously this can't happen if the two stages are transformer-coupled.)

The bias voltage-divider resistors should all be wirewound, since they not only draw a fair amount of power, they are less prone to thermal drift than are carbon-comp units.

Are there two 45s in push-pull? If so, are they both running unusually hot?

Be careful - the 45 tube(s) are probably worth more than the radio itself.

Ha Ha - Thanks Doug

No there is only one in this set.

Actually I have restored quite a number of TRF cathedrals from around 1930-31 that have the same audio output tube - so I guess I am rich.

Lou

:Are there two 45s in push-pull? If so, are they both running unusually hot?
:
:Be careful - the 45 tube(s) are probably worth more than the radio itself.
: