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Old 22nd Feb 2017, 8:29 pm   #188
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Restore a Beau Decca

What we want to check is the output valves are reasonably well balanced. Measure the voltage across R47 and R48 again but be extra careful as the HT voltage will be much higher. Then measure the voltage from chassis to the anode of the output valves.

The voltage across R47/R48 should be about 2.4V. If either reads above 3V switch off.

Valve amplifiers fail because the output valve overheats and thermally runs away increasing the anode current until something gives way. Your amplifier uses automatic cathode bias where a resistor is placed in the cathode path (R44 and a bit from R45 and R46) and the grids are maintained near zero so that the valve finds a point of equilibrium.

This design shares the bias resistor between both output valves. R44 is chosen to set the equilibrium point correctly if both valves are passing the same current. If one valve is weak the shared bias arrangement results in an equilibrium point where the good valve is conducting far more current than designed for.

This is why it is important to check the anode currents. Being a bit paranoid, I am about to install some cheap Chinese 7 segment volt meters to monitor the cathode voltage on my Beam Echo kit. These amps do not have common cathode bias so the cathode voltage will tell me all I need to know.
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