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turretslug · 9th Apr 2020, 11:15 pm

What is the value of R25 Ben?- I can't see it on the Trader sheet! (I may of course have overlooked it). That'll give you the cathode current in the output valve, which should be around 42mA- at the moment, it looks as though main HT current is somewhat excessive judging by the 139V across what is quoted as the 1400 ohm field coil. There's a bit of detective work needed to suss out whether it's because the output valve is taking too much current, or whether it's because of something else amiss. So far, the DC cathode voltages for the other valves earlier in the chain don't look too far out of kilter, except that I'm puzzled by the large AC voltage on V4 cathode (in particular)- even if there was oscillation resulting from instability somewhere, the cathode bypass capacitor should prevent too much AC component being apparent here.

Colin.

9th Apr 2020, 11:15 pm	#75
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,395	Re: Pilot U650 What is the value of R25 Ben?- I can't see it on the Trader sheet! (I may of course have overlooked it). That'll give you the cathode current in the output valve, which should be around 42mA- at the moment, it looks as though main HT current is somewhat excessive judging by the 139V across what is quoted as the 1400 ohm field coil. There's a bit of detective work needed to suss out whether it's because the output valve is taking too much current, or whether it's because of something else amiss. So far, the DC cathode voltages for the other valves earlier in the chain don't look too far out of kilter, except that I'm puzzled by the large AC voltage on V4 cathode (in particular)- even if there was oscillation resulting from instability somewhere, the cathode bypass capacitor should prevent too much AC component being apparent here. Colin.