05-12-2018, 03:30 PM
The buzz on LW may well be interference from other electrical equipment. Including LED and fluorescent lighting.
" Iintend to replace the capacitor on the output valve which is a 50 mu F at 12v. Does this sound in the right order of things? Have I got it right - it connects to the grid in parallel with a resistor 150 meg."
This doesn't make sense. There's a 150 OHM (not megohm) resistor from the CATHODE of the output valve to chassis. This is decoupled by a 50V electrolytic capacitor. All very normal. The cap may be OK but I'd replace it anyway. A modern part will be 47uF though if you used 100uF or even 220uF it wouldn't matter. Voltage rating can be anything more than 12V. 47uF 25V parts are very common. The critical capacitor is the the 0.005uF grid coupler that we mentioned in earlier posts. That one MUST be replaced as any leakage will put positive bias on the grid of the output valve. This in turn causes excess current in that valve which can kill both the valve and the output transformer.
" The detached resistor is designated R12 in the diagram and connects from the +ve end of C32 (2uf, 450 v.) to a terminal on the V3 (diode/triode EBC33; det. and A.F.amplifier). The curious thing is that there was already one resistor of the same value between those two points (red, red, yellow) which I took to mean 16k ohms. Thus two of the same value were originally connected in parallel. The circuit diagram says it should be 220,000 ohms so something doesn't add up/"
There should be a 220K resaistor from decoupled HT (the +ve end of that capacitor) to the anode on the EBC33 triode section. Red Red Yellow = 2 2 and 4 zeros. In other words 220K. I don't see how you got 16K from that colour combination. 16K would be brown/blue/orange.
I get the impression that somebody has been messing around in this part of the set before you. Possibly replacing R12 but not bothering to remove the old one.
" Iintend to replace the capacitor on the output valve which is a 50 mu F at 12v. Does this sound in the right order of things? Have I got it right - it connects to the grid in parallel with a resistor 150 meg."
This doesn't make sense. There's a 150 OHM (not megohm) resistor from the CATHODE of the output valve to chassis. This is decoupled by a 50V electrolytic capacitor. All very normal. The cap may be OK but I'd replace it anyway. A modern part will be 47uF though if you used 100uF or even 220uF it wouldn't matter. Voltage rating can be anything more than 12V. 47uF 25V parts are very common. The critical capacitor is the the 0.005uF grid coupler that we mentioned in earlier posts. That one MUST be replaced as any leakage will put positive bias on the grid of the output valve. This in turn causes excess current in that valve which can kill both the valve and the output transformer.
" The detached resistor is designated R12 in the diagram and connects from the +ve end of C32 (2uf, 450 v.) to a terminal on the V3 (diode/triode EBC33; det. and A.F.amplifier). The curious thing is that there was already one resistor of the same value between those two points (red, red, yellow) which I took to mean 16k ohms. Thus two of the same value were originally connected in parallel. The circuit diagram says it should be 220,000 ohms so something doesn't add up/"
There should be a 220K resaistor from decoupled HT (the +ve end of that capacitor) to the anode on the EBC33 triode section. Red Red Yellow = 2 2 and 4 zeros. In other words 220K. I don't see how you got 16K from that colour combination. 16K would be brown/blue/orange.
I get the impression that somebody has been messing around in this part of the set before you. Possibly replacing R12 but not bothering to remove the old one.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
