07-05-2013, 03:43 PM
Thank you Peter.
The UL41: I haven't checked out the 'unused / spare pin' yet, but the d.c. voltage at grid 1 is zero - mind you, it wasn't prior to changing the coupling cap.! Its un-bypassed cathode resistor is 150 Ω: there is about 6v. quiescent across it: that gives a standing current of about 40 mA, which sounds about right for a single-ended class-A O/P stage.
I hooked the 'scope up to the anode of the UL41. With the volume turned well up, I could see that although the tops of the speech waveform were fairly clean, the bottom of the waveform was clearly flattened, strongly suggestive that the valve has insufficient anode volts: i.e. insufficient HT.
Al.
The UL41: I haven't checked out the 'unused / spare pin' yet, but the d.c. voltage at grid 1 is zero - mind you, it wasn't prior to changing the coupling cap.! Its un-bypassed cathode resistor is 150 Ω: there is about 6v. quiescent across it: that gives a standing current of about 40 mA, which sounds about right for a single-ended class-A O/P stage.
I hooked the 'scope up to the anode of the UL41. With the volume turned well up, I could see that although the tops of the speech waveform were fairly clean, the bottom of the waveform was clearly flattened, strongly suggestive that the valve has insufficient anode volts: i.e. insufficient HT.
Al.
