02-08-2020, 01:17 PM
Too early or too late. They might make do as noisy grid leak resistors with a built in decoupler for a common grid amplier? They do burn well with excessive current. :D
I've cleaned the coal fire muck and the possible blood stain as well as possible, so photos soon.
Measuring the dropper cold (room temperature) and just after disconnecting power after five minutes (cold, hot) on the 250v tap:
279, 295 (12.1)
1546, 1622 (73.2)
630, 645 (29.5)
790, 822 (36)
The 250V tap is just a parking position; you get the same effect simply removing the jumper selection plug. Numbers in brackets are voltages with a 210V supply, though it's on the 250V tap. The shaver isolation transformer gives 220V on no load for 230V mains. The same current flows in each section if on the 250V tap. The lower voltage taps progressively short out starting at the 790 Ohms, which feeds HT and filament ballast resistor. The current is thus 46mA on the 250V tap with a 210 V supply. It's then 52mA at the next tap position, shorting out the 790 Ohms, at which point the set starts working (1R5/DK91 filament going from 0.91V to 1.1V). Between 1V and 1.1V is about the lower limit for any of the B7G 1.4V battery valve "pentagrid" mixer/oscillators. It's badly worn if it needs more than 1.1V.
I put 2 x 39K in parallel across the 1546 section and replaced the 180 Ohm on the 1R5 filament with 180 Ohms, because though 190 Ohms is in theory within spec, the choice of 180 Ohms is marginal because with the 190 Ohms the voltage was 1.42V when the total LT was 6.6V.
I've cleaned the coal fire muck and the possible blood stain as well as possible, so photos soon.
Measuring the dropper cold (room temperature) and just after disconnecting power after five minutes (cold, hot) on the 250v tap:
279, 295 (12.1)
1546, 1622 (73.2)
630, 645 (29.5)
790, 822 (36)
The 250V tap is just a parking position; you get the same effect simply removing the jumper selection plug. Numbers in brackets are voltages with a 210V supply, though it's on the 250V tap. The shaver isolation transformer gives 220V on no load for 230V mains. The same current flows in each section if on the 250V tap. The lower voltage taps progressively short out starting at the 790 Ohms, which feeds HT and filament ballast resistor. The current is thus 46mA on the 250V tap with a 210 V supply. It's then 52mA at the next tap position, shorting out the 790 Ohms, at which point the set starts working (1R5/DK91 filament going from 0.91V to 1.1V). Between 1V and 1.1V is about the lower limit for any of the B7G 1.4V battery valve "pentagrid" mixer/oscillators. It's badly worn if it needs more than 1.1V.
I put 2 x 39K in parallel across the 1546 section and replaced the 180 Ohm on the 1R5 filament with 180 Ohms, because though 190 Ohms is in theory within spec, the choice of 180 Ohms is marginal because with the 190 Ohms the voltage was 1.42V when the total LT was 6.6V.
