25-03-2019, 05:23 PM
Agree! It could have been seen as good practice - and knowledge of internal leakage developing over time was not known, because the time just hadn't been clocked up.
However the reasoning behind putting the resistor close to the valve, is because the length of lead between resistor and valve pin should be short (to minimise lead inductance, and capacitance to anything troublesome). Giving the instruction to mount the resistor as close as possible satisfies this. But so does swinging the resistor away from the valveholder, as long as that bit of wire is as short as possible.
With hindsight knowledge gained since, (Jeffrey's point), using pin 4 is now proved to have adverse effects. So, markandrewberry can decide to stay with the original design intention, but he'd be better advised to do what the designer would no doubt have done if he could have revisited his design after analysing failures of DAC90a's with 10 years service - and that's isolate pin 4! Unless he has found a secret stash of NOS UL41's!
However the reasoning behind putting the resistor close to the valve, is because the length of lead between resistor and valve pin should be short (to minimise lead inductance, and capacitance to anything troublesome). Giving the instruction to mount the resistor as close as possible satisfies this. But so does swinging the resistor away from the valveholder, as long as that bit of wire is as short as possible.
With hindsight knowledge gained since, (Jeffrey's point), using pin 4 is now proved to have adverse effects. So, markandrewberry can decide to stay with the original design intention, but he'd be better advised to do what the designer would no doubt have done if he could have revisited his design after analysing failures of DAC90a's with 10 years service - and that's isolate pin 4! Unless he has found a secret stash of NOS UL41's!
