06-12-2023, 05:22 PM
(06-12-2023, 05:05 PM)Mark Hennessy Wrote: I always leave R39 at 120 ohms.
If the HT is too high after a silicon rectifier swap, that's because the person who did it neglected to install a series resistor, or used a value that was too low. Looking at my notes, I've fitted anything between 33 ohms and 100 ohms. I've done loads over the years - mostly for a good friend who was selling them - but it's been around a decade since the last one, so without digging out my notes, I can't remember all the details.
The advantage of adding the series resistor is 2-fold: it saves changing the value of R39 and it avoids stressing the first of the capacitors (C36).
Totally agree that a separate resistor between rectifier and 1st cap is the proper thing. I didn't look at schematic and didn't realise R39 wasn't before any caps.
In any case the HT caps voltage rating ought to be good enough for the situation were the heater supply is disconnected or output valve dead/missing or O/P transformer O/C. Some makers skimped on voltage rating to save costs, assuming always normal operating conditions.
