06-12-2023, 05:05 PM
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). It is possible to include the resistor inside the original rectifier (Westinghouse FC 124), as the attachment shows. The resistor is in contact with the aluminium body, so gets a reasonable cooling effect from that.
Part of the trouble is knowing what the HT actually should be. It's not quoted in the manual. They do state the anode voltages at 250V (and 240V for the screens). But the RV20 manual quotes 270V (and 275V for the screens). The latter values make more sense to me - or at least, the fact the anode is lower than the screen seems logical. But otherwise, these sets are practically identical, and both have the same audio output specifications. So, perhaps the HT should be 275V after all? But I've tended to go a bit lower, and still hit the output power spec.
If you don't want to pick apart the rectifier swap then I guess it's OK to vary R39. Check the voltage rating of C36 isn't being exceeded, especially when the set is first switched on. I normally replace C35/C36 with parts rated at 450V - it's easy to cut open the original part and fit replacements inside (I'm not big on re-stuffing, but I make an effort with these).
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). It is possible to include the resistor inside the original rectifier (Westinghouse FC 124), as the attachment shows. The resistor is in contact with the aluminium body, so gets a reasonable cooling effect from that.
Part of the trouble is knowing what the HT actually should be. It's not quoted in the manual. They do state the anode voltages at 250V (and 240V for the screens). But the RV20 manual quotes 270V (and 275V for the screens). The latter values make more sense to me - or at least, the fact the anode is lower than the screen seems logical. But otherwise, these sets are practically identical, and both have the same audio output specifications. So, perhaps the HT should be 275V after all? But I've tended to go a bit lower, and still hit the output power spec.
If you don't want to pick apart the rectifier swap then I guess it's OK to vary R39. Check the voltage rating of C36 isn't being exceeded, especially when the set is first switched on. I normally replace C35/C36 with parts rated at 450V - it's easy to cut open the original part and fit replacements inside (I'm not big on re-stuffing, but I make an effort with these).
