I never looked at the 20 or 30. But the classic 100 has the same kind of dropping and filter ladder network as the 50 but then has a seperate 100 ohm resistor for each tube. 100 ohms makes little difference at these voltages, the real screen drop is in the shared supply to all the screens. Going down the ladder as each step reduces votlage, first the main output power tap, then the screens, then the pi and driver B, then the front-end B (first tube stage and also first tube stage in the switched-in 'overdrive' mode) '. Fixed bias on the output stage. So when the output section main power sags, the screen voltage will also dip proportionally because it's farther down the ladder, so it's probably important not to install huge electrolytics or chokes in some parts of that ladder, so the screen never momentarily exceeds the plate if the main sagged and the screen tap maintained voltage longer. That ladder has caps at each tap so there's little or no signal imposed on the supply or screens. Some people have conjectured the screen resistors on only one p/p pair but not the other may be like that to make any current limiting occur over a broader range instead of suddenly. But only 100 ohms doesn't make much difference. I think it's to allow the screens some independence from each other to accomodate tube differences (or perhaps tube failures, the fabled "resistor-fuse"?), like the resistors they often add on everything when putting output transistors in parallel? Maybe they figure letting one p/p pair float a bit from its sister pair is good enough? Maybe it prevents any signal imposed on the screens from affecting its sister screen? Anyway, I'm putting in another pair of 100 ohm screen resistors so both p/p pairs are the same, and I don't really expect it to make one bit of difference. I guess I should think of the original circuit as 100 ohms to make a 'springy' connection between the screens, rather than thinking of it as an additional voltage drop on one p/p pair of screens. Maybe the original designer didn't like the sound of screen resistors and found it combined system recovered from problem condtions with screen resistors on one p/p set? Most likely explanation I've heard is that the seperate resistors are to provide some kind of isolation between screens and two 100 ohm resistors cost a few pennies less to buy & insert than the more elegant 4 50-ohm resistors.
Last edited:
OK if the additional 5 watt individual screen resistors are intended to blow if a tube has a problem, I guess the resistor between tubes would prevent it from taking out an additional sister tube?
No, I'm not really OCD...so why does it bother me like a crooked painting would drive 'Monk' nuts?
No, I'm not really OCD...so why does it bother me like a crooked painting would drive 'Monk' nuts?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.