30-11-2023, 06:17 PM
No worries!
The next thing is to measure the other side - the side connected to the output transformer and the rest of the radio. That's the side that I'd call "HT". Others are more experienced with valve radios than I am, so they'll tell me if I'm wrong.
For your wire-wound resistors to fail, there must be quite a large voltage across them. That will be proved by measuring the voltage between the centre tap of the output transformer and chassis (fairly easily done from above the chassis) - it will be quite a bit less than 278V. If it's 240 volts, for example, that means that you would have 38V across R39. You could just put your voltmeter across R39 and measure that directly - saves the mental arithmetic, but does involve taking live measurements on an upside-down chassis.
When you know the voltage across R39, you can work out the power it is being asked to handle (V-squared over R). So for my fictitious 38V, across 120 ohms that's 12 watts. The original resistor is probably only rated for 2 or 3 watts, I'd guess...
Assuming you've changed C35, almost the only thing that can take enough current to damage R39 will be the output valves.
I'm not saying they're faulty. In practice, it's more likely that they are conducting too much current because of something else. That would usually be C31 and C32.
Re-reading your first post, I see you've changed the electrolytics. That suggests you haven't changed C31 and C32. If true, then do that before using the radio again. There is a risk of consequential damage until these have been done.
BTW, these "grid coupler" caps are another known trouble-spot on valve gear (not just the Mayflower). They almost always leak (electrically). This has the effect of making the output valve(s) conduct more than they should. They are so notorious that many people refer to them as "that capacitor".
With those replaced, you're probably sorted. Fingers crossed! They are 50nF, but 47nF is perfectly fine. Watch the voltage rating - they need to be rated for HT at least, so I'd go for a minimum of 300V, or 400V to be sure.
As a suggestion, these would do the job: https://uk.farnell.com/vishay/mkt1813347...dp/1166874
They are rated for 630V DC, and aren't too expensive. It's worth buying a handful.
The next thing is to measure the other side - the side connected to the output transformer and the rest of the radio. That's the side that I'd call "HT". Others are more experienced with valve radios than I am, so they'll tell me if I'm wrong.
For your wire-wound resistors to fail, there must be quite a large voltage across them. That will be proved by measuring the voltage between the centre tap of the output transformer and chassis (fairly easily done from above the chassis) - it will be quite a bit less than 278V. If it's 240 volts, for example, that means that you would have 38V across R39. You could just put your voltmeter across R39 and measure that directly - saves the mental arithmetic, but does involve taking live measurements on an upside-down chassis.
When you know the voltage across R39, you can work out the power it is being asked to handle (V-squared over R). So for my fictitious 38V, across 120 ohms that's 12 watts. The original resistor is probably only rated for 2 or 3 watts, I'd guess...
Assuming you've changed C35, almost the only thing that can take enough current to damage R39 will be the output valves.
I'm not saying they're faulty. In practice, it's more likely that they are conducting too much current because of something else. That would usually be C31 and C32.
Re-reading your first post, I see you've changed the electrolytics. That suggests you haven't changed C31 and C32. If true, then do that before using the radio again. There is a risk of consequential damage until these have been done.
BTW, these "grid coupler" caps are another known trouble-spot on valve gear (not just the Mayflower). They almost always leak (electrically). This has the effect of making the output valve(s) conduct more than they should. They are so notorious that many people refer to them as "that capacitor".
With those replaced, you're probably sorted. Fingers crossed! They are 50nF, but 47nF is perfectly fine. Watch the voltage rating - they need to be rated for HT at least, so I'd go for a minimum of 300V, or 400V to be sure.
As a suggestion, these would do the job: https://uk.farnell.com/vishay/mkt1813347...dp/1166874
They are rated for 630V DC, and aren't too expensive. It's worth buying a handful.
