03-01-2012, 05:55 PM
Thank you for your reply, Terry.
I have spent many years maintaining & designing high-voltage power supplies, with or without valves, and I am fully aware of the dangers and the necessary precautions that need be taken during the design and maintenance of such power units to reduce the chances of accidental electric shock.
When I produced my opening Post, it was late last night: too late to go outside and take some measurements: that I have now done. With the receiver's mode switch in the "TRANS" position (therefore, no load on the PSU by the receiver itself), I measured the HT at the point where those Rs are connected to the HT line. Then whilst monitoring that voltage, I switched off the incoming a.c. mains, and timed the rate of fall of that HT. Next, I repeated all of that, but with those resistors removed. Comparing the two results has now thoroughly convinced me that those resistors are indeed bleeder resistors. With them fitted, when the a.c. was switched off, the HT fell to 100v in 25 seconds. With those Rs disconnected, the time required for that HT to fall to 100v was 5 minutes: therefore, case proven.
In retrospect, there are 2 factors which were confusing me:
1. the ohmic value of those resistors: I'm more accustomed to expecting values round about 100 k-ohm;
2. their position in the circuit. It is far more usual to see a bleeder at the output of an L/C filter, rather than its input. Of course, since those 2 points are connected by the low resistance values of the smoothing chokes, it makes little difference.
Finally, on the grounds of safety, since with the passage of time carbon composition resistors of high ohmic value tend to rise in value, or even go O/C, I shall replace those two Rs. with one wirewound resistor of about 470 ~ 680 k-ohms.
Thank you everyone who contributed to this Thread.
Al.
I have spent many years maintaining & designing high-voltage power supplies, with or without valves, and I am fully aware of the dangers and the necessary precautions that need be taken during the design and maintenance of such power units to reduce the chances of accidental electric shock.
When I produced my opening Post, it was late last night: too late to go outside and take some measurements: that I have now done. With the receiver's mode switch in the "TRANS" position (therefore, no load on the PSU by the receiver itself), I measured the HT at the point where those Rs are connected to the HT line. Then whilst monitoring that voltage, I switched off the incoming a.c. mains, and timed the rate of fall of that HT. Next, I repeated all of that, but with those resistors removed. Comparing the two results has now thoroughly convinced me that those resistors are indeed bleeder resistors. With them fitted, when the a.c. was switched off, the HT fell to 100v in 25 seconds. With those Rs disconnected, the time required for that HT to fall to 100v was 5 minutes: therefore, case proven.
In retrospect, there are 2 factors which were confusing me:
1. the ohmic value of those resistors: I'm more accustomed to expecting values round about 100 k-ohm;
2. their position in the circuit. It is far more usual to see a bleeder at the output of an L/C filter, rather than its input. Of course, since those 2 points are connected by the low resistance values of the smoothing chokes, it makes little difference.
Finally, on the grounds of safety, since with the passage of time carbon composition resistors of high ohmic value tend to rise in value, or even go O/C, I shall replace those two Rs. with one wirewound resistor of about 470 ~ 680 k-ohms.
Thank you everyone who contributed to this Thread.
Al.
