Ultra Twin - R766

[ppppenguin]

You're right about the network of resistors around the filaments. I'd overlooked them. I'd need to do more work to see what would happen if one of the filaments went o/c. Effect would be different for different filaments. It might be OK. Or not.

[pwdrive]

Ignoring rectifier forward resistance I make the potential divider ratio with V2 heater o/c approx 9:1 if the mains tapping is set to 250 volts.

Lawrence.

[signalman4]

Yes it wouldn't go up to HT because of R19 (1k) which would hold it down a bit.

I did a quick calculation and I reckon the voltage across C21A could rise to about 50 volts or so through half the filament of V4.

There are some HT and screen decoupling caps that could also be at risk because the HT voltage rail would also rise quite a lot.

I think a clamping zener diode of suitable voltage (8.2v or 9v) might be a sensible addition from V4 pin 7 to chassis.

[pwdrive]

Yes it wouldn't go up to HT because of R19 (1k) which would hold it down a bit.

I did  a quick calculation and I reckon the voltage across C21A could rise to about 50 volts or so through half the filament of V4.

There are some HT and screen decoupling caps that could also be at risk because the HT voltage rail would also rise quite a lot.

I think a clamping zener diode of suitable voltage (8.2v or 9v) might be a sensible addition from V4 pin 7 to chassis.

Don't see the point of a zener diode, the electrolytics should be replaced anyways, I make the voltage across C21 with o/c V2 heater as approx 25 volts so 25 volt electrolytic should do.

Lawrence.

[Terry]

Ignoring, for the moment, R15, 17, 18 & 19, the current through R20 is given as 47mA.

This give a voltage drop across each filament of 1.41mA but some of this is shunted to earth by R17, 18 & 19  and, possibly, R15 - but we can't see from the schematic snippet where this goes.

This would leave an even lower voltage drop across the filaments. If it ends up as 1.3V per filament, that would be an ideal voltage for a long life - remember that a 7.5V battery will only produce this voltage for a short period when fresh, dropping quickly to a nominal 1.4V per cell and then dropping slowly until it is exhaused (~1.2V/cell, say) so a continuous equivalent to 1.3V/cell will be ideal.