So, when the amp is power up (after 2-5 minutes of filament warm up for the 866A rectifiers), the B+ ramps up nicely to ~460Vdc, then immediately reverses back down and settles at ~180Vdc where is holds steady. I expected the B+ to stay steady at 460Vdc. One of the 866A rectifiers is lighting up a nice blue glow, the other does not. I tried switching the tubes and the blue glow stays with the socket. This tells me this is not a tube issue. I also tried new 866A tubes and the same behavior. The filament voltage measures 2.5Vac, per spec. My filament wiring config is attached.
What might cause the ramp down in B+ ?
Thanks
What might cause the ramp down in B+ ?
Thanks
Attachments
Are there thermistor(s) for both the High Voltage transformer, and the filament transformer?
If there is only a thermistor for the High Voltage transformer, check to see if Both filaments are lit after the power supply warms up, and the B+ voltage is collapsing. If they are still lit, Both the filaments are still hot . . .
That would mean that either the B+ thermistor is bad:
Or the B+ primary windings or secondary windings has a bad wire,
Or there are bad connections from the circuit to the B+ transformer.
I guess L1 windings could have a bad wire, or the connections to it.
You are loading the B+, right?
Otherwise, the supply is operating as if it had a Capacitor input filter 1.414 x Vrms to the plate.
When loaded, a choke input filter only outputs 0.9 x Vrms to the plate.
1.414 / 0.9 = 1.57 ratio of cap input to choke input B+. Ratio = 1.57
Your ratio is: 460 / 180 = 2.56
(I did not account for the small voltage drop of the plate to filament of a Mercury rectifier).
I think the name of the device at your primary circuit might not technically be called a "thermistor".
I do not use series limiting devices in my power supplies.
I wire the power input this way . . . power Hot lead, fast blow fuse, slow blow fuse, power switch, and power transformer primary; going back to Neutral lead.
If there is only a thermistor for the High Voltage transformer, check to see if Both filaments are lit after the power supply warms up, and the B+ voltage is collapsing. If they are still lit, Both the filaments are still hot . . .
That would mean that either the B+ thermistor is bad:
Or the B+ primary windings or secondary windings has a bad wire,
Or there are bad connections from the circuit to the B+ transformer.
I guess L1 windings could have a bad wire, or the connections to it.
You are loading the B+, right?
Otherwise, the supply is operating as if it had a Capacitor input filter 1.414 x Vrms to the plate.
When loaded, a choke input filter only outputs 0.9 x Vrms to the plate.
1.414 / 0.9 = 1.57 ratio of cap input to choke input B+. Ratio = 1.57
Your ratio is: 460 / 180 = 2.56
(I did not account for the small voltage drop of the plate to filament of a Mercury rectifier).
I think the name of the device at your primary circuit might not technically be called a "thermistor".
I do not use series limiting devices in my power supplies.
I wire the power input this way . . . power Hot lead, fast blow fuse, slow blow fuse, power switch, and power transformer primary; going back to Neutral lead.
Update:
1. Thermistor was removed. No change in behavior.
2. B+ is loaded
3. Both 866A filaments are still heated when behavior occurs. I can see the glowing filaments
4. Only one of the 866A tubes has the blue glow when B+ is applied
5. Tried increasing the load by increasing the Rk. No change in behavior.
6. Install a 300k resistor from 866A to ground. No change in behavior.
6A3sUMMER - how would I test for a bad B+ winding wire?
Other thoughts?
1. Thermistor was removed. No change in behavior.
2. B+ is loaded
3. Both 866A filaments are still heated when behavior occurs. I can see the glowing filaments
4. Only one of the 866A tubes has the blue glow when B+ is applied
5. Tried increasing the load by increasing the Rk. No change in behavior.
6. Install a 300k resistor from 866A to ground. No change in behavior.
6A3sUMMER - how would I test for a bad B+ winding wire?
Other thoughts?
