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
It might be a faulty thermistor that I use on the incoming AC. I will test this tomorrow and report back. Tired.
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.
Try the PSU with ohmic (resistor) load.
BTW some MV rectifier not "lighting" when the load current is low. I usually use 50k-100k dummy load resistor after the MV rectifier.
BTW some MV rectifier not "lighting" when the load current is low. I usually use 50k-100k dummy load resistor after the MV rectifier.
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?
You could try loading each winding with a resistor. Check the required wattage according to voltage and current.
Regards, Gerrit
Regards, Gerrit
I recently had a transformer go bad on an amp using 866a rectifiers. Blew the fuses immediately on power up. Lasted about 18 months. The unit is on my bench. For layout purposes.
Update:
This is a monoblock. So, I replaced the PT with the other PT and everything works OK. The bad PT measures infinity on the resistance test (thanks astouffer).
So I will work to figure out what's wrong the the PT.
Thanks!
This is a monoblock. So, I replaced the PT with the other PT and everything works OK. The bad PT measures infinity on the resistance test (thanks astouffer).
So I will work to figure out what's wrong the the PT.
Thanks!
I just measured mine. From one HV lead to the CT = 47.8Ω. From the other HV lead to the CT = 3.5Ω. But it looks like you probably figured it out. What brand of PT id this? Mine were Edcor. Replaced with Heyboer.
Mine are from Muse Coils, but please note that it looks to be more of my soldering issue than the PT itself.
Can you run the rectifier without the LC filter directly over a resistive load such as a keetle, a heater, incandescent bulbs, etc., keeping in mind voltages and currents?
I can confirm that it was a PT problem that I caused by poor soldering one of the secondary ground wires. All is good now. Thanks to everyone for helping.
