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My power supply SS FWBR , full wave bridge rectifier, and i am getting too much voltage. The B+ is over 500vdc and i need it down to 440v. My wall AC is 118 so i don't want to put the CL on the primary. I was wondering about how much voltage i will lose putting 1 CL on each leg of the secondary high tension legs. It will be 2 x 50ohm Ametherm units and the secondary is outputting 330vac. Primary benefit is that it will limit the inrush current for a few seconds until the tubes start conducting. These 2 cl units will probably not take me down to the voltage i need but it will be a start. I noticed that they, Mousser, sells higher resistance units as well, would that work better? thanks for the help. regards, Dak
Edits: 08/29/17Follow Ups:
The other posters have given you reasonable options. A cLC filter, with a hefty inductor, allows you to tweak the small 1st cap. to get the rail voltage "just right". Bucking the power trafo primary is also good, provided no filament windings are present.
A 3rd possibility is switching to hybrid bridge rectification, to take advantage of the large forward drop vacuum rectifiers exhibit. Remember, the forward drop of a 5R4, at the rated 250 mA., is a whopping 67 V.
Eli D.
The voltage drop was too much. I have an extra 5v winding now that i dispensed with the tube rectifier. How do i use that in series or bucking the ac primary?
Wire the "extra" 5 V. winding in series with the primary and measure. One configuration raises the rectifier winding's voltage, while the 2nd configuration lowers said voltage. You want the reduced voltage configuration.
Eli D.
If the primary is 1, 2 and the 5v is 3, 4 . I connect 1 and 3 and 2 and 4 and measure the B+ at the secondary. If that is not lower than without the 5v winding connected then i reverse the order of the connections?
Wire in series, not in parallel. Try 2 to 3 and apply "120" VAC to 1 and 4. If the rectifier winding measures higher, connect 2 to 4 and apply the mains voltage to 1 and 3.
Eli D.
.
You have 118 vac at the primary with 500vdc at the end of the filter.
You would need 104 vac at the primary to get 440 vdc at the end of the filter.
A 14 volt bucking transformer will give you that.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'll just add that a standard 12.6V filament transformer should be close enough to do the job.
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Buy Chinese. Bury freedom.
Although i check it periodically, it could be higher nowadays. I soldered two CL90 right after the AC input from the IEC socket to each leg of the PT primaries and that actually made a big difference. The B+ after the filters, is now 460vdc. This is only 20 vdc more than i want but i might just leave it at that. Plate voltage is now 440 and G2 voltage is 420.
For single ended pentode, is this too much for KT88, KT90 , and KT120?
Using resistors to drop B+ to the output stage will ruin the regulation of the supply. This is OK for temporary test purposes, but it will turn a good amp into junk.
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Buy Chinese. Bury freedom.
My Power Supply is LCLC. Then there is a 3.3K dropping resistor for the G2 voltage and then another 33K for the B+2.
So as it stands now, i have 460vdc with 6550A power tubes. I am trying to get to 440vdc. I have a suspicion that with the kt120 power tubes the B+ will be around 440 volts. Do you think 410 volts on G2 is too high?
Adding resistance on either side of the transformer degrades regulation. This is the wrong way to accomplish the drop.About G2, I haven't used the KT120. Someone like Jim McShane might be able to advise you regarding the practical aspects of this tube.
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Buy Chinese. Bury freedom.
Edits: 08/30/17
Hey CB, the PS is choke loaded. The L is 8H, followed by 40uf, I added another 2H L, followed by 400uf. The amp is SE for KT120 x 2. Driver tube is 2 x 6j5.
I do have an unused 5v winding for the tube rectifier.
I also tried ss half wave rectified but that only got me 330vdc.
If the suggestion from Caucasian Backplate proves to be insufficient.
What comes after the FWB? You can always go with a small cap and then a choke to shed some extra voltage, heck sometimes just a small choke at the input can get you to your desired B+.
Also, if your power transformer has an extra 6V winding, you can arrange it in series with the primary to drop the high voltage on the secondary a bit.
How much current are you supplying with this power supply? Do you have a PSUD screen shot?
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