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Old 6th September 2013, 05:32 PM   #41
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Better?

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Old 6th September 2013, 06:03 PM   #42
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The diode is backwards. It will shunt the entire B+ supply to common ground.
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Old 6th September 2013, 06:09 PM   #43
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Holy crap! Yep. Good catch. That's a fuse blower

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Old 6th September 2013, 06:59 PM   #44
DF96 is offline DF96  England
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Better. I still think the diode is unnecessary, but at least now it won't do any harm.
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Old 6th September 2013, 07:25 PM   #45
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Both damper cathodes and both damper heaters should be at the same potential. Then, no issue of heater to cathode potential limit can surface.

Protection against inductive kick back spikes is needed, when SS diodes are mixed with choke I/P filtration. I don't see "sand" in this B+ PSU.

The voltage equalizing resistors shown will slowly bleed the filter caps. down.

If the B+ supply is charged up, so is the bias supply. Negative voltage on the O/P tubes' control grids provides electrostatic protection against cathode stripping.

Speaking of the bias supply, mention was made of having both a "free" 5 VAC winding and a free 6.3 VAC winding. Phase those windings up and wire them in series. Fewer stages of voltage multiplication will be needed, to obtain a satisfactory bias supply.

If a "fudge factor" cap. proves necessary, make sure you use a part rated for several thousand WVDC. The 0.47 μF. limit for the "fudge factor" part is to ensure that a cap. I/P filtered regime is not entered.
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Old 6th September 2013, 07:42 PM   #46
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Thanks Eli!

I also see that I wired the filaments in series in my schematic. Not really thinking there. If I opt to run the Damper filaments on 6.3V, do I just tie the cathodes to one side of the 6.3V winding? Thanks!
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Old 7th September 2013, 05:46 AM   #47
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I re-ran the sim with your current schematic but without the bleeders or the small input cap (both of which are unnecessary). I had to guesstimate transformer primary DCR, which makes a big difference (I used 5 ohms), and just for grins I added parasitic capacitances to the chokes. Rectifiers are 6D22S, since I don't have a model for the 6CJ3. They are very similar tubes, so the voltage will be accurate within a couple volts.

With this configuration you are looking at 503V at idle (110mA) with almost zero ripple. At 350mA load the output voltage drops to 427V. Regulation is 18%. The loss of regulation over my earlier sims is due to the DCR of the trans primary and the DCR of the second choke. If you measure the DCR of the trans primary I can get even better accuracy.

One area of concern is choke saturation on startup due to the large final cap size. With the choke parameters I have entered, the rectifiers must endure a peak current of 1.2A about 60ms after startup, with current over 1A lasting for several cycles before tapering off to idle current after about 250ms. If the chokes saturate the current will be much higher, potentially limited only by the DCR of the trans and chokes. This is something to look at once the supply is built. Damper diodes do not have a hot switching transient current rating on their data sheets like tubes designed for power rectifier service. They do have a "peak" current rating (2100mA in the case of the 6CJ3), which is called out as a "steady state peak current". This combined with the rugged cathode means the damper diode should handle a couple amps for 200ms. The only caveat is to make sure the cathode is hot before switching on the plate current. A lot of folks suggest using the slow warm up of dampers as a slow-start feature. This is really bad practice. Yes, they are tough, but they're not indestructable. If you want a slow start, use a step-start arrangement.

Oh, and on the subject of the bleeders. If you don't need 'em, don't use 'em. They are a waste of power (although they can improve regulation). If you are concerned about over-voltage, use an OVP circuit.
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Old 7th September 2013, 06:37 AM   #48
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Quote:
Originally Posted by Defiant View Post
Oh, and on the subject of the bleeders. If you don't need 'em, don't use 'em. They are a waste of power (although they can improve regulation). If you are concerned about over-voltage, use an OVP circuit.
I agree. With the exception of "if" in regard to concern. I can think of two circumstances where the voltage will soar to over 1kv without over voltage protection under common use and I don't even have any rigorous electrical training. Just a lot of hands on one-on-one bench experience as I was growing up.

Eli mentions the use of bleeders in choke input filtration as an important one.

Over-voltage protection of some kind with choke input power supplies is important. Bleeders are easy so that is why I suggest them.

I think keeping voltage where it is expected and in the amount it is expected is the only concern. Once that is taken care of, everything else can proceed and we can enjoy arguments of what is hifi etc etc.

I'm sure there is some over-voltage protection in the works and I'm just jumping the gun here. I can't imagine anyone here would recommend there would be none. -Fred
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Old 7th September 2013, 09:01 AM   #49
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Having the voltage "soar to over 1kv" would require a series of extremely unlikely events. Under normal conditions, loss of load on the filter would have the voltage reach 850V across the filter caps. If each cap is rated for 450V, there is no issue. The supply is essentially "passively safe". There is also the rest of the amp circuitry to consider. A 20mA draw from a series divider, for example, will hold the voltage down to 620V. There would be no need to burn another 20 watts in a completely unnecessary bleeder. That is why I said "if"

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Old 7th September 2013, 09:21 AM   #50
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What I want to know is would you suggest over voltage protection? -Fred
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