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In Reply to: RE: II. Power Supply posted by Garg0yle on December 06, 2014 at 18:04:02
The Classic Hammond 200 Series power transformers now are made with 115 VAC AND 125 VAC taps on the primary. If your home's line voltage is a typical 122 VAC, and you use the 115 VAC primary winding, you get a 1.0608 step up ratio. This makes the 350-0-360 VAC winding a 373.1 VAC winding, nominal.Also, you used 165 Ohms as the PSUD PT DCR. That, without looking, is likely the end to end ( 350 - 350 VAC ) total winding. On such a center tapped transformer, you only use EITHER end to center tap as the DCR.
But wait, it gets better, you can be more precise, and we should be. You can measure the DCR of the primary, and the step up ratio of the primary winding, to the High Voltage secondary winding, to determine the "best " DCR numbers to input into PSUD 2.
I will look for that Formula and post it herein.
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Hi Jeff!
When you work with Duncan amps PSU Designer II, please use the formula for Rtransformer:
Rtr. = Rsec. + (n x n x Rprim.)
Rsec. = DC Resistance secondary [Ohm], measured between (0V - high volt) of one 'leg'.
n = step-up ratio (= voltage secundary / voltage primary)
Rprim. = DC Resistance primary [Ohm]
This is the transformer resistance as it's 'seen' by the rectifier
tube.
Jeff Medwin
Edits: 12/07/14 12/07/14 12/07/14Follow Ups:
If you download the specs for the one particular transformer, you get the open circuit voltage and the winding resistances, as well as showing the two primary taps. 273X is 381v, 85 ohms; 125v primary is 2.889 ohms. I'm seeing 461v with 48mV pk-pk ripple on PSUD-II, assuming 125v in on the 125v winding.The current specs are now in rms current, so you can use PSUD and see the increased current available with a choke-input filter compared to cap-in. Very nice!
Hammond is really being responsive to DIY needs. My power line is 123.7vAC right now (10PM local time).
Edits: 12/07/14
"The Classic Hammond 200 Series power transformers now are made with 115 VAC AND 125 VAC taps on the primary"
I didn't know that. As with the filament transformer, I am out of date with my Hammond catalog - this is really good to know. Should solve a lot of "my transformer buzzes" posts!
Hiah Paul,Have I inputted this PSUD2 simulation correctly? If so, will the Driver stage be happy with 66.84 mVAC of B+ ripple? Am I looking at it wrong, simulating it wrong, or what... both ?? Oh yes, the driver stage B+ ripple will perhaps be divided in mVAC by those two high value dividing resistors, which I was unable to display in PSUD.
I usually try for 2 mVAC of ripple, or less, to the driver. Anything under 850 mVAC is fine to the non-fussy 2A3 Finals.
I keep looking at this, and decided to ask for your help.
Can you, or anyone simulate what the ripple will be at "idle" at the top of the choke load, the B+ that feeds the input / driver stage please??
Is higher driver stage ripple a "topology trade-off" on any "Monkey On A Stick" design, and / or the 1947 Jack Robin-Chester Lipman L-W amp, where the B+ to the 6SF5 is also derived from the Final's tube cathode, with minimal power supply decoupling ??
Jeff Medwin
Edits: 12/07/14 12/07/14
In my quick try I used a higher winding resistance and got more like 50mV peak to peak; otherwise my simulation was pretty much the same.
The driver is fed from its plate choke, whose impedance at 120Hz is 113K ohms - that gives a lot of reduction. The ripple at the choke takeoff is half that of the full PSU, from capacitive splitting - more help. Not having built this, or simulated it, I can't be sure. But another cap at the high voltage end of the plate choke will kill the driver hum, if it's a problem.
I didn't show the calculation, but here it is: 1mVrms at the speaker is 2.8mV peak to peak, which at the primary is 50mV peak to peak. The filament will make about 2mVRMS at the 8-ohm tap, so this seems like a reasonable spec. Got to leave some room for the reality check once it's running! :^)
"Also, you used 165 Ohms as the PSUD PT DCR. That, without looking, is likely the end to end ( 350 - 350 VAC ) total winding. On such a center tapped transformer, you only use EITHER end to center tap as the DCR."
I thought you would be shocked by that.
They are in fact 330Ohms from end to end.
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