On December 17, 2016 at 13:34, Ernie Gilman said...
That's all true. Previously you said there MAY be voltage drop. In truth there is always, as you now correctly put it, forward voltage drop.
See the italics:
Previously you referred to the output of the transformer as increased line voltage. Here you refer to the line voltage as "whether DC or AC." Since you have not separated the concepts of transformer input and secondary output, you have here said that a transformer can output DC.
I missed a whole other thing here: The output from the power transformer's secondary is already higher than the line voltage by the factor of the windings ratio and "output... must be increased" is not true.
There is NOTHING between the output from the transformer's secondary and the rectifier except a piece of copper wire. And this wire somehow increases that voltage?
Here you are very clearly saying that after going through the transformer, the transformer's the voltage must be increased in order to get to the 300VDC+ range. After the transformer, what device or occurrence increases the voltage further?
[There is an apparent increase after the rectifier because a sine wave, rectified and filtered, tends to increase from the RMS value of the sine wave up toward the peak voltage of the sine wave, minus, of course, the forward voltage drop of the rectifier. In tube equipment, that voltage drop can be substantial, way more than the drift from RMS voltage to peak voltage. I just looked up the 5U4 and its voltage drop at 275 mA per plate is 50 volts.]
See, you did say something about DC from the transformer, because at that point you were conflating the concept of "line voltage" with "secondary" and not mentioning that there's an entire circuit between the transformer secondary and the DC measurement point.
Your facts are right but they're expressed in a way that they can clearly mean something you did not intend. It's also hard to tell exactly what you do mean because of the way you put things together. I can't believe you actually intended to say that "the output from the power transformer's secondary must be increased before it reaches the rectifier." But it's what you wrote.
I should have written that the voltage from the primary increases in the secondary before it reaches the rectifier.
We know:
-the PT always works with AC and the ratio of secondary to primary winding causes the increased AC voltage at the secondary output (or decreased, since the heaters use AC voltage, too).
-the rectifier and filtering is where AC is changed to DC and the reason I wrote "may be" is because diodes don't drop the voltage much, unless Zener diodes or some other device/network is used to recreate the effect of the original tube(s) in the half or full wave rectifier.
-It's not DC until the filtering has been done.
For all of the precision in line voltage that we would like to see, it's not always necessary unless the equipment is built to similar tolerance and is operating at the upper limits of the device specs. If they leave enough headroom, normal fluctuations don't do much damage but the fast spikes can take their toll.
Here's an example of something that shows what would seem to be sloppy design, or maybe it was just accepting the fact that line voltage and manufacturing tolerances weren't very stable at the time- at the upper right corner, the diagram shows "voltages read to ground with electronic voltmeter values shown are + or - 20%".
In my amp of this model, I have read over 530VDC at the plates with a solid state rectifier that is sold as a direct replacement (pretty irresponsible, considering the fact that many people would never think of/bother with re-biasing), but the filter caps are rated for 600WVDC, so they still had some headroom. When I installed a tube rectifier, the voltage at the various test points was almost exactly as shown.
[Link: ampwares.com]
