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In Reply to: RE: 5R4GY and filter cap values posted by Tre' on March 21, 2017 at 17:07:45
"The max uf value for the input cap depends on the 'voltage, the current draw and the PT secondary DCR' "
That's mostly correct; the last part of that sentence should read: "...and the PT source resistance". As you know, that includes the PT's primary DCR and the formula is Rsource=Rsec+N^2*Rpri+Ra. Admittedly, I got lazy and didn't include the primary's contribution in the sim I posted. Had I done that, the PT's resistance would've come out to 74 Ohms. Which brings me to another point; the link you provided (which I can't view for the time being), that has the secondary resistance at 15 Ohms, seems low to me. I have 10 lb trannies that have higher resistance values than this seemingly small-ish PT. Unless someone took actual measurements of the transformer, I would remain sceptical. Bellow, I am including a link to the schematic that I found, showing current, voltage and resistance values for the psu/power transformer.
The other point you brought up about increased current and voltage, I will also partly agree with. Two reasons: one, is that this is a class AB amp and when at full swing one of the two output tubes is in cut-off thus drawing no current. Of course the tube that remains on and depending on the steepness of the load-line could reach three even four times its quiescent current draw; but it's one tube pulling current and even if it sees a five-fold increase in conduction, it should not exceed 200 mA in this amp - we're talking about a 6V6 tube here.
Second, the tube that's drawing max current, will have its plate voltage pulled down to a minimum value. This is to say that the two conditions of max current and max voltage do not occur simultaneously, so kicking up both at the same time is not warranted. When I ran the original sim with a step load, 210mA draw was the limit before peak current at the rectifier reached its 650mA limit. If the schematic I used is the correct one, then you could probably go a bit higher on the current. If the schematic that you used is the correct one, you'd go a bit lower say around 180mA max draw.
In any case, I still think that the OP could probably try the 5R4GY with no ill effects, assuming of course the the tube is in good condition with no gas, shorts etc.
It occurred to me after I posted my response to your post, that you were talking in general terms about possible conditions where a small first cap is necessitated when the 5R4GY is used and not necessarily about this specific application. In that case, I will agree with all you wrote.
I just posted a long response to you post above but after reading this one I deleted it.
Yes I was speaking in general terms, trying to make the point that the number given on the datasheet (any rectifier tube, any datasheet) is dependent on the circuit condition with which the rectified tube is being used and is not a max or "hard" number.
Making sure the peak repetitive current rating (for whatever rectifier tube we are using) is not exceeded is what guides us to the max uf value of the input cap based on the circuit conditions, case by case.
Thanks for being able to "read me".
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