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In Reply to: RE: 5R4GY and filter cap values posted by DewDude on March 21, 2017 at 07:49:04
I saw in your system description that you're using a pair of Motorola HS-619 mono's, so I'll assume that these are the amps in question. I did a quick search and found a schematic online that shows the amp to have a current draw of 90 mA and the voltage at the input of the filter as being 345V.
Based on these figures I ran a PSUD sim which shows that the peak current per cycle (peak repetitive) to be 350 mA using a 47uF cap at the input to the filter. This is well within the 5R4GY's safe operation envelope, since both current and B+ voltage are on the lowish side for this amp. As I had indicated in my previous post the max steady-state peak current for this tube is 650mA.
As a comparison, changing the first cap to 4uF reduces the peak current to 311mA - not that big of a drop, but also reduces the B+ at the output of the filter to 332V vs. 344V in the case of the 47uF cap, all else being equal. Hope this helps.
I attempted to write TWO replies last night and neither one showed up.
After your quick explanation and screen shots, in addition to hammering my brain to recall old information....things started to make a little more sense.
Stuck my nice 5R4GY tubes in the amps and ran them for several hours. Improvement? Not really...I just wanted some cool looking rectifiers.
Thanks again for the extra effort of mocking that up in PSUD. That looks like something I'll need when I start attempting to design/ripoff a quad 6l6 amp.
Glad it worked out. In retrospect, I should have taken a look at the 5Y3 datasheet to see that it has a 440mA peak repetitive current rating which is two thirds that of the 5R4GY. That would have put any concerns to rest since the 5Y3 has been running in your amp for a long time without issue.
At the very least, now you know how to check for compatibility with other rectifiers in this or other amps. PSUD might have its limitations, but is a very nice and simple utility for stuff like this.
In the future, if you ever get the inclination to do a mod/upgrade, you might want to consider replacing the 120 Ohm series resistor with a filter choke such as the Triad C-17X. It's a small choke and fairly inexpensive and I think that it would offer a decent improvement in sound over the resistor.
The schematic I found at the link below shows the PT secondary winding at 15 ohms per side of the center tap.
With 47uf the peak current increases to 370mA. Still well within the tubes rating of 650mA.
However if you kick the voltage of the power transformer up to 900vac center tapped and increase the current draw to 200mA. the rectifier peak current becomes 759mA.
The input cap needs to decreased to 4uf to keep the peak current below the rated 650mA level.
The max uf value for the input cap depends on the 'voltage, the current draw and the PT secondary DCR' of the particular circuit one is using the rectifier tube with.
Have Fun and Enjoy the Music
"Still Working the Problem"
The Sams schematic shows the secondary ad 52ohms and 57ohms each side of the ct of the secondary.
The Beitmanns or whatever they were linking to over at AK I found to be slightly less accurate in some cases than the sams.
"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".
Have Fun and Enjoy the Music
"Still Working the Problem"
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