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In Reply to: RE: update 3 posted by vinnie2 on October 25, 2016 at 13:25:26
The four 7k resistors are drawing 271ma. of current from the power supply.They are not needed.
Figure out what the total current draw of the circuit is (all the tubes feed by the supply) and then make sure the input choke has enough inductance to be critical.
Tell me the total current draw and I tell you how much inductance you need.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/25/16Follow Ups:
I sort of figured it was wrong from what I saw cruising around on the web tonight, but I could not find anything that directly addressed what I was trying to do. All filaments are supplied from separate transformers, so the total current draw would be 2 x 62mA for the 813's plate current and 6 mA for the 26 plate current (only one channel right now.) for 130mA total , yes? Also, I think I only need one resistor on the first cap and that's all right? Thanks!
Edits: 10/25/16 10/25/16
"130mA total "
Yes.
The math says 3.75Hy is critical for 475 volts at 130ma. so I would use 5Hy or maybe up to 7.5Hy.
"Also, I think I only need one resistor on the first cap and that's all right?"
With a 5Hy choke and 130ma. current draw you don't need a shunt resistor at all.
If you wanted a bleed resistor just so the cap voltage will bleed down when you turn the thing off, for safety, then a 220k would draw 2ma. and dissipate 1 watt (use a 3watt).
What I saying is this, if your choke is critical inductance, with the current draw of the circuit alone, then you don't need shunt resistors to increase the current draw.
Adding extra current draw will make the filter less effective in terms of ripple.
BTW "balancing resistors" are when you have two cap in series and you want to keep the voltage balanced across each cap. Each cap, in the series, would have a resistor across it. But never as small a 7k. 7K draws a lot of current. Those balancing resistors would be more like 100k.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What I saying is this, if your choke is critical inductance, with the current draw of the circuit alone, then you don't need shunt resistors to increase the current draw.
While this is true, it isn't the whole story particularly if using a solid state or directly heated rectifiers. In this case the B+ comes up much quicker than the tubes and without the tube current draw the B+ shoots up to the full cap input value for a moment. Sometimes this doesn't matter but consider a 400V choke input supply and some 300B's heated with coleman regs. Before the output tubes are hot, the B+ will jump up to over 600V which isn't too big a deal for the tubes since current draw will be 0 but it might be a bit of a surprise to the caps (both PS and coupling)
I have an amp here that uses damper diodes for rectification and coleman regs for the 300B's and they track each other perfectly on initial turnon but if you shut the amp off for a moment and then turn it back on, the dampers conduct almost immediately since they are still warm but the filament regs take the normal time to gently ramp up causing a temporary overvoltage situation that needs to be accounted for and I have some shorted 600V coupling caps to prove it.
dave
I have separate heater and filament transformers that come on with the main switch and after a minute I throw the switch for the plate transformer.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
To do the heater and B+ transformers. I would like to do this on my Aikido preamp or implement a timer circuit like the one counterpoint uses in their upper preamps and power amps.
Do you use a NTC like an CL90 inline on the switch side for the main transformer?
No I don't, hasn't been a problem.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's my standard method too, unless I am using a slow turn on tube rectifier like a 5AR4.
Sounds like my 5-25H swinging choke ought to do the trick. I will put a bleeder resistor on the last cap then. Also, when I get the other channel set up is it correct to assume the critical inductance becomes 7.5 Hy? Is there a link to the equations that covers the calculations for this? Thanks for the help!
Edits: 10/26/16 10/26/16
chapter 14
Thanks. Reading that ought to keep me out of trouble and off the streets for a while!
The formula for critical inductance is,
inductance = ((voltage/current) + dcr of choke) / 1000
Most books (including the one linked by elblanco) state that one should use 2 times the calculated critical inductance.
"..when I get the other channel set up is it correct to assume the critical inductance becomes 7.5 Hy?"
No, if you draw twice the current at the same voltage then critical inductance will be one half the value originally calculated but you have to figure in the DCR of the choke so it won't be exactly one half.
The math at 130ma. current draw
475/.13=3655+100 (drc of choke just a guess)=3755/1000=3.755Hy So twice that would be 7.51Hy
The math at 260ma. current draw
475/.26=1827+100=1927/1000=1.927Hy So twice that would be 3.845Hy
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's weird and counter intuitive, so it is probably true. Thanks, I will make sure I remember that when the time comes. Two more sheets for the notebook ......
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