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The specs call for 4 uf. How important is this value for the safety of the tube? I found a stash of Brimar 5R4G but my input condenser is 12uf. I don't see many Brimar's for sale so i don't want to blow it up. But, it would be nice to check them out in my SE amp. It is a nice tube looking at the construction.
Follow Ups:
" This is the Heath W5M and right off the rectifier they use a pair of 40uf in series,giving you an effective 20uf at 900 volts.Just get a 12uf to 15uf film cap and you will be fine.I have seen one 5R4g coke bottle fail out of all the W5Ms and that was because the cap was shorted and they jumped out the fuse.The W5Ms also had a thermal resistor with contact tabs but I just put in a CL90 and it does the job.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
It is likely true that he will be fine with a 12uf to 15uf input cap but neither you or I can know that for sure without first knowing the total current draw, the voltage ratio and DCR of the power transformer windings and the voltage.
The 5r4 has a peak repetitive current rating of .65 amps.
Here's an example where a 4uf cap is too large and the peak repetitive current rating is being exceeded 120 times a second (60 times per second, each diode).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It's not the value of the first cap per se.
It's the value of the repetitive peak charging current that happens at twice the line frequency.
The more resistance in the secondary the more capacitance you can get away with.
If you use a low DCR power transformer then you will be forced to use a low value first cap.
If you use a critical value input choke then you can use as big of a cap after the choke as you want.
BTW All of this information is on the datasheets. When the datasheet shows 4uf (or whatever) that assumes the required DCR is present in the secondary of the power transformer. If there is less than the required DCR then you can get into trouble with less than the stated 4uf (or whatever).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
what comes after the first cap is important. If you go 10uf to a 10 ohm resistor to a 100uf cap it will blow up. You need to have an adequate choke or resistor after the cap. PSUII can give you some kind of indication of acceptable loading.
C1 is 12uf Sprague VitQ to 6H to 40uf Panasonic DC link to 8H to C3a and C3b (100uf ASC 386) one feeding channel A and the other feeding channel B. The power supply sounds very dynamic this way with excellent percussion. The residual AC is .1 to .2 volts.
With the 5R4 the music seems slightly smoother than with a 5AR4.
I would have preferred choke input but the power trans did not have enough high tension for that.
Edits: 01/21/17
The reason it appears smoother with the 5R4 is because you are lowering your B+ with the 5R4 and therefore lowering peak power and raising distortion.Put the two tubes in and measure your comparative B+ at the first cap and the output stage.
Also,if there is a 12UF Vitamin Q for that first cap,leave it alone.I didn't realize you had two monstrous chokes.It reminds me of my Pedersen KT66 amps which are fantastic and I only have a 6uf VitQ off the 5V4 going into a pair of chokes.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 01/23/17
It's not hard to swap in a 4mf capacitor. At that size you could use a nice film cap. Or do an LC filter with a 10 henrie choke.
Either way I wouldn't push it with what you have in there. You might also think about a thermister on the line side of the power transformer to slow warm up.
As other have noted, it's peak repetitive current that's the actual limit. Cathode can only support so much current, and can be damaged it it's exceeded. As with most tube specs, you can usually get away with exceeding one limit if the others are well under limits. At maximum rated voltage, maximum rated DC current, minimum transformer winding resistance, your input cap should not exceed 4 uF.
"You might also think about a thermister on the line side of the power transformer to slow warm up."
The stated limit for the input cap value has nothing to do with turn on surge.
It has everything to do with peak repetitive charging current.
This is a "surge" that happen at twice the line frequency for as long as the unit is powered up.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
as the value of their first cap. For example, Heathkit did not bother with that cap limit and used a pair of 40uf x 400v (if memory serves) in series.
That net 20 μF. is in line with what some 5R4 data sheets show. Your 12 μF. should be just fine at the voltages found in most tubed audio circuitry.
You can make quite certain of safety by installing a CL-130 inrush current limiting thermistor between the rectifier and the PSU filter.
Eli D.
"You can make quite certain of safety by installing a CL-130 inrush current limiting thermistor between the rectifier and the PSU filter."
The max input cap value stated in the datasheets has to do with peak repetitive charging current rating.
The turn on surge current rating is much larger.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
10-12uF should be safe for either tube. Newer 5U4GB are rated 40uF and 5R4GB rated 20uF.
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