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Old 17th March 2014, 10:05 PM   #1
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Default Not using all the sections on multi-section cap

I'm replacing the power supply caps on my second Fairchild 260. I'm replacing the two section cap C6A=60uF, C6B=40uF. (listed in Sams as C1.) It is closest to the 5V4 rectifiers.

On my first amp my tech used a JJ 40/20/20/20 which when bridged provided the correct capacitance, but too low a voltage for what the amp sees at today's line voltages.

For amp #2, I've ordered a 20/20/40/40 550V cap for C6A & C6B (Sams C1 A&B) from the Ask Jan German website. I'm wondering if I should just leave one section unused, or bump up section A to 80uF, or go with 60uF for both sections.

Any thoughts? Would there be any risk up upping capacitances that much?

BTW, this is the only schematic I can find at the moment. I'll upload an updated EL34 version when I can find it.
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File Type: pdf Fairchild 260-R Schem.pdf (121.9 KB, 41 views)
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Old 17th March 2014, 11:49 PM   #2
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Originally Posted by WBS View Post
Any thoughts? Would there be any risk up upping capacitances that much?
60 ufd for the first capacitor right off the 5V4 rectifier is a lot for that tube even assuming the plate supply impedance is high enough to prevent exceeding maximum peak plate current. I would not go any higher for C6a and perhaps even a little lower. Increasing the value of C6b is doable because of the series resistor. A larger capacitor in this position will also help improve the reserve voltage for base response. So I'd make it 40+80. (C6a/C6b) Never waste a section if you can help it.
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Old 18th March 2014, 04:13 AM   #3
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60 ufd for the first capacitor right off the 5V4 rectifier is a lot for that tube even assuming the plate supply impedance is high enough to prevent exceeding maximum peak plate current.
AMEN! Check the 5V4 data sheet out. Notice that 10 μF. is given as "typical" at the PSU filter's I/P. Hold the capacitance in 1st position down to the minimum value consistent with keeping the rail voltage up. The 2nd position is where you pile up the energy storage.
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Old 18th March 2014, 05:19 AM   #4
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Default Now I'm really confused...

Thanks for the replies. The tube data sheet is indeed an eye opener.

I wonder why Fairchild would design the power supply this way, calling for 60uF on the side closest to the tubes and the 40uf section 50ohms away.
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Old 18th March 2014, 06:15 AM   #5
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I wonder why Fairchild would design the power supply this way, calling for 60uF on the side closest to the tubes and the 40uf section 50ohms away.
Fairchild was not alone in abusing rectifiers. Back in the day, they were inexpensive and frequent replacement was semi-OK.

Some recent Carey offerings beat the snot out of vacuum rectifiers too.
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Old 18th March 2014, 08:56 PM   #6
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Originally Posted by WBS View Post

I wonder why Fairchild would design the power supply this way, calling for 60uF on the side closest to the tubes and the 40uf section 50ohms away.
Actually they used a pair of 5U4's with paralleled elements, which allowed for higher peak currents and higher capacitance. If you include the total effective plate supply impedance (which limits current), 60 F should be still in the safe area for a pair of 5U4's, though very close to the limits. Also keep in mind that the 50 ohms resistor in the first RC filter is a very low value (probably much lower than the plate supply impedance, which is over 400 ohms) and adding too much capacitance after this resistor is not recommended if you want to stay within the maximum ratings of the 5U4.
The recommended 40/80F combination is OK , using 60/60 will also work but consider these as maximum values in this circuit.

Last edited by Tubologic; 18th March 2014 at 09:18 PM.
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Old 19th March 2014, 12:24 AM   #7
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I'm going to have to do the same job this summer,mine are still original
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Old 19th March 2014, 04:54 AM   #8
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Actually they used a pair of 5U4's with paralleled elements, which allowed for higher peak currents and higher capacitance. If you include the total effective plate supply impedance (which limits current), 60 F should be still in the safe area for a pair of 5U4's, though very close to the limits. Also keep in mind that the 50 ohms resistor in the first RC filter is a very low value (probably much lower than the plate supply impedance, which is over 400 ohms) and adding too much capacitance after this resistor is not recommended if you want to stay within the maximum ratings of the 5U4.
The recommended 40/80F combination is OK , using 60/60 will also work but consider these as maximum values in this circuit.
I'm not sure if it's a typo or not. You referred to 5U4, but the amp has a pair of 5V4G.

The data sheet indicates 10uF at maximum voltage. With a pair I should be able to go to 20-40 for the first section depending on voltage. I plan to measure it tomorrow to see where it is.

I would like to be able to make the first section closest to the tubes 40uF and the second section after the 50ohm resistor 80uF. How would this affect the second cap sections C7a&b which are now 40uF each? Would there be an issue with values being so far apart among the caps in the PS?
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Old 19th March 2014, 07:53 PM   #9
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Originally Posted by WBS View Post
I'm not sure if it's a typo or not. You referred to 5U4, but the amp has a pair of 5V4G.

The data sheet indicates 10uF at maximum voltage. With a pair I should be able to go to 20-40 for the first section depending on voltage. I plan to measure it tomorrow to see where it is.

I would like to be able to make the first section closest to the tubes 40uF and the second section after the 50ohm resistor 80uF. How would this affect the second cap sections C7a&b which are now 40uF each? Would there be an issue with values being so far apart among the caps in the PS?
Yes, It's a typo and should read 5V4G. In my (restored) Fairchild 260's I used the 60/60 F combination for years without any detrimental effects on the rectifiers life or reliability. A short simulation with PSUD II will tell you if the maximum ratings of the rectifier tubes are exceeded or not. At Utr= 450V , Supply source impedance of 375 Ohms, 2X60 F/50 ohms RC filter and 100mA load current the max. ratings are not exceeded even with a single 5V4. The only situation where the rectifier peak current may be exceeded is at hot switching which can't be accurately simulated with PSUD and never happens in real life anyway. Thus you may safely use 60/60 or 40/80 F combination without compromising rectifier tubes life, the difference (if any) being marginal due to the very low (50 ohms) resistance between the two caps. The C7 a/b decoupling capacitors are isolated by the 10K resistor and any value from 40F and up (the higher, the better) will fit.
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Old 20th March 2014, 03:39 AM   #10
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Thanks Tubelogic for your reply. It's nice to hear from someone who has had real life experience with this amp. I'll keep you posted on my progress!
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