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In Reply to: RE: Thanks! posted by Sherwood Forest on November 25, 2014 at 08:38:12
Ok, fixed. The 22mfd/500VDC filter cap that supplies the phase inverter failed. Interesting, as this is the second F&T 22mfd/500VDC which has failed on me in the last two years. A new one failed on a Deluxe Reverb.
I replaced the last two 22mfd caps with Sprague 20mfd/500VDC ones. All other caps were replaced one-by-one to rule out failures. Starting with the first 80mfd/450VDC cap.
Reversing the power cord didn't work, for this amp.
BTW... anyone try the 20mfd/600VDC Sprague Atom cap, yet? Very $$$ (nearly $20 a pop from AES).
No more hum. Thank goodness!
Thanks, for the replies.
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Follow Ups:
FWIW, if the B+ downstream from the choke was nominally 470 volts with a 117 volt primary it would be just under 500 volts at 124 volts on the primary. If the tubes weren't fully warm and drawing their normal current then if the standby switch was flipped too soon that could cause the caps to see over 500 volts. They are rated for 550 surge so that shouldn't do any harm, but note that the (2) 100 uf/350 volt B+ caps are in series so you have a 700 volt effective rating. They did that for a reason.
500 volts for those 22 uf caps seems pretty close to me. I'd be tempted to use a couple 47 uf 350 or 400 volt caps in series there just 'cause I'm a chicken! 23.5 uf at 700 or 800 volts sounds good to me.
Just my thoughts...
Cool, thanks for the followup. 120hz hum was the dead giveaway, good on ya for being able to tell the difference from 60hz hum. Most people get that wrong.
So, haven't looked at the schematic, but I'm fairly sure that the cap that failed would have had a resistor or two in front of it, and that it's one or two caps from the first filter stage. As such, the amount of current that it could leak was somewhat limited, and it probably didn't contribute all that much to your inrush situation. Unless the tranny "just does that" when it inrushes, if you want to rule out a cap as cause, pay most attention to the ones with the least resistance between them and the diodes; the first filter section really. Couldn't quite tell from your post if you replaced them all or not... Sounds like you got a bad batch of caps there...
Ended up replacing all the main filter caps. And the bias supply ones, too. I upgraded the bias supply to two 100mfd/100VDC.
I was thinking of stocking up on those 20mfd/600VDC Sprague ATOMS. For the reasons that Jim noted. Kinda $$$ at $20 each. Big, too. They will fit under a Fender amp "doghouse." Anyone try these yet?
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As for 60Hz versus 120Hz, I learned it, this way:
60Hz is lower than an open low E-string. Standard tuning.
120Hz is a little higher than an open A-string.
If you have a guitar, difference is apparent.
If those 600V Sprague Atoms are made like the old ones used to be, it's simply two 40uF caps in series. There are no equalizing resistors, so I would guess the two caps would be rated at 350 - 400 volts each to be safe. Since the case on the section with the positive terminal would be at approximately half the applied voltage, they put them in a cardboard tube before applying the plastic shrink sleeve for safety.
Prolly best to added the two 40mfd caps, in series, yourself. Side-by-side with say, 220k-ohm, 2-watt leveling resistors underneath.
Then, the long length of the 40mfd/600VDC "single" cap by Sprague won't be an issue.
Happy Thanksgiving!!!
You are right, those 600 volt rated Atoms are painfully expensive. I use the 450 v and 500 v Atoms mostly. Sprague seems to keep the same qualities up and down the voltage rating of the Atom line. What that tells you is that the 600v caps are simply more expensive to manufacture. That is why cheaper caps fail at +500 volts. Consider them an insurance policy.
I will use a Solen 630v as a first cap to take the beating and heal. I will also either use a vacuum tube rectifier or use a 20% voltage standby switch with silicon diodes. I hate the screaming. I need to silence the lambs.
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