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Old 1st February 2013, 10:55 PM   #1
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Default Tube rectifier and first capacitor

Hello!
I just found out that my headphone amp (6DE7-based made by a famous manufacturer in New York) is using 330uf for the first capacitor and it's using a Sophia 274B. I know some manufacturers go beyond the limit but I've never seen a value this high. I don't see any flash on start-up but I'm worried about the rectifier. I'm assuming it's okay since it's not drawing too much current (?). I tried replacing it with 33uf but I got unacceptable hum. I'm thinking of adding a C-R in front.
Thanks.
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Old 2nd February 2013, 01:09 AM   #2
benb is offline benb  United States
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I don't know a whole lot about this except knowing that a tube rectifier shouldn't have too big a capacitor as the very first one it connects to, unless it's got a choke input.. Googling the name and part #, I found this:

http://www.sophiaelectric.com/pages/se/274b.htm

Quote:
To fully utilize its benefits, it may require first stage capacitor (capacitor input) to be less than 10MFD, or would shorten the 274B life span (very much like smoking is bad for your health).
Later text implies using it to run a power amp (300B), instead of a (presumably lower power) headphone amp, but still, that looks like it would have large peak currents during warmup.
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Old 2nd February 2013, 04:31 AM   #3
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Thanks for your reply. I've read the max limit for each type of rectifier. Even the 5AR4 is only limited to 60uf.
I'm really not comfortable using the rectifier in this amp.
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Old 2nd February 2013, 08:40 PM   #4
Ketje is offline Ketje  Belgium
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Look what Tung-Sol say about the size of the capacitor.
How much is the plate-impedance in your amp?Probably ok if the manufacturer knows what he(she) is doeing.
Mona
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Old 2nd February 2013, 11:27 PM   #5
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Thank you. The maximum allowed for the Sophia is only 10uf. I think it's still high.
Which impedance do you mean? the 6DE7?
I don't think the manufacturer knows what he's doing.
I will just probably sell this amp.

Last edited by clowkoy; 2nd February 2013 at 11:34 PM.
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Old 3rd February 2013, 12:41 AM   #6
jrenkin is offline jrenkin  United States
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Do you have a schematic? That is so high that I wonder if it really is the rectifier, but it seems like a fancy tube for display...
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Old 3rd February 2013, 04:12 PM   #7
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Sorry I don't have a schematic. It uses a dissimilar triode (?) for each channel and an output transformer.
Yes, pin 8 of the rectifier goes directly to C1.
Can I put a resistor (~50 0hm) between the rectifier and C1?
My other idea is to put a ss diode instead of the resistor.
Any other ideas?
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Old 3rd February 2013, 07:16 PM   #8
jrenkin is offline jrenkin  United States
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Maybe draw a schematic. I don't know much about headphone amps and what might make the first capacitor value less of an issue. But if this is a production amp and many were made and none are blowing up the rectifier tube, do you need to worry about it?
If you really want to fix this issue, I think you would be better off changing the first cap to the largest value you feel comfortable with, maybe 10uf and adding a CR after the second cap. As lowering of the first cap will likely lower your available B+, as will adding a additional CR, so make sure you have headroom. Changing to a SS rectifier will increase B+, possibly a lot, and may really change the character of the amp.
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Old 22nd February 2013, 12:21 PM   #9
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Maybe the amp has a soft startcircuit (resistor/relais). If done properly the initial loading of the 330uF cap can be very slow. Then there is no danger.
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