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.
 

benb

Member
2010-04-24 1:52 am
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

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.
 

Ketje

Member
2012-12-19 7:24 pm
Flanders
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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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.