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In Reply to: RE: Found this schemo on Net posted by FenderLover on December 30, 2015 at 06:47:38
It looks like the first cap is 1200uF @ 450V (notice the B+ is 470V!) bypassed by a .22uF cap. The rectifier is a CV378/GZ37/53KU. Max input cap for the CV378/GZ37/53KU is not given in the data sheet I could find, but in "typical application" it suggests 4uF.
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...is suggested for CV378 but if you compare max repetitive peak current to that specified for 5AR4 you'll find the values identical. Since input cap value along with plate supply impedance are the significant determinants of max peak current, it's pretty safe to conclude that max cap value for CV378 is identical to that of 5AR4 at 60uF under identical conditions. Either way, 1200uF as called out in the schematic is grossly excessive...assuming of course that plate supply impedance is conventional.
I agree that in theory the GZ34 and GZ37 should be roughly equivalent. In practice however I have found the GZ37 to be much more robust. I had a pair of Quicksilver KT88s in the 1980s (they had a first cap of several hundred microfarads) and the GE GZ34s that were easily available at that time for a nominal cost would only last a few months. I eventually changed to the GZ37 that were $16 from Ned Carlsen and never had a failure with them.
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