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    5AR4 Rectifier Cmax

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    heyraz

    Posts : 58
    Join date : 2011-10-26

    5AR4 Rectifier Cmax

    Post by heyraz on Tue Jan 24, 2012 9:18 pm

    I recently completed a VTA ST70 using the upgraded multicap (40-80-20-30). To protect the 5AR4 from transformer flashback, two UF4007's were placed in series between each of the mains' secondaries and pins 4 and 6 of 5AR4 (4 diodes total) as recommended in another post.
    Then I read the following about the Cmax of the 5AR4 rectifier tube:

    "If you are running a 5AR4 or GZ-34 Cmax is usually 60uF

    Yes, according to the tube specification sheets 60 mfd is maximum. Real life practice would dictate that the first cap value be lowered since the level of quality between the older rectifiers and the newer ones leaves a lot to be desired. Lowering the first cap to 40-45 mfd lowers the chance of sparking/ark over that wrecks the 5AR4. Have used a 60 mfd value in the past which resulted in several failures even with name brand rectifiers. Past experience my recommendations would be to steer away from the JJ brand as well as those made in Japan and China. Note* JJ does make some very nice triodes and I am especially fond of the 12At7 that I used in my VTA board I had a few years ago."

    My question is would it be beneficial to put one or two uf4007's in series between pin 8 of the 5AR4 and the first section of the multicap to further protect the 5AR4 from capacitor flashback?
    I am wondering about this because a friend has suggested that I add another 100uf capacitance to the 80uf section to tighten up the bass on my amp. I am afraid to do this because of what I read above.
    Rich

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: 5AR4 Rectifier Cmax

    Post by j beede on Tue Jan 24, 2012 11:50 pm

    To find out whether the loose bass you are experiencing can be improved by adding more filtering why not try bypassing your 5AR4 with 1n4007 and then add the 100µF.

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: 5AR4 Rectifier Cmax

    Post by GP49 on Wed Jan 25, 2012 1:12 am

    On my Mark II amplifiers I've "disobeyed" the Cmax rating, with 120μF on the input capacitor. No problems whatever, though the rectifier on the Mark II isn't a 5AR4/GZ34, but a 5U4.

    This was something I tried the first time I upgraded the power supplies of these particular amplifiers; my notes say it was in 1987, and since it worked, I have not made any changes, ever since.

    The RCA data sheet for the 5U4 does not specify a Cmax, but does suggest a 40μF input capacitor on the smoothing filter. What probably has allowed the 5U4 to survive over the past 25 years (!) may be its peak inverse voltage rating: 1550 volts.

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: 5AR4 Rectifier Cmax

    Post by heyraz on Wed Jan 25, 2012 1:21 am

    Would placing UF4007's between the tube rectifier and the first capacitor section protect the tube rectifier from backflow from the capacitor or just add noise?

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