I have a 40/40/20/20 quad cap that I would like to use with a VTA board in an ST-70 rebuild. Will it be enough? What am I giving up? Thanks for any help.
5 posters
Quad Cap Question
Roy Mottram- Admin
- Posts : 1840
Join date : 2008-11-30
- Post n°2
Re: Quad Cap Question
as long as it's rated 525v or better, that's more cap than the original 30-20-20-20 so sure, why not?!
j beede- Posts : 473
Join date : 2011-02-07
Location : California
- Post n°4
Re: Quad Cap Question
I think the stock ST-70 used a 30uF segment on the rectifier side of the choke... If you are using tube rectification you might check to see what the Cmax is for your particular tube. If you are running a 5AR4 or GZ-34 Cmax is usually 60uF. Several poeple on this forum seem to prefer to keep that load lower than that. I have had very good results with pre-rectifying the B+ using a pair of 1n4006 diodes ahead of the 5AR4 in my Mark IIIs. Have you tried this?
quadaptor- Posts : 60
Join date : 2009-05-04
- Post n°5
Re: Quad Cap Question
What is the proper orientation for this cap in a VAT amp? I've heard the 40/40/20/20 quad cap is coded different than the originals. Also, can somebody explain the best way to drain the electricty from a quad cap to prevent electrical shock when replacing? Thanks
Roy Mottram- Admin
- Posts : 1840
Join date : 2008-11-30
- Post n°6
Re: Quad Cap Question
I would connect one of the 40uf sections to the output of the rectifier (input to choke) and the other 40uf section to the output of the choke.
You can use about a 100 ohm resistor to drain the voltage from the capacitor, do each section twice to fully remove any stored voltage.
You can use about a 100 ohm resistor to drain the voltage from the capacitor, do each section twice to fully remove any stored voltage.
quadaptor- Posts : 60
Join date : 2009-05-04
- Post n°7
Re: Quad Cap Question
Thanks - do you simply ground the capacitor to the chassis and then touch each post on the quad cap? I've listened to Dynaco amps for years and built one of your VTA ST-70s but I really don't consider myself an electrician. I also want to install a set of Russian capacitors in the board. Should I discharge the existing caps the same way. Again, many thanks!
Roy Mottram- Admin
- Posts : 1840
Join date : 2008-11-30
- Post n°8
Re: Quad Cap Question
yes to both questions
burnedfingers- Posts : 29
Join date : 2009-05-06
- Post n°9
Re: Quad Cap Question
quote
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.
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.
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