upgrade caps ? (in a st70 with solen and xxx)

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upgrade caps ? (in a st70 with solen and xxx)

Postby mati1979 » Sat Feb 13, 2010 12:45 pm

hi i want improve my st-70
i have a triode kit (with solen and
another normals caps)

Can I improve the sound with any upgrade caps?
i can obtain here in my country mundorf

Someone has changed this caps with auricaps, mundorf or
another better caps ?
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Postby EWBrown » Sat Feb 13, 2010 5:00 pm

I've used Auricaps with very good results.

Also the Russian K40y paper in oil caps are very good, there are readily available on e-bey. These are very similar to the vintage US made Sprague "Vitamin Q" PIO caps (which have not been produced for a very long time).

HTH

/ed B
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Postby Geek » Sat Feb 13, 2010 6:53 pm

- Illinois Capacitor MPW's
- 715 or 716 series Orange drops

Both will handle the heat well too (Solen's don't like any heat... they were originally designed for X-over caps)

Cheers!
-= Gregg =-
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Postby antiquekid3 » Sat Feb 13, 2010 7:05 pm

What is the minimum voltage needed for these capacitors? I might get some of those Russian PIO capacitors, but if I don't need 600V capacitors, I'll stick with something lower.

Kyle
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Postby Geek » Sat Feb 13, 2010 10:53 pm

No coupling cap in an ST-70 should be less than 600V.

Say your 6.3V dies or the tubes aren't warm with SS rectifier... Bias ~ -40V, unloaded rectifier voltage ~510V.... there's 550V across on the grid couplers alone.

Cheers!
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Postby antiquekid3 » Sat Feb 13, 2010 11:07 pm

I don't believe in solid state rectifiers...! :-) Does anyone add a standby switch for the B+ so you can give the 6.3V tubes time to warm up (with the SS rectifier)? Or, are there any ways to "smooth start" the B+ so that it creeps up over a few seconds?

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Postby TerrySmith » Sun Feb 14, 2010 3:16 am

On ST-70's the 5AR4 lives a hard life. My fix is SS rectifiers and a TV damper tube inserted between the diodes and the first cap so you have a slow turn on.

Damper tubes are only a single rectifier but handle a LOT of current, plentiful and cheap for NOS.

Here's a pic of a '70 with this mod. Kevco now owns it. The tube is a 6DW4B.

Image

If there were two rectifier sockets you could use two damper tubes as the rectifiers like in this amp.

Image
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Postby kevco » Sun Feb 14, 2010 6:50 am

Not only do I own that amp, but I've been running it daily for almost a year with that very same damper tube in place and not so much as a hickup. I bought a spare for $4 but it's gathering dust. That amp has taught me a lot about tube tech and electronics in general. A true war horse, thanks Terry!
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Postby Geek » Sun Feb 14, 2010 9:22 pm

Gorgeous build, Terry!

Cheers!
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Postby mati1979 » Sun Feb 14, 2010 10:32 pm

if i replace a 4 caps output coupling Solen
( 0.22uF 400V Output Coupling Capacitor)

with this:

http://www.madisound.com/catalog/product_info.php?manufacturers_id=170&products_id=8114

or this:

http://www.madisound.com/catalog/product_info.php?manufacturers_id=142&products_id=1433

this upgrade can be a good (choise) upgrade for improve my st70 ?
and... wh?t about capacitors board 390 and 270mf ?

the questions is: what changes are needed to improve the sound in st70?
.
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Postby Geek » Mon Feb 15, 2010 3:15 am

Hi,

mati1979 wrote:...... wh?t about capacitors board 390 and 270mf ?


There is no 270pF cap... only 390pF @ 1000V and 82pF @ 500V in the stock ST-70.

Cheers!
-= Gregg =-
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Postby Dyna70 » Wed Mar 31, 2010 11:31 am

Just realised my 390pF silver mica cap is rated 500V and not 1000V - should I change it ASAP?

- John
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Postby Geek » Thu Apr 01, 2010 12:35 am

Dyna70 wrote:Just realised my 390pF silver mica cap is rated 500V and not 1000V - should I change it ASAP?

- John


That would be a good idea.

It's tied to the screen witch sees some good peak-to-peak swing PLUS the B+.

I have some in stock if you don't have any.

Cheers!
-= Gregg =-
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Postby Dyna70 » Thu Apr 01, 2010 12:21 pm

Many thanks for the kind offer but it's easier for me to source them (and quicker) here in the UK.

Best,

- John
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