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Dynakit 70 filter cap question



 
 
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  #1  
Old December 9th 06, 09:26 PM posted to rec.audio.tubes
GuitarPsych
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Posts: 1
Default Dynakit 70 filter cap question

I am about to replace the filt capacitors on my dynakit 70 (I'm
guessing they have never been changed) and rather than open the amp
just to find out what they are, does anyone here know off hand what I
should purchase as replacements for the current filter caps, and how
many?
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  #2  
Old December 9th 06, 09:47 PM posted to rec.audio.tubes
Eeyore
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Posts: 8,474
Default Dynakit 70 filter cap question



GuitarPsych wrote:

> I am about to replace the filt capacitors on my dynakit 70 (I'm
> guessing they have never been changed) and rather than open the amp
> just to find out what they are, does anyone here know off hand what I
> should purchase as replacements for the current filter caps, and how
> many?


You'll need to look inside. Modern capacitors are likely to be considerably
smaller ( better materials and technology ) and you'll have to get something
that physically fits.

Graham


  #3  
Old December 9th 06, 10:37 PM posted to rec.audio.tubes
Bob H.
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Posts: 118
Default Dynakit 70 filter cap question


Triode Electronics has a SDS cap board made to fit under the ST70,
which has all the caps on board, including the bias rectifier caps and
diode. It's not that much more than the new can cap you can get for
that amp, and the caps are high quality. You can leave the can mounted
for looks, and pop this board under the chassis. I havent' used it
myself, but I wish it had been available when I recapped my ST70. I
ended up getting an expensive can cap, and wedging caps under the
chassis to get some larger values that the original 30-20-20-20 uf
caps.

http://www.triodeelectronics.com/sdss70parts.html


Bob H.



GuitarPsych wrote:
> I am about to replace the filt capacitors on my dynakit 70 (I'm
> guessing they have never been changed) and rather than open the amp
> just to find out what they are, does anyone here know off hand what I
> should purchase as replacements for the current filter caps, and how
> many?


  #4  
Old December 9th 06, 11:32 PM posted to rec.audio.tubes
Peter Wieck
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Posts: 2,418
Default Dynakit 70 filter cap question


GuitarPsych wrote:
> I am about to replace the filt capacitors on my dynakit 70 (I'm
> guessing they have never been changed) and rather than open the amp
> just to find out what they are, does anyone here know off hand what I
> should purchase as replacements for the current filter caps, and how
> many?


30/20/20/20uF @ 525VDC

It is the 525V (or better) that makes it awkward as that is not a
common value for electrolytic caps and so they are unusually expensive.


Modern caps won't fit into the existing can, so you will wind up
stuffing them underneath (rather than restuffing the can) unless you
want exposed HV above the chassis.

There are after-market add-on boards for this purpose as one other has
mentioned.

Peter Wieck
Wyncote, PA

  #5  
Old December 10th 06, 02:41 AM posted to rec.audio.tubes
Chris Hornbeck
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Posts: 1,744
Default Dynakit 70 filter cap question

On 9 Dec 2006 13:37:08 -0800, "Bob H." > wrote:

>http://www.triodeelectronics.com/sdss70parts.html


This is a good recommendation for folks like the OP
who may be unsure about options and just want to get
the elderly party up and running safely and well.

It's much, much preferable to the can cap replicas
of just a few years ago, which would explode (!)
more often than not, at only slightly higher voltages,
in Mk3's. Very disappointing...

For homebrewers, pairs of caps stacked for better
voltage rating are easy enough to do. Modern
electrolytics with 300 volt rating are very compact
and (new) leakage is low enough to permit balancing
resistors as low as a milliAmp or two draw.

Stacking for a 600 volt rating isn't really all that
extravagant in fault conditions, just good engineering.

All good fortune,

Chris Hornbeck
"Too soon oldt; too late schmardt."
  #6  
Old December 12th 06, 11:50 PM posted to rec.audio.tubes
- - : R A T B o y : - -
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Posts: 13
Default Dynakit 70 filter cap question

I've had one of these from Ned in my Stereo-70 for about 6 years:

http://www.triodeelectronics.com/40ufat52twis.html
  #7  
Old December 17th 06, 04:51 PM posted to rec.audio.tubes
wb
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Posts: 13
Default Dynakit 70 filter cap question

Antique Electronics Supply carries an original style 525 volt multi
section can capacitor that will work and look like the original. Also,
replace the selenium bias rectifier and the two capacitors in the bias
supply with something like a pair of 50-100 uf @ 50 or more volt
capacitors. You should replace those original coupling capacitors as
well since they love to leak DC voltage from the phase inverter circuit
to the control grids of the EL34's.
  #8  
Old December 18th 06, 04:16 AM posted to rec.audio.tubes
west
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Posts: 154
Default Dynakit 70 filter cap question


"Chris Hornbeck" > wrote in message
...
> On 9 Dec 2006 13:37:08 -0800, "Bob H." > wrote:
>
> >http://www.triodeelectronics.com/sdss70parts.html

>
> This is a good recommendation for folks like the OP
> who may be unsure about options and just want to get
> the elderly party up and running safely and well.
>
> It's much, much preferable to the can cap replicas
> of just a few years ago, which would explode (!)
> more often than not, at only slightly higher voltages,
> in Mk3's. Very disappointing...
>
> For homebrewers, pairs of caps stacked for better
> voltage rating are easy enough to do. Modern
> electrolytics with 300 volt rating are very compact
> and (new) leakage is low enough to permit balancing
> resistors as low as a milliAmp or two draw.
>
> Stacking for a 600 volt rating isn't really all that
> extravagant in fault conditions, just good engineering.
>
> All good fortune,
>
> Chris Hornbeck
> "Too soon oldt; too late schmardt."


I don't like using 525v replacements especially since the AC input has
increased from days gone by. Chris, why don't you provide us with real
values for your stacking caps and balancing resistors? I think this would
be more of a practical help for many.

Cordially,
west


  #9  
Old December 18th 06, 01:04 PM posted to rec.audio.tubes
Peter Wieck
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Posts: 2,418
Default Dynakit 70 filter cap question


west wrote:

> I don't like using 525v replacements especially since the AC input has
> increased from days gone by. Chris, why don't you provide us with real
> values for your stacking caps and balancing resistors? I think this would
> be more of a practical help for many.


Agreed. However, the balancing resistors are a nicety that is
unnecessary in this particular application. It is simple enough to use
450V caps, two-each in series of the appropriate capacitance to make up
a considerable safety margin. 2 x 60 @ 450V = 30 @ 900V. If one does
not want to spring for the (fairly) expensive 60s, go to 330V caps at
about half the price and have your rating at 660V. It is not as if you
are taking anything off the junction, and modern capacitors are far
more consistent than vintage types. If the balancing resistor makes you
feel better... sure. But the amp (and the caps) won't notice. However,
it is the 'right' thing to do so:

If you want to be elegant, it can be calculated:

For 2 capacitors in series: R = (2Vr - Vb) / (0.0015 C Vb)
For n > 2 capacitors: R = (Vr - Vb/n) / (0.00075 C Vb)
where R = resistance in Megohms

Vr = max rated surge voltage
Vb = max voltage across entire bank of caps
n = number of caps in series
C = capacitance in uF

With thanks to Cornell-Dublier for the formula.

DO NOT, repeat, DO NOT Mix voltages if installing caps in series. And
it is nearly infinitely better to use only identical caps to replace
any one section.

Peter Wieck
Wyncote, PA

 




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