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#1
Posted to rec.audio.tubes
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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? |
#2
Posted to rec.audio.tubes
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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
Posted to rec.audio.tubes
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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 |
#4
Posted to rec.audio.tubes
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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? |
#5
Posted to rec.audio.tubes
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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
Posted to rec.audio.tubes
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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 |
#7
Posted to rec.audio.tubes
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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 |
#8
Posted to rec.audio.tubes
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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 |
#9
Posted to rec.audio.tubes
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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. |
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