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Old 13th November 2010, 12:10 AM   #1
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Default Time to replace my 50 year old filter caps...

its about time to replace my aging filter capacitors for the B+ supply.

One of them finally leaked enough electrolyte out to rot the cathode lug off.

There are 4 filter caps in the B+ supply, values are 2@ 150uF and 2@ 200uF, both at 300V

They all appear to be single section caps (and appear so in the schematic), they are twist lock caps, but all of the twistable lugs are tied together (0 ohms between lugs, and stated value between main lug and any of the mounting lugs)

I am ok to go higher on capacitance correct?

Say, 250uF@500v JJ Tesla for all 4?

These are the only caps I can find that are equivalent or better in values.


(heathkit AA-121 btw...)
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Old 13th November 2010, 03:06 AM   #2
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Any thoughts about the higher capacitance on the replacement caps before I order them?
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Old 13th November 2010, 04:19 AM   #3
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from everything ive read you want to keep the same capacitance as stock on the filter caps
im a novice and am going only by what ive read in other posts ive seen
more specifically there are a few thread over on AK about recapping and upgrade a magnavox amp, both the 175 and the 185 and it was said there that filter caps should be kept at the same capacitance
I will head over there now and see if they explain why...
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Old 13th November 2010, 05:12 AM   #4
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Quote:
Originally Posted by kmcarpenter View Post
Any thoughts about the higher capacitance on the replacement caps before I order them?
Depends - what rectifier?
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Old 13th November 2010, 06:19 AM   #5
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470 uF snap in caps of 385 or 450V are common SMPS filter capacitors meaning they are cheap high ripple/ low esr and robust.

As for keeping filter caps the same as the originals why? by the looks of it the designer used the largest economic size of the time, 150 and 200uF 300 and 350V were the mainstay Television filter caps in the late 60's. The designer used what was common then for the same reason that I would be looking at the 470uF SMPS caps now. If in doubt run a simulation.
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Old 13th November 2010, 12:24 PM   #6
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Quote:
Originally Posted by metalsculptor View Post
470 uF snap in caps of 385 or 450V are common SMPS filter capacitors meaning they are cheap high ripple/ low esr and robust.

As for keeping filter caps the same as the originals why? by the looks of it the designer used the largest economic size of the time, 150 and 200uF 300 and 350V were the mainstay Television filter caps in the late 60's. The designer used what was common then for the same reason that I would be looking at the 470uF SMPS caps now. If in doubt run a simulation.
link?
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Old 13th November 2010, 02:03 PM   #7
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Default reason for cap limitation

Heathkit AA121 doesn't help much. Rectifier tubes have current limitations, especially cold. Abbreviated datasheets like the popular tung-sol's don't show them. The only one I know by memory is 5AR4/GZ34, 100 microfarad (now abbreviated uf by suppliers). If you go above that you are supposed to put 100 ohm resistor on the cathode end. If you don't you can get arcover at startup. If you have a solid state rectifier,it is probably less sensitive to cold current.
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Old 13th November 2010, 09:44 PM   #8
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I have all solid state rectifiers in this amp. The B+ rectifier is silicon, and the bias supply used to be selenium, but ive replaced it with a new silicon one.

The Heathkit AA-121 is a stereo push pull ultralinear EL-34 tube amplifier. The El34's are fed signal by a pair of 6AN8A's, the pentode side used to amplify incoming signal, and the triode side is used as a phase splitter to send one side of the wave to the control grid of one EL-34 and the other to the Grid of the other tube in a given channel.

Both channels use a shared power supply, are cathode biased through a series of potentiometers on the front of the chassis.

This amp runs 511 volts on the plate and about the same on the screen.
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Old 13th November 2010, 10:13 PM   #9
DF96 is offline DF96  England
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Increasing the first (reservoir) capacitor will increase the peak charging current. This may stress the rectifiers and increase transformer heating. It may also increase magnetic induction of buzz into nearby circuits. You could counteract this by adding some resistance in series with the rectifiers.

The second (smoothing) capacitor can usually be increased without any problems.
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Old 13th November 2010, 10:29 PM   #10
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the primary winding on the power transformer has a 100ohm 300w resistor that is removed from the circuit after a bimetallic blade (reacting from the resistor heat) shorts out a pair of contacts.

Would that mitigate the problem you describe? Or are you saying that the charging current is an all-the-time occurrence, rather than just on initial startup.
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