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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 [email protected] 150uF and [email protected] 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, [email protected] JJ Tesla for all 4?

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


(heathkit AA-121 btw...)
 

mackinthebox

Member
2010-10-19 11:03 am
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...
 
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.
 
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?
 
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.
 
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.
 
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.
 
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.
 
Increasing the primary filter capacitor will narrow the charging pulses somewhat, how much depends on the transformer design.

A link to a supplier of snap in caps, this was the first US supplier I tried and they had plenty of 450V caps, in europe the 385V ones are more common (380V supply vs 440V supply) <A href="http://au.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Electrolytic-Capacitors-Snap-In/_/N-75hr0?P=1z0wrk5Z1z0vl82">Mouser electronics</A>

They have plenty of different capacitors so you could use a smaller primary capacitor if the 470uF causes problems, I see no problems with 1A silicon diodes but there were a few 1/2 amp diodes getting around back then and they might be borderline with the longer power on surge of the larger capacitors.

No link as to the relative popularity of capacitors in the 60's I know from stripping dozens of televisions as a child what sizes were popular, I used to assemble the filter capacitors into arrays and use them for destructive purposes :)
 
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.

60uF is the rated maximum for 5AR4 in any data sheet I can find, and I wouldn't go over this with a vintage 5AR4, and current production are far worse. (I consulted the Amperex and Sylvania data sheets for reference.)

OP what amplifier are you referring to? It sounds like from your brief description that the supply might be a doubler. (Citation II, Citation V? Or ?)
 
Kevin, see msg #8. :)

..Todd

I was led astray by post #7... :D I don't see any issue with significantly increasing the capacitance in the doubler as long as the rectifiers are replaced with modern types that can handle the inrush current.

This amplifier IIRC uses fixed bias in the output stage, not cathode bias.
 
I was led astray by post #7... :D I don't see any issue with significantly increasing the capacitance in the doubler as long as the rectifiers are replaced with modern types that can handle the inrush current.

This amplifier IIRC uses fixed bias in the output stage, not cathode bias.


Yes, I was inaccurate.

This amp uses a variable negative input voltage on pin 5 control grid to adjust the bias.

I got wrapped around the axle about the details.
 
Carpenter,
post the pics of your 50 yrs old capacitors, i wanna see how do they look like:D

Cosmetically, they look like new. They have a case made from cardboard, are 1.5" in diameter, and about 4.5" tall. THey are black in color, with white printing in script, it reads Astron something orruther, and then the capacities.

I ended up buying that set of JJ teslas I was looking at. 250uF @ 500v.