I have an Ameritron AL-1200 amplifier, and it came (used) with a
capacitor problem. Upon inspection, 4 of the caps and 3 of the
resistors in the Plate supply have been replaced, and some of the caps
are in pretty sad shape (not to mention half of them are 220uf and the
other half 270uf...).

Upon a closer examination of the unit and schematics, I find the plate
voltage is expected to be at 3600 volts (more or less), the manual has a
warning about avoiding transformer taps that produce over 3700 volts,
and the total of the capacitor bank rating is 3600 volts. (8 caps in
series, 450volt rating. Each cap is 220 uf, so the entire pack is 3600
volts, 26.25 uf)

I was in high tech engineering (computers and peripherals) for 20 years,
and we never ran electrolytics at anywhere near their rated voltage.
Permissible margins for low voltage DC circuits were 50% or more
margin. Typical rule of thumb was 100% margin (a 10 volt cap running on
nominal 5 volt line, etc.). Even in an industry where every penny of
component cost was significant, submitting a design with electrolytics
running with as little 25% margin typically got a less than stellar
response during design reviews, to say the least.

So, I was very surprised to find the capacitor bank in the AL-1200
running at or slightly above the rating of the capacitors. It just
looks like a problem waiting to happen to me. Judging by the components
replaced by a previous owner, I think that at least some of the
capacitors failed in the past, possibly catastrophically... As I wish
to prevent (or at least reduce the likelihood of) this happening in the
future, I'm looking for capacitors that can replace the existing ones
with higher voltage units. Unfortunately, there just aren't many
manufacturers that have 500 volt electrolytics in their product line,
and probably few stocking distributors that would have any in stock (or
order them for a small order quantity).

The part numbers I have found that will work for me (in order of
preference) are listed he

Cornell Dubilier (CDE)
520C271T500AB2B 270uf 500 volts
500C311T500AB2B 31o uf 500 volts
520C341T500AJ2B
550C361T500AJ2B
500C381T500AJ2B
520C401T500AC2B
DCMC321T500AB2B
DCMC401T500AJ2B

ALS30/40 series BHC Aerovox
ALS334H331D3L500
ALS30A331DE500

All of these are "computer grade" with screw terminals, and range from
inverter duty down (in order, from the best are 550C, 520C, 500C,
DCMC).

Do you know of any distributor that currently stocks these? Price would
be good too...

I need 10 of any one part number. I am in process on gathering supplies
to make my own PCB to use 10 caps (instead of 8) so if I can get the 500
volt capacitors, the rated capacitor bank will be 5000 volts and 27 to
39 uf, which should provide a decent margin for a 3600 volt supply (and
not blow up if my line voltage rises and I see 3900 volts, which
happened to me at a former QTH). There's _just_ enough room for the
larger PCB for the 10 caps (and resistors) in place of the existing
board with 8 of each.

I know where to get CDE DCMC271T450AA2B (450 volt, 270uf) which would
give me a 4500 volt 27uf pack, but really would prefer to have the extra
margin of the 500 volt capacitors. I can wait a bit for reasonable lead
times, as the amplifier doesn't need to be in operation next week, but I
don't want to still be looking 3 or 4 months from now.

I'd appreciate any leads to suppliers of any of the above capacitors (or
their equivalents). I can't say that price is no object, or I'd just
buy a big QRO amp and sit back, but that's not the way I do things... I
should expect these caps are going to be somewhere north of $20 each,
even at Qty 10.

Thanks
--Rick AH7H

"Rick Frazier" bravely wrote to "All" (14 Oct 05 07:08:21)
--- on the heady topic of "High Voltage Caps for Plate Supply ??"

RF From: Rick Frazier
RF Xref: core-easynews rec.radio.amateur.homebrew:88241

RF I have an Ameritron AL-1200 amplifier, and it came (used) with a
RF capacitor problem.
[,,,]
RF I was in high tech engineering (computers and peripherals) for 20
RF years, and we never ran electrolytics at anywhere near their rated
RF voltage. Permissible margins for low voltage DC circuits were 50% or
RF more margin. Typical rule of thumb was 100% margin (a 10 volt cap
RF running on nominal 5 volt line, etc.). Even in an industry where every
RF penny of component cost was significant, submitting a design with
RF electrolytics running with as little 25% margin typically got a less
RF than stellar response during design reviews, to say the least.
[,,,]
RF Thanks
RF --Rick AH7H


Rick, it is okay to run electro's at about 80% of their voltage spec.
Electro's are formed at up to 140% of their final spec and can thus
tolerate turn on surges. It is the turn-on surge spec that one should
worry about because that is when the electro is vulnerable. At turn on
an electro re-forms a little bit which draws a larger current than
usual. If that current doesn't fall off rapidly enough the electro can
become damaged and leak permanently. In the old days electros used to
be stamped with both surge and working voltage ratings.

Isn't using an electro at 50% of spec rather conserative? I might
understand being this prudent with semiconductors. An electro also has
a leakage current spec at the rated working voltage. There isn't a
direct linear relation between voltage and leakage. The relation is
more like a reverse exponential and the leakage current drops off a
lot more at a lower percent of rated voltage.

A*s*i*m*o*v

.... Paul's Law: You can't fall off the floor.

"Asimov" wrote in message
...
"Rick Frazier" bravely wrote to "All" (14 Oct 05 07:08:21)
--- on the heady topic of "High Voltage Caps for Plate Supply ??"

RF From: Rick Frazier
RF [snip]
RF e never ran electrolytics at anywhere near their rated
RF voltage. Permissible margins for low voltage DC circuits were 50% or
RF more margin. Typical rule of thumb was 100% margin (a 10 volt cap
RF running on nominal 5 volt line, etc.).

Rick, it is okay to run electro's at about 80% of their voltage spec.
Electro's are formed at up to 140% of their final spec and can thus
tolerate turn on surges. ...

Isn't using an electro at 50% of spec rather conserative? ...
A*s*i*m*o*v


In Motorola, our practice on lytics ( & resistors) was 50% of rated Voltage
(power), but that was in the 12 v. area. I don't know where this came from,
but they had extensive accelerated test capability and field data said our
stuff is known to keep on ticking...

I seem to recall that in tube days you were much closer and 80% seems
reasonable.

73, Steve, K9DCI

Asimov and Steve:

I would agree, designs for lower voltage operations (at 50% of spec) tend to be
much more conservative, particularly when the actual cost differential isn't
that great, but it really does seem to cut down the failure rate. Also agree on
the 80% figure, and that's what I'm trying to achieve (at minimum) with the
change to higher voltage caps, (and relayout of the pcb to use 10 caps). Even
if I can't get the higher voltage 500V caps, I can still get nearly 20% margin
at the nominal 3600volt DC level, leaving some extra capacity for an occasional
surge... I remember testing 50 caps at a time in my "blast box" to see how long
they would withstand their surge rating. Yep, more than a few disintegrated
during the testing, and many bulged without actually coming apart, all within as
little as a few hours at the rated surge voltage. If I remember correctly, I
also saw about a 5% failure rating at rated voltage within a few hundred hours,
though the particular units tested at that time weren't computer grade or
inverter grade caps like the part numbers I'm currently looking for. Of course,
testing with a higher ripple content always broke the caps down faster than
nearly pure DC, but you'd expect that just from the heating effect of the ripple
current alone. Sometimes I don't even want to think about the hours that were
dedicated to finding caps with both a low leakage figure that had a decent
margin, and a long life so they could be used on part of a product that spent
about a third of it's time on (lithium) battery power.

Most caps are no longer marked for surge, but the datasheets typically show the
surge rating. Unfortunately, you have to be very careful with surge figures,
because some manufacturers seem to provide "absolute maximum" ratings instead of
safe "momentary" surge ratings. Of course, it's up to the purchaser to determine
whether the published ratings are satisfactory for the intended use (as I was
quoted on many an occasion when inquiring of the manufacturer in years past.
Just try to get a manufacturer to state in writing what they mean by
"momentary"...

Having performed "accelerated aging" tests to help determine the failure rate of
stressed components, and compared it to actual field failures, I know that it
isn't an exact science by any means. At least this is only a capacitor. Just
try to get decent information on a battery (numerous experiments with lithium
thionyl chloride types comes immediately to mind... ) is like searching for
chickens with teeth. If capacitor manufacturers treat their products like some
battery manufacturers did 10 years ago, it's a wonder they even stamp a rating
on them... Of course, a tolerance rating of -10%, +50% on a capacitor should be
a telling item to begin with...
[sigh]

Thanks for the response.
--Rick

Asimov wrote:

"Rick Frazier" bravely wrote to "All" (14 Oct 05 07:08:21)
--- on the heady topic of "High Voltage Caps for Plate Supply ??"

RF From: Rick Frazier
RF Xref: core-easynews rec.radio.amateur.homebrew:88241

RF I have an Ameritron AL-1200 amplifier, and it came (used) with a
RF capacitor problem.
[,,,]
RF I was in high tech engineering (computers and peripherals) for 20
RF years, and we never ran electrolytics at anywhere near their rated
RF voltage. Permissible margins for low voltage DC circuits were 50% or
RF more margin. Typical rule of thumb was 100% margin (a 10 volt cap
RF running on nominal 5 volt line, etc.). Even in an industry where every
RF penny of component cost was significant, submitting a design with
RF electrolytics running with as little 25% margin typically got a less
RF than stellar response during design reviews, to say the least.
[,,,]
RF Thanks
RF --Rick AH7H

Rick, it is okay to run electro's at about 80% of their voltage spec.
Electro's are formed at up to 140% of their final spec and can thus
tolerate turn on surges. It is the turn-on surge spec that one should
worry about because that is when the electro is vulnerable. At turn on
an electro re-forms a little bit which draws a larger current than
usual. If that current doesn't fall off rapidly enough the electro can
become damaged and leak permanently. In the old days electros used to
be stamped with both surge and working voltage ratings.

Isn't using an electro at 50% of spec rather conserative? I might
understand being this prudent with semiconductors. An electro also has
a leakage current spec at the rated working voltage. There isn't a
direct linear relation between voltage and leakage. The relation is
more like a reverse exponential and the leakage current drops off a
lot more at a lower percent of rated voltage.

A*s*i*m*o*v

... Paul's Law: You can't fall off the floor.

On Fri, 14 Oct 2005 07:08:21 GMT Rick Frazier
wrote:

I was in high tech engineering (computers and peripherals) for 20 years,
and we never ran electrolytics at anywhere near their rated voltage.
Permissible margins for low voltage DC circuits were 50% or more
margin. Typical rule of thumb was 100% margin (a 10 volt cap running on
nominal 5 volt line, etc.). Even in an industry where every penny of
component cost was significant, submitting a design with electrolytics
running with as little 25% margin typically got a less than stellar
response during design reviews, to say the least.

Your margin would be determined by your tolerance for failure. If
you're making 10,000 units per year, each with 10 caps in them and
want to keep your cap related failures to less than, say, 10 per year,
then you need to be a lot more conservative than if you have a single
device with 10 caps in it that you want to keep your odds for failure
under 1% per year.

For most of the items I fix around here, I like to approach the
problem in a way that I can think of the solution as permanent, but
when you have a sample of just one, a 1% annual failure rate is about
as close to perfect as you'll ever get,

I'd say that for 450V caps, you're being much too conservative. We
have banks of 450V caps at work, each bank with about 200 caps in it.
They are used for energy storage and charged slowly and discharged
rapidly about every 5 minutes all day long. We have about 10 such
banks. We have occasional failures, maybe 1 cap every 3-4 years. We
don't think that's too bad.

I'm currently working on some upgrade banks which will have about 90
16,000 uF, 450V caps per bank. These will be run at 450V, and we'll
have 16 such banks in the second phase of the project. United
Chemi-Con doesn't seem to have any problem with this. We expect
occasional failures, but we also realize that anything else is just
being unrealistic.

C-D lists computer grade caps rated at 500 and 550 V, each with surge
ratings higher than that. United C-C does, too, but they admit that
their etched alum foil for those voltages is not as advanced as at
450V, so the energy density is not as high. This does not sound like
it would be a problem for you.

Another thing we noted with interest was that the C-D catalog said
that grading resistors were not necessary when installing caps in
series for higher voltages, especially if all the caps were from the
same batch.

If I were you, I would just buy the caps you need/want at 450V, and
then buy a couple of spares from the same batch. You would want to
reform these if you ever needed them, but they would give you the
necessary assurance that you'd be set for life.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

Jim:

Thanks for the response.

Wow, with cap banks like you describe, there's some significant energy
there... What are you working on, a pulsed cyclotron?

Let's see, given your 10 banks with 200 caps each, that's 2000 caps and only
1 failure every three years, your failure rate is nearly insignificant. With
a failure rate that low and a cycle rate you described, I'm supposing you are
charging them with nearly pure DC, but without knowing the application, it's
hard to guess... Of course, 200 caps in parallel mean leakage of about
800ma, so you've got to have a pretty good supply charging those caps.

A bank of 90 caps at 16000uf each looks like 1.4 farads. That's an
incredible amount of storage at first look. The leakage alone on a bank like
that would be about 400ma. To charge them slowly, I'd be interested in
knowing the charge time, and what is being used to limit the charge current.
Is it an active supply with a ramped current, or as simple as really high
wattage limiting resistors?

In the linear amplifier, there are really a couple of things that tend to
kill the caps. First, the ripple current tends to heat the caps, which are
in a relatively warm location to begin with, sitting right next to a fairly
large transformer, and a full wave bridge just above them. The transmitting
tube is on the "other side of the wall" so to speak, but I'm sure it does
contribute to some heat in the area. I haven't had this particular amplifier
up and running yet, so can't instrument it to find out what the temperatures
are, but I wouldn't be surprised to find it at 50C or higher, as the cooling
air is all directed into the tube chimney and no particular attention has
been made to cooling the capacitor bank. Given the ripple current, I wouldn't
be surprised to see 80C, which is dangerously near the rated temperature
rating (at least for me). Next, we have the typical cycle service an
amateur amplifier puts these through, with a nearly uncontrolled charge
(limited somewhat by a series resistor in the primary supply for the first
second or so on an initial power up), and the demands as the amplifier is
used. Now, given that the capacitor bank is never fully discharged, and the
voltage across the bank is 3600 volts, the transformer rated at only 0.8
amperes, and the amount of current required during typical transmission, the
heating from ripple current is probably the biggest concern for the initial
design.

However, given that I live in an area with less than perfect regulation of
the incoming AC line, I tend to be a bit gun-shy about what I see personally
as a "marginal" design. At my last QTH, the line voltage per our local
Hawaiian Electric Light Company (HELCO) should have been between 230 and 240
volts. I was the last house on the end of a 1 mile run of utility poles, and
the transformer, which was over 300 feet from the building, served only my
house and workshop at the time. In my workshop, I ran a single phase to 3
phase solid state inverter for a 3 horsepower wood lathe. During a period of
two years, even with rather sporadic use after the first year, I had four
inverters fail, and the failure was always the input diode bridge. After the
first failure, I instrumented the AC line and kept graphs on it. The line
voltage ranged from 235 to 243 volts over the typical 24 hour period, and
tended to be around 239 volts much of the time, which I see as the nominal
voltage for that location. Unfortunately, there were times when the voltage
would rise to as much as 255 volts for brief periods of time (usually only a
few seconds, but one prolonged instance went over 15 seconds). I didn't
actually catch any high line spikes during operation of my lathe, but there
were times I forgot to turn off the inverter and would come back and find it
dead in the water a couple of days later. I finally eliminated the problem
by putting a buck/boost transformer in the circuit, reducing the nominal
voltage to 227 volts. Now there is a definite possibility the particular
inverter input bridge wasn't properly rated, or wasn't up to spec, but all
problems went away once I got the line voltage down (and the observed spikes
at or below 243 volts).

Now thinking about the amplifier, which is supposed to produce 3600 volts
with a nominal 240 volt input. Given some likelihood of seeing spikes to 255
volts, I could easily imagine the capacitor bank seeing 3825 volts for as
much as 15 seconds. OK, not a long time, considering the really big picture,
but looking inside the amplifier and seeing that three of the bridging
resistors (50K ohm, 7 watts) and 4 of the capacitors were replaced at some
time in its history, I am tending towards getting more margin into the
design.

My tolerance for failure is low. I would prefer to get this amplifier up to
snuff, put it into service, and never have to open the case again. I know
that's a fairly high goal, but the AL-1200 uses one of the longest lived
tubes in this type of service, and I'm unlikely to overdrive it so long as I
load it properly, as it can withstand 130 watts and both of my transceivers
only put out 100 watts..

I can purchase the 450 volt capacitors for about $10 each, so the 10pc bank
will cost me about a hundred dollars. Add another $25 for new resistors and
$20 for the supplies to layout the PCB for 10 caps, I'm looking at $145 to
have a 4500 volt capacitor bank. If I can get the 500 volt capacitors, they
will likely cost $25 to $30 each (lower volume, fewer dealers, etc.) and I
would have a 5000Volt capacitor bank for $295 to $345 out of pocket. If I
just replaced the caps and resistors on the original PCB, I'd be out $125,
but have a 3600 volt bank. With 500 volt capacitors I'd be at 4000 volts.
Given the circumstances of apparent previous failure(s) and my low tolerance
for failure 'on my watch', I would feel better having the extra headroom,
even with a price premium well over double. (Funny how it comes around to
the warm fuzzy feelings isn't it?) Of course, f I don't find a supplier for
the 500 volt capacitors fairly soon, impatience will take over and I'll end
up buying the 450 volt ones anyway!

Of course, in the back of my mind (and not previously mentioned) is the
possibility of running the amplifier from the 220 volt tap on the
transformer, which would give me 3925 volts nominal on the bank at 240 volts,
and 4010 volts on the bank at 245 volts, and 4175 if I see the same sort of
255 volt spike at the new QTH. I've done dumber things in my life and could
see a possibility of it happening in a moment of weakness, so I'm also buying
some insurance by going to the higher voltage capacitors and the new PCB.
The biggest reason to run a higher voltage would be to allow getting full
legal output with a lower drive from the transceiver. I would prefer to run
with less than 100 watts out from the transceiver, but thinking about it, it
would likely only be about 10% or so less drive, so perhaps it isn't worth
all the work for that reason alone.

Thanks
--Rick

Jim Adney wrote:

On Fri, 14 Oct 2005 07:08:21 GMT Rick Frazier
wrote:

I was in high tech engineering (computers and peripherals) for 20 years,
and we never ran electrolytics at anywhere near their rated voltage.
Permissible margins for low voltage DC circuits were 50% or more
margin. Typical rule of thumb was 100% margin (a 10 volt cap running on
nominal 5 volt line, etc.). Even in an industry where every penny of
component cost was significant, submitting a design with electrolytics
running with as little 25% margin typically got a less than stellar
response during design reviews, to say the least.

Your margin would be determined by your tolerance for failure. If
you're making 10,000 units per year, each with 10 caps in them and
want to keep your cap related failures to less than, say, 10 per year,
then you need to be a lot more conservative than if you have a single
device with 10 caps in it that you want to keep your odds for failure
under 1% per year.

For most of the items I fix around here, I like to approach the
problem in a way that I can think of the solution as permanent, but
when you have a sample of just one, a 1% annual failure rate is about
as close to perfect as you'll ever get,

I'd say that for 450V caps, you're being much too conservative. We
have banks of 450V caps at work, each bank with about 200 caps in it.
They are used for energy storage and charged slowly and discharged
rapidly about every 5 minutes all day long. We have about 10 such
banks. We have occasional failures, maybe 1 cap every 3-4 years. We
don't think that's too bad.

I'm currently working on some upgrade banks which will have about 90
16,000 uF, 450V caps per bank. These will be run at 450V, and we'll
have 16 such banks in the second phase of the project. United
Chemi-Con doesn't seem to have any problem with this. We expect
occasional failures, but we also realize that anything else is just
being unrealistic.

C-D lists computer grade caps rated at 500 and 550 V, each with surge
ratings higher than that. United C-C does, too, but they admit that
their etched alum foil for those voltages is not as advanced as at
450V, so the energy density is not as high. This does not sound like
it would be a problem for you.

Another thing we noted with interest was that the C-D catalog said
that grading resistors were not necessary when installing caps in
series for higher voltages, especially if all the caps were from the
same batch.

If I were you, I would just buy the caps you need/want at 450V, and
then buy a couple of spares from the same batch. You would want to
reform these if you ever needed them, but they would give you the
necessary assurance that you'd be set for life.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On Sun, 16 Oct 2005 02:19:32 GMT Rick Frazier
wrote:

Wow, with cap banks like you describe, there's some significant energy
there... What are you working on, a pulsed cyclotron?

Plasma physics research facility. 2-4 minute charge cycle, 70 mSec
discharge. These banks supply the crowbar, or "follow-on," current
once the big 5 kV banks have discharged down. Yes the charging
supplies are large, and the future ones will have to be even larger.
We tend to go for some kind of constant current scheme, up to the
preset voltage limit.

I don't think our leakage current is nearly as high as you suggest.
The caps tend to form nicely after awhile and the leakage current
ramps down with use. Until one fails, of course. ;-)

A bank of 90 caps at 16000uf each looks like 1.4 farads. That's an
incredible amount of storage at first look. The leakage alone on a bank like
that would be about 400ma.

I rounded. We're actually shooting for individual banks of .3F at
900V, so that takes 1.2F of 450V caps, times 16 of these banks.
Charge/discharge is as above, leakage should be less that what you
suggest, but I could be wrong about that.

In the linear amplifier, there are really a couple of things that tend to
kill the caps. First, the ripple current tends to heat the caps, which are
in a relatively warm location to begin with, sitting right next to a fairly
large transformer, and a full wave bridge just above them.

I don't know how much internal heating you'll get, but temp is
certainly something to be concerned with. Our application stays pretty
much at room temp.

However, given that I live in an area with less than perfect regulation of
the incoming AC line, I tend to be a bit gun-shy about what I see personally
as a "marginal" design. At my last QTH, the line voltage per our local
Hawaiian Electric Light Company (HELCO) should have been between 230 and 240
volts. I was the last house on the end of a 1 mile run of utility poles, and
the transformer, which was over 300 feet from the building, served only my
house and workshop at the time. In my workshop, I ran a single phase to 3
phase solid state inverter for a 3 horsepower wood lathe. During a period of
two years, even with rather sporadic use after the first year, I had four
inverters fail, and the failure was always the input diode bridge.

Did you consider upgrading the input bridge? I agree that excessive
line voltage is a problem, but the bridge was clearly your weak link.

After the
first failure, I instrumented the AC line and kept graphs on it. The line
voltage ranged from 235 to 243 volts over the typical 24 hour period, and
tended to be around 239 volts much of the time, which I see as the nominal
voltage for that location. Unfortunately, there were times when the voltage
would rise to as much as 255 volts for brief periods of time (usually only a
few seconds, but one prolonged instance went over 15 seconds).

I think the surge ratings for Al caps are for a 30 second surge.

OK, not a long time, considering the really big picture,
but looking inside the amplifier and seeing that three of the bridging
resistors (50K ohm, 7 watts) and 4 of the capacitors were replaced at some
time in its history, I am tending towards getting more margin into the
design.

I agree that there's often room for improvement in amateur gear.
Upping the resistor power ratings might be useful, too. If they run in
the same ambient, then they should also be derated.

My tolerance for failure is low. I would prefer to get this amplifier up to
snuff, put it into service, and never have to open the case again.

I agree with your goal, and I don't think running 450V caps at 450V
will come back to haunt you. Just make sure that the 30 second surge
rating is above the highest voltage you think you'll ever see.

Of course, in the back of my mind (and not previously mentioned) is the
possibility of running the amplifier from the 220 volt tap on the
transformer, which would give me 3925 volts nominal on the bank at 240 volts,
and 4010 volts on the bank at 245 volts, and 4175 if I see the same sort of
255 volt spike at the new QTH.

With 10 caps, that's still less than 420 V/cap. I don't think that's
any problem at all.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------