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Old 21st Apr 2024, 11:45 am   #1
Jan Zodiac
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Default Tantalum vs. electrolytic capacitor

Hi all.

I'm restoring an Ortofon valve amplifier for a friend. It's this model:

https://www.hifiengine.com/manual_li...n/ks-601.shtml

The owner prefers everything to look as close to original as possible. This gives some challenges when it comes to replacing quite a lot of axial electrolytic capacitors.

It seems that tantalum capacitors are available as axials, with some of the values needed. They will be used for cathode decoupling in the preamp/tone control section.
My question is, if I can simply replace the original electrolytic caps with tantalum ones? I have no experience with tantalums, and a lot of more or less religious stuff is written about them online.
I know they can be fragile when it comes to heat, overvoltage etc., but this will not be a problem in this case.
I also know a lot of the tantalum "drop" types that was common in the 70's, has gone bad over time and needed replacing. But that goes for a lot of regular electrolytic caps as well.

So is there anything to worry about, or can I just go ahead and order tantalum types?

Best regards,

Jan
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Old 21st Apr 2024, 12:22 pm   #2
Neutrino
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Default Re: Tantalum vs. electrolytic capacitor

Are you thinking of this type of axial lead tantalum capacitor?
https://docs.rs-online.com/efae/A700000009084056.pdf

In my opinion, the main reason not to use this type of capacitor is cost. Also, make sure that you get the polarity correct.

My employer fifty years ago specified similar looking axial lead tantalum capacitors. Employees were required to wear safety glasses when testing new circuit boards because mistakes with polarity had been made during assembly.

Recently I repaired a nearly 50 year old HP 5381A Frequency Counter. The only thing wrong with it was a short circuit axial lead tantalum capacitor.

David
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Old 21st Apr 2024, 12:49 pm   #3
Craig Sawyers
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Default Re: Tantalum vs. electrolytic capacitor

The standard trick is to remove the innards from the original part, and hide a modern electrolytic inside - inevitably smaller. The unit then looks physically original.

Same goes for the multi-section smoothing capacitor if that is a source of problems.

The main issue is likely to be the single sided board on brown phenolic base. Removing parts and clearing solder can easily lead to tracks lifting off the board.

Good luck with the restoration.

Craig
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Old 21st Apr 2024, 1:12 pm   #4
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Default Re: Tantalum vs. electrolytic capacitor

By the way, Tantalum capacitors are electrolytic. "Wet slug" tants use a liquid electrolyte, but there are also solid electrolyte variants. Similarly Aluminium electrolytics use a liquid electrolyte and there are now solid electrolyte variants. All perfectly designed to sow confusion!

David
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Old 21st Apr 2024, 1:21 pm   #5
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Default Re: Tantalum vs. electrolytic capacitor

The word tantalum sends me in to a shaking state of frenzy. Seen far too many bad one's to ever trust them in the slightest.
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Old 21st Apr 2024, 1:29 pm   #6
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Default Re: Tantalum vs. electrolytic capacitor

I don't know what the values & voltages the capacitors are (can't access hifiengine, they don't allow anyone to register to download).
Axial electrolytics are easy to get hold of, if you want a better selection of parts look on Mouser, I recently used a number of the Kemet PEG series to replace some in a HP 6920B (where they are visible through the covers), note the highest temperature ones lack any plastic sleeving.

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As for axial tantalums, there are two types, the 150D series linked in one post is a hermertic sealed solid tantalum, they don't seem to fail as often as the bead types, but remember to derate on voltage if using them.

The second type is the sealed wet tantalum, these either have a rubber seal (which perishes over time and leaks corrosive acid electrolyte) or a glass seal which never leaks, both of these are hideously expensive due to the use of silver or tantalum for the cases, which are materials resistant to the sulphuric acid within. I have a number of the rubber sealed 109D series to replace in my HP 432A power meter, thankfully no major damage from the leakage.

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Old 21st Apr 2024, 2:20 pm   #7
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Default Re: Tantalum vs. electrolytic capacitor

I thought about using tantalum caps for de-coupling valve cathodes some time back as I read that electrolytic caps in this function can cause distortion because there is insufficient bias voltage EG a 63v cap in a common cathode where the cathode bias is 1 or 2 volts. In reality I don't think this is an issue.

If you use a tant for cathode bypass there is always the danger it will fail SC, also decent axial caps are still available, see - https://uk.farnell.com/w/c/passive-c...ic%20capacitor I usually go for the Vishay caps.

Lastly I have a few NOS axial caps, I'm happy to pop some in the post FOC if that helps. Andy.
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Old 21st Apr 2024, 2:30 pm   #8
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Default Re: Tantalum vs. electrolytic capacitor

On reflection, I would not use tantalum electrolytic capacitors. In my experience tantalum electrolytic capacitors fail short circuit. A short circuit cathode decoupling capacitor could damage the valve and output transformer.

Aluminium electrolytic capacitors, in my experience, tend to go low capacitance and high ESR when they fail. This would cause a loss of performance but no damage to the amplifier.

David
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Old 21st Apr 2024, 2:34 pm   #9
Jan Zodiac
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Default Re: Tantalum vs. electrolytic capacitor

Yes, it's the solid tantalums from the link I refer to, and yes I am also aware that they are electrolytic capacitors. I know they are destroyed if the polarity is reversed or a too high voltage is being applied.

Values I need for the amplifier are 3u9/10V, 25u (27u) 6V and 15V. Yes, I can probably find 33u/25V as a regular axial electrolytic, but I had no luck with the 3u9 yet. Except for the tantalums of course.

There is no way I am going to spend hours of restuffing the original capacitors on something that is not even that rare. So I was just thinking if the tantalums really live up to the bad reputation they apparantly have, or if I could just go on and use them.
The higher cost is not a problem in this case, since only a handful are needed.

Jan
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Old 21st Apr 2024, 2:38 pm   #10
Jan Zodiac
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Default Re: Tantalum vs. electrolytic capacitor

I was not aware that when/if they fail, they are likely to SC. I can see that is a problem of course, but in this case they are not being used in the output stage. It is a fixed bias stage, so no cathode resistors/capacitors.
I was going to use them in the preamp stages, but a SC here is of course a bad thing as well, although it will not risc the output transformer.

Jan
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Old 21st Apr 2024, 3:16 pm   #11
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Default Re: Tantalum vs. electrolytic capacitor

Closest for 3.9uF at 10V would be the 5uF 25V from Vishay https://www.mouser.co.uk/c/passive-c...7C~Capacitance

And 25uF at 6V & 15V, could be replaced with 25uF, 30uF or 33uF at 16V or 25V from Vishay. https://www.mouser.co.uk/c/passive-c...7C~Capacitance

Check physical sizes and the actual voltage they are running at before ordering new parts, I've found older parts with the measured voltage exceeding the rating several times before.

David
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Old 21st Apr 2024, 3:47 pm   #12
Jan Zodiac
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Default Re: Tantalum vs. electrolytic capacitor

Yes I am sure it will work fine with 5uF. Thanks for the links, I will forget about the tantalums.

You are right about the voltages. I have the schematic with voltages written on it, and in some places the actual voltage is very close to the rating. We have a higher mains voltage now, so that needs to be looked into. I will make sure this will not be the case when I replace them.

Jan
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Old 21st Apr 2024, 3:50 pm   #13
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Default Re: Tantalum vs. electrolytic capacitor

I've never had a problem with tantalum capacitors except in 1984 when I was developing a base drive circuit for a pulsed power transistor, I used one as a speed-up capacitor where it was passing significant pulse current, and I found they'd last a couple of hours each.

Handle them with kid gloves, don't reverse the polarity, operate them at 75% of rated voltage maximum, don't let them handle significant current, and they'll last for a very long time. I routinely use them to bypass the supply rails of op-amps (with upstream 33 ohm resistors) and I've got equipment in the field working for 30 years with them in!

I did use an axial tantalum capacitor, 150uF 125V across the rails of a portable battery superhet (90V HT), from surplus stock (list price £66) and I left it with 100V on for a week just to get some confidence before wiring it in. It didn't let me down.
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Old 21st Apr 2024, 6:12 pm   #14
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Default Re: Tantalum vs. electrolytic capacitor

Many years ago - and I mean many, in the days when valve kit was prevalent and electrolytics tended to be much larger and a tad leaky, I was presented with a tape amplifier which had a small button as the cathode bypass. I could not believe the 50 mfd stamped on such a small device and when I checked it on the bridge, I could not believe the absence of leakage resistance.

It suddenly reminded me of the scene from 'This island earth' movie, where a perfect replacement for the huge "XC condenser" was supplied as a bead!

That was my first introduction to tantalum caps. - mind you it was a mil spec device.
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Old 21st Apr 2024, 6:22 pm   #15
Craig Sawyers
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Default Re: Tantalum vs. electrolytic capacitor

Quote:
Originally Posted by Jan Zodiac View Post
Y
There is no way I am going to spend hours of restuffing the original capacitors on something that is not even that rare.
Jan
Jan

In your original post you said "The owner prefers everything to look as close to original as possible"

So just stuff in any modern part that fits, and the unit will not look anything like "as close to original as possible"

If you search for that Ortofon model, you can find all sorts of bodge jobs, broken tracks with wire bridges, capacitors in mid air - real horror stories.

Craig
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Old 21st Apr 2024, 7:38 pm   #16
Jan Zodiac
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Default Re: Tantalum vs. electrolytic capacitor

Craig,

Yes, maybe I did not explain myself well enough. The owner would like the original axials to be replaced with new axials, not radials. The large filter cans are being gutted and new caps installed. That is not a hard task. But to restuff all the small yellow Jensen caps is not something I'm gonna spend time on. In my opinion it is perfectly fine with new axial types, and the owner of the amplifier agrees with that.
If it should be kept completely original, it was best off unused on a shelf.

And yes, there are many horrible examples of work that has been done to those Ortofon amplifiers, but it is in general a bit of a horrible amplifier to work on already as it is. The large pcb is of bad quality, often burnt around the output valves that has been running hot due to a badly designed bias circuit.
Tracks loosen easily and it is very difficult to replace components without damaging them. I'm certainly not a fan of that model, but the owner likes it, and he even has the matching tuner for it.

I'm sure all will work out fine, but it will of course not be 100% original when finished, because parts have been replaced with new ones. I also have to redesign some of the bias circuit to make it more reliable and not burn the new set of valves to the ground.

Jan
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Old 21st Apr 2024, 7:48 pm   #17
Jan Zodiac
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Default Re: Tantalum vs. electrolytic capacitor

Quote:
Originally Posted by lesmw0sec View Post
Many years ago - and I mean many, in the days when valve kit was prevalent and electrolytics tended to be much larger and a tad leaky, I was presented with a tape amplifier which had a small button as the cathode bypass. I could not believe the 50 mfd stamped on such a small device and when I checked it on the bridge, I could not believe the absence of leakage resistance.

It suddenly reminded me of the scene from 'This island earth' movie, where a perfect replacement for the huge "XC condenser" was supplied as a bead!

That was my first introduction to tantalum caps. - mind you it was a mil spec device.
That's interesting. I actually thought tantalum caps were a relatively "new" type of capacitor.
Maybe I should do some research on them to get a little background knowledge.
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Old 21st Apr 2024, 8:19 pm   #18
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Default Re: Tantalum vs. electrolytic capacitor

Quote:
Originally Posted by Jan Zodiac View Post
Quote:
Originally Posted by lesmw0sec View Post
Many years ago - and I mean many, in the days when valve kit was prevalent and electrolytics tended to be much larger and a tad leaky, I was presented with a tape amplifier which had a small button as the cathode bypass. I could not believe the 50 mfd stamped on such a small device and when I checked it on the bridge, I could not believe the absence of leakage resistance.

It suddenly reminded me of the scene from 'This island earth' movie, where a perfect replacement for the huge "XC condenser" was supplied as a bead!

That was my first introduction to tantalum caps. - mind you it was a mil spec device.
That's interesting. I actually thought tantalum caps were a relatively "new" type of capacitor.
Maybe I should do some research on them to get a little background knowledge.
It was one of those stud-mount disks, around the size of a sixpence. I don't think the red blob variants had yet been invented.
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Old 22nd Apr 2024, 12:11 am   #19
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Default Re: Tantalum vs. electrolytic capacitor

Wet tantalums are not really prone to shorts, but are ****** expensive...

Closest to original axial electrolytics are made by Vishay/BC. Electrically anyway.
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Old 22nd Apr 2024, 7:09 am   #20
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Default Re: Tantalum vs. electrolytic capacitor

I've got 3.2u & 4u7 but no 3u9, maybe a 2u2 & a 2u5 in parallel might do, but then electrolytics aren't bang on spec anyway. I also have 25u but all these would need checking first.

You might got 3u9 in film caps, Andy.
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