NOS Mallory TC-72 a bad idea for my La2a?

[Zim]

So, I picked up a NOS Mallory TC-72 capacitor without really thinking it through...

Being an electrolytic and therefore more likely to be affected by age, it only occurred to me after buying that maybe a NOS cap wasn’t the best idea to put in my La-2a. Well, it’s arrived and seems to measure 14.95uf, so it’s out of spec for a 10uf cap, but...

Just wondering what the consensus is - is it too far out of spec, is it just a bad idea in general, or is the mallory tc-72 such an intrinsic part of the La2a sound (as I’ve seen mooted somewhere or other) that it’s worth a try?

appreciate any thoughts...

 

[soapfoot]

You probably won’t hurt anything.

could be a good chance to try this against a polypropylene cap of some sort (Solen “fast” 10uF will just about fit) and tell us which you prefer and why

 

[Zim]

thanks soapfoot,

quick bit of reading and seems that the higher value is not necessarily that surprising, particularly in an older cap like this. Reading could also be down to measuring With an LCR and not having 450V going through it which I’ve got to assume would have some affect on the measurement right?


(Currently have a sprague atom in the La2a)

 

[abbey road d enfer]

In this position, a larger value is not an issue, however I would be concerned with leakage, since DC in the primary can generate distortion.

 

[Zim]

In this position, a larger value is not an issue, however I would be concerned with leakage, since DC in the primary can generate distortion.

Concerned to the extent that it would be best to not try it?

(I understand that there’s really no ‘right’ answer, just trying to get a sense of risk vs reward and benefit from any experience others may have when it comes to NOS caps)

 

[soapfoot]

You can measure it for leakage.

connect the high voltage side and leave the transformer side floating.

clip your meter in place and set it to read DC volts

if it only reads a few fractions of a volt or less, it’s fine to try.

If you’ve already got a Sprague electrolytic in place, I doubt you’ll notice (or measure) an earth-shattering difference. With a film cap, maybe something slight.

 

[abbey road d enfer]

Concerned to the extent that it would be best to not try it?

You can easily evaluate the leakage current by connecting the positive of the cap to the anode of V2 (pin 1) and the negative to a 1k resistor that goes to ground. Then measuring the voltage across the 1k resistor will give the value of the leakage current in mA (1V read=1mA leakage).
Beware that the measurement must be taken after a few minutes for stabilizing.
A modern electrolytic cap should have less than about 100uA leakage. OTOH a film cap would probably have many times less.

 

[soapfoot]

(That’s better than my advice)

 

[Zim]

Thanks guys - just to check... would I need to remove the Sprague that’s there already or am I good just connecting positive to pin 1 V1 at the same time and measuring as per the instructions?

 

[abbey road d enfer]

Thanks guys - just to check... would I need to remove the Sprague that’s there already or am I good just connecting positive to pin 1 V1 at the same time and measuring as per the instructions?

You don't have to disconnect the Sprague.

 

[beatnik]

I don't really see the advantage in using an electrolytic in this situation - other than chasing some sort of holy grail magical vintage dream tone.
Even though the sound difference might be irrelevant, the audio performance of a polypropylene and perhaps even a polyester cap is better - on paper at least. and for sure they last longer.

 

[Zim]

I don't really see the advantage in using an electrolytic in this situation - other than chasing some sort of holy grail magical vintage dream tone.
Even though the sound difference might be irrelevant, the audio performance of a polypropylene and perhaps even a polyester cap is better - on paper at least. and for sure they last longer.

For me it’s not really about chasing a holy grail, as a relative newcomer to the world of DIY audio it’s just about getting an understanding of what does what. This was my first ‘proper’ build and used the Analog Devices chassis, guide and BOM - electrolytic was just what was specced there. I tried to be as true the original specs as possible (using carbon comp resistors where possible, NOS allen Bradley If i could find them, also with the pots). This is just an extension of that really.

I just picked up another chassis from Hugo on here and was planning to go with all metal film resistors and ‘modern’ parts when I do that one. If a polypropylene or polyester cap is a possible alternative there (I’ve asked but that hasn’t been suggested so far) then I’m happy to give it a try. Like I say - just learning at the moment and, though I heard of polystyrene caps being great for audio when I built a don 250eq, I’ve no idea yet when one type of cap could / should be used in place of another.

I just figured with that first La2a build it could be used as a ‘control’ rather than trying different things and modding before I’d even heard a hardware la2a. The next build, also planning to try the 12au7 mod with different resistor values I’ve seen suggested by (I think) CJ.

All suggestions welcome - what sounds best wins in my book (as subjective as that can/will be)

 

[abbey road d enfer]

I don't really see the advantage in using an electrolytic in this situation - other than chasing some sort of holy grail magical vintage dream tone.
Even though the sound difference might be irrelevant, the audio performance of a polypropylene and perhaps even a polyester cap is better - on paper at least. and for sure they last longer.

There may be a potential advantage, since electrolytics are usually less bulky than film caps, wich helps limiting parasitic capacitive coupling.

 

[andYz00m]

Ive been told you need to "reform" old caps that have been in storage for a long time. And was recommended to just put it on the becnh supply overnight. Havent done it yet. I am also building a LA-2A (waiting for a case) and have an older Mallory cap.

 

[soapfoot]

For me it’s not really about chasing a holy grail, as a relative newcomer to the world of DIY audio it’s just about getting an understanding of what does what. This was my first ‘proper’ build and used the Analog Devices chassis, guide and BOM - electrolytic was just what was specced there. I tried to be as true the original specs as possible (using carbon comp resistors where possible, NOS allen Bradley If i could find them, also with the pots). This is just an extension of that really.

I just picked up another chassis from Hugo on here and was planning to go with all metal film resistors and ‘modern’ parts when I do that one. If a polypropylene or polyester cap is a possible alternative there (I’ve asked but that hasn’t been suggested so far) then I’m happy to give it a try. Like I say - just learning at the moment and, though I heard of polystyrene caps being great for audio when I built a don 250eq, I’ve no idea yet when one type of cap could / should be used in place of another.

I just figured with that first La2a build it could be used as a ‘control’ rather than trying different things and modding before I’d even heard a hardware la2a. The next build, also planning to try the 12au7 mod with different resistor values I’ve seen suggested by (I think) CJ.

All suggestions welcome - what sounds best wins in my book (as subjective as that can/will be)

Polyester or polypropylene film will work great, and modern caps can fit.

Polyester tends to be smaller than polypropylene, and metallized film tends to be smaller than film-and-foil.

The theoretical best would likely be polypropylene film-and-foil, but at 10µF this will be rather bulky--perhaps too bulky to use.

I used a Solen "Fast" metallized polypropylene in a recent LA-2A build. It fit well (though only just!) but was slightly bulkier than an old electrolytic (and much bulkier than a modern electrolytic).

You won't find a polystyrene cap in the proper value and voltage rating, and if you somehow could, it would definitely be impracticably large.

The Allen-Bradley pots are worthwhile if only because their log tapers were a bit different than tapers found on most modern parts. For the pots to behave as you'd expect an old LA-2A to behave, it's worth using those older parts (or their expensive PEC descendants). If you don't care what the knob pointer says and only care about the sound, it's likely not critical.

The electrolytic output coupling cap, the class 2 ceramic (Z5U) coupling caps, and carbon comp resistors are all about trying to mimic the distortion characteristics of the old LA-2A amplifier.

This assumes two things: 1) the distortions introduced by the cheap old parts are audible, and 2) the distortions are desirable.

That gets into the realm where things like confirmation bias and personal taste play a part, so I won't editorialize on either point.

I will say that an LA-2A is a bit of a moving target, anyway. We have three vintage ones at our place, and all sound just a bit different from one another. The one I built (with metal films, film-and-foil polypropylene coupling caps and a metallized PP output cap) isn't any exception--but it's very much in the same family, and I'd happily use any of the four in place of one another.

The transformers and tubes will probably have a much bigger impact on the sound of the amp than any of the caps and resistors, and the T4B will have a far bigger impact on the gain reduction behavior than will anything else.

 

[Johnny1234]

It's a firecracker waiting to explode. No way in hell I would install it in an LA-2A.

 

[soapfoot]

It’s a 450v rated cap with about 105 volts on it.

High ESR is a possibility, which could make it more likely to overheat and build pressure, but the conservative application would somewhat improve the odds against it reaching an explosive failure point.

But it is a valid concern, and ESR can be measured with the right test equipment. A multimeter won’t do it, but the $26 transistor tester sold on Amazon can.

The unaddressed elephant in the room is that the majority of vintage LA-2As in the wild would’ve probably had this capacitor replaced long ago.

All three of our old ones have more-recent electrolytic replacements (two Sprague, one no-name radial lead cap bodged in ages ago—it sounds fine so we’ve left it)

 

[leadbreath]

Jesus, all this hand wringing... just stick the thing in and see if it burns...???

 

[andYz00m]

Think it will blow? Haha.

 

[murrayatuptown]

Newark/Farnell has the wrong picture(frustratingly common)showing a 4-lead 1100V part, but this is a 2-pin 10 uF, 500 VDC MKP type for US$2.69.

https://www.newark.com/kemet/c4aqlbu5100a1xk/cap-10-f-500v-10-pp-radial/dp/50AC9233
I recently bought 10, 22 40 & 60 uF ranging between 650 and 800 VDC. They ARE huge, and have 6 mm long PCB leads, but other than those issues, they will likely never need replacement. Just need the real estate to park them.

Some values have rotating promo prices.