Vintage Japanese HiFi amp restoration issue.

[TimMills]

Can I ask for a little help please?
I need to replace the bi-polar caps that feed current from the vol pot to the grid of a 12AX7 preamp valve on my Japanese vinatge hifi amp. C19 on the schematic attached. My confusion is; which side should the outer foil lead out be connected to, for the lower noise floor? I'm thinking it should go to the valve but someone else has said to put it the opposite way round and attach that lead to the pot. I've tried looking on line but the info I've found on an amp building site has the pot feeding the anode and not the grid

[cathoderay57]

I don't think it is going to make any significant difference. Both ends of the cap are at high gain parts of the circuit and so vulnerable to pickup from any adjacent AC wiring. I suppose on balance the end attached to the pot is likely to be the more "earthy" end assuming that you usually run the amp at low volume therefore the slider of the pot will normally be near the ground terminal. Therefore, the outer foil lead should be connected to the pot. If you are really worried why not try it both ways and suck it and see?

[Sideband]

Bi-polar is usually a reference to non-polarised (reversible) electrolytics. That looks like a standard polyester or polypropylene type capacitor....probably would have been paper dielectric originally. I suppose it depends what style capacitor you are replacing it with. Possibly unless you are going for expensive 'audiophile' type capacitors, the outer foil is probably not marked and I doubt it was important when the amp was built otherwise the original cap would have been marked and the question wouldn't arise. The grid is the most likely place of hum pick-up (since the other end is closer to earth/ground) so if it was important I'd say outer foil to grid.

I doubt it will make any audible difference. Whether you could measure any difference is another thing.

[bikerhifinut]

Conventional wisdom has it that you have the outer foil on the input side if I remember correctly.
Practically it makes no audible difference in an Audio amplifier and indeed a lot of plastic film capacitors give no indication of which end is which.
By bipolar I presume you mean non polarised capacitors as certainly the one in your circuit diagram should be. These days that means plastic film capacitors eg polyester or polypropylene etc. Avoid paper in oil is my advice.

A.

[Nuvistor]

Some older capacitors have the outer foil marked on the body of the capacitor, if yours has follow that, if not like others say it will probably not make much difference.

[TimMills]

Many thanks for your thoughts.

This is the blurb I am trying to adhere to:

"Auricap film capacitors are not polarized. However, the outer foil is marked by a red leadout with a black mark - Position this lead out to the lower impedance end (lower DC voltage end). This will improve the noise floor. "

Grid or Pot? there seems to be conflicting advice here?

[Herald1360]

Pot is definitely lower impedance.

[bikerhifinut]

Agreed.
Think about it. You normally feed a lower impedance source into a high impedance input therefore the lower Z end is the input of the capacitor.
But it really won't make a jot of difference i reckon.

[valveaudio]

I cannot believe that the polarity of this capacitor will make any difference to the noise, given that it is at tone controls stage, if it was at the input stages possibly.

Trevor

[TimMills]

Originally Posted by bikerhifinut

Agreed.
Think about it. You normally feed a lower impedance source into a high impedance input therefore the lower Z end is the input of the capacitor.
But it really won't make a jot of difference i reckon.

Thank you for this insight!
That's how I had it, and was thinking it sounded quieter tonight on my cans. but was reading this: https://robrobinette.com/How_To_Buil...tor_Outer_Foil
And it states outer goes to the tube.

[Techman]

Originally Posted by Sideband

I'd say outer foil to grid.

There seems to be a lot of confusion going on here. The inner foil goes to the grid and not the outer.

By the way, we live in the UK, so it's valves, not tubes.

That link is confusing as it shows a previous stage where the outer foil of the capacitor is correctly connected to the anode of the valve in that stage. Confusingly, what it doesn't show in that diagram is the inner foil connected to the grid of the valve in the next stage, just part of the circuitry.

As previously said, it often doesn't matter anyway.

[TimMills]

Thanks Techman.
Hey you know what, I'm still unsure what's best, so will do a bit of experimenting when I next have it on the bench.
One side of the cap is presented with a 1 million ohm pot which I have at around 750k ohm when listening on headphones. (that's when the noise of RF is often a problem) . The other side of the cap is presented with 470k resistor to ground off the grid tab.... so maybe the lowest impedance is to the grid of the tube/valve?

Goodnight all. I have to go and put the trash out and check I locked the trunk of my car.
;-)

[Herald1360]

Originally Posted by TimMills

Goodnight all. I have to go and put the trash out and check I locked the trunk of my car.
;-)

[SiriusHardware]

Originally Posted by Techman

By the way, we live in the UK, so it's valves, not tubes.

..And there goes the special relationship.

TimMills, I'm guessing from your location you may be an American serviceman currently marooned in England against your better judgement, although of course you don't have to say.

As far as I am concerned, you are as welcome to use your native terms here as you would be if you were posting from the USA.

I hope you get your amp working.

[Radio Wrangler]

That is a coupling capacitor. It has to be sized to efficiently carry all audible frequencies, and to privide a low impedance at AC, so whichever way round you put it, any difference in noise pick-up will be vanishingly small.

It doesn't really matter.

David

[PaulR]

Originally Posted by TimMills

Goodnight all. I have to go and put the trash out and check I locked the trunk of my car.
;-)

As a matter of interest "trash" is a very English word. Shakespeare used it!

As far as the outer foil of capacitors is concerned I have tried to determine which it is by connecting one lead to an oscilloscope probe and the other to earth,holding the cap, then reversing the connections and I have never been able to find which gives the lower ripple so I would agree that it makes little difference which way round it is connected

[Diabolical Artificer]

"As a matter of interest "trash" is a very English word. Shakespeare used it!" I've just been reading Bill Bryson's book "Made in America" where he discusses word etymology; lot's of word's I was sure were "English" and sometimes got a bit sniffy about, turn out to be American in origin. The exclamation "Gordon Bennet" for instance is a case in point; I thought he was an English footballer!

That aside after following often contradictory advice about which way the foil should go, I found it makes little difference in audio amplifiers. Best way to see if it does make a difference in a particular circuit is to try and measure it Tim and make up your own mind.

Sometimes when I tried measure the difference I was still non the wiser - what's the difference between 5uV and 10uV where noise is concerned in a low gain stage for instance??

Andy.

[Sideband]

Worrying about things like this you start getting into 'audiophool' territory and listening for the faults rather than enjoying the music. Remember also that this amp was made back in the 60's and is unlikely to have the s/n ratio of a modern amp.

[TimMills]

Well, what an interesting thread this has turned out to be!
Thanks again everyone for your input.
Truth of the matter is I love this little valve amp more than a man should love an object.
I've spent the best part of a year firstly bringing it back to life, then subsequently upgrading components here and there.
After it's little trip to the bench today I've settled for having the outer foil running to the grid and it's grid resistor... Why?... because that side of things measured lower DC voltage, which, according to the instructions should be the outer foil end. I've had to ignore the impedance directive for 3 reasons, firstly it's attached to a 1meg pot, thus having variable resistance, which, I think, is closely related to impedance. Secondly; I don't completely understand 'impedance'. Thirdly, as some of you have pointed out, it's going to make ****** all difference to the overall sound.

I'm experiencing a lower noise floor either way round compared to the caps that I first put in during resurrection.

Big love to all you techy gents.
good night one and all.

PS. I'm not a Septic, but have probably picked up the 'tube' thing from Uncle Doug on YouTube. not to mention 1 or 2 facebook groups which use the term. Valves it is!!

Tally Ho!

[GrimJosef]

Originally Posted by TimMills

... One side of the cap is presented with a 1 million ohm pot which I have at around 750k ohm ...

Things are likely to be a bit more complex than that I'm afraid. First, the 'lower' section of the pot will be in parallel with whatever signal source is driving it, so the net impedance will be less (very probably a lot less) than the impedance of that part of the pot. Second, since it's a volume control pot it's likely to have a 'logarithmic' resistance profile. This means that if you rotate it through 75% of its range of travel you won't have 75% of the resistance in the bottom section. The log profile is often approximated by two linear sections. The first of these, up to 50% of the rotation, covers 10% of the resistance and the second, over the remaining 50% of the rotation, covers 90% of the resistance. So at 75% rotation you have the whole of the first section (=100kohm) plus half the upper section (=450kohm) which adds up to 550kohm.

Cheers,

GJ