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Old 5th Dec 2015, 9:47 am   #1
Super99
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Default Capacitor Use

I need to replace a capacitor that goes from the +ve of a full bridge rectifier straight to earth. It is currently a 300V electrolytic.

Is there any reason I can't or shouldn't replace this with a polypropylene film capacitor?
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Old 5th Dec 2015, 9:59 am   #2
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Default Re: Capacitor Use

If you want the same capacitance, it may be bigger than the equipment it is fitted in.
Do you have a tardis?
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Old 5th Dec 2015, 10:37 am   #3
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Default Re: Capacitor Use

I'm tempted to ask 'why would you want to replace an electrolytic with a polypropylene film capacitor in the first place?'
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Old 5th Dec 2015, 10:38 am   #4
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Default Re: Capacitor Use

Technically, unless it's part of a pi filter with an inductor in it (when it's possible that the ESR of the electrolytic was factored in to the damping) it should be fine. Large can paper or film in oil types are (were?) used for this in high quality equipments.
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Old 5th Dec 2015, 11:24 am   #5
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Default Re: Capacitor Use

Replacing electrolytics with film caps in a PSU is something that some 'audiophiles' do because someone told them that it is a good idea. In my view it simply wastes space and money, and gives no advantage.
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Old 5th Dec 2015, 11:31 am   #6
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Default Re: Capacitor Use

Quote:
Originally Posted by Super99 View Post
I need to replace a capacitor that goes from the +ve of a full bridge rectifier straight to earth. It is currently a 300V electrolytic.

Is there any reason I can't or shouldn't replace this with a polypropylene film capacitor?
You don't say what the capacitance of the original capacitor is. Being a capacitor following a bridge rectifier it will be a smoothing, or more correctly a reservoir, capacitor with a high capacitance value.

The great thing about electrolytic capacitors is that you get a lot of capacitance in a small package. I have some 1uF polypropylene types here and they are huge.
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Old 5th Dec 2015, 7:47 pm   #7
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Default Re: Capacitor Use

The only reason I ask is I'm finding it difficult to find a 2uF (2.2uF) axial electrolytic at a suitable voltage, where as I've found a couple of different film caps.

The size difference isn't that large at this low capacitance.
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Old 5th Dec 2015, 7:59 pm   #8
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Default Re: Capacitor Use

Is that a small 'reservoir' capacitor to make a hybrid between capacitor input and choke input PSU? A film cap will be fine if it will fit.
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Old 5th Dec 2015, 8:21 pm   #9
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Default Re: Capacitor Use

It's always helpful to know the application and circuit details when specifying a capacitor, as this is what decides the suitability of a particular construction or dielectric. Please can you fill us in...?
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Old 5th Dec 2015, 9:55 pm   #10
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Default Re: Capacitor Use

Quote:
Originally Posted by Super99 View Post
The only reason I ask is I'm finding it difficult to find a 2uF (2.2uF) axial electrolytic at a suitable voltage.
http://www.cricklewoodelectronics.com/2U2A450.html
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Old 6th Dec 2015, 2:11 am   #11
Super99
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Default Re: Capacitor Use

Apologies, very new to this still.

I don't think its a choke input as there are no inductors in the circuit, and its not the reservoir or smoothing cap as I've identified those two as 50uf.

I don't actually understand the application of this capacitor unless its part of the smoothing circuit (I'm guessing here), but its part of a Dynatron GR8 record player amp. It just links the + DC wire coming off the rectifier to the chassis earth.

Thanks Station X, I've put my entire order together so far on farnell. But it looks like I may have to purchase that one from cricklewood.
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Old 6th Dec 2015, 2:32 am   #12
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Default Re: Capacitor Use

The only 2.2F in the Dynatron GR8 forms the H.T. decoupling to the pre-amplifier and phase splitter stages together with a 10k resistor.
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Old 6th Dec 2015, 2:48 am   #13
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Default Re: Capacitor Use

That 2(.2) uF capacitor should not connect to rectifier +. The rectifier + should go to a 50uF then via 3.9k to another 50uF and then via 10k to the 2(.2)uF.

Now we know where it is, it is safe to say that it could happily be replaced by the film cap, but equally happily by any electrolytic of adequate working voltage and larger capacitance that would physically fit. In other words, its capacitance isn't critical. 2.2uF was probably the smallest and cheapest value which would give the required performance in the design.
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Old 6th Dec 2015, 5:41 am   #14
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Default Re: Capacitor Use

The theory of valve amps is still very new to me, and as much as I'm trying to learn it is still very difficult for me to pick out which part of a circuit does what just by looking at the drawing.

I understand that anything that comes into contact with audio signal the capacitance is really important because it changes the signal. But is this not the case with the rest? Were power supply capacitors just specified to be as low a value as possible and still do the job? Or if you replaced 50uF smoothing and reservoir caps with 5F would it change anything else?

What about grid coupling capacitors? Do these effect the audio or are these more like power supply and just specced for cost?

Just to confirm, because it is a decoupling capacitor its value isn't as important as others in the circuit? Would 10uF be acceptable? Or is this pushing it too far?

Thanks for the help!

Last edited by Super99; 6th Dec 2015 at 5:55 am.
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Old 6th Dec 2015, 5:56 am   #15
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Default Re: Capacitor Use

Yes, this would be fine! It may give a lower hum level- all to the good.
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Old 6th Dec 2015, 7:36 am   #16
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Default Re: Capacitor Use

Some capacitors are used as energy stores in the power supply. The mains supply is AC, so its voltage is constantly changing. Much of the time it is too low to power the amplifier, so large capacitors are used to take extra current when the voltage is high, and to give out current when the AC voltage is too low.

This means that the reservoir capacitor voltage varies, with a quick charge, and then a slower decay, repeated 100 times per second. The greater the capacitance of the reservoir, the smaller the voltage variation, which is good. But a larger capacitor demands larger charging current (for a shorter time) from the rectifier. So you can increase the reservoir capacitance, but you need to limit this by considering the peak current rating of rectifiers and transformers.

The variation of reservoir voltage is still too much for most radio/audio applications, so either a choke or a resistor is used to feed a second stage of storage capacitor, usually called the smoothing capacitor. Splitting the storage capacitor this way is more effective than just having one bigger capacitor.

Preamplifier stages are much more sensitive to noise and hum on their power supplies, and they usually need a lower voltage, so you will find that they are usually powered via a dropper resistor, and another capacitor.

The smoothing capacitors can be increased to give less hum, and they don't stress the rectifier, so the limitations are cost and physical size.

Leaving the power supply and looking at the amplifier, some capacitors are used to couple signals from one stage to another... well a piece of wire could do that, but the point is to block DC voltages where the DC level of the output of one stage is different to the DC level needed at the input of the next stage. If the coupling capacitor is too small, you lose some bass response. If it is increased, there probably isn't much perceptible difference. If it is too large, you may get more rumble from the turntable coming through the speaker, and you may get more thump on turn on in transistor amplifiers.

There are also capacitors in tone control circuits and in equalisers used to correct for record and cartridge characteristics. These do need to be the right value, and their value directly affects the sound.

In radios there are capacitors used in the circuits which select the station to be heard and which filter out adjacent stations. These are in the RF, mixer and IF stages, their values are critical.

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Old 6th Dec 2015, 8:08 am   #17
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Default Re: Capacitor Use

Cheers David! That helps a lot!
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Old 6th Dec 2015, 10:33 am   #18
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Default Re: Capacitor Use

Quote:
Originally Posted by ukcol View Post
The only 2.2F in the Dynatron GR8 forms the H.T. decoupling to the pre-amplifier and phase splitter stages together with a 10k resistor.
Agreed. As Herald 1360 says in the following post, that cap isn't connected to rectifier positive, but in his first post the OP states that it is.

Has someone "had a go" at this player, or is the OP mistaken?
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Old 6th Dec 2015, 9:08 pm   #19
Super99
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Default Re: Capacitor Use

No I am mistaken, it does go through all the rest. I've just misinterpreted the circuit diagram.
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Old 9th Dec 2015, 6:40 pm   #20
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Default Re: Capacitor Use

I restored a Dynatron GR8 in mid 2014 and had to replace all the electrolytics, not just C4 - the 2.2 uF one, as they all had very high ESRs and were clearly duff on a capacitance meter.

I also had to replace ten of the fifteen resistors as they were were more than 25% high - some very much higher. I had to replace the volume control as the mains switch was open circuit. The ones sold by Bowood electronics were identical in size and with the flat on the shaft in the right place, albeit the new one had a plastic shaft whereas the original had a metal shaft.

C4 is on the top side of the chassis - most of the Cs and Rs are beneath the chassis. The contact cooled rectifier still functioned fine.

Unimpressed by the flimsy solid core inter-wiring, and no grommets where wires passed through holes.

Works well enough, and at least it has a proper mains transformer - not live chassis.

First pic shows the electrolytics (& one Hunt's mouldseal) and the resistors that I had to replace. Second pic in the original volume control with the O/C mains switch, third pic is the replacement pot - same size and style of body but with plastic shaft). Last pic is the underside of the chassis after restoration.

(I won't mention the fun and games with the Garrard AT6 turntable - not relevant to the capacitor issue in this thread).
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