That Capacitor in a home made superhet

[Neil Purling]

The usual line up: 6K8, 6K7, 6Q7 & 6V6 with Eddystone IFT's.
That Capacitor is a 0.2uf plastic coated by RS/Dubilier.
I know it is probably untrustworthy.
Is there any reason to chose that value?
I have a 0.1uf already, so what'd be the probable effect of using that?

[oldticktock]

2 x 0.1uF in parallel would give the 0.2uF, I try to stick to the original value where I can, 0.22uF would be ok too. However being homemade perhaps that's all the engineer had to hand at the time, so you could be right 0.1 maybe fine

[Steve_P]

0.22uF will be fine. In those days the tolerances were large on capacitors like this.

Restoring home made sets is good fun. No chance of a manual...

Cheers,

Steve P.

[kalee20]

0.22µF does seem very large (although it will work). 0.022µF is more likely. Assuming 470kΩ grid resistor for the 6V6, you'll still get a bass response extending downwards to cutoff frequency of 15Hz.

Down side of really large value here is a physically large component (hence higher capacitance to chassis, hurting treble response although probably not noticeable unless you do hi-fi frequency response measurements); more likelihood of mains variations being fed to the output valve; more likelihood of the output transformer not being able to cope with the low frequencies; ditto with the loudspeaker.

I used massive capacitors like this in a 3-stage amplifier I made when I was 16. It oscillated at about 0.2Hz because the HT decoupling wasn't similarly OTT!

[Sideband]

.2uF does seem very high and as suggested, .02 is much more normal. If you look at most valve radio circuits you will find anything from .01uF to .05uF in this position. If my memory serves correctly, you can get an effect called 'grid blocking' if the coupling capacitor is too high in value. If you get a loud sound or the volume is turned up quickly, you get no sound for a few seconds until the capacitor has discharged and then normal sound resumes....at least I think that's what can happen. It's been 40 years since I did valve amplifier theory!

I don't see why the capacitor has to be such a high value in a domestic radio. 0.1uF is more likely to be used in Hi Fi amps for better bass response (see Mullard 5-10) and I've also repaired a guitar amp where 0.1uF was used.


SB

[Tony1951]

The manuals for both my Philips radios give the value for that capacitor as 4.7n in the 371A and 15n in the 353A which translates as 0.047uF and 0.015uF. The biggest of these is only a thirteenth of the capacity of the .2uF one.

[kalee20]

Originally Posted by Sideband

If my memory serves correctly, you can get an effect called 'grid blocking' if the coupling capacitor is too high in value. If you get a loud sound or the volume is turned up quickly, you get no sound for a few seconds until the capacitor has discharged and then normal sound resumes....at least I think that's what can happen.

Yes, it can happen.

If there is DC on the volume control (and if the volume control is used as the detector load then there will be), then rapidly turning the volume up will couple a negative voltage to the following grid - if the DC blocking capacitor is large enough relative to the speed you can turn the knob. This can cut off the valve briefly. If you turn the knob the other way, the opposite occurs but this is not usually so noticeable because grid current can equalise the capacitor's voltage quicker.

This happens with excessively big capacitor between the volume control wiper and the following grid - Neil's capacitor is actually the next coupling capacitor along. But here, with larger amplitudes, the other effect you mention can happen - a loud sound or interference bang can drive the 6V6 briefly into grid current, the coupling capacitor then charges to a level it shouldn't, and after the pulse the 6V6 will be biased severely back until things have equalised. With a smaller capacitor, it'll still happen but the recovery time is much quicker and it's hardly noticeable (because the human ear's AVC is also recovering following the bang).

[Sideband]

Originally Posted by Tony1951

The manuals for both my Philips radios give the value for that capacitor as 4.7n in the 371A and 15n in the 353A which translates as 0.047uF and 0.015uF.

4.7n=4700pF which is .0047uF not .047uF. One of your zero's disappeared

0.047=47nF.


SB

[Neil Purling]

The grid resistor is indeed a 470K & there's a 10K stopper too.
The volume pot is fed via a 0.01uf capacitor from the diodes in the 6Q7.
The o/p transformer is a RS Midget transformer, which surely is more properly intended for a universal replacement in a valve portable. So there's the other peculiarity in the o/p stage.
Someone took some care in this radio because the HT rail has the usual twin section can, one section being 32uf and there's a choke between them.
I am amazed by how well the radio works. The coils are Denco Red and Blue chassis mount type. The IFT's are by Eddystone, not Gov't Surplus.
The first order of business is the rather tired looking braided mains lead and then That Capacitor. Would you believe that a change down to 0.1uf would actually be beneficial?

[Tony1951]

Originally Posted by Sideband

4.7n=4700pF which is .0047uF not .047uF. One of your zero's disappeared

0.047=47nF.


SB

Ouch!

I am going outside to birch myself for that.

[kalee20]

Not 0.1µF, but even smaller - especially considering the small output transformer. Have a play - I'd recommend 0.022µF but if you have 0.047µF use that.

The capacitor coupling to the volume pot sounds about right. This will actually give some low-frequency roll-off, so there's little chance of distortion in the output transformer due to it being fed frequencies it can't cope with.

As it's a 2-stage amplifier, there won't be any chance of oscillation (motor-boating) via the HT rail. However, mains 'flicker' (slight drops or rises in mains voltage) will get through the HT smoothing and translate into a slight unsteadiness in the rail, and if this is coupled to the 6V6 grid (as it will be via the 6Q7 anode load and 'that' capacitor) you'll get amplified variations of anode current here. I've seen an extreme case (with a big output transformer) where the loudspeaker cone has randomly wandered about its mean position. Good design dictates that the HT smoothing or decoupling is effective to lower frequencies than the intervalve coupling, so that the HT rail can't change rapidly enough to cause this sort of trouble.

[Neil Purling]

0.047uf eh? Well, that's just great. I have got 0.047 @400V Mullard Mustards and I have a selection of Ex Soviet stuff, like 0.01 @750V sealed military spec.
A rake through the capacitor box should produce most items I need in addition to the grid coupler.
Can you believe that the radio actually works fine at present and on it's original rectifier too.

[kalee20]

I'd trust the Mullard Mustard C296 without question. Not quite so sure about the others - use with caution and check for positive grid voltage on the 6V6 (with it removed).

What is the rectifier - is it selenium? I've never had any problem with them.

Post up the results - with a few piccies!

[PJL]

Originally Posted by kalee20

0.22µF does seem very large (although it will work). 0.022µF is more likely. Assuming 470kΩ grid resistor for the 6V6, you'll still get a bass response extending downwards to cutoff frequency of 15Hz.

As Kalee20 says it is the cutoff frequency that is important and not the capacitor. f=1/(2*pi()*R*C) = 100Hz should be fine where C is your capacitor and R is the grid resistor. So C=1/(2*pi()*R*f) or next value up.