Audio Coupling Cap - value critical?? - UK Vintage Radio Repair and Restoration Discussion Forum

TNC · 22nd Aug 2007, 4:37 pm

Just checking over a friend's HMV 1117 - it's clearly been worked on and to a very reasonable standard - but I have noticed that the audio coupling cap. to the KT61's grid is 0.1 mf whereas the circuit specifies .047 mf.
Just to save the bother of turning on the iron(!!) and knowing how tolerant valve radios are, will it really make a great deal of difference...or should I swap it out?

Thanks in advance,
Trevor.

paulsherwin · 22nd Aug 2007, 4:45 pm

It normally makes very little difference, and if the radio is working well I'd leave the larger cap in place.

Too small a value will lose some bass, and too large a value can cause odd circuit damping effects, but in general the value isn't at all critical. In fact you could probably replace all the waxies in a radio apart from the tone control components with 0.1uF caps and not notice any difference.

Paul

Steve_P · 22nd Aug 2007, 5:36 pm

I'll second that, Paul. In the good old days, capacitor values varied greatly. E.G. a 0.05uF capacitor can be anything from 0.03 to 0.1 !!

Leave it, if it's not leaking any volts, it'll be fine.

Cheers,

Steve P.

TNC · 22nd Aug 2007, 11:47 pm

Thanks for your thoughts - I'll leave well alone!

Trevor.

quantum · 24th Aug 2007, 4:59 pm

As a general measure, when I'm replacing an audio coupling cap if the grid resistor for the output valve is 470k or above, I'll only replace the cap with a maximum value of 0.05uf, regardless of what the circuit diagram says. I've come across too many instances of 'grid blocking' by high value caps at decent volume levels and as previous posters have stated, the loss of any bass is minimal anyway. I'll use 0.1uf caps only if the grid resistor is 220-270k or so.

22nd Aug 2007, 4:37 pm	#1
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Audio Coupling Cap - value critical?? Just checking over a friend's HMV 1117 - it's clearly been worked on and to a very reasonable standard - but I have noticed that the audio coupling cap. to the KT61's grid is 0.1 mf whereas the circuit specifies .047 mf. Just to save the bother of turning on the iron(!!) and knowing how tolerant valve radios are, will it really make a great deal of difference...or should I swap it out? Thanks in advance, Trevor.

22nd Aug 2007, 4:45 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: Audio Coupling Cap - value critical?? It normally makes very little difference, and if the radio is working well I'd leave the larger cap in place. Too small a value will lose some bass, and too large a value can cause odd circuit damping effects, but in general the value isn't at all critical. In fact you could probably replace all the waxies in a radio apart from the tone control components with 0.1uF caps and not notice any difference. Paul

22nd Aug 2007, 5:36 pm	#3
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Audio Coupling Cap - value critical?? I'll second that, Paul. In the good old days, capacitor values varied greatly. E.G. a 0.05uF capacitor can be anything from 0.03 to 0.1 !! Leave it, if it's not leaking any volts, it'll be fine. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

22nd Aug 2007, 11:47 pm	#4
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Audio Coupling Cap - value critical?? Thanks for your thoughts - I'll leave well alone! Trevor.

24th Aug 2007, 4:59 pm	#5
quantum Retired Dormant Member Join Date: Mar 2003 Location: Birmingham, West Midlands, UK. Posts: 785	Re: Audio Coupling Cap - value critical?? As a general measure, when I'm replacing an audio coupling cap if the grid resistor for the output valve is 470k or above, I'll only replace the cap with a maximum value of 0.05uf, regardless of what the circuit diagram says. I've come across too many instances of 'grid blocking' by high value caps at decent volume levels and as previous posters have stated, the loss of any bass is minimal anyway. I'll use 0.1uf caps only if the grid resistor is 220-270k or so.