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ms660 · 13th Jan 2017, 9:25 pm

"Meantime hope you will indulge me as I embark on some circuit tracing and that someone can offer some advice on the attached picture. Hopefully this shows the V5 (Mullard CV 31) valve base viewed from underneath the chassis - the pencil tip, I think, is pointing at Pin 8 which is the Cathode? If this is correct then the + ve end of the new electrolytic capacitor 47 μF, 450 V is connected to the Cathode while the - ve end is connected to the chassis (more on that connection later). Is this a replacement for C29 which should be 32 μF, 275 V??"

"I can also see that Pin 5, the Anode, has connections as do Pins 2 and 7 for the Heater. I do, however, note that there are connections to Pins 1 and 3 which according to my research for the CV 31 are not used?? This is what confuses us beginners!"

The 47uF, if connected to pin 8 of the rectifier will be a replacement for the original 32uF reservoir capacitor, the fact that it's rated voltage is higher is no problem, the fact that its value is higher would probably be no problem either, if you look at the CY31's data sheet it gives the maximum reservoir capacitance for an anode voltage of 250 volts fed from a feed resistance of 125 ohms, in your receiver the anodes feed resistance is given as 150 ohms, any series resistance provided by the transformer can be added to that 150 ohms, that coupled with the fact that the rectifiers anode voltage should be 225 volts when the mains voltage selector is set correctly would probably offset the effects of that extra capacitance.

CY31 data sheets:

http://www.r-type.org/pdfs/cy31.pdf

Regarding the extra connections to the CY31's valve socket, unused connections are often used as extra connection points for other parts of the circuit so that's not unusual, but be aware, then can sometimes be a rake in the long grass, typical example is the old WW2 R1155 aircraft receiver, a particular valve in that one did not have an externally painted metalized screen but one of the common substitutes did, unfortunately that metalized screen was connect to a pin on the valve base which corresponded to one of the valve holder tags that was being used as an anchor point for some of the HT related circuit, if the power was on then grabbing the valve with one hand and the chassis with the other resulted in a shock...the rake had sprung.

Lawrence.

13th Jan 2017, 9:25 pm	#43
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush A.C.91 "Meantime hope you will indulge me as I embark on some circuit tracing and that someone can offer some advice on the attached picture. Hopefully this shows the V5 (Mullard CV 31) valve base viewed from underneath the chassis - the pencil tip, I think, is pointing at Pin 8 which is the Cathode? If this is correct then the + ve end of the new electrolytic capacitor 47 μF, 450 V is connected to the Cathode while the - ve end is connected to the chassis (more on that connection later). Is this a replacement for C29 which should be 32 μF, 275 V??" "I can also see that Pin 5, the Anode, has connections as do Pins 2 and 7 for the Heater. I do, however, note that there are connections to Pins 1 and 3 which according to my research for the CV 31 are not used?? This is what confuses us beginners!" The 47uF, if connected to pin 8 of the rectifier will be a replacement for the original 32uF reservoir capacitor, the fact that it's rated voltage is higher is no problem, the fact that its value is higher would probably be no problem either, if you look at the CY31's data sheet it gives the maximum reservoir capacitance for an anode voltage of 250 volts fed from a feed resistance of 125 ohms, in your receiver the anodes feed resistance is given as 150 ohms, any series resistance provided by the transformer can be added to that 150 ohms, that coupled with the fact that the rectifiers anode voltage should be 225 volts when the mains voltage selector is set correctly would probably offset the effects of that extra capacitance. CY31 data sheets: http://www.r-type.org/pdfs/cy31.pdf Regarding the extra connections to the CY31's valve socket, unused connections are often used as extra connection points for other parts of the circuit so that's not unusual, but be aware, then can sometimes be a rake in the long grass, typical example is the old WW2 R1155 aircraft receiver, a particular valve in that one did not have an externally painted metalized screen but one of the common substitutes did, unfortunately that metalized screen was connect to a pin on the valve base which corresponded to one of the valve holder tags that was being used as an anchor point for some of the HT related circuit, if the power was on then grabbing the valve with one hand and the chassis with the other resulted in a shock...the rake had sprung. Lawrence.