GEC BC5445 C13 question

[cernem1alt]

Having read the closed topics on this set forgive me for posting a question here.

Is Capacitor C13 0.04uF an X2 coupler as well as C15 which is "that capacitor?

I am currently musing over one of these as an Autumn project. I have all the usual online circuits but need a bit of help clarifying that Cap.

Thanks Mike

[mister valve]

The Capacitor often referred to as "THAT" Capacitor, is the interstage one coupling the anode of the (usually) Triode amplifier Valve, to the control grid of the output valve; which in this case is C15 which is a 0.01mfd (on "GEC service bulletin 167" service sheet), and should be rated at about 350 VDC or above. Do change this Capacitor if the original one is still in situ, as it is bound to be electrically leaky by now.

I don't quite understand what you mean by "X2 coupler" ? If you mean a class X2 capacitor, then no it does not. Only Capacitors meant for use across the Mains need to be class X1 (or X2).

C13 is the 0.04uF input coupling capacitor to the Triode Amplifier and can be a fairly low voltage capacitor (say) 50VDC or above. If this is also a paper capacitor, I recommend changing this one along with all other Paper Capacitors. If they are not electrically leaky now, they soon will be. I personally always change all Electrolytic Capacitors also during restoration work.

Good luck with your restoration project !

Regards, John.

[cernem1alt]

Thanks John all clarified in my mind now. Yes C13 will go.

[cernem1alt]

I will throw this one open to anyone while on the subject of capacitors on the GEC BC5445 on C18 it is rated at 0.005uF 1000v just wondering could I risk 630v as I have a packet full of 0.0047uF sitting around?

[ms660]

630v is plenty.

Lawrence.

[cernem1alt]

Thank you Lawrence

[cernem1alt]

just found in my spares a new 32+32uF can, the GEC 5445 has 32+16uF, the U78 valve is rated at the 32+16uf I was wondering if it would be happy with the 32 or should I stick to the specs?

[Nuvistor]

The specifications list a 16uf so I would stay with that value, it doesn’t state a max C, just the 16uf.
http://www.r-type.org/pdfs/u78.pdf

[cernem1alt]

Yes I had thought that yet somewhere I think I saw 22uF in use via a cobbled together terminal strip arrangement with a 33uF+22uF individual capacitor arrangement..??

[cernem1alt]

Replacing U78 with a CV4005 KB/ZA would give a 32uF max, according to the spec so would that be a way to go?

[Nuvistor]

Were did you get the specifications from, I couldn’t find them? If you have one you could try but new from Langrex they are £45, be cheaper just buying a capacitor.

[Nuvistor]

I can find that a CV4005 is a ruggedised 6X4, same operating parameters, probably extra mica supports.
The KB/ZA could be different.

[cernem1alt]

CV 4005 links

http://www.r-type.org/exhib/aai0314.htm

http://www.r-type.org/pdfs/cv4005.pdf

MOD designations

K designates a valve manufactured to specification K1001 or K1006; B denotes qualification by a UK authority,

ZA MOV, Gateshead, pre March 1957

so KB/ZA

[Nuvistor]

Thanks for those links, well if you have one or can find one at a reasonable price. If your U78 is good I would just just stick with the original circuit.

[mister valve]

If your 32uF + 32uF Capacitor is in first class condition, I would say that it would be ok to use. I feel sure that the U78 valve will be able to cope with the slight increase in the initial charging current. There should be a slight reduction in the hum level too.

You could put a surge limiting resistor of say 100 ohms in series in the HT line between the U78 Cathode and the Reservoir Capacitor, if you are worried about the Rectifier's ability to cope.


Regarding C18 being 0.005uF @ 1000VDC. There should not be any standing DC voltage there at all. Only audio signal voltage transferred from the previous stage (via DC blocking capacitor C15) should be at that point, so a 630VDC is more than adequate.


I would also recommend that you check R18 (150 ohm) is within tolerance. If the resistance value is less than this, do change it. N78 is a very high slope output pentode which runs very hot indeed. The valve and its electrode construction is really too small to dissipate the heat satisfactorily, so it is usually run very conservatively by those manufacturers that used that valve in their sets. However, if R18 has dropped in resistance value, it will cause the valve to pass excessive current and get even hotter and eventually destroy it. "THAT" Capacitor C15, I spoke about in an earlier post, will also cause the same problem in the N78 valve if it is electrically leaky.

[cernem1alt]

Thank you for that information the 32uF+32uF is brand spanking new, I will check R18 as a matter of course, it was coming out, this is turning into a very interesting and informative project most of the "bits" I have in the component draws so that makes it lighter on the wallet. I presume the Reservoir Capacitor is the 16uF side on the original?

[Nuvistor]

I would reform the 32uf+32uf before using it, the extra load of an unformed reservoir capacitor could damage the rectifier.

[mister valve]

Hi, yes the 16uF would be the Reservoir Capacitor & the 32uF would be the smoothing Capacitor.

Has one or both of the original 16uF + 32uF capacitors in the can failed ? If not, you may be able to reform them and continue to use them.

If it hasn't failed but the set hasn't been used for many years, I do recommend reforming both capacitors and any other electrolytic capacitors that you will be retaining in the set.

[cernem1alt]

I must admit I am presuming the old 16uF+32uF is dead. I know nothing of the radios history or storage so I am completely in the dark on that. Reforming Caps is a thought but is it worth it? All for a quick fix me, As I posted earlier I am leaning towards a 22uF+33uF on a terminal strip I seem to remember reading a 22uF is ok, or was it on YouTube a guy has restored one?

[Mike. Watterson]

The Electrolytics either reform in a few hours or even minutes and are then fine. While the paper dielectric are / were ALWAYS poor unless hermetically sealed, oddly even 1930s Electrolytics can be fine, yet they are the least reliable part on CFL lamp ballasts, LED SMPSUs, setbox/player/console PSUs, phone chargers and motherboards! Ironically they can be running cooler in a valve set than the limited space in a lamp or setbox / charger. Also the 50Hz or 100 Hz ripple is less severe on them than the 17kHz to 1MHz of SMPSUs and electronic Ballasts. Sad it's hard to get florescent tube fitting with the last forever laminated iron choke rather than short life Electronic ballast which also often generates RFI!

I check them / reform with DMM autorange in series with 30V Bench PSU set to current limit at 10mA. If the current drops to under 1mA AND they have enough capacitance (seeing how fast they discharge if you have no meter) then OK.
Make sure set is unplugged / no heater power.

The only cap I've had to regularly replace is the 4uF on the FM discriminater. If in doubt I disconnect cathode side of any cap across the cathode resistor (most common the audio out) and check it. I've replaced those a few times. Actual HT Electrolytic caps quite rarely. Instability on a Battery valve set usually means the sole Electrolytic across the HT battery has dried out (zero capacitance and no leakage).