Custom HV Capacitor Replacement Advice

[Latin Luke]

Hi everyone.

I require a capacitor on a 6C4 rectifier valve, it currently has a 500nF vintage wax capacitor rated at 1000VDC. According to the schematic it will see -680VDC.
My question has 2 parts-

A. As no direct replacement is likely, will it be fine to use a 470nf capacitance in its place but of a polypropylene type.

B. Achieving the desired 1000VDC required with the space restrictions I have within the chassis, I wonder if using 4x 470nF 630V. Fitted two in series / two in parallel is allowable?

Thanks in advance, Luke.

[Terry_VK5TM]

Well, you could do it that way or just buy a 470nF 1000V polypropylene type.

Although you possibly won't get an axial type, radial ones are plentiful (Wima, Wurth are a couple of brands that come to mind).

Just make sure to extend the capacitor lead connected to earth/ground rather than the one carrying the high voltage.

[TrevorG3VLF]

Do not use capacirors in series to get high voltage capability, they will need voltage sharing resistors which need to have high resistance and voltage capability and waste power.

[David G4EBT]

CPC/Farnell can supply single Brtish made 1,000V DC 0.47uF axial polypropylene capacitors at £5.88 each inc VAT:

https://cpc.farnell.com/icw-industri...xoCDEYQAvD_BwE

(From the datasheet, the physical size seems rather large at 46mm long x 25mm diam).

Hope that might help.

[Herald1360]

What's a 6C4 rectifier when it's at home? Something Russian? 6C4 is normally a B7G medium mu triode.


0.5uF sounds rather small for rectifier related goings on, too. What is it doing in the circuit?

[julie_m]

6C4 could be a typo for 6X4, a.k.a. EZ90.

Go on, let's see a snippet of the circuit diagram!

[AC/HL]

https://reverb.com/uk/item/1963830-g...rectifier-tube

[Sideband]

Originally Posted by Herald1360

0.5uF sounds rather small for rectifier related goings on, too. What is it doing in the circuit?

I suspect it's RF bypass across the mains rather than anything to do with the rectifier.....In which case an X type component may be more appropriate.

Alternatively it might be just a decoupler across the main HT supply.......yes, lets see a circuit so we can be sure.

[Latin Luke]

Thank you for all the helpful advice,
And sorry for the vagueness in what I’m actually trying to do.
Basically I have an old oscilloscope dated around the early 50’s, it is extremely compact and I’m trying to renew the capacitors. Whilst replacing the caps I’m attempting to move them to one side of the chassis, leaving space for a few modifications.
The 6C4 valve is being used as a negative rectifier, the filament section is connected to the cathode to one end of the secondary winding, and puts out -680v to the cathode of the CRT.
I have been told that this use of the 6C4 valve was common in older oscilloscope builds. Section of schematic included to hopefully better explain its use.

[trobbins]

Moving capacitor locations will alter the loop area of the charging pulses for that half-wave rectifier, and may increase capacitive coupling to other sensitive areas.

A 330nF 1kV MKP10 (in my spares) is 30x25x15mm, so how does that size compare to the original wax capacitor? I also have S&M 0.47uF 1kV poly caps (P654 series but I don't have a datsheet) from UPS applications that are 30x24x12mm, so a bit more appropriate.

[Radio Wrangler]

The schematic is labelled 6C4 and shows a triode with zero grid/cathode bias, so that would work as a diode.

I never knew the little 6C4 (half an ECC82 I seem to recall) could do high voltage rectification.

I think the schematic pre-dates a guitar valve company using that designation on something quite unrelated. A stupidly confusing thing to do. They either don't know or don't care that they've hit on a number of a well-known pre-existing part.

David