6P25 screening

[brymac]

I'm restoring a 1950 Murphy which uses a 6P25. The original valve has lost quite a bit of its original silver screening around the bottom third of the valve. The valve tests OK on my Mullard HSVT, but I'm concerned that the set's performance will be affected by the lack of screening on the valve. Should I be, and what can I do about it??

[PJL]

It's an audio output valve so screening probably makes little difference...Peter

[brymac]

Thanks - I was moved to post the question because despite my best efforts (recapping and aligning) this set only really performs well with my huge outdoor aerial. I'm having far less luck with 10 feet of wire! Oh - it's a 1950 Murphy 168 (Trader 1089)

[Hermit6345]

No, I am sure it won't make any difference. The original 6P25s in my A146CM were a bit grotty in that department but it didn't affect the operation of the set. Not sure why they did it or why some versions of the UU6 were metalised either. Perhaps that is something for another thread though.

Ian

[G8UWM-MildMartin]

I agree that it's unlikely to be anything to worry about, since the KT61 didn't have any.
Martin.

[Leon Crampin]

I think the screening on the lower part of some of the Mazda output valves was a carry over from the types with two diodes built in (eg. Pen45DD and similar types). With these types, the diodes were at the bottom of the valve and could be susceptible to hum as these valves have a high slope. The Mullard EBL1 and 31 were similarly partially metallised.

I would not be worried about losing this from an ordinary Pen45; as has been pointed out, most other output valves are non-metallised (as were the later UU6 rectifiers).

Murphy sets of this period usually work well - if all of the components are up to scratch, check the alignment.

Leon.

[brymac]

Thanks - I'm now trying to track down the cause of the HT being low, which may have a lot to do with the poor performance with a short wire aerial! I've got as far as changing the smoothing caps (all the usual wax caps have been replaced) but that hasn't done the trick so I'm going to focus on the other electrolytics next.... Bryan

[Steve_P]

There were 2 versions of this : One for AC Mains and one for AC/DC Mains. Which one have you got?

Anyway...

The 6F25 should have 235v on the anode - measured with an Avo 8.

If V5 is OK, then for the Dropper Resistor version, check R35, C45, The coil between the smoothers (270 Ohms), R20, C44.

If it's the Mains Transformer version, then if V5 is OK, check R24, R20, C25.

Cheers,

Steve P.

[Top Cap]

I have an A168 with its original 6P25, not much metallising left on it but it still works ok. Only other faults encountered were leaky main electrolytic s giving bad hum, short waves cutting out due to low emission 6C9 and a nasty HT short on the waveband/Gram selector switch. The HT is removed from the front end when set to Gram but the wafer was arcing badly to earth so I disconnected this part of the switch so that the radio is hard wired to receive.
Hope this helps
Les

P.S. Extremely good sensitivity on this radio and excellent short wave reception, even on a small piece of wire.

[brymac]

Thanks - the radio is the AC version with a transformer. I'll work through your suggestions! Bryan

[brymac]

Thanks - R20 was very tired! It's fine now. Bryan