25a6 & 25z6 - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 7th Aug 2016, 9:25 pm

Output & rectifier types 25V, 300mA heaters. Are they uncommon or hard to find today?

yestertech · 7th Aug 2016, 10:52 pm

wouldn't have thought so..
I have both listed if youre stuck

Andy

jonnybear · 8th Aug 2016, 12:10 am

If the 25z6 is being used as a half wave rectifier, you could always use the cheap and plentiful PY33 29 VOLT .3 AMP heater and at a push even a PL36 can be used for audio 25 volt .3 amp heater. these are both TV valves often found in old boxes of spare valves for pennies. though 25z6 and 25a6 are quite cheap anyway, 25L6 is another contender for 25A6 though most of these are in the States.
John

Neil Purling · 8th Aug 2016, 6:14 am

A 25L6 was mentioned as a potential substitute, although it does not look like an equivalent.
The output is greater and so is the apparent gain. Although that may not be so if used with the anode load of the 25A6.

What johnnybear said of the PY33 and PL36: Would the PY36 work when being under-run by 4V.
I am also not sure about the PL36 in place of the 25A6. Do either work in real life, rather than comparing data sheets?

Neil Purling · 8th Aug 2016, 8:21 am

I meant PY33 when referring to post #3 by johnnybear.
The under-run looks significant, but the valve would plug in without changes to a position intended for a 25Z6.
As the PL36 is a line o/p valve the high voltage is sensibly led out to a top cap. Adding a fly-lead with a insulated cap would be an easy change.

jonnybear · 8th Aug 2016, 10:22 am

The PY33 would not be under run by 4 volts, the whole heater chain will be but that would not make a lot of difference, as the mains tolerance is 230v + 10%
- 6 %, it is not that critical.
John

Neil Purling · 8th Aug 2016, 8:19 pm

Would a PL36 be alright with 160V on the anode?

Herald1360 · 9th Aug 2016, 1:03 pm

Yes. Max value suggested by Philips is 250V for the anode and the curves in the data go up to that. Just choose a cathode resistor to keep the standing anode current down to less than 40mA. It's resistance may be somewhat greater than the original! Don't forget the joys of the anode top cap, either

ms660 · 10th Aug 2016, 5:14 pm

There's what's described as a new unused 25A6G up for auction as I type.

Hope it's ok to post this.

Lawrence.

Neil Purling · 13th Aug 2016, 11:17 pm

I have obtained a PL36 to substitute for the 25A6. It will work, however.....
be aware that pin #3 of the PL36 is an internal connection to pin #8 (k,g3).
If the 25A6 is in a radio with the o/p transformer nearby you need to remove the wire between pin #3 and the o/p transformer primary and solder a short wire and the top cap connector to the transformer terminal.
Don't have too much stray wire' it doesn't like it.

The PY33 works fine in place of the 25Z6 without any changes being necessary.

7th Aug 2016, 9:25 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	25a6 & 25z6 Output & rectifier types 25V, 300mA heaters. Are they uncommon or hard to find today?

7th Aug 2016, 10:52 pm	#2
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,135	Re: 25a6 & 25z6 wouldn't have thought so.. I have both listed if youre stuck Andy

8th Aug 2016, 6:14 am	#4
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: 25a6 & 25z6 A 25L6 was mentioned as a potential substitute, although it does not look like an equivalent. The output is greater and so is the apparent gain. Although that may not be so if used with the anode load of the 25A6. What johnnybear said of the PY33 and PL36: Would the PY36 work when being under-run by 4V. I am also not sure about the PL36 in place of the 25A6. Do either work in real life, rather than comparing data sheets? Last edited by Neil Purling; 8th Aug 2016 at 6:21 am. Reason: extra text

8th Aug 2016, 8:21 am	#5
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: 25a6 & 25z6 I meant PY33 when referring to post #3 by johnnybear. The under-run looks significant, but the valve would plug in without changes to a position intended for a 25Z6. As the PL36 is a line o/p valve the high voltage is sensibly led out to a top cap. Adding a fly-lead with a insulated cap would be an easy change.

8th Aug 2016, 10:22 am	#6
jonnybear Octode Join Date: Nov 2010 Location: Cullompton, Devon, UK. Posts: 1,435	Re: 25a6 & 25z6 The PY33 would not be under run by 4 volts, the whole heater chain will be but that would not make a lot of difference, as the mains tolerance is 230v + 10% - 6 %, it is not that critical. John

8th Aug 2016, 12:10 am	#3
jonnybear Octode Join Date: Nov 2010 Location: Cullompton, Devon, UK. Posts: 1,435	Re: 25a6 & 25z6 If the 25z6 is being used as a half wave rectifier, you could always use the cheap and plentiful PY33 29 VOLT .3 AMP heater and at a push even a PL36 can be used for audio 25 volt .3 amp heater. these are both TV valves often found in old boxes of spare valves for pennies. though 25z6 and 25a6 are quite cheap anyway, 25L6 is another contender for 25A6 though most of these are in the States. John

8th Aug 2016, 8:19 pm	#7
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: 25a6 & 25z6 Would a PL36 be alright with 160V on the anode?

9th Aug 2016, 1:03 pm	#8
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: 25a6 & 25z6 Yes. Max value suggested by Philips is 250V for the anode and the curves in the data go up to that. Just choose a cathode resistor to keep the standing anode current down to less than 40mA. It's resistance may be somewhat greater than the original! Don't forget the joys of the anode top cap, either __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Aug 2016, 5:14 pm	#9
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: 25a6 & 25z6 There's what's described as a new unused 25A6G up for auction as I type. Hope it's ok to post this. Lawrence.

13th Aug 2016, 11:17 pm	#10
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: 25a6 & 25z6 I have obtained a PL36 to substitute for the 25A6. It will work, however..... be aware that pin #3 of the PL36 is an internal connection to pin #8 (k,g3). If the 25A6 is in a radio with the o/p transformer nearby you need to remove the wire between pin #3 and the o/p transformer primary and solder a short wire and the top cap connector to the transformer terminal. Don't have too much stray wire' it doesn't like it. The PY33 works fine in place of the 25Z6 without any changes being necessary.