Replacement EL33 - UK Vintage Radio Repair and Restoration Discussion Forum

coopzone · 7th Mar 2019, 3:01 pm

Hello,

Can i get away with replacing an EL33 with a 6p25 valve. I can see there are some differences but they look fairly close to each other.

Derek

Herald1360 · 7th Mar 2019, 3:18 pm

Yes....

I'd be inclined to up the cathode resistor a bit to wind back the anode current to 35mA or so and check that the mains TX isn't bothered by another 0.2A heater current.

paulsherwin · 7th Mar 2019, 3:32 pm

Agreed.

You can usually swap these different pin compatible output valves without problems. You just need to keep an eye on the anode current, adjusting the cathode resistor appropriately, and make sure the heater isn't drawing too much current from the mains transformer.

American valves like the 6V6 generally have a lower slope than European types, but they will typically work well enough in a standard superhet. The 6P25 is European of course, despite the American sounding name.

coopzone · 7th Mar 2019, 3:36 pm

Thanks, that's about what i thought and yes already got a new cathode resistor ready. just thought i would ask. Better safe than sorry!

G6Tanuki · 7th Mar 2019, 5:15 pm

In the past I've replaced an EL33 with a N78, building an adapter using the base off the dead EL33 and a B7G valvebase supported on stiff wires soldered into the pins of the old EL33 base then filled around the wires with car body-filler.

Done with care - adding some dye to the filler so it's black to match the old EL33 base - it can look rather neat.

Refugee · 8th Mar 2019, 12:19 am

I built an adapter using stiff wire and a slice of an old ski pole that I found.
It is made of some kind of composite so a small bead of epoxy was all that was needed to fit it to one of those octal plugs that can be bought in quantity from China.

Silicon · 8th Mar 2019, 9:35 pm

If you make a valve base adaptor you could use an EL84 or EL41.

Mike. Watterson · 9th Mar 2019, 12:50 pm

A 6L6 works. Note the datasheet shows it at much higher SG & Anode voltages than the EL33. It's actually similar cathode current in an EL33 circuit. Screen voltage as well as control grid sets cathode current.
Obviously you'd only do that if you had one spare.

Heatercathodeshort · 9th Mar 2019, 5:32 pm

The Mazda 6P25, MOV KT61 and EL33 valves are all high gm valves that are very similar maybe only requiring a slight alteration to the cathode resistor.

The 6V6 and KT63 types are much lower gm and will give a noticeable reduction in output
particularly when used in circuits that do not employ a driver stage such as the EBC33 and similar valves. All very robust valves that rarely fail. John.

7th Mar 2019, 3:01 pm	#1
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Replacement EL33 Hello, Can i get away with replacing an EL33 with a 6p25 valve. I can see there are some differences but they look fairly close to each other. Derek

7th Mar 2019, 3:18 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Replacement EL33 Yes.... I'd be inclined to up the cathode resistor a bit to wind back the anode current to 35mA or so and check that the mains TX isn't bothered by another 0.2A heater current. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

7th Mar 2019, 3:32 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Replacement EL33 Agreed. You can usually swap these different pin compatible output valves without problems. You just need to keep an eye on the anode current, adjusting the cathode resistor appropriately, and make sure the heater isn't drawing too much current from the mains transformer. American valves like the 6V6 generally have a lower slope than European types, but they will typically work well enough in a standard superhet. The 6P25 is European of course, despite the American sounding name.

7th Mar 2019, 3:36 pm	#4
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Replacement EL33 Thanks, that's about what i thought and yes already got a new cathode resistor ready. just thought i would ask. Better safe than sorry!

7th Mar 2019, 5:15 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Replacement EL33 In the past I've replaced an EL33 with a N78, building an adapter using the base off the dead EL33 and a B7G valvebase supported on stiff wires soldered into the pins of the old EL33 base then filled around the wires with car body-filler. Done with care - adding some dye to the filler so it's black to match the old EL33 base - it can look rather neat.

8th Mar 2019, 12:19 am	#6
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Replacement EL33 I built an adapter using stiff wire and a slice of an old ski pole that I found. It is made of some kind of composite so a small bead of epoxy was all that was needed to fit it to one of those octal plugs that can be bought in quantity from China.

8th Mar 2019, 9:35 pm	#7
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,167	Re: Replacement EL33 If you make a valve base adaptor you could use an EL84 or EL41.

9th Mar 2019, 12:50 pm	#8
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Replacement EL33 A 6L6 works. Note the datasheet shows it at much higher SG & Anode voltages than the EL33. It's actually similar cathode current in an EL33 circuit. Screen voltage as well as control grid sets cathode current. Obviously you'd only do that if you had one spare.

9th Mar 2019, 5:32 pm	#9
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Replacement EL33 The Mazda 6P25, MOV KT61 and EL33 valves are all high gm valves that are very similar maybe only requiring a slight alteration to the cathode resistor. The 6V6 and KT63 types are much lower gm and will give a noticeable reduction in output particularly when used in circuits that do not employ a driver stage such as the EBC33 and similar valves. All very robust valves that rarely fail. John.