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Radio 6A3 in place of PX4

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freya
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The PX4 in my daily radio has given up this morning with the heater hanging in bits, a replacement is going to be silly money so my question is can one of the many 6A3's in my box be used in place,  can I the 6volt filament be run from a rectifier / capacitor from the 4 volt supply or is this suitable for directly heated valves.

thanks

Stephen 

 
Posted : 12/12/2023 10:17 am
Till Eulenspiegel
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Replacing the PX4.  How about considering the Mullard PM24M directly heated pentode connected as a triode?  PM24M has the B5 base and has a four volt filament.

PM 24M, Tube PM24M; Röhre PM 24M ID3053, Vacuum Pentode | Radiomuseum.org

Till Eulenspiegel.

 
Posted : 12/12/2023 12:55 pm
freya
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Its a good idea Till, was just thinking on how to use up these 6A3`s i have in seeming abundance rather than buying a PM24M

 
Posted : 13/12/2023 2:00 pm
Till Eulenspiegel
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Info about the 6A3:  6A3, Tube 6A3; Röhre 6A3 ID2686, Triode, vacuum | Radiomuseum.org

The PX4: PX 4, Tube PX4; Röhre PX 4 ID2787, Triode, vacuum | Radiomuseum.org

From the Continent, the AD1: AD 1, Tube AD1; Röhre AD 1 ID211, Triode, vacuum | Radiomuseum.org

I mentioned the PM24M as a replacement for the PX4 because I considered using the Mullard directly heated pentode in the Beau Decca mirror lid TV set.

Till Eulenspiegel.

 
Posted : 13/12/2023 3:04 pm
Cathovisor
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@till AD1s fetch as outrageous prices as the PX-valves in the UK so I wouldn't suggest them. Just ask any Telefunken/Koerting owner!

 
Posted : 13/12/2023 7:07 pm
sideband
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Surely the PM24M is tiddly compared to the PX4? I've got a PM24M in my Superinductance radio and it's only good for a couple of watts at the most.

 
Posted : 13/12/2023 9:17 pm
Cathovisor
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Posted by: @sideband

Surely the PM24M is tiddly compared to the PX4? I've got a PM24M in my Superinductance radio and it's only good for a couple of watts at the most.

The original PX4 wasn't that powerful: the first AC/PEN was only rated for about one watt.

The PM24M has the same heater power (4W) but a slightly more modest anode dissipation of 9.5W against the 12W of the PX4. The Cossor 2P (the wartime "ten watt" valve) certainly makes a good noise in my Cossor sets fitted with it!

 

 
Posted : 13/12/2023 10:59 pm
freya
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Thanks for all the ideas,

I managed to wind a 6 volt heater onto the existing transformer to use a 6A3 this was far easier than to do than i first thought. It does work but it needs the Hum cancellation pot adding, which i dont have time to do before christmas.

As a temporary solution i have swapped the everyday radio to the GEC All wave 8, another real favortite set of mine.

 
Posted : 16/12/2023 10:57 am
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Cathovisor
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Posted by: @freya

As a temporary solution i have swapped the everyday radio to the GEC All wave 8, another real favortite set of mine.

Glad to see you've got that rather splendid GEC running - I have a few regrets selling mine but truthfully I needed the space (and the money) more. I'm trying - not very successfully - to disperse the bulk of my collection now.

I do have another 8-valve GEC, but it's the later model (and buried in the shed) - the BC3890.

 
Posted : 16/12/2023 11:01 am
freya
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@cathovisor 

That 3890 is in my top 3 sets to try and find, its doubtful i could afford it knowing its pedigree though.

 

Stephen

 
Posted : 16/12/2023 5:46 pm
Cathovisor
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@freya You have a PM.

 
Posted : 16/12/2023 8:43 pm
Boater Sam
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The PM24M would seem to be a good sub but the ra is incredible at 43kΩ. Against the PX4 with ra 830Ω, possible output transformer difficulties?

The heater is OK at 4v and the base is the same.

Boater Sam

 
Posted : 14/02/2024 4:02 pm
Till Eulenspiegel
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Have no figures for a triode connected PM24M but the ra will be be much less than 43kohms, the figure for the valve is operating as a pentode. 

An example: the Mullard EL37 ra is 13.7Kohms @ Va 300V, but when triode-connected the ra is 1.4Kohms @ Va 300Volts.

Till Eulenspeigel.

 
Posted : 15/02/2024 1:49 pm
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Boater Sam
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Posted by: @till

Have no figures for a triode connected PM24M but the ra will be be much less than 43kohms, the figure for the valve is operating as a pentode. 

An example: the Mullard EL37 ra is 13.7Kohms @ Va 300V, but when triode-connected the ra is 1.4Kohms @ Va 300Volts.

Till Eulenspeigel.

Which raises the question as to how one would measure the ra of a PM24M valve connected as a triode.

This is a point that I had not considered.

Indeed, which of the valves that could be used in triode mode, pentodes and beam tetrodes such as 6V6, EL84, EL34 etc. would then have an ra approaching that of a PX4?

 

Boater Sam

 
Posted : 15/02/2024 2:08 pm
Till Eulenspiegel
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The thing is it is just a radio receiver. Why not just take a chance and fit the PM24M in lieu of the PX4? If doesn't sound too good it will be an easy matter to return the circuit to it's original state. It's that or pay daft money for a PX4.

The valve holder might be made for a four pin valve but if a 6A3 is to be fitted the socket will need replacement anyway, unless an adaptor is made.

Till Eulenspiegel.

 
Posted : 15/02/2024 4:44 pm
Boater Sam
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I would think about implanting a modern miniature valve inside a failed PX4. Once the base is off it is easy to remove the pinch and the internals.

Then a new valve, say EL84, secreted inside the envelope with connections into the original base. No one would know!

Getting the 6v for the heater is not a problem, a tiny autotransformer or rectifier and capacitor or even adding a winding onto the mains transformer.

Boater Sam

 
Posted : 15/02/2024 7:14 pm
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