Triode for 4XP replacement

[boater sam]

Old sets that had 4XP, 2P or PX4 output valves are almost invariable lacking these valves when I get hold of them, the valve robbers have been there.

I am thinking of making up plug-in replacements using possibly a 6V6 valve on a B4 base with a purpose wound 4v to 6.3v transformer  to feed the heater.

Any comments or advice on this idea with a pentode or beam tetrode valve connected as a triode?

[Cobaltblue]

I have had reasonable success on Cossor sets using a 6V6 triode connected.

I haven't formalised the solution yet as I have been busy the last couple of years and still haven't finished my house rebuild.

I put the 2V supply for the 2P or 2XP in series with the 4V supply to the other valves in the set.

This isn't ideal as it increases the load on the 4V winding but in practice it doesn't seem to overheat the transformer as there is a lower load on the 2V winding.

In many sets there will be a humdinger that you may need to disconnect.

Didn't find the adaptor alone worked for me as I would otherwise have to make too many changes. Ideally you would like a solution where you just plug in a 2P when one turns up.

So I had a couple of trailing wires.

Be pleased to hear how you get on with this Sam.

Cheers

Mike T

[boater sam]

I have had reasonable success on Cossor sets using a 6V6 triode connected.

I haven't formalised the solution yet as I have been busy the last couple of years and still haven't finished my house rebuild.

I put the 2V supply for the 2P or 2XP in series with the 4V supply to the other valves in the set.

This isn't ideal as it increases the load on the 4V winding but in practice it doesn't seem to overheat the transformer as there is a lower load on the 2V winding.

In many sets there will be a humdinger that you may need to disconnect.

Didn't find the adaptor alone worked for me as I would otherwise have to make too many changes. Ideally you would like a solution where you just plug in a 2P when one turns up.

So I had a couple of trailing wires.

Be pleased to hear how you get on with this Sam.

Cheers

Mike T

I am pleased to know that someone else has had the same thought.

How did you connect the 6V6 as a triode?  G2 to Anode?  Any parasitic stopper resistors?
The beam plates are already internally connected to the Cathode.

 Any thoughts on the 4v to 6.3v transformer? I have a winder, I would hope to make it small enough to fit in an adaptor base.

Sam.

[Mike Watterson]

How did you connect the 6V6 as a triode?  G2 to Anode?  Any parasitic stopper resistors?
The beam plates are already internally connected to the Cathode.

The main reason to connect g3 as well as g2 to triodize a pentode with wound g3 is HF/VHF capacitance. A secondary reason is if it's a spiral wound grid. So EF80 or DF97 at VHF will have g3 and g2 tied to anode to work as a self-oscillating mixer. It's not a problem and possibly better that beam Tetrodes (Philips called their Kinkless Tetrodes aka Beam Tetrodes, pentodes) have "g3" to cathode or 0V. Because a Rod Pentode has no spiral grids, they too have g3 to cathode or 0V. They are staggeringly low capacitance compared to 1L6, DF97 or EF80, so they actually work with g3 at 0V triodised.

Elsewhere I discussed the 1p24b as a possible PX4 and similar replacement. The filament can be series and will work at 2V @ 110mA (but decouple centre tap). Or you can use ballast resistor for 4V supply to give 2.2V, or have two in parallel with filaments in series for 4V @ 110mA. You can put three in parallel and ballast resistors for 2.2V max. If the 4V is centre tapped or otherwise center referenced (like a pot to 0V), then 1/2 the ballast resistor at each end. The data sheet specifically discusses triode mode and power.

I do think 6V6 or similar triodized is a good idea if you can find 6V. The grid bias should be considered. Any KT or beam tetrode is fine with g3 to cathode when g2 is to anode. c.f. UL windings on EL84 etc.

 
Any thoughts on the 4v to 6.3v transformer? I have a winder, I would hope to make it small enough to fit in an adaptor base.
Sam.

It's 450mA. One idea might be a 4V to 2V (6V is enough) transformer and put that in series with the 4V then it only needs to manage about 0.9W rather than 2.8W, but still might not fit.

[ppppenguin]

Remember that it only needs to be an autotransformer. I think Ed Dinning has designed these and they're pretty small.

[Mike Watterson]

Remember that it only needs to be an autotransformer. I think Ed Dinning has designed these and they're pretty small.

Yes, of course and still only 0.9W instead of the 2.8W if it was a regular 6V transformer

[Cobaltblue]

I am pleased to know that someone else has had the same thought.

How did you connect the 6V6 as a triode?  G2 to Anode?  Any parasitic stopper resistors?
The beam plates are already internally connected to the Cathode.

 Any thoughts on the 4v to 6.3v transformer? I have a winder, I would hope to make it small enough to fit in an adaptor base.

Sam.

Sorry that was a bit lazy of me to describe it as triode connected.

Just G2 to anode.

I didn't need any additional stoppers in my particular case.

There was a bit of a mismatch ISTR with the output transformer but I still had adequate output.

The existing cathode resistor caused the 6V6 to run a lower anode current as well ISTR you need to be careful of that as sometimes the Cathode bias arrangement is also used for biasing other stages and AGC delay.

One day I will get around to making measurements.

This was all prior to 2018 I claim no originality for the idea I bought a set at Harpy with a 6V6 already installed but a proper job with the socket replaced and the biasing sorted it worked quite well.

Cheers

Mike T

[Mike Watterson]

Also the 6AQ5 = EL90 is essentially the same spec as a 6V6, but B7G base. Even same heater current of 450mA yet allegedly 4.5W. I had one German AM/FM set with the EL90 and it ran pretty hot!

The 6M6, EL11, EL3, KT61 and EL33 (various bases) are all similar to each other, about 4.5W but 900mA heater. Possibly due to size you can run a bit more power than the EL90/6V6.

The EL41 (Rimlock) and EL80 (Noval) are roughly similar to each other and 710mA heater, but about 3.9W

The EL42 (Rimlock) and EL95 (Noval) are roughly similar to each other and lower power, only 200mA heater, so about 2W

Any of these will work in a triode circuit with g2 to anode. Check bias for the HT volts on g2. I think all are really Beam tetrodes, so the "plates" are tied to cathode.

[Jez]

6BW6, KT63.... many options. FWIW there was article in Electronics World about making up a replacement output valve using a mosfet on an octal base.... 

I'll get my coat

[Geordie McBoyne]

My Beau Decca mirror-lid TV set employs a pair of PX4 triodes in the audio amplifier. Have considered experimenting with triode connected Mullard PM24M directly heated pentodes. PM24M has the five pin B5 base.

Geordie McBoyne.