P-Series Valves, what to do with them?

[ppppenguin]

P series valves were used extensively in TVs. All the heaters were wired in series so you add up all the voltages. Then use a big resistor to drop the difference between that and the mains. Same idea as using U series valves in radios.

A number of valves, such as EF80, are 6.3V 300mA heaters. So you can use them in series chains at 300mA or in parallel at 6.3V. The heater is just a resistance, albeit one that varies a bit with temperature. So it doesn't matter whether you think of it as 300mA heater or a 15V (or whatever) heater. If you have a number of 300mA 15V heater valves you can run them in parallel from a 15V supply.

For the valve audio folk, take a bunch of PL519. 300mA, 40V heaters. Bung 6 in series and connect direct to mains. No problem apart perhaps from safety. Or use a 40V 2.4A transformer. Makes no real difference to the valves.

You can do anything with an EF80 (or EF91). People have built radios using only these valves. You can just about do a small TV though line scan is a bit tough.

[Murphyv310]

Hi.
as Jeffrey says you just add up the heaters and use a suitable resistor to drop the difference between mains and the valves. Ideally the 300ma heater current should be as near as possible but Mullard and the other valve designers build in a bit of tolerance, around 7-10% dependent on the actual valve. "P" valves also in the main have better Heater to Cathode insulation too so are less likely to blow if there is 100v or more between them, a bit like an EZ80 that can have 250v between cathode and heater. The PY33 half wave rectifier valve also has exceptional heater to cathode insulation as do Boost diodes and line output valves as these are always the first valves in a heater chain. Just think of these valves wired like a set of old Christmas lights.
Again I agree with Jeffrey, the EF80 has a multitude of uses, in at least one TV it was used as an Audio output, it is obviously good at RF and as an oscillator, it's got countless uses and quiet enough for a magnetic cartridge pre-amp.

[ppppenguin]

Come the apocalypse, the cockraoches will be using EF80s to build their kit The EF80 is a tough critter.

[Terry]

I've always been uncertain what the 300mA means as regards a P type valve - 300mA max? And how the Dickens would you regulate a series chain? A bit higher mains and current goes up no?

Andy, as you say, if the mains voltage goes up, so does the current but only by the same percentage as a 6.3V transformer fed valve.

Is the 300mA figure a maximum figure? Is 6.3v for an EL34 a maximum figure? In the same way that all the valves in a parallel fed design pass different currents to suit the valve's purpose, all the valves in a series chain have different voltages but the current is 300mA for all of them, it is as simple as that.

Apart from any increase in heater power that might be needed in a series chain because if the heat insulating effect of increasing the heater cathode insulation voltage rating, the heater power of both series and parallel valve designs should be the same (although I've never checked the data and put this to the test!)

[Diabolical Artificer]

Thanks for the clarification lads. Will have to dig out the P types I've got and build an amp at some point, though I gave away all the P519's I had I still have a few useful valves.

Andy.

[ppppenguin]

I think the audio boys love the EL519, the 6.3V version of the PL519. Both are ideal for valve amps without output transformers. EL519 was hardly used in the UK where series heater chains were used in almost all valve era TVs. The E/PL519 is also used in low cost RF linear amps: https://pa0fri.home.xs4all.nl/Lineairs/F...400eng.htm

I assume that both EL519 and PL519 are now getting expensive.

[Terry]

Andy, I left something out of my previous post that will be important to you if you build something using these valves in a series heater chain.

In a parallel heater chain, all valves receive the same heater voltage at switch on, irrespective of how long they take for the heater to reach its operating temperature. Because the resistance is initially lower at switch on, the current will be higher than specified, but the manufacturer will have factored this into the valve design so that no surge limiting is required.

In a series heater chain, however, it is the voltage, rather than the current that is split between the valves, so there may be just one valve that gets to its operating temperature quickly and several that take much longer.

Because these slower valves have, in total, a much lower resistance, the voltage drop will be much lower so that the fast heat-up valve(s) get a much higher share of the voltage drop than is good for them.

To negate this problem, a thermistor is placed in series with the valves which slows down the process because its initial high resistance allows only a small current to flow but, as it gets hot, its resistance reduces and continues to do so until it has reached its final operating temperature. This causes the heating up process to be slowed down and slowly speeded up which makes all of the valves much friendlier to their neighbours.

I hope that makes sense to you.

[ppppenguin]

Not all sets had a thermistor, many just assumed the valves were tough enough for it not to matter. For the most part it probably didn't matter. EB91 double diodes in TVs often lit up like a small lamp at switch on. They were noted for having a long life. Post apocalypse the cockroaches will be using EB91s along with EF80s.

[Ed_Dinning]

The line output valves, not being favoured by the audio fraternity were not all very linear in their G1 characteristics as they were designed for pulse operation and fast switching.
This has been overcome by some experimenters with circuits that drive G2 with the signal. G1 is simply used to set a fixed bias point.
G2 is lower impedance and needs to be driven at a higher voltage by (often) a cathode follower. You will see several designs on the web if you fancy trying something different.

Ed

[Murphyv310]

Hi.
Due to its nature a capacitor dropper will stop the the likes of the EB91 glowing like a 100watt bulb on switch on. On my TV22 project I have a cap dropper and a 10 ohm resistor to the heaters, even with the 10 ohm removed none of the valves light up like a beacon.