PL802 as audio valve?

[jay_oldstuff]

Hi all i've been going through my valve stocks and have come across quite a quantity of PL802 pentods both NOS and used. Now I know there where intended as video output pentodes in early colour TV's but I was wondering if anyone had tried these as low power audio output valves? what I am pondering is a small single ended amp with an output of 1-2 watts.
the heater voltage and currient is easy. I could go PL84 as i know these work as audio output valves but I have a lot more PL802's than PL84's whitch do come in for my vintage TV's.

Jay

[paulsherwin]

You can use pretty much any power pentode or beam tetrode as an audio output valve if you're just lashing something up. You can't (easily) use them in high end hifi designs because you will need high quality output transformers with the right turns ratio.

[Maarten]

The PL802 happens to be the one (or maybe one of the few) TV valve that is actually a bit rare and in demand amongst collectors of early colour sets. Nowadays less so than in the early 1980's when Philips even supplied a transistor equivalent with mediocre performance, but still so. Philips was the only manufacturer and the valves were usually driven quite hard so they didn't last very long. Factory stocks were exhausted a few years after production ended, but some repair shops kept a few in their own stock for maintaining 'that one last set' - supply problems may have existed before the last production run.

It's not particularly suited (but will work) for audio since it was optimised as a video output valve (at least 5MHz usable bandwidth for a crisp picture and relatively low power - for audio anyway). Maybe it would be suited to build a small AM-transmitter, but I'm sure a colour-television collector will be happy to take those valves off of your hands or trade them for valves more suitable for audio purposes.

[jay_oldstuff]

Thanks for your thoughts guys. I think I will keep the half dozen boxed NOS ones to one side to pass on for their intended purpose. I ran the pile of used ones through the valve tester most where knackered but i've ended up with half a dozen that are free of shorts and test good for emission.

I do still fancy having a play with these valves, I am going to try one at audio. I have a little amp chassis I think it originally used EF86 EL41 valves and has a solid-state rectifier. I'll fit a B9A base for the PL802 and for now power the heater from my bench PSU. Not sure how well matched the EL41's output transformer will be to the PL802.

I'm not looking for Hi-Fi quality but something pleasant to listen to, what i'm intending eventually building with this is a stereo I pad dock. I want to use TV valves because I have lots sat about doing nothing.

Jay

[Studio263]

An interesting thing about the PL802 is its staggeringly high mutual conductance, 40mA/V. The previous record for European valves was held I think by the "L" section of the PFL200, but that was "only" 21 mA/V. Because of this very high mutual conductance it is difficult to get an idea of how good these valves are using a valve tester; most do not have the range to accurately display such high figures. Even a mediocre example should bury the needle, putting them in a set and seeing if there is enough brightness and "punch" to the picture is the only way to really assess them.

As has been pointed out, making an audio amplifier out of them is a bit of a waste of time as a suitable output transformer won't be availalbe. Since the key challenge in valve audio amplifier design is to produce a transformer which allows enough feedback to be applied to mask the non-linearities of the valves without running into phase inversion issues this means that only lo-fi performance will be possible. It is best to stick to something which decent transformers are easily obtainable for, e.g. EL84s.

The heater voltage is a nuisance too, so that's another special transformer you will need!

[julie_m]

It can't hurt to try. Your main difficulty with using any "non-standard" valve is going to be the speaker matching transformer.

Some people have reported success using power transformers as speaker transformers; it depends how much DC the core can withstand.

Should be a good opportunity for experimentation on the long Winter evenings, at any rate .....

[Peter.N.]

I have seen an EF80 used as audio output in a TV, really depends on how much power you want.

Peter

[jay_oldstuff]

Originally Posted by Studio263

An interesting thing about the PL802 is its staggeringly high mutual conductance, 40mA/V. The previous record for European valves was held I think by the "L" section of the PFL200, but that was "only" 21 mA/V. Because of this very high mutual conductance it is difficult to get an idea of how good these valves are using a valve tester; most do not have the range to accurately display such high figures. Even a mediocre example should bury the needle, putting them in a set and seeing if there is enough brightness and "punch" to the picture is the only way to really assess them. As has been pointed out, making an audio amplifier out of them is a bit of a waste of time as a suitable output transformer won't be availalbe. Since the key challenge in valve audio amplifier design is to produce a transformer which allows enough feedback to be applied to mask the non-linearities of the valves without running into phase inversion issues this means that only lo-fi performance will be possible. It is best to stick to something which decent transformers are easily obtainable for, e.g. EL84s. The heater voltage is a nuisance too, so that's another special transformer you will need!

the valves I discarded had inter electrode shorts or where running into grid current although I did find one that was fine in that respect but had zero emission. I first ran the valves through my Mullard high speed valve tester to weed out the valves with obvious problems then ran the passes through my AVO CT160 (I allays test unknown valves like this i'm a bit protective about my CT160) under test in the CT160 they are just about making it into the green so I’m guessing they are well below par. when I'm next up at the workshop I'll try testing one of the NOS ones I have for comparison. As for the heater voltage my intention was to run the heaters from DC using something like a LM317 variable voltage regulator, that's good for over an amp and even if i ran two for single ended stereo output that's only 600ma heater current. I know there are much easier ways to build a simple valve amp but this is about tinkering and seeing what can be done and solving the output transformer problem is part of the fun.

Originally Posted by ajs_derby

Should be a good opportunity for experimentation on the long Winter evenings, at any rate .....

That's the whole point behind it. And if i end up with a nice little amplifier at the end of it so much the better.

Originally Posted by Peter.N.

I have seen an EF80 used as audio output in a TV, really depends on how much power you want. Peter

I did build a little amp using an ECL80 that sounded very good but only at about 500mw maximum output power if i tried to push any more out of it it would distort and the valve got very unhappy. I wonder how much output you could get from an EF80 i have 100's of them. I was also wondering about the Ecc82 i repaired a little harmonica amp a while back that used one in push pull as the output valve. I'm rambling now so will stop Jay

Some people have reported success using power transformers as speaker transformers; it depends how much DC the core can withstand.

Go push pull and it goes away. As to feedback, simply use an unbypassed cathode resistor.

[G6Tanuki]

If you're using any seriously-high-slope valve for audio output duty you should expect to get HF/VHF parasitic oscillations unless you design to avoid these!

"Stopper" resistors in the grid and screen-grid feeds - I like something of the order of 5K or 10K wired directly in series with the grid feed [use the shortest possible length of lead from the resistor to the grid pin] and something like a 500-Ohm *wirewound* resistor on the screen-grid [the idea of using a wirewound is to make it somewhat like a 'lossy choke' at VHF].

Sometimes you may need another 100Ohm wirewound connected between the anode and the speaker-transformer as well to tame it.

Same rules apply with push-pull.

Also remember that a valve which is nicely stable under static conditions may become wildly unstable at some point during its conduction-cycle when it's driven with real-world signals. I've had this issue with a transmitter-modulator that used a pair of PL519s in push-pull.

"Inside every amplifier there is an oscillator just waiting to get out".

[Peter.N.]

I can't remember the TV that used an EF80 for audio o.p. but I do remember that the sound was quite adequate, I was quite surprised.

Peter

[FERNSEH]

Although not a direct equivalent to the PL802 the American 12BY7A could also be considered as an audio valve.
Evidently it had lots of other uses other than a video amplifier valve. Wideband applications for instance.

http://www.r-type.org/exhib/aaa0086.htm
http://www.radiomuseum.org/tubes/tube_12by7a.html

Originally Posted by Peter.N.

I can't remember the TV that used an EF80 for audio o.p. but I do remember that the sound was quite adequate, I was quite surprised.

Hi Peter, It was certain EMI TV sets made in the early fifties. HMV and Marconi. EG the HMV1814. Actually, the earlier EF91 was used in the same application, in the HMV1807. The EF80 was given an MO type designation: Z152 or Z719. The Z152 was made by Mullard and branded as Marconi.
Some Osram Z719 valves were made by MO as well as Mullard.

DFWB.

[G6Tanuki]

Originally Posted by FERNSEH

Although not a direct equivalent to the PL802 the American 12BY7A could also be considered as an audio valve.
Evidently it had lots of other uses other than a video amplifier valve. Wideband applications for instance.

12BY7A is well-known as a driver-valve in amateur HF transceivers, usually driving a couple of 6146 or similarly-re-engineered ex-TV "Sweep-tubes" as the power-amps.

The 6CH6 has seen similar use in times past.

http://www.r-type.org/exhib/aaa0400.htm

[FERNSEH]

The 6CH6 was used as the audio output valve in KB radios.
As the developer of the 6CH6 was Brimar which was a STC company as was KB it doesn't come as a surprise that the valve found a domestic application.
I can't think of any domestic TV set which uses the 6CH6 as the video amplifier.

From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_6ch6.html

DFWB.

[Maarten]

If presented with a choice of TV valves to make a small audio amplifier, I would probably go with the PCL805 or PCL86. You can even go OTL using both tubes, the ECL86 in place of the EL84, the PCL805 in place of the EL86. Philips actually used this combo in a few continental B&W chassis.

[jay_oldstuff]

I do have a quantity of NOS PCL805's, I most likely have many used ones too in one of my boxes of mixed used tv valves. It would also reduce the valve count as I’m guessing I could use the triode section as preamp to the pentode. All possibilities and suggestions welcome at the moment.

I'd like to use this thread as a melting pot for ideas.

As an interesting aside regarding TV valves use in other equipment, I build custom Harmonica amps, small units with 3-5 watt output with a circuit very loosely based on the old Fender Champ and I regularly use EF91's in the preamp stage.
They tend to be microphonic and overdrive very easily and give a distorted sound that harmonica players just love.

Jay

[FERNSEH]

Another video amplifier valve that can also be employed as an audio output valve is the PL83. Bush used it in the video and audio output stages in the models TUG26 and TUG36.

From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_pl83.html

Pye used it as a frame output valve in many mid fifties TV sets.


DFWB.