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Old 28th Jan 2010, 10:29 pm   #1
FERNSEH
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Default EL or PL509 as an audio output valve.

In the world of audio there is a great interest in the use of the PL509 as an audio output valve. Evidently, the trick is to supply the audio signal to the screen grid of the valve. The control grid can be connected to ground if auto cathode bias is used. Alternatively GI can be supplied from a negative bias source, in which case the cathode is returned to ground. Has any forum member tried the EL or PL509 in such an application?

DFWB.
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Old 28th Jan 2010, 11:11 pm   #2
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Default Re: EL or PL509 as an audio output valve.

Sounds like a poor man's version of a high power zero bias triode.
I've never understood the (verging on hysterical) obsession with triodes for audio amp's.
I haven't tried this in an audio o/p stage, though it's a popular way of modulating an RF amp.
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Old 28th Jan 2010, 11:27 pm   #3
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Default Re: EL or PL509 as an audio output valve.

Here's a circuit of an amplifier that employs an EL509. Note the triode connected pentode section of the 6BM8/ECL82. It operates as a power cathode follower. Also note worthy are the direct coupled stages.

DFWB.
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Old 28th Jan 2010, 11:38 pm   #4
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Default Re: EL or PL509 as an audio output valve.

Have a search of some of the audio forums such as diyaudio, you'll find a few examples. Many years ago I built a single ended PL509 amp, fixed bias and driven from g1 - needed about 100Vpp+ if I remember. I've not come across driving the PL509 from the screen though; what is the advantage for audio?
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Old 28th Jan 2010, 11:53 pm   #5
Michael Maurice
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Default Re: EL or PL509 as an audio output valve.

There is one huge disadvantage and that is Safety.

A lot of people like to show off their valve equipment by leaving their bottles freestanding above the chassis.

This is OK with EL84, EL34, KT66, KT88 and similar valves because the anode is brought out at the valve base so you cant (in theory) get a shock.

But the PL509 had their anode coming out of the top cap, and you can't leave that exposed.
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Old 29th Jan 2010, 12:01 am   #6
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Default Re: EL or PL509 as an audio output valve.

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Originally Posted by Michael Maurice View Post
But the PL509 had their anode coming out of the top cap, and you can't leave that exposed.
Swetlana make a version of the EL509 without a top cap. I has the standard octal (K8A) base. But the idea of using the PL509 is to make a cheap to construct high power audio amplifier.

DFWB.
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Old 29th Jan 2010, 1:00 pm   #7
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Default Re: EL or PL509 as an audio output valve.

Tim de Paravicini's EAR company has been using PL/EL509s in its (surprise, surprise) EAR509 amplifiers for 25 years now ; and several German integrated valve amplifiers from the 60s used push-pull pairs of EL500s in their output stages.
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Old 29th Jan 2010, 3:39 pm   #8
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Default Re: EL or PL509 as an audio output valve.

Quote:
Originally Posted by FERNSEH View Post
Quote:
Originally Posted by Michael Maurice View Post
But the PL509 had their anode coming out of the top cap, and you can't leave that exposed.
Swetlana make a version of the EL509 without a top cap. I has the standard octal (K8A) base. But the idea of using the PL509 is to make a cheap to construct high power audio amplifier.

DFWB.
How does that work, is it specifically designed for audi applications? The PL509 has several thousand volts on its anode in its application as a line output valve, how would that work with a base?


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Old 29th Jan 2010, 4:12 pm   #9
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Default Re: EL or PL509 as an audio output valve.

Quote:
I've not come across driving the PL509 from the screen though; what is the advantage for audio?
The idea is improved linearity in triode mode. g2 is further from the cathode than g1, and is more likely to be vaguely similar in shape to the anode. This means that the g2 characteristic will be similar to the anode characteristic. Running it as a g2 input triode then means that the negative feedback from the anode electric field is the same shape as the g2 one.

Line output valves are beefy and cheap, but not very linear when driven from g1 as they were only intended to be switches. g2 drive improves linearity, but at the expense of much lower gain and the need for a power driver. At least, thats the theory but I've never tried it myself.
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Old 29th Jan 2010, 4:48 pm   #10
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Default Re: EL or PL509 as an audio output valve.

Quote:
Originally Posted by Peter.N. View Post
How does that work, is it specifically designed for audio applications?


Peter
The Svetlana EL509-2 is intended for audio applications only.

DFWB.

Last edited by Brian R Pateman; 29th Jan 2010 at 6:32 pm. Reason: Quote fixed.
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Old 29th Jan 2010, 8:42 pm   #11
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Default Re: EL or PL509 as an audio output valve.

If you want a high power triode how about Darius' Triodelington?
http://triodelington.blogspot.com/

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Old 30th Jan 2010, 10:13 am   #12
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Default Re: EL or PL509 as an audio output valve.

Quote:
Originally Posted by FERNSEH View Post
Quote:
Originally Posted by Peter.N. View Post
How does that work, is it specifically designed for audio applications?


Peter
The Svetlana EL509-2 is intended for audio applications only.

DFWB.
Ah!
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Old 30th Jan 2010, 3:56 pm   #13
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Default Re: EL or PL509 as an audio output valve.

I tried an EL38 in a scrap radio. It was only running on 110v with a dodgy speaker but it didn't sound to bad. Probably scope for improvement. Top cap anode put me off a bit though.

http://www.youtube.com/watch?v=-kxUeFEqCTA
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Old 30th Jan 2010, 4:08 pm   #14
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Default Re: EL or PL509 as an audio output valve.

Quote:
Originally Posted by Lee de Forest View Post
I tried an EL38 in a scrap radio. It was only running on 110v with a dodgy speaker but it didn't sound to bad. Probably scope for improvement. Top cap anode put me off a bit though.

http://www.youtube.com/watch?v=-kxUeFEqCTA
They hurt a lot more in TVs with about 3kv on them, I've got the burns to prove it.
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Old 30th Jan 2010, 9:00 pm   #15
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Default Re: EL or PL509 as an audio output valve.

G8HQP Dave, thank you for answering my question; the 'why' of a design is not always obvious from a circuit diagram.

Ref the anode cap. There were[are?] some close fitting insulated covers around when I used the PL509s. I too, have brushed my hand against the PL509 top cap in a TV - big ouch, lesson learned.
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Old 30th Jan 2010, 9:18 pm   #16
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Default Re: EL or PL509 as an audio output valve.

In the late 1950s Philips made a PA amplifier that employed EL81s. I've got one upstairs. I can't remember if two or four EL81s were employed.
I can check that out on Monday.
The P/EL81 was originally designed as a line output valve.

DFWB.
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Old 30th Jan 2010, 10:20 pm   #17
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Default Re: EL or PL509 as an audio output valve.

Personally I think you would be better off sticking with the trusted EL34 or the beam tetrode equivalent .The PL509 (EL509) was designed to run in the rather harsh environment of a colour tv line output stage rather than that of the more sedate conditions found in HiFi outputs. As they are getting rather rare I would suggest there are better choices of valves with better characteristics. I have never seen the PL509 used in an audio output stages but have seen one perform really well in a home built transmitter on the 4m or 70Mhz band.
Going back to the EL509-2 it sounds like Svetlana have totally redesigned it for the audio world which I find intriguing as there are already good audio output valves which will take some beating.
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Old 31st Jan 2010, 12:26 pm   #18
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Default Re: EL or PL509 as an audio output valve.

Didn't the Futterman amps, about 25 years ago, use PL509's in a transformerless output stage?

Cheers

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Old 31st Jan 2010, 12:48 pm   #19
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Default Re: EL or PL509 as an audio output valve.

Dear Hybrid Tellies, you can see one now in this attachment.
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Old 24th Feb 2010, 8:49 am   #20
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Default Re: EL or PL509 as an audio output valve.

Quote:
Originally Posted by FERNSEH View Post
In the late 1950s Philips made a PA amplifier that employed EL81s. I've got one upstairs. I can't remember if two or four EL81s were employed. The P/EL81 was originally designed as a line output valve.

DFWB.
I have a Philips FZ977A Bi-Amp radiogram chassis that has 4 PL81s in the output stages. It was from one of their top-of-the-line radio/tape/record player consoles and was out for rubbish collection. Sadly the cabinet had been vandalised beyond recovery before I scored the chassis and speakers. but amazingly the chassis, dial scale and valves were all still intact.

The PL81/EL81 is rated for use as an audio o/p valve, TV line-output valve, and a series voltage regulator.

Cheers

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