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Old 7th Sep 2020, 8:59 pm   #1
vinrads
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Default An amplifier using EL508's

Having purchased four EL508's I searched for a cct , this is the only one I could find , it's a bit unusual to say the least, it needs a 100 volt neg supply ,anyway I built it, this one is a proper rat's nest , but surprise surprise , it worked first time .it's not without it's problems though , but it's not oscillating no strange rf floating about ,I think I need to set the bias pot correctly , I thought about fitting a 10 ohm resistor in the cathode of each of the output's to measure the volts ie current , notice the low 200 volts on the screen's thoughts please. Mick.
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Old 7th Sep 2020, 9:30 pm   #2
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Default Re: An amplifier using el508's

Interesting comment about the screen voltage and anode voltage on the valve spec sheets to keep the G2 watts in a safe area.
http://www.r-type.org/pdfs/pl508.pdf
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Old 7th Sep 2020, 9:34 pm   #3
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Default Re: An amplifier using el508's

Those valves aren't designed to be particularly linear, so the low screen voltage may be to move the kink down a bit.

The use of grounded cathodes and negative grid pias is to make the most of the available HT and not waste any in cathode resistor drop.

These two statements seem to be opposing.... hmmm.

I'd wonder where it came from and the background of whoever posted it. It'll work but it could be in the 'could do better' category. Some detailed investigation would be needed to evaluate it for sure.

David
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Old 7th Sep 2020, 9:59 pm   #4
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Default Re: An amplifier using el508's

You can obtain the 400 V and 200 V by using a power transformer with a centre-tap. Over the whole winding you connect a bridge rectifier; the 200 V is than taken from the centre tap. Philips used this in a range of PA amplifiers with EL36's.

The datasheets of the EL508/PL508 are not very detailed, but to a certain degree they resemble the EL86.

The curves in triode mode:
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Old 8th Sep 2020, 8:24 am   #5
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Default Re: An amplifier using EL508's

Hi I used the Mullard 5-10 amplifier circuit with PL508s and it works well. The power transformer I had gave about 290v HT and the low loading bias resistors came to about 780 ohms.
This was my telyamp so it had EF80s strapped as triodes in place of the Ecc83 a new use for them.
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Old 8th Sep 2020, 9:13 am   #6
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Default Re: An amplifier using EL508's

The curves are a bit limited, Robert, because they've only been drawn as far as the continuous anode dissipation limit. In class A/B push-pull audio and also in TV line output service they go appreciably outside this region in every cycle of the signal.

In a push-pull stage with say 300v HT, if one valve pulled its anode down to 80v at 100mA, the non-conducting valve would see 300 + (300-80) = 520v on its anode.

For full class-A, you'd have a load line from 80v, 100mA to 520v, 0mA so you're going well off of the plotted chart mean dissipation would be with a quiescent current of 50mA at 300V in each valve = 15W so my figures aren't too far off the mark.

For that back-of-a-fag-packet calculation the peak anode span is (520 - 80)=440v and (100mA-0) = 100mA so an anode to anode load of 440/0.1 = 4400 ohms would make sense.

Just rough figures to illustrate that an audio amplifier will go well outside the graphed area, and then you can look at the steady compression of the grid voltage valuse as the different plutted curves cross the load line. This amplifier will give quite a lot of distortion, but it will at least be smooth, low order distortion.

It should have plenty of 'valve sound'.

In TV line output duty it will get worked much harder in terms of transient anode voltage far, far off the graph to the right during flyback, and during the scan, the current can be rather large, some of the colour TV line output bottles were spec'd to reach 1 amp. Clever design tried to make the trajectory between all-the-way-on and all-the-way off not a straight line, but to go downwards first, then along near the axis to the right, so the valve tried to skirt away from the high dissipation locus. The same tricks are done with switch mode power supplies, even with transistors... um make that especially with transistors. Early bipolars were only just up to the job, so you needed to grab every advantage you could get your hands on!

Thinking about it, copying the Mullard 5-10 doesn't sound like a bad idea. Ra-a won't be far out. The valves can do the voltages and currents. A distortion measurement might not be the best. but it should work. These valves won't believe what a nice easy life they're getting

David

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Old 8th Sep 2020, 11:51 am   #7
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Default Re: An amplifier using EL508's

Strange thing I noticed this morning , when I switch on or off my audio gen or soldering iron I get let's call it a squeging noise through the speakers, very odd , I have to switch off , to discharge the smoothing , switch on again all ok .Also when adjusting the bias pot , towards one end the output fluctuates ,I noticed this on the bass spk the cone was moving slowly in and out , on the scope the waveform is moving up and down slowly .Mick.
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Old 8th Sep 2020, 12:03 pm   #8
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Default Re: An amplifier using EL508's

Quote:
Originally Posted by vinrads View Post
... when I switch on or off my audio gen or soldering iron I get let's call it a squeging noise through the speakers ... Also when adjusting the bias pot , towards one end the output fluctuates ...
Quote:
Originally Posted by vinrads View Post
... this one is a proper rat's nest ...
Could be a connection. "Inside every amplifier there is an oscillator struggling to get out !".

Cheers,

GJ
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Old 8th Sep 2020, 8:32 pm   #9
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Default Re: An amplifier using EL508's

Hi Mick, a trick with this type of not very linear scan valve is to feed the signal into G2, direct from the driver anode and use G1 to set the standing bias.
Circuits on the web

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Old 8th Sep 2020, 10:01 pm   #10
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Default Re: An amplifier using EL508's

Thank's for that Ed, I had heard of that ,but couldn't find any info , will give it a go . Mick.
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Old 8th Sep 2020, 11:16 pm   #11
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Default Re: An amplifier using EL508's

ISTR that the EL508 was pitched at field output duty, so presumably acts more as an amplifier than a switch? That would imply an inherently reasonably linear transfer characteristic. If it's simply a PL508 with a different heater, the screen grid may only have been rated for usage with directly rectified/filtered mains, i.e. circa 200VDC anyway, so perhaps the designer was playing it safe.

Good to see that someone's built a circuit with this valve, I suppose it represents a half-way house between EL84 and EL34 power capability. (I've long thought that the EL84 would have been better presented in a B9D bottle anyway!)
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Old 9th Sep 2020, 7:39 pm   #12
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Default Re: An amplifier using EL508's

Update on the amp , following info from Ed I re designed the cct and fed grid 2 with the signal direct from the anode of the phase splitter with neg bias on grid 1 . It's been working all afternoon , not sure if it sounds as good as it did with the g2 at 125 volts but it's behaving itself .
The 125 vols on the screen was picked as the sweet spot ,and work's well Mick.
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Old 11th Sep 2020, 6:32 am   #13
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Default Re: An amplifier using EL508's

Lots of info on DiyAudio on "special drive" I think they call it over there. I'd have thought you'd have to have more V swing feeding g2 though.

If it's "squeeging" it might be an idea to test it 1) without load at very low OP & 2) with a cap as load, try a 100n first, see if it behaves.

Andy.
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Old 11th Sep 2020, 9:33 am   #14
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Default Re: An amplifier using EL508's

Thank's Andy , feeding g2 goes against everything that I was taught , but it works ,there is certainly enough output ,I had to make g1 less neg it's running about -7volts seems quite happy, anode current is about 12/15ma Mick.
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Old 13th Sep 2020, 6:29 pm   #15
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Default Re: An amplifier using EL508's

Quote:
Originally Posted by Radio Wrangler View Post
In TV line output duty it will get worked much harder in terms of transient anode voltage far, far off the graph to the right during flyback, and during the scan, the current can be rather large, some of the colour TV line output bottles were spec'd to reach 1 amp.
The PL508 was specifically developed for field deflection, so the flyback pulse was mostly suppressed.
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Old 13th Sep 2020, 6:35 pm   #16
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Default Re: An amplifier using EL508's

A very popular line output valve,not audio.Some times used in RF linear s.
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Old 14th Sep 2020, 6:49 am   #17
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Default Re: An amplifier using EL508's

The EL508 is not a line output valve but a frame output valve (vertical deflection). I never saw them being used in RF linears.

Most frame output valves can also be used for audio. Examples: E(C)L80, E(C)L82, E(C)L85, EL82 and EL86.
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Old 14th Sep 2020, 8:56 am   #18
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Default Re: An amplifier using EL508's

My fault,I am thinking of PL504!
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Old 14th Sep 2020, 9:39 am   #19
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Default Re: An amplifier using EL508's

There are audio amplifiers with line output tubes though. The Braun CVS 60 and the Telewatt VS 71 A use EL500's (identical to EL504 except for lower dissipation ratings). Kruundaal used the EL504 in several PA/instrument amplifiers. Philips made several PA amplifiers with the EL36.

Addition: I use the EL504 in an otl amplifier for 800 Ohm loudspeakers. See: http://otl800.blogspot.com/2014/02/o...2-x-el504.html
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Old 14th Sep 2020, 10:52 am   #20
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Default Re: An amplifier using EL508's

Quote:
Originally Posted by Robert Gribnau View Post
There are audio amplifiers with line output tubes though.
........................ Philips made several PA amplifiers with the EL36.
Also the Philips AG 9006 which used EL81s.
https://www.radiomuseum.org/r/philips_ag9006ag_900.html

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