PL504/EL84 SE vs parafeed - diyAudio
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Old 21st July 2006, 06:55 PM   #1
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Default PL504/EL84 SE vs parafeed

Gents,

I'm willing to build a new amp, using the following valves: 2xPL504, 3 (or is it 4?)EL84, 1xECC83, 1xECC88.
Now, regarding the PL504, here are the circuit values i have computed, due to very little info/use of this tube on the net:

Triode connection (very linear!)
B+=125V
Iidle=150mA
Grid bias= -20V
Rload=800 Ohm

With these values i estimate 1,29W of power output at around 4,5% 2nd order distortion.
My problem with these little current beasts is exactly that - one SE output tranny to handle 125mA of current at idle (it peaks around 200~230mA) and not be the size of a living room. PP would be also considered, but i still haven't learned on how to do it/caculate it :/

EL84... a SE amp would be nice too, but again, more expensive and harder to find. The "Compact 6W" is being considered at the moment.

So here comes a question: how do i choose the plate resistor for a parafeed amp? Either that or SSCCS loading - but i haven't found one that sources 200mA! I've read almost all posts regarding parafeed on this forum, searched all over the net, but couldn't find an answer! Choke loading is a bit out of the question because of it's size.
I have a 230/230V 300W tranny, so power dissipation isn't a problem . Of course, a simpler lower wattage isolation tranny is also found nicelly.

Any input would be greatly appreciatted

Thank you
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Old 21st July 2006, 09:24 PM   #2
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Start here:

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Old 21st July 2006, 10:05 PM   #3
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Default Re: PL504/EL84 SE vs parafeed

Hi Simpleton,

Quote:
Triode connection (very linear!)
B+=125V
Iidle=150mA
Grid bias= -20V
Rload=800 Ohm

With these values i estimate 1,29W of power output at around 4,5% 2nd order distortion.
Estimating Po up 1/100 Watt precision and H2 to 1/10% precision certainly requires extremely sharp eyes and precise rulers (among lots of other factors to consider) to measure out loadlines drawn on plate curves of extreme precision

If you have the triode strapped plate curves of E/PL504 at hand, I would be most interested to have a look at them (I am maintaining a small collection of such triode strapped pentode/tetode curves on my homepage).

Tom
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Old 21st July 2006, 10:22 PM   #4
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Let's just say my printer is very good
Here is the datasheet with the curves:

http://www.mif.pg.gda.pl/homepages/f...10/p/PL504.pdf

Dave, i've read that page before (maybe not with the maximum attention), but i still can't figure it out very good. I admit i'm a dunce...
I'll give it another read tomorrow, thanks.
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Old 21st July 2006, 10:32 PM   #5
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Quote:
Here is the datasheet with the curves:

http://www.mif.pg.gda.pl/homepages/f...10/p/PL504.pdf
Excellent pointer, thank you very much!

Tom
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Old 21st July 2006, 10:33 PM   #6
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Think of building it as a SEPP with another PL504 as the triode load. Otherwise as you say the SSCCS could be a little tricky. The DIYAUDIO CCS popularised by SY could be configured to do duty. Finding a transistor capable of delivering 200mA with 150V across it shouldn't be to difficult, though heatsinking it might be (25w of transistor heat ).

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Old 21st July 2006, 10:57 PM   #7
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The only objection i see so far to compute the SEPP/SRPP topoly is that i don't know the mu and transconductance of the PL504!
Seems that the El84 is the way to go, though i'm not conformed with it and will try to make something out of the PL504!
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Old 22nd July 2006, 03:10 AM   #8
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Quote:
Originally posted by Simpleton
i've read that page before (maybe not with the maximum attention), but i still can't figure it out very good. I admit i'm a dunce...:
The main thing i got from that is that if you use a CCS or an R as a load instead of a choke is that you need a lot more B+...

dave
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Old 22nd July 2006, 08:42 AM   #9
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Resistor loading would be insane (I use it in a headphone amp), and a CCS only slightly less so.

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Old 22nd July 2006, 09:20 AM   #10
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If you used the PL504 as a SEPP you could configure the top PL504 as a Pentode. Because the Pentode will act more like a CCS this will simplify the tuning of the circuit. Also creating a good screen supply at such low voltages should be relatively simple. If doing this I wouldn't try it as a MU-follower, because the speaker load will effect linearity more than if taken from the anode of the triode. Because the output load is so small, excellent results can be achieved with using mains Power toroidals as output transformers. I have built a similar circuit and they sound good. This is definately the way I would go.

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