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Old 24th March 2005, 10:41 PM   #1
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Default Plate Resistor

I am constructing an RIAA preamp based on the attached schematic from Svetlana. My question is, surely the plate resistor value is to high? I'm reading a healthy 360V at B+ but by the time it hits the Plate it's dropped to 65V, surely way low for this tube?

I am only seeing a voltage gain of about 10 (4mV in/40mV out) which is pretty useless.

Or is there an issue with the B+ supply?

Being a newb I'm cautious to experiment too far with plate resistors for fear of setting my hair on fire.

Thanks for any input.

Philip
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Old 24th March 2005, 10:47 PM   #2
SY is offline SY  United States
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See the data sheet:
http://frank.pocnet.net/sheets/030/e/EF86.pdf

It looks like you can swing a volt or two with that low of a plate voltage.

Not that I'd recommend this tube for RIAA service, at least not in pentode mode.

Do double-check the cathode and screen grid wiring- your gain is suspiciously low.
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Old 24th March 2005, 11:02 PM   #3
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Thanks - this is one of those...I've started so I'll darn well finish projects!

I've got to say that the Svetlana layout is pretty obtuse, everytime I look at it it's different but my wiring appears correct.

I am having a hardtime understanding why they would have you build a beefy B+ only to drop it down to 65V at the plate? Doesn't seem right to me or maybe I'm missing something?
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Old 25th March 2005, 01:38 PM   #4
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They say in the notes that it needs to see a load of 500k or greater. What load is your riaa section presenting to it? If it's significantly lower than this, it might be part of the problem.
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Old 25th March 2005, 01:48 PM   #5
MelB is offline MelB  Canada
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Morning Phillip:

I have a book here "Audio Valve Amplifiers" by elektor. It has 3 examples of an EF86 wired up as a Pentode. In one example they are using 100k plate resistor 2.2k cathode resistor and 470k to G2 It is wired the same as your example. The voltages they show are about 4.6v on the cathode and around 208v on the plate with B+ of 350v. (G2 voltage shown as 192V) Hope that helps. MelB
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