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Old 6th July 2004, 12:47 AM   #1
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Default is 5mA too low???

Is 5mA current really too low in a pre-amp??

I have currently a resistor loaded stage (20K anode resistor, 120v plate, 3.5v bias, on a EL84 triode connection)..

is this too low to drive a SS amplifier or tube amp??

also, would changing the 20K resistor to a choke load improve things??

finally, what value of choke should I use for a EL84 connected in triode mode???

there is no cathode follower on the output.

Thanks!!!
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Old 6th July 2004, 01:43 AM   #2
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More current isn't going to help you if your source impedance limits your drive. I'd be thinking cathode follower or some other low-source-Z configuration.
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Old 6th July 2004, 03:30 AM   #3
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Yes. EL84 should be upwards of 20mA. If your PSU can't handle it, I suggest a 12AU7 or AT in CF. If you need gain, use the other triode in front.

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Old 6th July 2004, 12:39 PM   #4
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Hi,

A trioded EL84 has a Ri of around 1K.

Surely that should be able to drive al but the lowest Zin Sucky State amp?

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Old 6th July 2004, 12:58 PM   #5
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For a 10K input Z, that's pretty marginal.
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Old 6th July 2004, 01:23 PM   #6
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Errrr, so a SET amp running a 2A3 circa 800 ohms Ri into a 2.5kohm load is utterly useless? I don't see the connection.

Now, at 5mA, yes it most certainly is.

Nice thing about small signals though, you can get away with nasty hookups because it's so small, it encounters only small amounts of nonlinearity. You can easily drive 600 ohm lines from a... 6DJ8 (huh, now that's odd, you never see that guy as a line driver), plate loaded. You might only get 5 or 10V, but it's there. I personally drive 32 ohm headphones with 12AU7 CF's at 5mA or so, it gets the job done but that's about it (only a few mW, but thus it shall remain as the PSU can't spare any more bias current).

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Old 6th July 2004, 01:32 PM   #7
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Tim, I suppose it depends on what you think is an acceptable level of distortion. Just curious, since I don't have triode-connected EL84 curves to hand- what's the change in rp when going from 19 to 21 ma?

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Errrr, so a SET amp running a 2A3 circa 800 ohms Ri into a 2.5kohm load is utterly useless?
As a signal processing device, possibly not. As a neutral amplifier, yes. But my bigotry in that regard is well-known.
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Old 6th July 2004, 05:54 PM   #8
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Default hmmm....

well funny thing...

if you look at the curves and specs....

triode EL84 looks very similar to ECC99...

and have similar "sound"

running at 120v 5mA 3.5v bias.. the triode EL84 has a very natural sound to it.. very good bass.

and this is with a 18K plate load with NO CF... driving a 80K input impedance of a tube amp..

which btw, doesn't have a very good gain and needs about 4v or so for max output.

So if for "regular" amps SS or tube.. with the 1v sensitivity I think that 5mA should be ok.

question is now:

should I put an active load on the EL84 or choke load it with a 200H choke???

thoughts?
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Old 6th July 2004, 10:19 PM   #9
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Well yeah, 4V, you'll never notice the .02% THD in it. Up around max. output of the stage, particularly at high frequency into a capacitive load and/or a low impedance load, you'll see a real mess however.

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Old 6th July 2004, 11:03 PM   #10
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Hi,

Never built this one but maybe worth tinkering with...

Cheers,
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