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Old 2nd November 2006, 04:18 PM   #1
Klimon is offline Klimon  Belgium
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Default EL84 bias point

Hi,

I've been looking around a little (on the web that is) for a good bias point for el84 triode-strapped SE; currently my amp is running at 300V with 200ohms cathode resistor. I can't seem to find an el84 datasheet with loadlines beyond 300V, most of the amps + all datasheet recommendations I found have a bias point around 250volts except for the RH84 and this design (http://www.geocities.com/dmitrynizh/6p14p-twzsh-pio.htm), both 300v, both of them probably used spice loadlines for designing.

With the knowledge and available graphs I have I think (atleast from a theoretical point of view) a lower B+ bias point of around 265v, -9V, 45mA would be (much) better. Any comments in favor of a high or lower B+ point?

Cheers,

Simon
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Old 2nd November 2006, 04:39 PM   #2
SY is offline SY  United States
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Well, 300V is the maximum, so you're pretty much limited to that. There's no reason that you need the datasheet curves to go over 300V just to set up your loadlines- a ruler and pencil will do fine to extrapolate.

Looking at Mullard's info on SE triode connection, I'm not optimistic about achieving high fidelity performance at anything more than flea power (0.5 watt or so).
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Old 2nd November 2006, 07:42 PM   #3
rdf is offline rdf  Canada
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Default Re: EL84 bias point

Quote:
Originally posted by Klimon
Any comments in favor of a high or lower B+ point?

Hi Klimon. All the EL84/6bq5 variants I've measured in triode-SE (into 5Kohm) trade off lower second harmonic distortion for increasing higher harmonics as the bias current increases into the 40s. Data sheets typically recommend in the low to mid 30 ma range for triode. The low end of that range is where I usually find the best balance (for my tastes) between second harmonic and above as well. At 300 volts plate-cathode it's still at a comfortable ~10 watts plate dissipation and you'll get more power out of it.

Due to its low drive requirements, if you're comfortable with local feedback and a couple millvolts DC on the output the EL84 also responds well to fixed biasing and using the ouptut transformer secondary as the cathode load. In my amp (from memory) the combination was good for about 1.5% second harmonic, under ~0.1% third, ~0.01% fourth at 1 watt out with a damping factor in the 5 range. In the under-1/4 watt range where it would normally play harmonics above the third fall beneath the noise floor. Cathode secondary feedback is also possible with cathode bias but I didn't experiment with it much.
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