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Old 17th July 2004, 11:48 PM   #1
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Default New EL84 SE schemat.

I'm building a stereo EL84 SE amp with a chassis I just received and a pair of EHRL - A-472 10k SE opt's at a royal 3WV each and a single 350-300-0-300-350 (I'm assuming 100mA each side but will back it up with real measurements soon...)

I was wondering if I could use this schematic with those pair of 10k opt's?
*** SE EL84 ***

I need to know what is the best B+ tap 300 or 350? I'm hoping to run two sides off the same trafo is this possible if there is enough current? Should I consider using the CT as common for both sides
in a bridge design? or build a full-wave rectifier and feed two sides off the final capacitor when it's DC? Both sides are in the same chassis and the power supply is not seperated.

And finally any suggestions for modifications?
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Old 18th July 2004, 06:38 AM   #2
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First, your OPTs will give too high an impedance for EL84 in pentode mode, which is what your schematic uses. It needs between 5k and 7k load, depending on the conditions under which it is run. Pentodes don't perform well with too high a load.

You could run the EL84 successfully in triode mode with a 10k load and 270 ohm cathoide bias resistor. This would give you lower distortion (2.5% instead of 10%) and less power (3W instead of 5W). Also, the triode distortion would be mostly 2nd harmonic, which is pleasing to the ear, instead of the pentode's 2nd and 3rd harmonic, which is not. You would also get a higher damping factor from a triode (punchier bass).

B+ for the EL84 in triode mode should be 250v, so you could probably use your 300-0-300 volts winding with full-wave rectification and a capacitor-input filter. A 2-stage C-L-C filter would help to reduce hum. You can use a tube rectifier (e.g. EZ81) to get slow turn-on and drop a few volts too, so the B+ hopefully won't be excessive. If it is, try adding a small resistor, say 100 - 150 ohm 5 watt, in series with each plate of the rectifier.

You should be able to run both channels from the same supply provided, as you say, the transformer is up to it. Don't use a bridge rectifier, because that would enable you to use only half of the secondary winding to get the same B+ but with only half the available current (using the full winding would double the B+ voltage AND halve the available current).
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