Simple Hybrid SE EL84 UL amp.. comments please.

AudioGeek · 8th September 2006, 04:33 AM

thinking:

1:1 input transformer -> source follower mosfet -> 1:10 step-up -> 6 to 8 parallel EL84's SE UL.

Running 300v 39mA a piece. maybe a 600:8 output tranny.

fixed bias on output stage to secondary of 1:10.

very simple... no coupling caps.. all major voltage gain by 1:10 and output stage.

power supply: cap multiplier circuit. suggestions?

DC on EL84 filament supply.

comments? suggestions?

will this work and what should I look out for?

thanks!

rdf · 8th September 2006, 02:00 PM

If it were me the first consideration is confirming a 1:10 doesn't balk at driving the grid capacitances of 8 parralled el84s. Have you considered a source follower, maybe DC coupled, to the el84 grids and using the 1:10 as the input stage? Drops one tranny, replaces it with a smaller signal, higher quality unit and the follower will better drive the output. Since you're going fixed bias you already have a negative supply handy.

Johan Potgieter · 10th September 2006, 11:04 AM

I would not go to the complication of dc EL84 heater supply. If the heater wiring does not go near an input, you will certainly not get interference from 6VAC (which I presume is the reason for dc).

Regards

8th September 2006, 04:33 AM	#1
AudioGeek diyAudio Member Join Date: Jan 2004 Location: Minnesota	Simple Hybrid SE EL84 UL amp.. comments please. thinking: 1:1 input transformer -> source follower mosfet -> 1:10 step-up -> 6 to 8 parallel EL84's SE UL. Running 300v 39mA a piece. maybe a 600:8 output tranny. fixed bias on output stage to secondary of 1:10. very simple... no coupling caps.. all major voltage gain by 1:10 and output stage. power supply: cap multiplier circuit. suggestions? DC on EL84 filament supply. comments? suggestions? will this work and what should I look out for? thanks! __________________ It is not you... you are it.

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8th September 2006, 02:00 PM	#2
rdf diyAudio Member Join Date: Jun 2004 Location: big smoke	If it were me the first consideration is confirming a 1:10 doesn't balk at driving the grid capacitances of 8 parralled el84s. Have you considered a source follower, maybe DC coupled, to the el84 grids and using the 1:10 as the input stage? Drops one tranny, replaces it with a smaller signal, higher quality unit and the follower will better drive the output. Since you're going fixed bias you already have a negative supply handy.

10th September 2006, 11:04 AM	#3
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	I would not go to the complication of dc EL84 heater supply. If the heater wiring does not go near an input, you will certainly not get interference from 6VAC (which I presume is the reason for dc). Regards