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robertc 18th November 2006 02:10 PM

EL34 as preamp
anyone here know whether i can use pair of my EL34 in hand to build preamp. i am thinking to try B+250v with plate resistor 25w 5k in triode and auto bias with 40ma. and cap 10uf output.

GK 18th November 2006 02:39 PM

Perfectly possible, but it would be very inefficient for a preamplifier though. :confused:
I don't know what kind of output voltage swing you want from this amplifier, but one thing to keep in mind is the voltage drop across the plate resistor. When an EL34 is running with a 250V supply and a speaker transformer as a load, the average voltage of the plate will be only a negligible few volts less than the 250V supply (due to the small resistance of the transformers primary winding). With a 5k resistor and a plate current of 40mA the plate voltage will only be 50V as there is a 200V (5000*0.04) drop across the resistor.
In order to obtain the same operating characteristics with a 5k resistor load for the EL34, as with a transformer load with a 5k primary impedance, you’ll need a supply voltage of 450V to compensate for the voltage drop across the resistor. That will get you back to 250V on the plate.
An alternative to raising the supply voltage would be to shunt the 5k resistor with a big choke of several henries (but that would limit the bass response).

SY 18th November 2006 02:43 PM

For the cost of using ridiculously too large and inappropriate components, you will be rewarded with a preamp of mediocre linearity and output impedance. And your numbers don't seem to make much sense (50V on the anode?). You might start with the Mullard datasheet and lay out some loadlines if you're determined to do something like this.

robertc 18th November 2006 02:58 PM

infact i need 250v after the plate resistor.
i saw one pre amp with 6l6 without tran out drive a se 300B power amp with very good result but i did not have the schematic so i like to try myself to build one with EL34.

GK 18th November 2006 03:18 PM

infact i need 250v after the plate resistor.

You mean on the plate? If you want 250V on the plate with 40mA plate current and a 5k load resistor, then you will need a supply voltage of 450V (I'm ignoring the screen current here, which you want to have triode strapped, as its contribution is negligible.)

That would give you a preamp with around 18W of power dissipation per channel (not including the heaters) :eek: ! That being said, it would still work though.

phn 18th November 2006 03:47 PM

Why not stick with a more reasonable power pentode like the 6F6? And I think OTs are essential. B+ 250V will give you a signal swing of 310V if I remember.

robertc 19th November 2006 12:00 AM

thats why i ask whether possible the build a preamp with EL34.
like i mentioned above someone has made one with 6l6 with a good result.

Burnedfingers 19th November 2006 12:38 AM

Sure its possible but is it going to deliver performance that will offset the trouble? I don't think so personally. It would be a conversation piece that would deliver so so performance. The time and money would be better spent with a design that would deliver good performance. Do a search and locate some of the better designs here on the forum.

Just my .02

robertc 19th November 2006 05:12 AM

i got the parts and the El34. what i need just more information.
i did search all over the net but unfortunately no one try to look for trouble themself.

i think sometime somebody may try something new. (i copy this from a japanese who design the fully auto toilet.:D :

rdf 19th November 2006 05:26 AM

It's not exactly what you're looking for but visit Pete Millett's site. He used EL34s as CCS tubes in a low mu preamp. His experiences might be a valuable guide.

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