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Old 19th November 2006, 07:43 AM   #11
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robertc, when you say preamp, do you really mean driver? A power pentode acting as driver for a power triode is fairly common.
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Old 19th November 2006, 11:13 AM   #12
robertc is offline robertc  Malaysia
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hi ray_moth,
linestage.
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Old 25th November 2006, 02:24 PM   #13
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Default Re: EL34 as preamp

Quote:
Originally posted by robertc
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.
possible??
I have used a R120 as linedriver at one time; this is a triode with an indirect heated cathode. It is pin-compatible with the EL34 in triode mode (just connect the extra grid). The funny thing was that it has exactly the same operating point and amplification between the two tubes. So I sometimes interchanged them. The circuit had a 6k load from 280Vb. The frequency range was just marvelous. About the distortion I can't say much - I never measured it. The Rk was something like 820 ohm. This can be decoupled. Decoupled the amplification was around 10* if I remember correctly.
About the sound then: There was a difference I noted at low and high level playing. It was rather warm sounding.
I said I used it as a linestage: I actually used it to drive a set of Mosfet output devices (Toshiba SK135/SJ50).

There are quite some people who use an EL84 as triode as line stage buffer. The EL84 has a better linearity.
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Old 26th November 2006, 12:21 AM   #14
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Hey-Hey!!!,
Run the EL34 as a pentode.

I just built a linestage with a 6AV5GA running full pentode. Regulated g2 at 75V, used a 100R cathode resistor and a 750R plate load. B+ is ~190. Little more with a GZ34, little less with a 5R4.

It is my first such experiment, and I am abandoning triode linestages based on the results.
cheers,
Douglas
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Old 26th November 2006, 01:00 AM   #15
rdf is offline rdf  Canada
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Hmmmm, I see what you're doing there. Very clever, and more than a little .... eccentric. What quiescent current is the pentode running? Have you done any audio measurements?
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Old 26th November 2006, 01:07 AM   #16
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Hey-Hey!!!,
Each 6AV5 is running 55 mA. G2 voltage was chosen to keep g2 current low, and to leave g1 bias achievable with a low value resistor. It is bypassed at the moment with a 1000 uF/25V cap. I plan to remove the cap and see if it still keeps my attention.

The heater is running AC, provided by a dedicated TX, and has its CT grounded. Supply is LC-LC with 600 uF of photo-flash capacitors.
cheers,
Douglas
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Old 26th November 2006, 02:06 AM   #17
rdf is offline rdf  Canada
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Thanks Douglas. I spent a lot of time recently with pentode load lines investigating operating points for cathode folllowers. My guess, after a quick look at the plate curves, was 50ma. Appears to be nice and linear, very interesting.
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Old 26th November 2006, 02:22 AM   #18
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Hey-Hey!!!,
The OP got picked with a lot of guesses. The circuit cannot swing a lot of volts, and if I had 680R plate loads, they'd be in now... So now I am looking at building an amp to sit behind it.

The 6CB5A with a 6k6 a-a load, B+ of 300V, and g2 at 75 V idling at 85 mA delivers ~20W of Class A power. Do a bit of local FB to get output Z manageable it should sing. Of course with 5A of 6.3 to the finals, it will be taking more heater power than it delivers to the voice-coil.

...or I could just try the 6CB5A with a 500R plate load idling at 75 mA from a 200V B+ as the new linestage...heh-heh-heh
cheers,
Douglas
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