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EL84 driving EL84 SET with fixed bias

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cotdt

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Joined 2005
#1
I would like to make a(nother) low-powered EL84 amp.

Rationale:
-EL84 has just the mu we want to drive another EL84
-With the same tube, we get distortion cancellation under a specific bias and voltage setting
-It is a VERY linear tube, and distortion should be extremely low with a CCS load
-Can make a low powered speaker amp, preamp, headphone amp, or driver stage for a 300B, 845, GM70, etc.
-The GE and Sylvania EL84 sounds very similar to a 45 DHT, but has the advantages of indirect heating. The Russian and Chinese tubes won't give that DHT sound, unfortunately.

To accomplish these goals I will use a CCS loaded driver stage and fixed bias on the output EL84. One can use a 12V battery or an adjustable power supply to adjust bias current.

An externally hosted image should be here but it was not working when we last tested it.
 
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#4
Two of my goals for this project are:
(1) to compare the typical cathode bias for EL84 versus fixed bias. I really like the idea of a 9V battery, but have never tried.
(2) to evaluate the sound quality of EL84 as driver compared to others I've used to drive EL84 in the past such as 6DJ8, 12A*7, 12BH7, 5687, etc.
 
#8
EL84 drive EL84 could be something you do for fun
though I guess there would be better ways to build EL84 amp

funny, but yesterday I thought about using EL84 in a QUAD II style amp using the new Amplimo trafos with cathode feedback winding
 
#10
Based on my tests with many triode connected pentodes with CCS loading I think that the first EL84 will work well with smaller anode current, say 10 mA.
It's linearity is improved if the grid resistor of second EL84 is increased.
The maximum recommended by Philips is 300 k.
 
#11
You should include a resistor (1 or 10Ohm will do) between cathode and ground just to measure current in the power tube easily.
0.22uF might be a bit short for a 100k load...
Looks simple, meaning it will work fine and sound good!
looking forward to hear how it goes
 
#14
Combo 7189 Bias on Leslie 10x Amps . .

Hi, I'm a little late to this discussion, but came to it while looking at Leslie's model 10x amps.

They have a rather interesting arrangement of Biasing; Cathode loads of 33 Ohm/500uF 6V and -12V applied via 270K to the G1 grids.

I would love to hear the views of the Cogniscenti on this design!

My idea is to use the unit as a 3.1 Home Theater system, with the Pedal amp (large OPTX) as the bass driver, the C and Tibia amps as left and Right channels, and the C# as the Center.

I do understand fully that these are NOT Hi-Fi amps by any stretch of the imagination, but then neither is my auditory sensor subsystem.

I am freshly retired and now finally able to spend as much time as I want playing with valves.

Reference: http://www.captain-foldback.com/Leslie_sub/Leslie_schematics/103.gif
 
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