Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on May 28, 2014, 10:22:20 am
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Hi ! Just a word to a better understanding of current and voltage occuring in two parallel tubes in a SE amplifier. One of the pics shows 2 X EL34 in parallel, biased through a common cathode resistor of 240 R The equilibrium is perfect and both tubes operate at 24 watts. On the second pic, I have swapped one of the EL34 for a 6V6. Look at what happened: the 6V6 is absolutely happy, at 12 watts but the EL34 is left with too much power that the 6v6 can't consume and is now redplating. You change something in a circuit and all of the rest is affected.
Colas
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... One of the pics shows 2 X EL34 in parallel, biased through a common cathode resistor of 240 R The equilibrium is perfect and both tubes operate at 24 watts. On the second pic, I have swapped one of the EL34 for a 6V6. Look at what happened: ... the EL34 ... is now redplating.
"You change something in a circuit and all of the rest is affected."
Yep.
With the same applied plate voltage, you expect the 24w tube to be able to pass double the current of a 12w tube before redplating. That plate current is being pulled through the cathode resistor, and Tube Current * Cathode Resistance = Bias Voltage (across the cathode resistor).
If the 6V6 pulls less current through the cathode resistor than the EL34 it replaced, then total current through the ressitor dropped. That means the bias voltage also dropped, idling the tubes hotter. Because the EL34 has higher Gm, it would respond the most to the changed bias voltage, which seems to have resulted in redplating.
It would be interesting to see how much current each tube passed by using a 1Ω resistor per tube between the cathode and the cathode resistor, but I suppose that wouldn't do much other than satisfy curiosity.
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I can satisfy your curiosity right away: plate voltage: 390V ( from plate to cathode ) , EL34 drew 82mA and 6V6 33mA through the same K resistor of 233 ohm ( measured ). With 2 X EL34, each tube is dissipating 24watt ( plus or minus dust due to tubes not being perfectly matched......does this happen ? ). My idea first was to create a new design with 2 dissimilar parallel tubes in class a configuration. The tone was great but I had to bias each tube separately. I ended up using 2 X el34 which tone was a bit brighter and more defined. I put back the dissimilar pair after to compare and to be really sure of what I was liking better, but too lazy to replace the K resistors......
Colas.