Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: turtle441 on December 16, 2017, 09:19:42 pm
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I recently picked up an epiphone Valve Junior. Planning to gut it and try one of the plexi SE pcb's. While I'm waiting to finish my current project, I've been messing around some with the stock circuit. (Schematic available here: http://www.sewatt.com/files/sewatt/StockVJ_1-3.pdf )
Just for giggles, I decided to check out what the bias on the EL84 looked like. Got the following measurements:
Cathode Resistor Value = 216 ohms (measured without lifting one leg, not sure how much this matters as there is a bypass resister in paralell)
Voltage Drop across cathode Resistor = 10.4V
Anode Voltage (pin 7) =336V
Cathode Voltage (pin 3) = 10.4V
So, by my math, cathode current = V/R = 10.4/216 = 0.048 = 48mA
Anode to Cathode voltage is 325.6.
325.6 x .048 = 15.6W
Am I missing something here, or is that poor EL84 cranking out in excess of 15W? (Which may explain why it sounds like it needs a new power tube.)
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"Cranking out" 15+ Watts of heat. Not audio output.
You have computed P+G2 power. G2 is maybe 15% of total cathode current. You may be nearer 13.3 Watts in a 12 Watt plate. Not something to panic over. "Everybody" has abused EL84 for decades. The tube makers may build them to take it. OTOH, the EL84 is about the cheapest power tube around, so maybe they don't. Maybe you should just treat them like vacuum-cleaner bags.