by EWBrown » Thu Dec 02, 2010 11:05 am
The four EL84s and the VA & PI tubes consume a total of around 3.7 amps, and the slight amount of resistance in the PCB's copper etches will "lose" a few tenths of a volt by the time it reaches the filament connections.
The biasing voltage on the cathode (pin 3) sets the total current passing through the tube, which is a combination of the plate and screen grid currents, which is easily measured at the cathode, since that carries the total of the two curents. The grid voltage is always negative in relationship to the cathode voltage, whether in cathode, or in fixed bias situations.
Typically, with a B+ around 365VDC, a cathode-to-G1 voltage differential of approximately 14VDC will operate the tube at or near 35 mA current, this voltage is not a set-in-stone "golden" value, but an average figure. I've seen EL84s and 6P14Ps which can vary from as low as 10VDC to as high as 16VDC to achieve 35 mA with 365 VDC B+ (in a typical ST35 design).
I've had matched quads of J/J EL84s which were dead-on at 35 mA, with my initial pre-setting of the cathode resistor networks to 400 ohms from pin 3 to ground, and then the "generic" Russian 6P14Ps can vary to the extreme high and low voltages (16V and 10V, respectively).
(I suspect that is how Sovtek and other tube vendors "grade" their tubes for guitar amp usage, the hotter tubes give better "grunge" , the lower gain ones are cleaner)
That would translate to cathode resistor values as low as 285 ohms, and as high as 457 ohms. THe "10V" tubes have higher gain than do the "16V" tubes.
I save these, "extreme voltages" tubes for SE projects, and as voltage regulators, and keep the 13V to 15V" tubes for ST35 / SCA35 usage.
The plate and scren grid (G2) voltages will be determined by the DC resistance of teh output transformer primary windings at teh plate and SG taps, and the tube's actual operating current.
35 mA isn't an absolute, ideal value, it is determined by the actual plate voltage and B+ , for lower B+ voltages (300 to 325VDC) the current can be up to and over 40 mA, and for higher voltages (380 to 400VDC) it should be set lower, around 32.33 mA, in order to stay within the safe maximum PD ratings (typically 13W total). With fixed bias operation, the total current per tube can be set lower, 25 to 30 mA.
For Class A SE operation, a constant currernt sink (CCS) can be used, to set the (cathode) current at an fixed value, and then any random EL84 / 6P14P / 6BQ5 can be used and it sill operate at teh pre-determined current. CCS can be designed using LM317, TL783, 7805, etc r three-terminal egulator ICs.
/ed B
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