triode operation

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triode operation

Postby overdrajv » Mon Dec 24, 2007 1:37 pm

Hi!

I want to quiet down my simms-watts pa 100 head http://www.drtube.com/schematics/simms/100w-amp.gif ,and the first thing I did is that I cut the signal from the two of EL34. Now I want to convert it to triode and bias it class A. But I dont know exactly how to do it. How should I drop B+ from 500v to 420v ?

Thanks!
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Postby EWBrown » Wed Dec 26, 2007 6:28 am

Converting to triode mode is pretty easy, disconnect each tube's screen grid (G2) from the ultra-linear taps and re-connect (using the original resistors) to their plate.

Reducing B+ from 500V to 420V can be accomplished in two ways.

The simple, and "brute force" method is to replace the 25 ohm resistor directly after the positive side of the FW bridge, with a higher resistance (and wattage capability) in order to drop the 80VDC. The actual value depends on the tubes' plate current / bias settings, I'll presume 50 mA for the EL34s (I'll also ignore the input stages' current consumption is it is only a few milliamps at most) :
80V reduction at 200 mA would equate to 400 ohms, with about 16 watts being dissipated through the resistor. The "rule of thumb" is to have at least three times the actual dissipated power, so the 400 ohm resistor should have at least 50 watts capability. The easiest way to accomplish this is to use five 2000 ohm, 10 Watt resistors in parallel. These will still dissipate a LOT of heat, so it is best to mount them on top of the chassis in order to allow air flow and heat dispersal.

Second method is to select a choke with the necessary inductance and current handling capability.

Once again, presuming a total of 200 mA, the choke should have some additional "reserve" so 250-300 mA minimum. SInce the choke is being fed from the rectifier, with no input capacitor (choke-input filter), the choke's inductance, in combination with its DC resistance, can drop the B+ from 500 to 420VDC.

The formula for inductive reactance is Z = 2*PI*F*L, F is in HZ (50 or 60)
L is in Henries, and 2*PI is 6.28 (also can be expressed as 377/60).

Making the assumption that the AC line voltage in Slovenia is 50 Hz,
2* PI*50 is 314, so the inductance in Henries would be: 400 (ohms) / 314 or 1.27 Henries. If you use a higher inductance value, then the actual B+ voltage can be fine-tuned by means of a low-capacitance (a few uF) polyprop or similar non-electrolytic capacitor connected in front of the choke, this will increase the output voltage in fairly small and controllable increments.

HTH

/ed B in NH
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