Members gusfinley Posted September 3, 2005 Members Share Posted September 3, 2005 Hi, I'm building a custom 1987 50W plexi. One of the options I am putting in it, is the ability to switch between adjustable/cathode biasing. I need a little clarification on the equations. Mainly, what is the proper common cathode current for a pair of EL34's with a plate voltage of 438V? My handy dandy spreadsheet tells me for 438V I should get a plate current of 42.8mA - but is that for one tube or two? And I know that I should account for the screen current since this will also travel through the cathode resistor (I will only be using one cathode resistor for both tubes). Also, is -40V the standard bias voltage for a plexi 50W? I am also installing test points for bias voltage which will be taken from a 1ohm resistor placed in series with the plate of one of the tubes. What kind of reading will I be looking for as far as reading only one tube ( I have a matched set) is this the 42.8mA? Most of my confusion is coming from an old 60's EL34 datasheet with very different operating conditions that any current EL34 amps use..... Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 3, 2005 Author Members Share Posted September 3, 2005 up. Link to comment Share on other sites More sharing options...
Members BlackElk Posted September 3, 2005 Members Share Posted September 3, 2005 volts x amps = watts That's it...Just plug and chug. 438volts x Yamps = .75(25) - for 75% of max el34 diss, ~25 watts Solve for Y, there's your ballpark max bias setting that people usually talk about, 75% of max dissipation. If that's what you were asking about, anyway, sorry if it wasn't... Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 3, 2005 Author Members Share Posted September 3, 2005 Originally posted by BlackElk volts x amps = wattsThat's it...Just plug and chug.438volts x Yamps = .75(25) - for 75% of max el34 diss, ~25 wattsSolve for Y, there's your ballpark max bias setting that people usually talk about, 75% of max dissipation. If that's what you were asking about, anyway, sorry if it wasn't... solying for Y gives 42.8 mA.... a figure that I already have... Is that the plate current for one tube then? I think the pairing of the class A/B tubes is throwing me for a loop.... Link to comment Share on other sites More sharing options...
Members BlackElk Posted September 3, 2005 Members Share Posted September 3, 2005 Yeah, that would be for each tube. For the cathode resistor, you'd double it with two tubes. If your amp has -40v on the grid, then double it for the two tubes, then figure for the cathode resistor. Edit, grr, not the grid, the current.. Sorry. R= -40v / (42.8ma*2) or ~ 470ohm resistor Or that's how I'd figure it, but I'm pretty green at this, so don't listen to me, heh. Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 3, 2005 Author Members Share Posted September 3, 2005 yeah, its making more sense now.... I've been laying this thing out all day, and my mind was a little boggled.... It seems I forgot the simple stuff..... -40V is the bias voltage for a 50W plexi, isn't it? Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 4, 2005 Author Members Share Posted September 4, 2005 I suppose the best thing to do would be to would be to just build the thing, fire it up , test the bias voltage and then design the cathode bias from that.... Link to comment Share on other sites More sharing options...
Members igge Posted September 4, 2005 Members Share Posted September 4, 2005 Hmm, you're using EL34s and want them cathode-biased. I did the same with my JCM800 2205, but I have 428 volts on the plates. So I have a 1200 ohm resistor. Gives me 37 volts across the resistor = ~31 mA which is a nice ~50 % . You have to take into consideration that different cathoderesistor values gives different platevoltages & compensate for that. Good luck. Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 5, 2005 Author Members Share Posted September 5, 2005 I do have some good book to reference, when the time comes.... Radiotron designers handbook RCA tube manual Link to comment Share on other sites More sharing options...
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