Calculating ra and gain

[Suusi M]

Do you mean the coupling cap? Or the decoupling cap (filter cap)? Just to be clear. I assume you mean coupling cap.

Yes coupling cap my mistake sorry it was a long day yesterday

[Merlinb]

If not lets try this a different way. Don't mean to be stubborn here but I still do not understand how to calulate a gain stage before actually wiring it. If you guys were designing a simple triode stage and wanted to calculate gain before wiring the actual circuit for testing what equation/s and calculations would you make if you had 300v supply voltage, 12ax7, 100kplate resistor, and 1.5k cathode resistor values? The ra value is what is confusing me here.

You would generally use:

A= muRa/(Ra+ra)

ra should always be given on the datasheet. In American datasheets it will usually be "rp" or sometimes something else, but it will be there along with transconductace and mu. Taking a random datasheet: http://www.mif.pg.gda.pl/homepages/f...93/1/12AX7.pdf
Under "Characteristics and typical operation" you will see amplifcation factor (mu), transconductance (gm) and plate resistance (ra or rp). In this case 62500 ohms. BUT, if you happen to know that you're going to use a low supply voltage, or really cold bias, then it's wise to increase that to 70k to 80k.

[GibsonLover]

Another good argument for TubeCad.... In 15 seconds I entered your data and found: Gain = 56.88 (35.1dB) ImA = 1.05mA, Vcathode-plate=193V Vplate resistor=105V and Vth=1.71V. I like the Vth data because that is the input at which the output starts to clip. now you can go to a spreadsheet with that data and determine the interstage attenuation you need (want) to overdrive the next stage (take your output after amplification and divide that by the Vth of the next stage > overdrive=20log(Vout/Vin) where Vin = Vth of next stage. If you're good with Excel, you can get a lot of milage out of that. I've found you can get pretty close with a gain structure on paper this way before fine tuning with you ears.