• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

simple triode gain stage

hi all
according to pdf i've sent i have a problem with it
in page 11 the author uses -1v as a grid bias voltage
before he ploted the dc load line on the curve

of course we know that -1v means that -1 to zero for positive and -1 to -2 for negative cycle of swing

but in page 13 as u can see he used -1.5v as grid voltage in order to calculate the cathode resistor to plot ac load line
using -1.5 has a special reason or the authur prefered to use it??

and i wanna know that the way to achieve the optimum bias point is that we should plot ac and dc load line in triodes and the cross point of them is our optimum?


  • Common_Gain_Stage.pdf
    1.5 MB · Views: 114


2011-04-29 8:37 pm
The -1.5V bias point is just an example. The AC load line crosses the DC load line at the bias point,
since it must coincide when the input is reduced down to zero. That is, the AC load line is plotted
starting with the Q point, using the AC slope instead of the DC slope. From geometry, recall that
a point and a slope determine a line. The optimum bias depends on the requirements, keep reading.
Last edited:


Paid Member
2011-10-31 1:00 am


  • Screenshot from 2019-11-30 21-12-04.png
    Screenshot from 2019-11-30 21-12-04.png
    526.7 KB · Views: 386


Paid Member
2003-06-12 7:04 pm
Maine USA
....in page 11 the author uses -1v as a grid bias voltage.....but in page 13 as u can see he used -1.5v as grid voltage...

Perhaps he picked 1V as a simple round number just to see how the stage worked.

Then when he went to use it, he realized his signal might be more than 1V peak, and changed it to avoid gross distortion.

Like: I design a house 1 furlong wide. Then I sketch all my furniture and tools in it, see if they fit. I realize I really need a house 1.5 furlongs wide. (A furlong is 16 feet.) We can change our plans.


diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
if you are too lazy to plot load lines, the RCA resistance coupled design charts give you a table of tubes and resistor values, pick one that suits your requirements...

common cathode circuits can be designed for maximum gain, or maximum output swing and anything in between...

i can say that gain is affected by cathode currents since transconductance increases as cathode currents increased, all within reason as plate dissipation is your show stopper..

the plate load resistor determines your operating point, you can get the plate voltage and grid bias from your plate load resistor of choice...

that valvewizard blog is a good source of information imho..

i downloaded it,thank u
searched a a lot in net but the infos were not veryb useful
could u plz guide me to find some refrences to learn about
what is Srpp and advantages/disadvantages and how to bias it...
and a cathode follower and everythings about this two topic??

of course i'm familiar with these but i wanna know more about them
such as biasing and gain and where should we use each one
c = 299,792,458 m/s …

1 fur = 220 yd × 3 ft/yd × 0.3048 m/ft
1 fur = 201.168 m

c = 299,792,458 m/sec ÷ 201.168 m/fur = 1,490,259 furlong/sec

( 1 fortnight = 14 day ) × 24 × 60 × 60 → 1,209,600 sec

c (fur/fort) = 1,490,259 fur/sec × 1,209,600 sec/fort
c (fur/fort) = 1,802,617,500,000 fur/fort​
and the rest can be found in El Sid's comment, above, № 14

Just Saying,
GoatGuy ✓