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eddog 19th January 2006 07:48 PM

Load line frustration
(this is for 6L6-gb tubes)I have a schematic that shows plate voltage to be 390, connected to output transformer in a push pull config with 310 v on the screen(grid 2) and 27.5 volts on the control grid 1.
Do I need the DC resistance value of the primary OPT to determine the load line of this setup? Looking at the tubes data sheet the tube manufacturer recommends a max of 360 volts on the plate and 270 volts on the screen and 27.5V on the controlgrid. What effect was the designer of this circuit hoping to achieve by biasing the tube past maximum recommended values?

SY 19th January 2006 07:59 PM

What you need is the plate-to-plate impedance of the transformer with the intended load. If you're doing a single-tube loadline, it should be slope at 1/4 the total plate-to-plate impedance and intercept the quiescent point (i.e., idle). The DCR is just error and should be pretty small compared with the AC impedance.

SY 19th January 2006 08:08 PM

BTW, the circuit may well have been designed for the 6L6GC, which has higher max voltage ratings.

eddog 19th January 2006 08:11 PM

Thanks for your reply, I downloaded a 5 part article from Audiocraft that explains load lines and how to arrive at them so I will study that this weekend and hopefully I will understand the idea better. Can you point me to an idiots guide to determining tube load lines?

SY 19th January 2006 08:14 PM

If those are the Crowhurst articles, you've got gold nuggets. The exposition in the RCA tube manual is pretty good, too; that's where I learned it originally.

eddog 19th January 2006 08:28 PM

It is from a harmon kardon model HK250. The schematic shows them to be 6L6GB's. I was origonally going to try and re-create this amp, but now think that I will start with something more basic for my first tube amp and learn from the ground up. Got caught up in the excitement...:xeye:

cerrem 19th January 2006 10:42 PM

Just to clear up a misconception concerning MAX plate voltage...
With a Class AB Push-Pull circuit you can "safely" operate way beyond the MAX plate voltage specified in the data sheet.....
Typically the MAX plate volts in a data book are for Class A operation....For example lets say MAX plate voltage is 400V for Class A .... Now examine what is actually happening with voltage swing... The DC Q-point is at 400V DC, but the Load-Line will swing "modulate" this down to close to 0 volts all the way up to say 800 volts... ideally of course..... So by defintion your tube can safely handle 800V without arcing over.... When they designed tubes and speced them in the data sheets they typically Spec them for Class A at roughly half of 90% arc-over or 80%...
For example, lets say they find a 6L6 to arc over in lab conditions at 1000 they derate by 20% for process variation now you get 800V which is the "real" safe upper limit... Then cut that number in half to get the Class A MAX voltage of 400V ......
What this means is that the real MAX voltage limit is the sum of the DC voltage + the AC swing voltage..... So, an amp idling at 400V DC and has an additional 400V AC swing will reach 800V and you are safe... Now you have a 500V DC idle swing 300V to 800V are are still safe...
The trick is controlling the load-line swing in addition to the DC idle voltage to not surpass 800V in this example....
You can roughly DOUBLE the voltage in the data sheets as your safe upper limit...since that spec is mostly a Class A MAX voltage...
Sometimes it will be speced for Class AB and this requires some experience to know what the tubes limitations are..sometimes datat sheets can be conservative..
Now, getting to Push-Pull circuits....
If you look at Class AB push-pull voltage swing it really doesn't go much beyond the DC Q-point....each tube will swings DOWN in voltage more than up in voltage..this is because the tube will go into cut-off when the other is swinging down in the tube that is cut-off will get a voltage across it in opposition the voltage swing on the other half of the primary..BUT since the tube is in cut-off durring this time, it is not in any danger...
The biggest rookie mistake with doing load lines with pentodes is getting the wrong curves, i.e the wrong screen voltage.... Majority of curves found in data sheets will show 250V screen curves....this is useless if your screens are at other voltages... Getting the correct screen voltage curve sheet is key....
When setting up load-lines on pentodes for no grid current, the most important curve is the 0-bias curves.... This is where the load-line must properly meet to get the optimum load.....
For a P-P load-line you start at the plate voltage and draw a straight line up to the 0-bias curve ...making sure to just stay a hair above the knee, i.e to the right of the knee, staying mostly in the saturation region....if you choose to big a value for load-line you will hit the 0-bias curve in the TRIODE region , this will cause a drastic reduction in plate resistance and massive non-linearities will occur..also the screen current will go through the roof... Not for Hi-Fi application......
One of the best sheets to use for doing P-P load lines for a 6L6 is the sheet that shows only 0-bias curves for various screen voltages.... choose the screen voltage curve and your set...if your screen voltage falls between the two curve...then interpolation is fine....since it is fairly linear.....
Remember that the load-line is 1/4 the value of the plate-to-plate load....


eddog 20th January 2006 02:27 PM

Eureka!! Thanks so much Chris! I get it!!(well much more than before anyway). Much more reading to do. Please in regards to the 1/4 of plate to plate load , do i use the rating as listed in the tube datasheet(ie. 9000 effective ohm when operated in push pull) or the value obtained by dividing the voltage on the plate by the impedance values supplied by the output tranformer manufacturer(ie 7600 ohms).

Sheldon 20th January 2006 03:29 PM


Originally posted by eddog
Thanks for your reply, I downloaded a 5 part article from Audiocraft that explains load lines and how to arrive at them so I will study that this weekend and hopefully I will understand the idea better. Can you point me to an idiots guide to determining tube load lines?

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