Tube bias calculation

#1 hi allyesterday I asked people for helping me to learn how to bias a srpptoday I found sth usefulforgive me cause that this type of question maybe very common and simpleSRPP Amplifier Theory / Pictures | TTRadiothe following website showed sth very useful for people who wants to know about srpp and how to bias and calculation and....everythings that the author wrote on his website is okno problemmy question is his bias pointin the last page he wrote that:R=RAK=RK=3.3K ohmwe know that in srpp for two identical tubes,the current for both is equal when we input no signal,and the voltage for each one will be half of the B+I wanna know that for finding the optimim point of bias,he used 150v for the exact point of bias? or the same 300v? IMG_20191202_064528.png 315.4 KB · Views: 351 Last edited: 2019-12-02 3:36 am #2 having the same cathode resistors values on the lower and upper triode gets you to 1/2 B+... #3 I suspect that analysis is flawed. He replaces each valve in turn by a short circuit. He should have replaced each by their anode impedance, in my opinion. Hence it is likely that all his formulas are wrong. See the Valve Wizard site for a better analysis. #4 typical newbie reaction is to look at tubes as if exact values to the decimal point is possible..a triode common cathode will seek its plate current with a given cathode resistor, tube variations will not make exact results if that

Tell the bias story. Don't worry, and keep doing the math until you get in the neighborhood of 70% max dissipation then tweak to taste (or sound, in this case). "...pray do not imagine that those who make the noise are the only inhabitants of the field; that, of course, they are many in number; or that, after all, they are other than the little, shriveled, meagre, hopping, though loud and troublesome, insects of the hour." -Edmund Burke 04-25-2008, 07:05 AM #3 Forum Member Re: Biassing GrooveTube 6L6B I checked the Fender HRD schematic and verified that factory bias is indeed 60mV, its noted right in the schematic.Using Weber's bias calculator: the top calculator, for class AB and a 6L6GC tube at 424V plate voltage the bias current at 70% plate diss. should be 49.5ma.This x 2 = 99mA = 99mV across the bias measurement resistor, so that would seem to jive with your calculation.Working it backwards in the lower most calculation window yields 20W dissipation or 67% of 30W, so once again the numbers seem to bear that out.I should tell you that I am not familiar with biasing a HRD, so I'm only going by what I see on the schematic and the Weber calculator. So why would Fender be biasing it so cold? I don't know. Maybe there is something we are missing and someone else can weigh in. Ahoy, Kap'n? "Time is an illusion, lunchtime doubly so" -- Douglas Adams"If something has a 1 in a

Simple Bias Current Checker: Basically the way this thing works is that pins 1,2,3,4,5 and 7 pass uninterrupted through the device from tube base to tube socket. Pin 6 is not used and so there is no need to wire it up. Pin 8 from the tube base is sent out to a Multimeter set to dc milliamps and sent back to pin 8 of the tube socket. You are basically reading the current flowing through pin 8 by putting your meter in line with it. You can see the current flow in the diagram above. The current flows up from pin 8 of the tube base, out through the multi meter and then from the multi meter to pin 8 of the tube socket. The meter is set to DC milliamps and the range is set to under 200 ma on the meter. How to build the Bias Current Checker: Sand out the inside of the tube base if necessary so that the tube socket will fit down into the tube base. Do not try to force the tube socket into the base, the base is Bakelite and will crack. Trim the tube socket pins to about half their length so the pins do not hit the bottom of the tube base. Solder a 2 inch or longer bare wire to pins 1 through 7 on the tube socket. A small hole has to be drilled in the side of the tube base down near where pin 8 meets the base. You need a two conductor wire to exit out that hole and go to a double banana plug. I use the mini shielded cable for the wire. Solder the two conductors of the cable, one to pin 8 on the tube base and one to pin 8 on the tube socket. Push the whole contraption together so that wires soldered to pins 1 through 7 on the tube socket come out the bottom of pins 1 through 7 on the tube base. (They are hollow) Solder pins 1 through 7 at the tips of the tube base pins. Solder a double banana plug to the other end of the two conductor cable. I poured epoxy down inside the sockets to make them solid on mine, but this in not necessary. You do not want to get epoxy inside the area where the tube plugs in or your socket will be ruined. Click here to enter my parts catalog shopping cart. Materials list for each Bias Current Checker These items are located on my Tube sockets page 1 - 8 Pin ceramic Marshall style tube socket. 1 - 8 Pin tube socket base. This item is listed on my Wire/Cable page. 2 - feet of mini shielded cable. This item is listed on my Jack/Plugs page. 1 - Double banana plug. How to use the Bias Current Checker: * Plug your Bias Current Checker into a tube socket on an amp. * Plug your power tube into the Bias

Plate current. However, since the screen current is only a few mA, it can usually be ignored, and the error will be in the conservative direction, i.e., less plate current than expected, which is good for tube life. This method of biasing is the most highly recommended. There are a few bias meters on the market that use this method, including the SwAMProbe (it doesn't measure across a resistor, rather, it breaks the cathode circuit so you can insert a milliammeter for direct cathode current measurement), and the Bias King. These devices have socket adapters that go in between the tube and the socket to make it easy to measure the bias current on an amplifier without having to modify it.How much current?The other important factor in setting the bias is to know the acceptable range of currents at which the tube can be safely operated, known as the "safe operating area" of the tube. Unfortunately, without some technical knowledge, you cannot usually look at a schematic or tube type and tell what the correct bias current range is. The manufacturer should provide a recommended bias current or range of currents for each particular amplifier. Unfortunately, this is not usually the case, as most manufacturers want you to have to buy their tubes which have been specially "matched" to the amp in question, usually at a high markup. Other manufacturers are merely copying existing designs, repackaging them, and don't know the correct way to bias an amplifier.Some "gurus" will tell you that an EL34, for example, needs around 40mA of bias. This is inaccurate information, because it doesn't take into account the class of operation, the output transformer primary impedance, or the plate voltages involved. For instance, an EL34, in class AB operation at 400V, with a 4K plate-to-plate primary impedance, would be biased at 40mA, while the same EL34 tube, used in a true class A circuit at 250V, might be biased at around 100mA.A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube (to account for the higher dissipation that occurs under signal conditions), while true class A amplifiers generally run right at the maximum plate dissipation (the dissipation at full power is lower than the dissipation at idle in a true class A amplifier). For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 400V class AB operation, it should be biased no higher than (0.7 * 25/400) = 44mA. This doesn't mean you should automatically bias all tubes to 70% of max dissipation! They can be biased at any lower current if desired, and many people prefer a point of around 50% to 60% of the max plate dissipation, which contributes to longer tube life. In addition, the "70% rule" falls apart as you use very high plate voltages, unless the primary impedance is increased accordingly. In some cases, if the voltage is high enough, there

Current Checker. * Plug the banana plug into your Multimeter. Make sure you plug into the two holes that are for current measurement in the under 200ma range. You do not want the high amp range. * Set your Multimeter to DC milliamps (under 200ma range). * Turn on the amp and let it warm up in standby mode. * Take the amp off standby. (Be ready to quickly put it back on standby if the current is too high.) * Turn the bias pot till you get somewhere around 35 milliamps of current. (Generic setting) * Check next power tube and split the difference if they are off just a bit. Notes: If you have 38ma on one tube and 34 ma on another, they are still close enough to be called a matched pair. If you are off more than 8 or 10ma, that is not a real good matched pair. Differences in the tube sockets can be the culprit also, so you must swap the tubes around and read them in different sockets to figure out what is going on. If you do not have a bias pot, you will have to figure out where the fixed resistor is in the bias circuit that controls the bias. It is a resistor that will be able to read the negative bias voltage on one end and is connected to ground on the other end. You have to change the bias resistor to a larger value for less bias current (more bias voltage) and a smaller value for more bias current (less bias voltage). The end