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Biasing the amp
|Types of biasing:
Power tubes have a throttle, like an engine. This throttle is pin five on most common power tubes, the control grid. The control grid must be a negative voltage in relation to the cathode, pin eight. Some amplifiers have the cathodes of the power tubes connected directly to ground and need a negative voltage on pin five to hold them at a certain idle current. Think of it as setting the idle on an engine to run at so many rpm. This kind of system is called, fixed bias. There is usually a separate bias voltage tap on the power transformer that is rectified into a negative voltage, compared to the chassis ground of the amplifier. This negative voltage goes through a network of resistors and capacitors and usually a pot so that it can be impressed upon the control grid to hold the tube at a certain idle current. If there is a wire connected to the cathode, pin eight, that is soldered to the chassis then the amplifier is a fixed bias amplifier.
The other type of power tube bias you will encounter is cathode bias. In a cathode biased amplifier there is not a separate bias network like in a fixed bias amplifier. The bias is achieved by placing a resistor from the cathode, pin eight, to ground. All the tubes current must flow through this resistor. When current flows through a resistance a voltage drop occurs. The high voltage present on the plate, pin three, is dropped across the internal resistance of the tube itself and the cathode resistor. The top of the cathode resistor that is connected to the cathode will read a higher voltage compared to ground. If there was 400 volts DC present on the plate and 25 volts DC present at the top of the cathode resistor then 375 volts were dropped on the internal resistance of the tube and 25 volts were dropped on the cathode resistor. The other end of the cathode resistor is connected to ground, so the voltage drop stops there. Since the cathode was reading 25 volts compared to ground and the control grid is connected to ground, usually through a 220k resistor, then the control grid is 25 volts more negative than the cathode. This is the bias voltage that keeps the tube idling at a certain current. Just like in a fixed bias amplifier the control grid must be negative in relation to the cathode.
|Biasing a cathode biased amplifier
The only way to adjust the bias on a cathode biased amplifier is to change the value of the cathode resistor. A smaller value will flow more idle current than a larger value. There are limits to the size of the resistor on the low end and the top end. If the resistor is too small then too much current will flow in the tube and the tube plates will glow cherry red. If the resistor is too large the amp may lose some power, have more compression and distort the output signal. Experimenting with different values and listening to the amplifier always works the best.
|Biasing a fixed bias amplifier
If there is a pot in the bias circuit, this pot is usually used to raise or lower the negative voltage on all the power tubes at once. Some circuits do not let you raise or lower the voltage, they only allow you to balance the voltage so that the power tubes all have the same voltage. This system is not as flexible as the one that allows the bias voltage to be raised or lowered. The balance system can be converted to the adjustable system if desired. See rewiring the bias system section. Some amplifiers do not even have a bias pot, they have a fixed value resistor that sets the bias. This system is the least flexible since the bias resistor must be unsoldered and a new value replaced to change the bias voltage. This system can be converted to an adjustable bias system. See converting the bias system section. Look on the schematic to see which system your amplifier has and then use the bias method that relates to your amplifier.
Least desirable way to bias:
To check the bias on a fixed bias amplifier you must first have a schematic with the bias voltage that the manufacturer recommends. This is not always the best place to set the bias but it will work. With your multi-meter set on DC volts, connect the black probe to a suitable ground. The chassis screw where the power transformer center tap is grounded works fine. Turn on the amplifier and let it warm up a few minutes. Turn on the standby so that the amplifier is up and running. Keep one hand behind your back because you will be dangerously close to some very high DC voltages on pins three and four. Touch the red probe of your meter to pin five of the first tube. Notice the voltage and then move the red probe to pin five of the next tube. If the voltages are the same for each side of the power amp and are close to what the schematic says they should be set at, then you are done. If the voltages are not the same on pins five of every power tube or do not match the schematic, then go to one of the next sections that applies to your amplifier. This is the generic way to set the bias. The best way is to actually check the current flow through each power tube and set the bias pot accordingly. Using 1 ohm resistors on the cathode is the easiest and safest way to do this. See the section on installing 1 ohm resistors.
|Fixed bias - adjustable bias:
If you have an adjustable bias system, the bias on all the tubes can be raised or lowered to match the voltage on the schematic. With the red probe of your meter touching pin five, slowly turn the bias pot to see which way the voltage goes. Set the bias voltage to the schematic voltage and your done. Improvements can also be made to an adjustable bias system. See rewiring the bias system section. See the section on installing 1 ohm resistors. Setting the bias voltage to a schematic voltage is a generic way of setting the bias also. It is best to measure the tube current level and set the bias accordingly.
|Fixed bias - bias balance system:
If the voltages on pin five are not the same between the two sides of the power amp, or are not balanced, then you must first adjust the balance pot to make each side the same voltage. Now check the voltage on pin five of all the power tubes again. If they are all the same and you have a balance system bias then you are done because you cannot raise or lower the bias voltage without rewiring the bias system. See rewiring the bias system section.
|Fixed bias - non adjustable bias:
If you do not have a bias pot at all then you must examine the bias circuit to find a resistor that is connected to ground at one end and has the negative bias voltage present on the other end. This resistor can be changed up or down in value to change the bias voltage. If there is too much voltage on pin five then you must drain off more voltage to ground by making the resistor smaller in value. If there is not enough voltage then you must raise the value of the resistor so that less voltage is drained off to ground. This is what a bias pot circuit does. It is a variable resistor that can be changed from zero ohms to the full value of the pot. At zero ohms you would have less bias voltage and at full on you would have more bias voltage. A non-adjustable bias system can be changed to an adjustable system. See rewiring the bias system section.
|Rewiring the bias system:
Looking at the AB763 super reverb layout diagram here, we will examine the fixed bias network. The power transformer has a Red wire with a blue stripe. This is the AC bias voltage winding. The bias winding leads to a 470 ohm one watt resistor. This winding is the winding that will be rectified into a negative voltage compared to chassis ground. First the AC voltage coming from the winding is reduced a bit by the 470 ohm resistor. The AC voltage then is rectified into a pulsating DC voltage by the diode. This diode is inserted so that it rectifies the bottom half of the AC sine wave thereby creating a negative DC voltage compared to chassis ground. If the diode were reversed it would rectify the top half of the AC sine wave and the DC voltage would be positive compared to chassis ground.
After the diode, there is a 25 uf / 50 volt electrolytic capacitor connected to chassis ground. This capacitor is there to smooth out the pulsating DC voltage. It is installed with the negative end towards the bias voltage and the positive end connected to chassis ground. This is because the bias voltage is negative compared to chassis ground. The capacitor charges and discharges slower than the pulsating DC voltage and holds the DC at a more constant DC voltage level. It also has the effect of raising the DC voltage level a bit because the valleys and peaks of the pulsating DC voltage have been averaged up to a smoother less pulsating level. After the filter cap there is a 10k bias pot. The negative DC voltage enters one end of the bias pot and exits the other end through a 27k resistor. The 27k resistor is connected to ground on the other end. A voltage drop occurs across the bias pot and then across the 27k resistor. The center tab of the bias pot is able to sweep across the voltage drop of the bias pot and select different negative voltages. The 27k resistor sets the lowest point on the pot that the bias voltage can go. If the 27k resistor were not there the entire bias voltage would be dropped across the bias pot and the bias voltage could swing from zero volts to the maximum volts produced by the bias network. The bias voltage could short out directly to ground if the 27K resistor was not there and the bias pot was set to zero ohms. This could cause harm to the bias winding if the bias voltage was shorted directly to ground and the tubes would go into runaway current.
The center tab of the bias pot then goes to the junction of two 220k resistors. where it passes through the resistors then through the 1500 ohm control grid resistors and ends up on the control grids of the power tubes as a negative bias voltage. This is basically the way all fixed bias systems work. If your amplifier does not have a bias pot then a fixed value resistor is substituted for the bias pot and the 27k resistor. If the 10k bias pot in the super reverb was set at 6k resistance to achieved proper bias you could substitute a 33k resistor for the bias pot and 27k resistor. (27k + 6k = 33k).
|Changing non adjustable fixed bias to adjustable fixed bias:
To set up an adjustable bias pot we will use the BASSMAN reissue schematic as an example. (page 372 of the tube amp book / volume 3). The first thing you should do is decide what is the lowest voltage you will need to bias the power tubes. This is difficult to know without knowing how much current is flowing through each power tube. See the section on installing a 1 ohm resistor to the cathode of each power tube and decide if this is something that you would like to do. If not then you can guess and experiment with different resistors to find a suitable bias range. Back to the BASSMAN reissue schematic, you can see that the bias voltage should be set at -54.5 volts. Lets say that you decided that a range of -30 volts up to some value higher than -54.5, would be a good range. R41 is a 56k resistor that bleeds off some of the voltage to ground and controls the final bias voltage. By lowering the value of R41 until you find the value that will give you -30 volts, you would be setting the lowest point that the bias voltage could go. You will have to do this with the amplifier on but in the standby mode. You do not want to take the amp off standby with a bias of -30 volts. It is possible that the power tubes may run too much current and be damaged. On most amplifiers the bias voltage is present on pin five of the power tubes when turned on but in the standby mode. To find our -30 volts bias, lets just say that resistor r41 ended up being 22k in value. This 22k resistor will have one end soldered to ground and the other end soldered to one tab of our bias pot. Now we must decide what value the bias pot should be. The old resistor was 56k so we need at least 56k total resistance. We actually need more than 56k to be able to have more range on the high end. If you subtract, 56k - 22k = 34k, this is what the bias pot would have to be to be able to get at least -54.5 volts for our top end voltage. But we need more of a range than -54.5 volts. If we went up in value to next readily available bias pot size, we would choose a 50k pot. Now our bias range would be able to go from 22k to 72k because the bias pot can be adjusted anywhere from zero ohms to 50k ohms. The bias pot must be installed somewhere on the chassis. If you are using a trim pot you can epoxy or glue one end down on the chassis or circuit board. Some trim pots have a threaded barrel, a hole can be drilled somewhere and the pot can be mounted this way. The bias pot is now connected to the point where the bias voltage leaves capacitor c23 on one tab and our 22k resistor is connected to the opposite end. The other end of the 22k resistor is soldered to ground. The middle tab of the bias pot now goes to the junction of the two 220k resistors. Go back and double check all connections before going on to the next section. After checking all connections go to (powering up the amp after changing the bias system) section.
|CHANGING BALANCE SYSTEM TO ADJUSTABLE BIAS:
The balance system is very easy to change over to the adjustable bias system. We will use the Super reverb schematic AB 568 here. If you compare this bias circuit with the super reverb AB763 circuit on page 463 you can see that the all we have to do is rewire a few things and change some resistor values. Basically we are just going to copy the AB763 circuit. First you must disconnect all wires that are attached to the bias pot. Some bias pots have a center tap on the back of the pot, across from the three normal tabs. We will not use this center tap when we rebuild the bias pot. Any resistors that are soldered to the shell of the pot can be unsoldered from the pots tabs but do not unsolder them where they are attached to the bias pot can.
After all wires have been disconnected from the bias pot, you must find the wire that comes from the bias circuit board where the bias diode and bias filter cap are soldered. Solder this wire to the most clockwise tab on the bias pot. We will call this the right hand tab. The resistor that used to be soldered to the middle tab must now be soldered to the left hand tab. The other end of this resistor is soldered to the pot can itself. Now we must rebuild the section of the board where the two .1 caps will join with the two 220k resistors that we will install.
We must first unsolder the two .1 caps on the end that is towards the front of the amp. Unsolder one end of each cap a lift it up out of the way. Any resistors and wires that are attached to these two points must be unsoldered and removed from the board. There should be a hole that is between the two eyelet's where the .1 caps were attached. It is between the two eyelet's but closer to the middle of the board. Remove all components and wires from this eyelet also. Use a solder sucker to remove all solder from the three eyelet holes on the board.
Now you will need two 220k 1/2 watt resistors. Both resistors should be inserted into the middle hole on one end and each resistor should go to one of the eyelet's where the .1 caps where removed. The resistors are shaped like a V when finished. Now a wire must be inserted into the point where the two 220k resistors come together. This wire will go to the middle tab of the bias pot. Solder the wire into the board at the two 220k resistor point and solder it to the middle tab of the bias pot. Now find the two wires that used to be soldered where the .1 caps were. These two wires lead to pin one on two of the power tubes. Sometimes these wires go under the board and come out near the power tubes. If these wires were under the board and never removed they may already be poking up through the two eyelet's. You can unsolder and pull both wires out from under the board just to make sure that you are making the right connections and no mistakes will be made.
Once you have located the two wires that lead to pin one on both halves of the power amp, insert each wire into the point where the two 220k resistors come together. Do not reverse the wires. The wire leading to the right power tube must go to the right 220k resistor and the left wire to the left 220k resistor. Now insert each .1 cap back into the eyelet where the 220k resistor and the wire leading to pin one on the power tube are located. At this point you should have a wire that comes from the center tab of the bias pot and goes to the junction of two 220k resistors. Each 220k resistor should go to a eyelet that has the 220k resistor, a .1 cap and a wire that goes to pin one on each half of the power amp. If this is all correct then you may solder the two eyelet's where the .1 caps and resistors and wires all join. After everything has been checked you may go to the (powering up the amp after rewiring the bias) section. To double check everything, you are just copying the bias system found on the AB763 super reverb layout diagram here
POWERING UP THE AMP AFTER REWIRING THE BIAS:
After rewiring the bias system you can dial your bias voltage up or down on all power tubes at the same time. If you have installed the one ohm cathode resistors you can monitor the current flowing in each power tube and set your bias more accurately than where the schematic says the bias should be. If your power tubes are matched they will be flowing the same amount of current and you will not be guessing about your bias anymore.
See the (checking the power tube current flow section) for more information on matching.
To test the bias circuit after rewiring you should first remove all the power tubes from their sockets and power up the amp. Do not take the amp off standby. Take your multi-meter and connect the black probe to ground and touch the red probe to pin five of each power tube. The same voltage should appear on each pin. If you did not get a bias voltage reading you must go back and start from scratch and re-check every step carefully. If the voltages are all the same then go ahead and set the bias voltage to whatever the schematic says it should be set at. If you have not installed the one ohm cathode resistors then go ahead and power up the amp, let it warm up and then take it off standby. Re-check the bias voltage on pin five of all power tubes and fine tune it if necessary. If you don't know where to set the bias voltage and you have installed the one ohm cathode resistors on the power tubes, then go ahead and set the bias voltage to as high as it will go and we will power up the amp and check the current flow. Setting the bias voltage very high will keep the tubes from running too much current when you take the amp off standby. If all power tube sockets now have a bias voltage reading on pin five, you can install the power tubes and power up the amp. Set your multi-meter on DC millivolts and connect it across one of the one ohm cathode resistors.
See the sections on "installing one ohm cathode resistors" and "checking power tube current flow" for more details.
Take the amp off standby and let it warm up a few minutes so it is good and hot. Check the multi-meter to see how much current is flowing in the power tube. It should be very little if any since we set the bias voltage very high. Now slowly turn the bias pot in the direction that makes more current flow. Slowly bring the current flow up to about 35 milliamps of current. Check the other power tubes to find the highest reading. Set the bias to the tube with the highest current flow. If the power tubes vary in their current flow readings more than five milliamps then they are not a perfectly matched set, but they will work.
See "checking power tube current flow" section for more details.
After the current has been set to 35 milliamps on the tube with the highest reading you are done. The bias of the amplifier should always be checked using the one ohm cathode resistors. When changing power tubes you must check the current flow on all power tubes again. Every set of power tubes are different and must be checked.
*** INSTALLING ONE OHM RESISTORS ON THE CATHODE ***
This is a mod that is done on fixed bias amps so you can see exactly how much current is flowing through each power tube and set your bias very precisely. All you need is a multi-meter that reads millivolts. Basically the mod involves cutting the connection between pin eight and ground and inserting a one ohm resistor between pin eight and ground. Most 6V6 and 6L6 power tubes have a wire that is soldered to pin eight and then soldered directly to the chassis next to the tube socket. The other type of wiring you may encounter is, pin eight and pin one are connected with a jumper wire and then soldered to ground. Both types of wiring are wired the same way.
First you must unsolder the wire at pin eight, that is soldered to the chassis. If there is a jumper wire from pin eight to pin one, leave that wire there. Now solder one end of a one ohm three watt resistor to pin eight. Solder the other end to the wire that is soldered to the chassis. If the wire that is soldered to the chassis looks weak or frayed you can solder a wire from one end of the resistor to the closest ground lug next to the power transformer. Use heat shrink tubing on the second method and keep the resistor away from other components. After soldering the one ohm resistors to all power tubes you will be ready to check for proper current flow in the power tubes. By reading a voltage drop across a one ohm resistor we can see the exact current flow without doing any math. If your meter reads .035 volts this would translate to .035 amps or 35 milliamps of current.
See the (checking power tube current flow) section.
|*** CHECKING POWER TUBE CURRENT FLOW ***
After installing the one ohm cathode resistors you can now check the current flow through each power tube and set the bias much more accurately. First set your multi-meter on DC millivolts. Clip the black (negative) probe to the chassis near the one ohm resistors. Turn the amp on and let it warm up so that the power tubes are hot. Take the amp off standby. Be careful now because you are working near some high voltages. Touch the red probe (positive) to pin eight on a tube socket and observe the meter. Lets say that the meter reads .024 volts or 24 millivolts. This translates to a current flow of .024 amps or 24 milliamps. Now check the next power tube by moving only the red probe to pin eight. Lets say this tube reads .030 volts or 30 milliamps.
One tube is running 24 milliamps at idle and the other tube is running 30 milliamps at idle. The first thing this tells you is that your power tubes are not matched exactly or there is a problem somewhere in the bias circuit. I like to see a difference of only 2 milliamps at most between power tubes. Some matched sets of tubes are not very closely matched and this is one way to check that. If the power tubes were all very close then you have a good matched set. Now you can set your bias according to current flow instead of some general figure printed on a schematic.
It is hard to say where to set the bias, I say you should let your ears tell you where. A good general place to start is to bias the power tubes to 35 milliamps of current apiece at idle with no signal. From there you can go up or down and see how the amp sounds. If you go too high, the power tubes will glow cherry red from too much plate current and may be damaged. Some tubes will take as much as 50 to 60 milliamps and be fine, it depends on the individual tube. Also when setting the plate current this high you have to check to see if the tubes are glowing when playing full blast. If the room is slightly darkened and you crank it up to ten and notice any cherry red glowing on the power tubes you must back down the current. Also be aware that some output transformers are not built heavy duty enough to handle this kind of current and may overheat or be damaged.
Running the idle current up higher than 35 milliamps may give you a fatter, more saturated power tube tone so don't be afraid to experiment. Just keep a close eye on the power tubes and output transformer and make your adjustments in small increments, say 5 milliamps at a time. Play it a while, listen to the amp, feel the output transformer. If the output transformer is getting hot, back the current down and try it there for a while.
There are no set rules here so use common sense when raising the idle current on power tubes. By lowering the idle current on power tubes the amp may clean up and have more overhead. At some low setting, the power tubes may begin to cut off and produce distortion. This type of distortion does not sound very good so it is not advisable to lower the idle current this far. Again you must lower the current in small steps and listen for a while. There is not the same danger when lowering the idle current as there is in raising it. So don't be afraid to experiment with this. If you just want to play it safe, set the idle current for each power tube at 35 milliamps and forget about it.
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