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Bias Instructions for MANLEY Reference 600W or Ichiban

MANLEY Reference 600W or Ichiban - Built 1988 - 1993 (see picture below)


All Manley amps have individual trimmers that are used to adjust each output tube's bias voltage. "Bias" is a negative voltage applied to the grid of each output tube which controls the quiescent, or standing, current draw of each tube. It is like setting the idle on your car. When we "measure the bias" we actually measure the voltage drop across the cathode resistor of that tube. When we "set or adjust the bias" we actually adjust the amount of negative voltage going to that tube's grid. But don't worry about all that if you don't want to... we just need you to take a reading with a meter and turn a trimpot to maintain optimum conditions for the tubes and verify that all the tubes are working fine.


1) Optimum bias provides the best balance of tube life and sound quality. Manley Lab's amp designs operate the tubes cool at low current so that the they typically last 5 years or longer if the bias is correct. Although, sometimes tubes have a short life and others may be fine for 20 or 30 years.

2) Individual bias trims eliminate the need for expensive matched sets of tubes or the need to replace all the tubes if one tube has a problem. We do recommend that tubes with "reasonably similar" characteristics are used and we batch and label them at the factory. When you need tubes, give us a call and let us know the number that is hand written on the tube as we have some special tube batching jigs we've been using for years. Yes, we do sell tubes - the good ones. After all, we're only as good as our worst tube....

3) The test points allow the user to check that each tube is working and that the amp is performing well (even if you don't intend to trim). Sometimes if one or two of the 12 tubes is dead the amp may be operational and sound pretty much OK, checking bias voltages will tell you more than your ears will sometimes. It also will tend to show a tube on its way out. You would see excessive voltage drift or a tube be out of trim range. This means replace that tube.


1) Definitely, when output tubes are changed.

2) On a regular schedule, you should at least check every three months but we recommend once a month.

3) Not every day and even once a week is probably excessive.

4) With new amps, the bias has certainly been factory adjusted but with all new tubes, it's a good idea to keep an eye on these newbies for the first month or so. If you replace some tubes that have not been "burnt in" or pre-tested or batched we also recommend you check the bias every few days for a few weeks watching out for "infant mortality".

5) If you see a red glow on the plate structure of the tube (inside the tube) then you need to check that tube's bias.


1) For this procedure you will need: a common voltmeter (a hand-held auto-ranging DMM (Digital MultiMeter)), and a small 1/8" flat screwdriver. With all tube power amps an output load is absolutely required. Easiest, is just leave your speaker connected to the amplifiers. If you have your amplifier on a test bench or away from the speakers you can use a 5 Ohm to 10 Ohm dummy-load resistor (5 or 10 watts is ok) attached across the binding posts. If you are using a low wattage dummy load resistor, do not play any tunes into the amplifier while it is hooked up this way! If you don't know what we're talking about, call for assistance or just leave your speakers hooked up.

2) The amp must be well warmed up (an hour will do). When you get ready to check the bias, there must no music playing through the amplifier. Silence please so you will get a valid measurement. Turn down the volume control on your preamp. (And hopefully you don't have any nasty DC offset off your preamplifier causing other problems... )

Note: an open input to an (high impedance) amplifier will pick up buzzing noise. A shorted input is great.

Hey, what a concept: Turn the INPUT MUTE ON!

3) Set the meter to read 'millivolts' DC or use the 2 Volt DC scale on a non-autoranging multimeter. The voltages you are looking at are under one volt.

4) Locate the two black bias measurement tip jacks on the faceplate marked "READ BIAS". Connect the red or ( + ) probe to one of the "Read Bias" test points and your black or ( - ) meter probe to the other "Read Bias" test point. It doesn't really matter which probe goes in which test point, but facing the front of the amplifier, you will get a positive reading if your red probe is in the right jack and the black meter probe is in the left jack. So do that.

5) The 12 position switch in the center of the black insert on the faceplate selects each tube for reading the bias. Start at the One O'Clock which corresponds to Tube #1. This switch reads like a clock so if you are selected at 6 O'Clock then guess what? You're at tube #6.

If the meter is on and set to read low volts it should read between positive 150 and 350 milli-volts DC or 0.150 to 0.350 volts DC. The value should correspond to the tube used.

275 mV
275 mV
275 mV
275 mV

If the voltage is too high, the tube will be running "hot", and will have a shorter life. In extreme cases a tube plate may glow "cherry red" - which usually kills the tube if it has been this way for more than a few minutes. Too low a voltage reading and the tube is "starved" and may cause some crossover distortion apparent during quiet passages. A higher voltage reading corresponds to a higher tube current draw. Ohm's law tells us V/R=I, so a reading of 275 mV DC divided by the 10 Ohm cathode resistor we are reading across gives us a quiescent (standing) current draw of 27.5 mA. Within a range of 20% there should be almost no change in the sound but probably a change in predicted tube life. Expect some drift - especially from month to month. 10% drift is quite normal and 20% begins to raise the warning flags. This is an early warning to watch this tube-- it may be beginning to go south.

4) Check the next tube(s) by selecting the next position on the switch. See the picture below for which tube corresponds to which adjustment trimmer. Continue until all points are checked. A tube that is "way out" often indicates it should be replaced or watched and checked over the course of a week or so.

5) NOTE: If you get some strange or erratic reading, swiftly switch back and forth the read bias switch cycling through each position rapidly. These switches are known to oxidise a little over the years. By switching rapidly through each position you stand a good chance of cleaning the silver contacts inside the switch. If your switch has bummed out entirely, you'll note the wires from the switch go down to the top of the amplifier deck and are plugged into more jacks. You can always use these jacks as secondary emergency readout points. They read left to right facing the front of the amplifier, ie; the left most jack corresponds to tube #1. Facing the FRONT! They are labeled in the picture below in VIOLET.


You need that small screwdriver now, so locate the holes in the top deck of the top chassis under which the trims are visible. They are small square blue trimmers with white centers with a slot for the screwdriver. In the picture below, they are labeled in YELLOW. The tubes themselves are labeled in GREEN. Be extra-careful not to miss the trimmer and short out the screwdriver to a circuit board trace. If you have a plastic tweaker, great, use it. These are single turn trimmers with a rotation of only 270 degrees. You should never force-turn one of these trimmers. See the picture below for where these trimmers are located and which one corresponds to which tube.

1) This is the same as the above check procedure but at each measurement you can adjust a trimmer to achieve the recommended voltage. You should perform a check regularly but only adjust or trim the bias on tubes that are reasonably out of calibration or when tubes are changed.

2) Repeat as necessary. You have to go back and do it again if any of the trimmers needed major tweaking. You may notice that the first points have changed. There is some interaction. If just some minor 10% or 20% tweaks were needed you won't see much change.

3) Done - Sit back and enjoy some music. That wasn't too tough.

4) With new tubes, we recommend checking the bias daily for the first week of operation, and once a month thereafter.




The most common problem is that you tend to see all the tubes drifting up and down together. This is caused by AC power line fluctuations. Probably nothing we can do about that.

Goofy and erratic readings: see above about flaky bias select switch.

Sometimes you may see one test point only read "Zero" no matter where the trimmer is tweaked. Usually this indicates a dead tube. 9 times out of 10 replacing the tube solves that. Sometimes a tube is not too graceful as it dies - it shorts out and burns out the 10 Ohm cathode resistor. You can check the state of these cathode resistors by much like measuring bias except that the amplifier is OFF and the meter is set to read OHMS. Each should read close to 10Ohms. If one reads much higher, in that case a technician, with a soldering iron and a few 10 Ohm 5 watt resistors is needed. Easy fix. Better to burn up a resistor than a transformer. It does double duty as a fuse. Usually one of the "real" fuses goes first but not always.

"Don't worry, its rare, department"

If all the tubes suddenly start glowing bright cherry red it means the amp has lost the bias power supply. Turn off the amp and get it to a technician. Suggest "that he should remove the tubes before turning the amp on".

A few other obvious tube symptoms

"It makes a lot of noise if I tap this tube". It happens. The first preamp tube (12AT7) will generally be somewhat more microphonic because it deals with the smallest signals and amplifies most. It is a judgment call whether any of the tubes have become too microphonic. You may need several tubes to "pick out" the best one.

"This tube has turned white and cloudy". We like to joke that the vacuum leaked out. Air has gotten into the tube, probably through a small crack in the glass. Its not at all well - replace it. It's a goner.

"This seems like a lot of maintenance and technical mumbo-jumbo". Its all pretty easy really and on your own schedule. Remember that in the "old days" your folks could fix the family TV by changing a tube. This is the next easiest thing to changing a light bulb. You might also consider that when a solid state amp has a problem - it's FED-EX time. Your local tech wizard probably doesn't have the schematics and parts, and you probably just lost a few speakers in the process. Could be worse. Most people have less technical problems with our amps than with their CD player(s).

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