_instructions_for_manley

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 10 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.

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

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) 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 5ohm to 10ohm 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.

3) For this procedure you will need: a common voltmeter (a hand-held auto-ranging DMM (Digital MultiMeter) like the cheap & cheerful Radio Shack #22-810 variety is the easiest. It's only $15 bucks.), and a small 1/8" flat screwdriver. If you have a "240, 250, 440 or 500W model" a 3/32" Allen key is also required to remove the black front cover plate hiding the bias controls. You don't need elaborate tools - Radio Shack has all this. If you have the original owners manual, get it out (or go find it here) because it may answer specific questions that may arise.

1) Set the meter to read 'millivolts' DC or use the 2 Volt scale. The voltages you are looking at are about 1/4 of a volt. If it is a SE/PP 300B or SE/PP 807 then these tests are done in SE (Single Ended mode). If your amps has a TRIODE/TETRODE switch (250 or 500) it should be set for TRIODE.

2) Most of the amps have a number of little RED test jacks (one per tube) and a single BLACK test jack. The tip of the Black test lead from the volt-meter should fit in the amp's BLACK test jack. It is simply "ground" and you could also use any exposed (not anodized) part of the chassis or the negative speaker binding post, if you need.

3) Insert the tip of the Red test lead into the most left RED test jack. 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.

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). Move the red test lead to the next red test point moving right. Tubes & test points read left to right. The left-most test point (and trimmer) is for the left-most tube. 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.

You need that small screwdriver and locate the holes in the chassis under which the trims are visible. They are small square blue trimmers with white centers with a slot for the screwdriver. 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.

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.

The "250" and "500" use a slightly different bias for TETRODE mode. It should read about 300 mV in TETRODE - but do the bias procedure described above in TRIODE MODE for 275 mV. There is an internal "master trim" for TETRODE mode that we do not recommend changing. It is located right on the TRIODE/TETRODE switch in the amp. But if you must change the master bias pot, here's how to adjust the bias difference between Triode and Tetrode mode:

Turn the amplifier on its side and remove the bottom cover; set the switch
in the "triode" mode, and make sure the tubes measure about .275V each
(after at least 30 minute warm up). Now switch to "tetrode" mode, and
carefully adjust the trimpot mounted on the triode /tetrode switch so that
the bias reading in tetrode is within about 20mV of triode. Be careful, as
this adjustment is quite sensitive. It is normal for this to change a
little over time just as the individual tube bias changes over time. Unless this
adjustment is >30 mv (.03V) off, leave it alone.

On the "older 350, 500 or 600 double-decker amps" rather than individual test points there are only two points and a 10 position rotary switch to select the tube. The switch reads like a clock with the first position being at 10:00 which corresponds to the left-most tube/left-most trimmer. On both the "Reference and Neo-Classic 250" and "Reference and Neo-Classic 500" models the bias trimmers are hidden behind the black engraved horizontal insert associated with the TRIODE/TETRODE and STAND-BY toggles in the "Reference" models and labelled "To Set Bias Remove This Cover Plate" on the "Neo-Classic" models. You do not have to open up the amp. Simply use a 3/16" Allen key to remove the screws at the corners of the black insert - then undo the (cosmetic) plastic nuts on those two toggles (if present and don't worry, these are not the nuts that hold the switches in place). When you pull the black insert plate out all 10 trimmers will be clearly visible.

The older SE/PP 300B Compacts and Retros (before serial number 199) should be first checked in SE mode to get 740 mV and then additionally checked in push pull. The reading should drop to between 550 mV and 600mV. To ensure maximum hum rejection in PP, the two tubes should be slightly re-trimmed to be equal. Conversely, if it hums, check or do the bias procedure. Please go to the 300B SE/PP Bias page for specific details.

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.

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 10 ohms. 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.

Yeah it happens... we just never know when... Not to worry. There's a nice page here about checking cathode and screen resistors in a Manley Neo-Classic 250 or 500 and replacing them if need be. This page is helpful in many ways and pertains to all of our amps, actually.

Some of you have trouble identifying which trimmer is associated with which tube. We always start at the left and go right (facing forward). On the bigger amps the tubes may be staggered but you still read from left to right. On the 500 watt monoblock the most CCW switch position is the left-most tube and the switch is read clockwise. For the 600W or Ichiban models, you have been on this page so go there now to see which trimpot controls which tube.

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".

"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.

"I think my pet chimpanzee turned one of the bias trimpots a little too hard and broke it. And then a tube went red and I smelled smoke."

One check you can do is with the amplifier turned OFF, put your meter probes into position like you are getting ready to read bias. Except set your meter to read ohms. You should read 10 ohms at each bias measuring point. (What you are reading is the cathode resistor itself.)

When you are reading "the bias", you are actually reading a voltage over this cathode resistor and volts divided by ohms gives the current draw of that tube in amperes. So the secret math is:

200mV divided by 10 ohms = 20mA of quiescent current draw for that tube (if that's what it is). The real bias voltage is a negative voltage fed into the grid of each output tube that tells the tube how much current to draw. You don't get to measure the actual bias voltage just the cathode reference voltage as described above, but when you turn the trimpot, you are changing the negative bias voltage at the grid of each tube. That's how that works.

If the cathode resistor got damaged by Mr. Tube running away, then the math will be off. So it is essential that the cathode resistor is 10 ohms. Check that. It is a 3watt wirewound 10 ohm resistor for EL84 tubes and 10 ohms 5 watt wirewound resistor for every other tube we use that will not be hard to find if you have to replace it.

Second, pop off the bottom and do a visual looking for signs of burnt or discolored somethings. There is also a little 47uF 100V axial electrolytic capacitor in parallel right next to each 10 ohm cathode resistor. Look to see if any of these are puffy or if it looks like the plastic covering is pulled back, or if there is shiny goo all over the place or little bits of brown paper that looks like a mouse was living inside. If you see any of this, then Mr. Cathode Bypass Capacitor got hurt and he should be replaced too.

Once that gets sorted out, you need to check that your pet chimpanzee didn't break the bias pot. Since he won't admit to breaking it, you'll have to check it yourself. That can be checked once you have a known good tube in that socket, with for sure 10 ohms cathode resistor and good cathode bypass capacitor and see if now you can change the bias. If the voltage readout still stays stuck, maybe he did break the bias pot after all, and it is a pain to change, but can be done. It is a 10Kohms trimpot.

Contact our service department for more help. And tell your pet chimpanzee to not play with your amplifier.

"I can't get a bias reading on any of my tubes". We know you were really rocking out with that Def Leppard CD when that happened... So, is your amplifier turned on? Yes of course it is. Lights on but nobody's home. No readout at any of the test points usually indicates the B+ fuse blew. So turn off and unplug the power to your amplifier and locate your B+ fuse. Usually it is the fuseholder that needs a flat screwdriver to remove the fuse from its housing, but sometimes we are sneaky and put the fuse inside the amplifier, like in the Stingray, Mahi's and Snappers. Most of the time we use a ceramic bodied B+ fuse that you can't tell if it's blown or not by just looking at it. In this application glass fuses tend to go nuclear and cause all sorts of other damage when they blow, so that's why we use those sneaky ceramic fuses. Well it *looks* perfectly normal so you'll have to break out your trusty multimeter and measure this thing. Now, don't be a bozo and try to remove this fuse with the amplifier turned ON or you'll be in for a shocking surprise. Power OFF and UNPLUG your amplifier. Go have a cup of coffee and wait for the capacitors to discharge themselves. Now remove the B+ fuse, stick it on the floor, (presuming you do not have a conductive metal floor,) and place your test probes on each end of the fuse. Meter is set to read OHMS. You should read zero ohms for a good fuse and OPEN for a bad fuse. Or you should read whatever your meter reads when you touch the two probes together for a good fuse, and you should read whatever the meter reads when you hold the probes apart for a bad fuse.

So you got a bad fuse? RTFM (read the frikkin' manual) or RTFF (read the frikkin' fuse) to figure out what fuse you need to obtain to replace this guy with. Replace it with what you're supposed to. No, you're not allowed to jam chewing gum foil or 1 1/4" drywall screws or small clutch springs into the fuseholder. In Fuseology, there are SLO-BLO types and FAST-BLO types. And there is an Amperage rating for that fuse. And also a Voltage rating. In any case, get the correct fuse, remembering we like the ceramic bodied ones best for this application, and then replace the B+ fuse.

Before powering back up get ready with your multimeter to check the bias for all your tubes as they come back on-line. Besides maybe you rocked out too hard during your bachelor party, there might have been one bastard tube who was running away and it is your job to find him. So reading all the tubes as they warm up, if you see one who is winning the Most Volts Contest, try turning down the bias for that tube. If that tube is insisting on keeping to climbing, and you can't keep him in range, and now he's starting to turn red, and the fire department is busting down your front door, well he's just going to blow your brand new B+ fuse again, so turn off the amplifier and get a new tube. Check your resistors, re-read this page, etc.

Contact our service department for more help if this is all beyond you or you just need a little friendly hand-holding.

"This seems like a lot of maintenance and technical mumbo-jumbo". It's 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).