Biasing The JCM-900
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DISCLAIMER: LORD VALVE (W. WHITTAKER DBA NBS ELECTRONICS) WILL TAKE NO RESPONSIBILITY FOR ANY INJURIES OR DAMAGE SUFFERED BY ANYONE AS A RESULT OF THE MISINTERPRETATION OR MISAPPLICATION OF THE INFORMATION CONTAINED IN THIS ARTICLE. PROPER HIGH-VOLTAGE SAFETY PROCEDURES AND PRECAUTIONS MUST BE PRACTICED AT ALL TIMES WHEN SERVICING VACUUM TUBE AMPLIFIERS!! IF YOU ARE NOT COMPLETELY FAMILIAR WITH THESE PRECAUTIONS, SEEK GUIDANCE FROM SOMEONE WHO IS!
This is a revised version of an article I posted to the alt.guitar.amps newsgroup some time ago, in response to another poster's question on how to bias his JCM-900 Marshall. The biasing method presented herein is known as the "cathode resistor" method; it is fairly accurate, although there is a more accurate (and much more dangerous) method known as the "output transformer shunt" method. The OPT shunt method is dangerous because both meter probes "float" at the amplifier's plate voltage; if you drop a probe and it lands on your arm or leg, you could be electrocuted. In addition, many meters cannot successfully perform this reading; a high-quality instrument such as a Fluke or Wavetek DMM is usually necessary to obtain meaningful results. For this reason, and for safety considerations, newbies should stick to the "cathode resistor" method until they have acquired sufficient experience (and sufficiently expensive testgear) to use any of the other methods. Additional procedures such as the "crossover notch" method (signal generator, oscilloscope, and test load required) or the "harmonic distortion" method (signal generator, scope, THD analyzer, and test load required) do not offer significant advantages over the "cathode resistor" or "OPT shunt" methods, and, in the case of the "crossover notch" method, can actually result in bias settings which will destroy the power tubes.
Comments relate to a JCM 900; the procedure is substantially
the same for any 6L6 or EL34 amp. E me if you have questions.
I've added a few things for the Fender guys since I originally
posted this. Tech support is available via e-mail sent to
if you need realtime assistance, call me at 303-778-1156 AFTER 1:00 PM Denver (Mountain) time, Monday through Saturday. Ask for Lord Valve.
Lord Valve Speaketh:
Believe it or not, the factory recommended procedure on this amp is to put it on an AC current meter and adjust the bias control for 850 mA - 1A AC draw from the wall. (I have this secondhand from a Marshall warranty tech...) I'm going to have my bias-adjusting kits ready in a couple of weeks; it'll still cost you some bucks, but at least you'll have some test equipment when you've forked over your cash. If you're not familiar with the insides of a tube amp (or if you don't have any idea what high-voltage safety procedures entail) you'd be better off not to go poking around inside your amp. However... if you don't want to pay someone to do it for you, and you are determined not to send me any bucks for my bias-kit gizmo, listen up: What you'll need is two 1-ohm, 1-watt resistors, a DMM, and a soldering iron. If you look at the power tube sockets, you'll notice that pins one and eight are connected together, and that there is a wire leading from them to a ground lug that's attached to one of the tube socket mounting screws. (On a Fender, only pin eight will be grounded. DON'T ground pin one, or you'll get a big surprise...) CLIP this short wire from the ground lug and REPLACE it with a 1-ohm resistor on each socket. Turn your amp on, but leave it on STANDBY. Set your DMM to the highest DCV scale, ground the black probe to the chassis, and take a reading from pin FIVE of either socket. You should see a negative voltage in the -35 to -50 volt range, if the amp has EL34s, or in the -45 to -60 volt range if the amp uses 5881s (or 6L6s). First, locate the bias trimmer. (Possibly a little square blue thingy with a screwdriver-adjust slot in the center, or a round black thing that stands on three legs, or, for an old Fender, a full-size pot with a screwdriver-adjust slot on both sides; newer PCB-type Fenders use three-leg horizontal trimpots, if they have a bias-adjust pot at all.) Next, adjust the bias control until you have MAX NEGATIVE voltage on pin FIVE. (In other words, rotate the bias trimmer until you obtain the highest negative voltage that the bias supply is capable of delivering.) Install your tubes (the amp is still on STANDBY, remember) and wait a few minutes for them to warm up. Take the amp off STANDBY and make sure your DMM is still set to the highest DCV scale; take a reading between the chassis and pin THREE on either power tube socket. Write this voltage down; you'll need it later. Now, set your DMM to the lowest DCV scale (usually 200 mV) and take a reading across the 1-ohm resistor(s). This reading can be interpreted directly in milliamperes, for reasons I won't go into here. It'll be pretty low, because you have the bias trimmer set to max neg voltage. NOW...adjust the bias trimmer until you get a reading across the 1-ohm resistor(s) somewhere in the 30-40 mV range. MULTIPLY the voltage you read on pin THREE earlier by the reading you just obtained from the 1-ohm resistor. (Example: 450 Volts times 35 milliamps, or .035 Amperes.) This will give you the STATIC DISSIPATION WATTAGE at which the tube(s) is idling. (It'll be wrong, but more on that later.) The above example gives a static dissipation of 15.75 WATTS, which is well within specs for an EL34 (fairly cold, in fact) or a 5881/6L6. Take another reading from pin THREE (remember to set your meter on the HIGHEST DCV scale before you do!) and write it down. This new reading should be LOWER than the first reading you took, because the tubes are drawing more current now and the supply will sag somewhat. Multiply this new reading by the value you measured across the 1-ohm resistor(s); this will give you the idling wattage. If you have 5881s or 6L6s, you should shoot for 18 watts or lower. (The cooler you run the tubes, the longer they'll last. If you dig the way the amp sounds when the tubes are idling at 12 watts, fine...don't worry about it.) For EL34s, go for 21 watts or less. Remember, each time you adjust the bias control, you'll have to take a new reading from BOTH the 1-ohm resistor AND the plate (pin THREE) and multiply them to see what the tube is dissipating. You can play your guitar through the amp each time you adjust the bias, and see how you like it. You can even adjust the bias by ear, and then take readings as outlined above to see if the tubes are being operated within their ratings. If you find that you only like the tone when the tubes are operating near their limits, you may decide to trade some tube longevity for the tone you need. If you like the tone with the tubes running cold, you'll obtain significant extra tube life that way. It's your call! If you see a few milliamps difference between the two 1-ohm resistors, don't sweat it; this could be due to poor matching (not a factor if you bought 'em from me :), differences in screen current between the tubes, or differing leg impedances in the output tranny's primary. (All of those things are fairly common in guitar amps.) If you see a large difference between them (say, 8-12+ milliamps) this means you need to find out why this difference exists. One thing you can do is SWAP the tubes into the opposite sockets and take new readings. If the readings are consistent on the SOCKETS, then you'll need to look at the amp and see why. If the readings MOVE with the TUBES, you can be fairly sure you have a poorly-matched pair. REMEMBER...THERE ARE VOLTAGES PRESENT INSIDE EVEN THE SMALLEST TUBE AMPLIFIER WHICH WILL KILL YOU JUST AS DEAD AS A HAND GRENADE WILL!! If you're not familiar with high-voltage safety, seek guidance from someone who is. BTW, an oven mitt (real men like me use welding gloves) will come in handy for handling hot power tubes if you need to switch sockets; you don't want to let the tubes cool off too much while you swap them before taking new readings.
From the diskette Lord Valve sends out with his orders. Used by permission.