BIAS TRIM POT: Your amp may have one like this or a regular trimmer. This is where (with a non-conductive screwdriver!) you will adjust the bias voltage for the output tubes. We'll discuss how to do this and provide links to other sites which discuss this further. FEEDBACK RESISTOR: This resistor along with the presence control potentiometer control how much negative feedback is employed by the power amplifier circuit. Since negative feedback affects not only gain and frequency response but also dampening, a lot of tonal variation can be achieved, relatively safely, simply by changing out the value of this resistor or by changing which output transformer tap it is connected to. Some even disconnect the feedback resistor. We will discuss all of these options. BIAS SPLITTER RESISTORS: This is where the bias (or C- for the old timers) is fed into the output circuit. These resistors are also the grid resistors for the output tubes and as such have an impact on the gain of the output stage. The "standard" value is 220K ohms, and you should not exceed this value. However, many players lower the value of this component in order to get more grind from the amp, or what is sometimes referred to as a "browner sound". It is also a popular place to install a master volume circuit. We'll go into how to do that shortly. TONE CAPS: These three capacitors, the two bigger ones and the one smaller squarish one to the right (beneath the Slope Resistor) determine how the Bass, Treble & Mid controls work. This is one of the areas of the amp where you can do a lot with a little. One difference between a Super Lead Plexi and a bass head are the values of these three capacitors (and the Slope Resistor which works with them.) If you're interested at all in your tone controls and what they're doing, OEI HIGHLY reccomends tracking down Duncan Monro's Tone Control Simulator program which you can download FREE. A link to his site is on the Cool Links Page. MIXER BYPASS CAP: This capacitor in parallel with the mixer resistor for the bright channel forms a "short circuit" at high frequencies and lets the highs come through the mixer with less dB loss. The original Fender Bassman circuit used no bypass cap here. Changing the value of this cap, or removing it altogether can give a wide range of tonal options. MIXER RESISTORS: These two 470K resistors form a resistive mixer in which Channel's I and II are joined. There is also some loss of gain in the mixer, and in fact the original Bassman 5F6-A circuit used 270K resistors in this position. For a slight gain boost, you can use values lower than 470K here. To maintain a consistant tonality, if you change this resistor value you will need to "scale" the bypass cap. We'll show you how. 1ST STAGE Rk/Ck: My shorthand for First Stage cathode resistor and cathode bypass capacitor. (k for cathode). This is a big part of the difference in tone between the early amps like the JTM45 and the later lead amps such as the model 1987. The early amps and later bass amps used the circuit you see above: a single resistor and bypass cap shared by both sections of the first pre-amp tube. The plexi lead amps and later amps used two seperate resistor/capacitor combinations to give the two channels more divergent tonalities. Needless to say, a lot can be done here. 2ND STAGE Rk: This is a "classic" place for modifications and is home to part of the "plexi mythology". In some plexi amps (obviously not the one above!) there is a .68uF capacitor bypassing this resistor. This boosts the gain of the stage and gives a mid-high boost which can thicken up leads nicely. You can install this capacitor with a switch to bring in in and out of the circuit. SLOPE RESISTOR: Is part of the tone control circuit and has an influence over how the tone controls work. We'll talk about how. OUTPUT COUPLING CAPS: Are another component which gets modified or replaced a lot. The lead amps used a .02uF here, where the bass amps used a .1uF. Here are some of the different details between the '69 and the '73: Presence Control: This control changes from the single 5K-lin taper pot with a .1uF cap in the '69 to a 25k pot in parallel with a 4.7k resistor. This change was made to stop the control from making a "scratchy" noise, since in the original circuit there is some DC across the control. Some folks say there's a tonal difference and prefer the earlier circuit. It is very simple and inexpensive to retrofit if you want to try it out. Bias Trimmer: The PCB amp has a standard bias pot, which is a welcome change! Bright Cap on Lead (Ch I) Volume: This capacitor acts to make the lead channel very bright at lower volume levels. As you turn up the control the effect lessens. By about halfway up the tone does thicken up a lot. You can remove this cap or change out it's value to lessen the effect. Rk/Ck Lead Channel & Channel II: Here we see that the Rk/Ck combination has been seperated into two circuits with different values in the model 1987. Channel II keeps the values from the older amps (and 1986 amps) while Channel I gets a 2.7k paralleled with a .68uF. This leads to a brighter & "gainier" tone for the lead channel. Screen Resistors: The more astute "amp dog" will have noticed immediately the profound lack of any screen resistors in the '69 amp. This is typical of the older 1960's era Marshalls. It is important to remember that they had Mullard EL34's being made back then which could take 800V on the plates and were considerably more rugged than their modern counterparts. Here at the turn of the century installing 1K 5W resistors in between the B+ supply and the screens of the output tubes is an excellent "ounce of prevention" whatever the vintage of your amp. Some folks even claim they even improve the tone. When you lose an output tube it will typically fry this component. Older amps will have screen resistors that show burn marks or other discoloration. They often drift way off spec as well due the stress they endure. It's a good idea to replace them if you have any doubts as to their condition. USING A VARIAC: Much has been made of the alleged use of a Variac by one Edward Van Halen (among others) as part of the "brown sound". Perhaps no other piece of test equipment is so misused, misunderstood and maligned. Curious about whether to use one and if so, how? Then read on.. First off, the term 'Variac' is a trade mark for autotransformers made by the General Radio Corp. You can have a variable transformer made by many different firms, and the only ones that will say "variac" on them will be the General Radio ones. (they all do the same thing so buy the one that best suits your needs and budget). What they do is allow you to alter the amount of AC voltage your amp uses as its power source. This can be very helpful as many vintage amps were designed for 110VAC, and modern voltages can exceed 125VAC. While at first glance you might think, "so what".. that extra 15VAC becomes higher heater voltages (bad) and much higher plate voltages (also bad, in general). You can use a Variac to feed your vintage amp 110V, as it was designed for, and get the "correct" operation and tone from it. (not to mention longer life!). Of course you should have your amp biased at whatever source (AC) voltage you plan to run the amp from. There is absolutely no harm done by operating your amp this way. Taking voltage reduction still further, below vintage voltages, you can enter new tonal realms - which is at the heart of the "EVH Legend". As the story goes(according to some accounts), EVH lowered his variac to 90VAC and found sonic nirvana. What did this do to the amp? Rather than theorize, I subjected my own 50W head to a variac test. The results are below: AC in Filament B+ Bias V Idle Curr PIa PIb Pre B+ Va Brite Va Nrm 120 6.34 432V -36.7 33mA 204.2 203.7 253 189 140 110 5.78 394V -33.3 28mA 188.6 188.9 234 177 134 100 5.24 360 -30.4 24.7mA 177.8 177.7 217 166 130 90 4.71 325 -27.4 20.2mA 165.8 165.5 200 157 125 "AC in" is the variac voltage that the amp is running on. "B+" is the voltage at a power tube plate. "Bias V" is the bias voltage on pin5. "Idle current" is the plate+screen current measured across a 1-ohm resistor from cathode to ground on either power tube. "PIa" & "PIb" are the two phase inverter plates. "Pre B+" is the source voltage for the first stage of the pre-amp. "Va Brite" is the voltage at the plate of the lead channel Va Nrm" is the voltage at the plate of the darker channel. What happened: Lowering the voltage into the amp lowered all the voltages, including the filament, B+ and bias. The bias setting does not track linearly with B+ so as the B+ drops, so does the idle current and the amp is biased colder and colder as we drop the variac. What we could do (what some DO do) is to find a setting, say 90VAC, and then re-bias the amp. Doing this allows you to take advantage of the lower plate voltage (B+) and run the output tubes at higher current levels. IF YOU DO THIS HOWEVER YOU CANNOT OPERATE THE AMP WITHOUT A VARIAC!! If you set the amp for say, 40mA idle current with the Variac on 90VAC, then when you plug the amp back into unchecked wall voltage of perhaps 120V, the power tubes will be badly overheated and destroyed. If you want to stop using the Variac, you need to have the amp set up to run on full wall voltage again first. I really can't stress that enough! Another thing to be aware of is that running tubes on reduced filament voltages is ok up to a point. It actually increases tube life to run filaments about 10% low. Too much lower and you can decrease tube life by causing cathode stripping. With the Variac at 90 VAC, my filament winding was down to 4.71, which is 25% lower than normal. 10% under is 5.67 which would be around 105VAC. What (I think) people enjoy using a Variac is the smoother more compressed tone you get with reduced plate voltage. Many later JMP amps (mid 70's) already have lower plate voltages (in the 380V range) and if this is the tone you enjoy, perhaps you should be looking for one of these amps. For what it's worth, I get my "brownest" sounds running the bright channel wide open and using the bridge pickup on my SG. [OEI - Home] [Transformers] [Ordering] [Marshall Tech] [Fender Tech] [The Archives] [Cool Links] [A