Negative feedback is a commonly misunderstood subject. Global negative feedback refers to the "feeding back" of a small amount of signal from a later part of the circuit to an earlier part, usually from a tap on the output transformer back to the phase inverter. The use of global negative feedback does several things: it flattens and extends the frequency response, it reduces distortion generated in the stages encompassed by the feedback loop, and it reduces the effective output impedance of the amplifier, which increases the damping factor. All of these things affect the tone in some manner. The flattened, extended frequency response obviously changes the tonal character by removing "humps" in the output stage response and producing more high and low end frequencies. The distortion reduction makes the amp sound cleaner and more "hi-fi", up to the point of clipping. Perhaps the main difference for the "feel" is the increased damping factor produced by the negative feedback loop. The decreased effective output impedance causes the amp to react less to the speakers. A speaker impedance curve is far from flat; it rises very high at the resonant frequency, then falls to the nominal impedance around 1kHz, and again rises as the frequency increases. This changing "reactive" load causes the amp output level to change with frequency and changes in speaker impedance (a dynamic thing that changes as the speakers are driven harder). Global negative feedback generally reduces this greatly. This can be good or bad, depending upon what you are looking for. Negative feedback makes the amp sound "tighter", particularly in the low end, where the speaker resonant hump has the most effect on amplifier output. This is better suited for pristine clean playing or a tight distorted tone, while a non-negative feedback amp has a "looser" feel, better suited to a bluesy, dynamic style of playing. The other disadvantage of a negative feedback amplifier is that the transition from clean to distorted is much more abrupt, because the negative feedback tends to keep the amp distortion to a minimum until the output stage clips, at which point there is no "excess gain" available to keep the feedback loop operating properly. At this point, the feedback loop is broken, and the amp transitions to the full non-feedback forward gain, which means that the clipping occurs very abruptly. The non-negative feedback amp transitions much more smoothly into distortion, making it better for players who like to use their volume control to change from a clean to a distorted tone. The amount of voltage fed back determines the amount of gain reduction and the amount of distortion reduction, as well as the effective output impedance. The more voltage fed back, the less distortion, the lower the effective output impedance, the higher the damping factor, and the lower the gain of the stages enclosed by the feedback loop. Typically, in a guitar amp, somewhere around 6-10dB of feedback is used. If you have 6dB of feedback, for instance, and it takes 2V at the phase inverter input to achieve output clipping, if you removed the feedback, it would only take 1V at the phase inverter input to achieve output clipping. In other words, there is a voltage gain reduction of 6dB, or a factor of two, in the stages enclosed by the feedback loop. This is achieved by feeding back a certain percentage of the output voltage to an earlier point in the circuit, the phase inverter. The more voltage fed back, the more the voltage gain reduction, as mentioned previously. The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer. The actual resistor values used in the feedback attenuator aren't that important, as their ratio determines the amount of feedback. The shunt resistor value is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output. Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms. You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair. In addition, if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap. Note that the tap voltages are not linear with respect to the impedance, it varies linearly with the square root of the impedance, that is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap. It helps if you think of the equation for power: P = V^2/R. If you have 100W into 16 ohms, the voltage is V = sqrt(100*16) = 40Vrms. If you have 100W into 8 ohms, the voltage is V = sqrt(100*8) = 28.28Vrms. If you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20Vrms. Closely related to the subject of negative feedback is the use of frequency-dependent elements in the feedback loop to shape the overall response of the amplifier. Most guitar amps have a "presence" control, which boosts the high frequencies. It accomplishes this not by actually boosting the highs in the forward path of the output circuit, rather by cutting the amount of high frequencies being fed back. This effectively reduces the amount of negative feedback at those higher frequencies, which results in a boosting of the highs at the output. Some guitar amplifiers have a "resonance" control, which does a similar thing, by cutting the amount of low frequencies present in the feedback loop, thereby boosting the low frequencies in the output. The amount of boost is equal to the amount of negative feedback. If the amp has 6dB of feedback, there can be at most a 6dB presence or resonance boost. This means that if you reduce the amount of feedback for more gain, you will also reduce the effectiveness of the presence and resonance controls, likewise, if you increase the amount of feedback, you will increase the effectiveness of these controls. There is a danger in using too much negative feedback, however, as the amplifier can become unstable and oscillate, particularly with reactive loads. In addition, the more stages the feedback is applied around, the more likely the chance for oscillations, as there are more phase shifts within the forward path, due to coupling capacitors and other circuit capacitances. Some amplifiers, most notably the Marshall Valvestate transistor amps, use negative current feedback in lieu of negative voltage feedback, or a combination of the two. Global negative current feedback has a similar effect on distortion reduction, but instead of decreasing the effective output impedance and increasing the damping factor, it actually increases the effective output impedance and decreases the damping factor. This makes the amplifier's output voltage vary with variations in speaker impedance. Since a speaker's impedance varies radically with frequency, a current feedback amplifier will tend to feel more "tubey" than a voltage feedback amplifier, because of this speaker/amplifier interaction. In addition to global negative feedback, amplifiers usually have some form of local negative feedback, but sometimes this is not as apparent. A cathode follower is an example of an amplifier stage with 100% negative feedback. This is what gives it the high input impedance and low output impedance, and the near-unity maximum gain. Some amplifiers will use a single-stage inverting amplifier circuit with local feedback from the plate to the grid via a large resistor and coupling cap. The gain of these inverting stages is set by the value of the feedback resistor in proportion to the value of the input resistor. TRIODE Nothing complicated about wiring a triode switch. Just need a double pole switch. Disconnect the usual screen supply from the screen pin and connect it to one end of the switch, connect the plate via a 100 ohm 1 watt resistor to the other end of the switch, and then connect the center of the switch to the screen pin. ie. you either feed the screen from the usual source, or from the plate via the 100 ohm resistor. Only warning, and you should heed it, is that the ratings for triode connected pentodes are usually much less than the usual full pentode ratings. I've run triode/pentode in Champs without any trouble, but if the amp you're working with is already pushing the limits of the pentodes (high plate voltage, abundance bias current) then you could be in for some trouble just flipping it over to triode connection. Measure the normal screen voltage supply. Compare it to the plate supply voltage(s) at the leads of the OPT. If they are largely the same, then the current through the tube will not change when you switch from P to T. This is because Vg2 isn't changing much, and that has the most effect on Ip. I suggest you leave the screen stoppers right on the sockets, and connect THOSE to the center of the DPDT. Those will serve to limit overloading the screens. It is important to note, however, that screen current will drop quite a bit when switching to triode mode. This is because the screen is NEVER more positive than the plate when connected as a triode. Contrast that to the pentode connection where the screen is usually higher in potential than the plate when signals are applied. For this reason the usual series Rg2 is much lower for triode mode, say hundreds of ohms as opposed to thousands. As usual though the increased series resistance won't really do anything but increase reliability. I've done this recently with EL34's with excellent results, only the amp is setup with cathode bias and no feedback. I used 1k-5w resistors to connect the screen to the plate. I've run the amp at both 355V and 405V on the plates,(5Y3 and 5AR4 rectifiers) again, good results with both. The currents were about 38mA in pentode and 48mA in triode @ 357V on the plates. (Transformer shunt method)And about 44mA @ 405V in pentode mode, don't remember the triode readings @ 405V, I think it was around 56mA. Just a note, for my amp triode mode rolled the highs way off, so much that I installed a bright switch to compensate, although I kinda like the way it sounds with the highs rolled off too! MORE BIAS Bias balance pots are usually wired thusly: Ends of tapped pot to grid resistors of outputs...tap of tapped pot to raw bias supply, through some series resistance (this provides a voltage drop i.e. less negative bias when current is shunted to ground by next rheostat to ground). Wiper of tapped pot to another pot, wired as rheostat (i.e. wiper tied to one leg of pot) rheostat pot wired to ground through some series resistance (this limits the amount of current which can be shunted to ground, which thereby limits "hot" range of bias control) I should mention that the scheme I've described is both bias RANGE as well as BALANCE. The tapped pot provides the balance, and the pot/rheostat provides range. Ken, Thanks for the info. The question is how do you wire in a bias balance and bias adjust into the BFPR circuit. Right now the "raw" bias supply goes into a bioas adjust pot. The output of the bias adjust goes to one side of the "Intensity" control. The wiper of the "Intensity" control is wired to the junction of the 220K resistors. Except for the addtion of the bias adjust pot this is the normal bias wiring in a BFPR. I would have no problem adding a balance pot if an amp that did not have this type of vibrato set-up (where the effect is generated by modulating the bias supply). What I am unclear on is how you wire in a balance control in a circuit with this particular Vibrato design...if it can be done at all. Just get enough small value 5w+ resistors to dial it in. 500ohm 5w to 10w resistor and move up from there a little. 15W wirewound rheostat is easier... with a 500ohm 10 watter, check the voltage drop across it to determine the total curent through the tube. If the tube is running less the 80% of it's rating (including screen current), I think you should just listen to it to determine how you like it.Fixed Bias measurements: Plate Voltage: 335V Cathode Voltage: 0V Voltage Drop Across Tube: 335V Grid Voltage: -25.9V Tube Current Flow: 19 ma per tube Using a variac to adjust the voltage, I switched to cathode bias and measured the following: Cathode Bias Measurements Plate Voltage: 361V Cathode Voltage: 26.0V Voltage Drop Across Tube: 335V Grid Voltage: 0V Tube Current Flow: 19 ma per tube I then measured the cathode resistance value : 660 ohms Check against current 661 x .038 = 25.1V pretty close I was attempting to demonstrate that if you know the current you want, there is a way to hit the resistor value on the first shot. If converting a fixed bias amp, you would lower the voltage by the bias amount, twiddle the fixed bias pot to get the current you want, record bias voltage, then calculate the resistor. The rheostat is easier. I'm not sure what you consider a reasonable cost for a rheostat, but there are two types that work. Rotary Rheostat - Looks just like a pot, mounts in a hole, etc. From Mouser; Part 588-RH12-1K $18.73 From Newark; Clarostat CR-12.5-1500, Part 87F6388 $12.80 (price may be out of date) Adjustable Power Resistor - This is a wirewound resistor where a collar slides along the resistor to adjust the value. Part of the exterior coating of the resistor is "missing", and the collar can contact the resistance wire. Therefore, as you slide the collar, you change the effective length of the resistance wire. From Mouser; Part 588-AR10-1K $7.11 From Newark; Ohmite 210 Series -1K, Part 13F522 $5.74 (price may be out of date) The mounting feet and the collar sold seperately, another $2. I have found both types these in surplus stores for $1-2. Ok.. the first thing I would do is, find a way to get the plate voltage down a little so you don't have underbias the tubes to get the sound you want. This can be done with three 12v/5 watt zener diodes in series, inserted in series with the center tap of your power transformer...cathodes to ground. Now you can get the tubes to idle at a reasonable current level to keep them happy and sounding good. Oh, I think you'll need a bypass cap if you want it to sing. You a normal valued cap for the Rk bypass will be fine. The magic number is variable, but keep in mind about the golden rule of thumb: cap's reactance being equal to or less then 1/10 of the bias resistor ohmage for max gain at the lowest operating frequency. That means a cap who's reactance is <50ohms at 50Hz-80Hz. But, if you use a 500 ohm resistor to start, then I think anything between 25uF and 220uF will be fine. It's not that there is bass rolloff... the amplifier section just has more gain at higher frequencies with it's bias resistor bypassed with a cap. The cap makes the cathode think it's grounded at AC. IOW... the AC is bypassing the resistor via the cap. The smaller the value cap, the higher the frequency that is really bypassed ) around the resistor and grounded.(More gain at higher freqs). large value cap will ground the AC around the cap at all audio freqs. A general boost everywhere... still a flat response, but now twice as loud. No bypass cap means the amp has a flat response with no extra gain above near DC. It will sound like half as much gain. A real big value cap means more gain above near DC. If the amp is biasing up where you like it and there is a DC potential of say 30v across the resistor then you should be using 50-60v caps. One of the things I've noticed in the last year or so is old tweed amps with 25v cathode bypass caps. When I measure the cathode resistor, it might be sitting way over 25v. That might be strain on a 35 to 40 year old cap! A user-adjustable bias trim, panel mounted adjustment over a limited safe range after the nominal cathode bias resistance was determined, could be fashioned by taking a standard 2w carbon pot in series with some large value 1/2w fixed resistor, and placing that adjustable high resistance network in parallel with a slightly oversized main fixed cathode bias resistance. This would provide an adjustable "load-down" of the fixed resistance, allowing a bias trim adjustment. The low wattage pot won't see much actual current compared to the main bias resistor, maybe 10% or 20% or thereabouts, depending on what adjustment range made practical sense. pickups Another little tip... Wire the tone pot like Gibson did in the 50's- instead of running a a wire to the tone pot and then running the cap from a pot terminal to ground.Substitute the cap, for the wire, going to the tone pot, then ground the proper pot terminal. I realize that this is getting a little fussy But, the result is that only the frequencies which can pass thru the cap are loaded by the pot. Try it , guitar volume control If you feel there is something magic about 300K of loading, AND you are one of those folks who rarely turn the volume down all the way, howzabout just sticking a fixed resistor between the ground lug of a 250k pot and ground? Given that you're looking at an estimated fixed resistor of 43K-68K (given the usual variation in values for pots) that still leaves plenty of potential range in volume settings, and a minimum setting that should be relatively quiet (e.g., 250k/47k is lots of attenuation), and possibly quiet enough for pinky volume swells.