Mike Sulzer

A lot of tbe amps avoid putting op amps in the main signal path, but use them to drive the reverb and amplify its signal, etc. Solid state can be extremely clean and transparent, while tubes have a "sound", even when played clean. You have to work hard to get good distortion from SS, while tubes pretty much just do it themselves.

I think that it is best to keep a tube guitar amp all tubes, too.

Depends upon what you mean. If the goal is to add gain so that one can overload one or more triode stages without introducing a lot of solid state distortion, this circuit does not do that The distortion part, in the middle of the drawing has an op amp which overloads in either of the two gain positions since it is driven by at least a few volts of signal from the first tube. (I am assuming that the plate reisistor for this tube and its connection to the B+ have been omitted by accident.) The output of this op amp is connected to the output of another op amp configured as a voltage follower with grounded input (no input signal). This should mostly short out the gain stage, reducing the output level and increasing the distortion, but I do not know exactly what the intent was. The output then is clipped by a pair of GAS diodes (although it might not reach clipping level under some conditions) and then drives a tube stage with a cathode follower, with some negative feedback around this pair of tubes. The gain control in the feedback path of the op amp is really an adjustable low pass filter. It does not affect the low frequency gain. Additional tube gain follows this circuit.

Active pickups are usually an advantage when it comes to adding signals in adjustable amounts, and EMG provides you with a circuit for two volume controls with switch, and each volume independent for many of their pickups. Check the web page.

That's a good idea.

The problem with a clean solid state booster is that it you cannot keep it clean and get a lot of gain; you can get a moderate amount with typical battery voltages, but the final SS stage will overload if you use a lot. You could put a tube followed by a pot in front; then the current volume control becomes the master gain.

Glad to help, but some of us do not look here every hour! An amp with a single volume control and designed in the "correct" engineering manner is set up so that that preamp does not overload (except may be with really high output pickups). This one has the volume control after the first stage on each channel as one would expect. Getting preamp distortion is not as simple as just adding another pot; you need more gain. For example, on one of the two channels, you could add another preamp tube stage, followed by another pot, which would become the "master volume". Then the current volume control for this channel would be the "gain". The interesting thing is that I see nine preamp tube sections on the schematic, and so one of the dual triodes should have an extra section. So it is possible without drastic surgery, I think, but this would have to be examined, designed , and executed by someone who knows what they are doing. Another approach would be to throw out the reverb; then you could use the existing stages associated with it for a really high gain preamp channel.

Building a very small preamp to go in a pickup is not so easy, so the first thing to think about is: why do I want to put the preamp in the pickup? From a circuit point of view, putting the preamp a few inches away is not really any different as long as you connect using shielded cable. Here are a few things you could do if you design a pickup and preamp to go together: 1. Wind the pickup with a center tap that goes to ground. Then the two ends feed the two inputs on a preamp that is a differential amplifier. This cancels electrical pickup noise very well. 2. Wind a dummy coil* and amplify it with a separate preamp stage, and then add the pickup and dummy signals electronicly. This cancels magnetic hum, but without the treble loss that occurs when you put the dummy coil in series with the pickup. *Dummy coil design is sometimes not so easy because you need to make a coil with the same sensitivity to magnetic fields as the pickup, but it is best to use a core or cores that have no permanent magnetism.

And the other advantage is that the output impedance of the cathode follower is low, determined by the tube, not the cathode resistor. For example, it would be good to drive a long cable that has high capacitance to avoid losing high frequencies. In some circuits it is used to drive the tone stack. The impedance looking into the tone stack could vary with frequency and change as the settings of the controls are changed. If you drive it with a low impedance, it behaves in a more ideal manner, and you get the full boost and cut that you should.

That is a very good article; I really like the esplanation of travel stretch versus fretting stretch, and how fretting stretch is compenstaed for at the nut.

That's looking really good now. One comment on the reduced bandwidth with the large resistor and small drain current: The transconductance decreases as you lower the current. You can tell this is happening because the curves of drain-source voltage vs. drain current get closer together at lower current or more negative VGS. The output impedance of the source follower rises as the transcondance goes down, and so the bandwidth into a capacitive load decreases. One thing is still confusing me. With the original circuit with the 200K resistor, it looks to me like the output at 5000 Hz with a 1.5 v p-to-p waveform input would be very triangular, kind of like slew rate limiting with an op amp; that is, lots of distortion. Does Spice show this? You might have to put a large resistor in series with the input voltage generator.

Very interesting, Lovekraft. I learned electronics before there was Spice, but I guess I should get a copy, Did you measure the distortion at 500 Hz, or 5000 Hz? This could make difference. I think the only reason for using the larger source resistor would be to get nearly infinite battery life. By the way, the 1 pf cap probably does not do a lot for rf suppression without a choke in the signal line before it. But I guess it cannot hurt. The 4.7 microf cap is power supply bypass, always a good idea, but I bet it work fine without it. I think that you could get lower distortion (if anyone wants it) with this FET by removing the 10 Mohm resistor from ground and connecting to a voltage divider coming from the battery and providing about 2 or 3 volts. Or use the FET JohnH did.

OK, this is a bit more extreme than I thought. The drain current differs by almoat a factor of ten in the two cases. The obvious difference is the clipping level when driving a 40K load. With the larger resistor, you are starved for current, and so you can swing only about .3 volts in the negative direction. With the higher curent, you can get closer to the 1.5 volts. The more subtle difference occurs short of clipping. The same magnitude of change in VGS in the positive and negative directions does not give the same change in drain current. If we make VGS more negative with a set of equal increments in voltage, each additional increment results in less decrease in drain current than the previous one. This is non-linear behavior. The farther you move off the operating point, the greater the effect of the non-linearity. Suppose in the lower current case and the 40 K ohm load, we drive with signal that requires the FET to nearly turn off. The non-linearity is large. Suppose we go to the high current case, same signal voltage. Now the FET does not get anywhere close to turning off, and the non-liinearity is smaller.

How so? ← From the specs, with 22K in the source, you have a drain current of about .1 ma, or about 2-2.5 volts across this resistor. At low frequencies you can swing about 2 volts in the negative direction, not ideal, but more than you need. Driving the 40K load of the cable cap at 5 KHz reduces this swing somewhat, but not too much. Remember the curves for FET; as you lower the drain current, a given increment in VGS has less effect in further lowering the drain current. This is the essential non-linearity of the FET. So with 200K you probably get about 4 or 5 volts or about .02 ma, but the specs do not go this low, so I am not sure. To drive the same voltage and current into the cable cap requires a five times larger relative* current change than with the 22K. Thus the non-liearity is greater. *That is, relative to the no signal operating current.

You are probably hearing a difference in the small amount of distortion produced by the circuit, not its linear freqency response. With the 22k resistor, you have changed the operating point of the FET.

In evaluating the performance of this source follower, there are two concepts to keep in mind: output impedance and the ability to drive current into a load. Output impedance just tells you how much the output voltage changes when you put a load on the circuit, assuming you are driving the circuit with a small signal. With the source follower, the output impedance is determined by the FET characteristics (the inverse of the transconductance). The 200 K source resistor is in parallel, but it is so large that it is the FET that counts. The output impecance of the FET source follower is low enough to prevent high frequency loss. The 200K source resistor limits how much current flows through the FET, and thus how much signal voltage it can deliver into a load. Driving the 1 Mohm input impedance of your amp is not a problem. Driving the cable capacitance might be. If your cable is 800 pf, this is about 40 Kohm at 5 KHz. You can probably only get about a volt peak to peak before clipping with this circuit at 5KHz, but several volts at 50 Hz. Since guitars do not have much output at 5 KHz, this is not a problem. The unmodified Tillman circuit uses an unbypassed source resistor that makes the circuit pretty stable against variations in the FET.

http://www.buildyourguitar.com/resources/lapsteel/ That's right, but some people compensate any way.

This really sounds way worse than it should be even with single coil and no shielding. Does this single coil pickup have a shield that, maybe, you have connected to the hot? Ground lops are a minor problem compared to shielding; humbuckers get rid of hum from magnetic fields, not so much of an issue for playing clean, so something is wrong.

The problem, maybe: When you are in bypass mode, C1 is uncharged. When you switch to preamp, this capacitor has to charge to 4.5 volts through the volume control and the 470 K resistor, producting a voltage at the input of the preamp. I do not know if this overloads the op amp, or perhaps the next preamp in the chain, but if this is the trouble it can be cured by connecting a 10 meg ohm resistor from the left side of C1 to ground. Then C1 is always charged after you apply power to the circuit. If you are using a FET input op amp, you can make R1 and R2 much bigger if you want to, for a higher input impedance.

"I don't see how that's what you want to happen. If I managed to get ahold of a guitar that had the same low action at the first fret at all the rest of the frets without buzz, I'd hold on to it forever. Galen." Galen, I think I am not saying what I mean very clearly. Since the string is angled, rather than running "straight down the pipe" on a constant radius fingerboard this provides part of the proper height of the action (gap between string and fingerboard increasing towards the bridge). This is something that does not happen with a flat finger board. "Thing is, the surface beneath the string isn't flat, and the distance between string and fingerboard doesn't increase evenly." Mattia, I will think about this, but it looks to me as though it does increase evenly to a very good approximation.

Mattia said: "To make this nice and extreme: Take a section of pipe, any pipe. Take a straightedge. If you run it parallel to the central axis, it'll lie there, on the pipe, niiiice and flat, along the entire length. Now try angling that straighedge a little bit, so it's no longer parallel. So it's like a guitar string on a fixed radius board. What happens: it won't lie flat, without gaps." True, the gap between the string and the fingerboard (or staightedge and pipe) increases from the nut towards the bridge. But this is exactly what you want to happen anyway, and the radius of the fingerboard is just one factor that determines the height of the bridge saddle for each string. So there is no problem if there is no bending. "Fact is, all guitar strings, when laid out normally, 'trace out' a conical path (compound radius) and not a cylindrical one (single radius). That's what a compound radius gives you: the arc of the fingerboard changes as you go up the neck." I think that is a rally good way of understanding how the compound radius helps with bending.