Power amps

Reading the charts

Loop gain. Break the feedback loop at the feedback resistor, push a signal in, and see what comes back through the inverter, output tubes and transformer. Where loop gain is well above 1, the feedback network sets the gain, and distortion and the speaker’s ups and downs drop by about that much. Well below 1, feedback does next to nothing. The top chart is its size in dB, the bottom its phase.

Phase margin. Past the ends of the band, the coupling caps (bottom) and the transformer’s leakage and winding capacitance (top) turn the phase. If the loop gain is still 1 or more once the phase has turned 180°, feedback adds instead of subtracting and the amp oscillates. Phase margin is how far short of 180° the phase is where loop gain falls through 1. Under about 45° the amp rings on a square wave. At 0 it oscillates. If loop gain never reaches 1 at the top, nothing rings there.

Response. Volts at the speaker per volt from the preamp, with the loop closed and broken (the feedback resistor still on the speaker, so the load matches). The gap is the feedback. Presence opens it back up in the treble, resonance in the bass.

Output impedance. What the speaker sees looking back into the amp, from the output into two resistor loads. Damping factor is the speaker’s rated impedance over it. With little or no feedback, a tube amp’s output impedance is near or above the speaker’s, so the speaker’s bass bump and treble rise come through. Feedback pulls it down. The reactive load tool shows what a damping factor does to the voltage across a speaker.

Which clips first. The inverter’s two outputs are drawn with the speaker, each with its distortion. When the inverter runs out of room, its outputs flatten first. When the output tubes run out, their grids pull current on the peaks and the speaker flattens while the inverter stays clean. Feedback hides the first few percent of either, then the clipping comes on fast. Find full power on Clipping stops just short of the first part to run out: the output tubes judged as in the power load line tool, the inverter by one side cutting off or its grid reaching 0 V. With feedback, THD there is usually 1.5 to 3 %. With a speaker load the distortion goes by the speaker’s current, which pushes the cone. The voltage reads higher, since a speaker’s impedance climbs with frequency.

The three inverters. A long-tail pair is two triodes on a shared cathode resistor and a long tail. The preamp drives one grid, the other sits still, and the tail swings the second half the other way. It has gain and the most headroom. Blackface Fenders use a 12AT7 with a small feedback leg. The brownface values come from Fender’s 6G7-A Bandmaster drawing: a 7025, 47 pF across the plates, and a presence pot in series with .1 µF across the feedback leg. The Marshall and tweed Bassman values use a 12AX7, with the whole leg a 5 k pot and a cap from its wiper to ground. A cathodyne (split load) is one triode with equal resistors in its plate and cathode, so the plate swings down as far as the cathode swings up. Its gain is slightly under 1 each side, so a driver stage sits ahead of it and has the feedback in its cathode. The cathode rides high, so it runs out of room sooner. The Princeton Reverb values come from Fender’s AA1164 drawing: fixed bias, 2700 Ω of feedback into a 47 Ω leg. The 5E3 Deluxe values come from the 5E3 on the bench: no feedback, the 6V6GTs on one shared 250 Ω cathode resistor, and a sagging supply. A paraphase is two plain gain stages. The first drives tube A, and a divider feeds a small share of its swing to the second, which turns it over for tube B. The share has to match the second stage’s gain, and it drifts as tubes age or the signal gets big, so the sides wander apart. Its values are typical, not from one amp. The blackface and brownface values start on a 4 Ω load with the feedback from the same tap, the Princeton on its single 8 Ω speaker. The Marshall values use feedback from the 4 Ω tap whatever tap the speaker is on, as most Marshalls do. Values not from a drawing are typical of the kind.

Presence and resonance. Either goes on any inverter. Presence sits across the feedback leg and lets less treble back in. Each kind is sized for a leg of about 5 k. On a smaller leg, like the blackface or Princeton 47 Ω, it’s scaled to fit with a smaller pot and a bigger cap, so it covers the same frequencies and leaves the DC alone: 250 Ω and 10 µF on 47 Ω. Styles that come with presence use their own amp’s values. Resonance does the same in the bass. It’s the SLO-100 depth mod, from a drawing on the SLOclone forums: .1 µF and a 1 M audio pot in series with the feedback resistor, 4.7 nF across the pot. The cap cuts the deepest lows from the feedback. Turning the pot up adds resistance there while the 4.7 nF carries the mids and treble around it, so only the lows get fed back less. Those values suit a 100 k feedback resistor, as in the Marshall-style amps. On a blackface’s 820 Ω or a Princeton’s 2.7 k it’s scaled the same way: 10 k, 10 µF and .68 µF on 820 Ω. Left at the 100 k values, the cap cut feedback up to about 2 kHz and the knob lifted the mids more than the bass, so it worked backwards. Scaled, it raises the bass on every style. It also raises the output impedance in the bass, so the speaker’s bass peak comes through much more. Try it into the generic open-back or closed-back speaker and watch the output impedance chart. The pot is a variable resistor (lugs 1 and 2) on an audio taper: little change in the first half of its turn, most near the top.

Inductor and gyrator resonance. These two work from the other side of the loop: a branch from the feedback node to ground, across the leg, of 2.2 µF, 2.2 k, a 25 k reverse-audio pot (less resistance as you turn it up) and a coil. Where the cap and coil cancel, the branch pulls the leg down, so less bass is fed back and the bass comes up around there. The inductor kind uses a 500 mH coil with a typical 40 Ω winding, centered near 152 Hz. The gyrator kind is a TL071 follower with .47 µF, 68 k and 47 Ω, acting like a 1.5 H coil with 47 Ω in series, centered near 88 Hz. It’s simulated as the op-amp circuit, so a real gyrator’s quirks show: well above resonance it looks like the 68 k, the op-amp can run out of swing or current, and above a few kilohertz its lag moves the treble by about a tenth of a dB. Both are sized for a 4.7 k leg. On a 47 Ω leg they’re scaled down a hundred times, which leaves the gyrator a 0.47 Ω output resistor no real TL071 could drive on a loud note, so treat that one as a what-if. Both lifts are broad. A smaller branch resistor or a bigger coil narrows them.

Supply. The page opens with a sagging supply on a power transformer sized for the tubes. Stiff holds B+ steady whatever the tubes pull. Sags with the signal puts B+ behind a resistance and a reservoir cap, as in the power load line tool: the power transformer and rectifier on average, with no ripple. The inverter is on the same chain, so it sags too. As B+ drops, the output tubes and the inverter both lose room, and a cathodyne on a lower supply runs out sooner. The 5E3 values start with 650 Ω behind 16 µF, matching the drop through the 5Y3GT in the full 5E3 on the bench, about 12 V for 18 mA more current. A solid-state rectifier sags much less, a few tens of ohms.

Inverter and preamp supply. The inverter and preamp run off a chain of dropping resistors and filter caps from the screen node, or from B+ when the screens sit on it. Each amp’s chain comes from its factory drawing: two 10 k steps on a Deluxe Reverb, 1 k and 4.7 k on a Twin or Pro Reverb, two 18 k on a Princeton Reverb, one 10 k on a 2203 or 2204, and so on. Resistors stand in for the preamp, at its idle current, and for anything else on the chain (a reverb driver, a vibrato, a second channel), so the inverter sits and sags where it would in that amp. With B+ set to sag, B+ still idles at its value with the chain on.

Screens. The screens are fed from B+ through a dropping resistor or a choke onto their own filtered node (B+2 on many drawings), straight off B+, or from a separate supply. Ultra-linear feeds each screen from a tap on its half of the primary (the tap’s share of the turns is under Output stage). Triode ties each screen to its plate through a small resistor. Both have less gain, power and distortion than pentode, so the same feedback resistor lowers the gain less. A choke has little DC resistance, so the screens sit closer to B+ and drop less when pushed. Ripple isn’t modeled, so its filtering doesn’t show, only its inductance and resistance. Under Screen resistors (not shown for ultra-linear or triode), one per screen holds down each screen’s current when its plate swings low. One shared resistor with no cap after it carries both screens’ current. The fundamental cancels in it, so its drop follows the even harmonics and grows with drive. Each amp starts with its own screen parts. A pick in either menu stays when you change tubes, pairs or inverter, until Reset amp. On fixed bias a change is rebiased to the same idle dissipation, as with Set bias. On cathode bias the resistor stays put and the idle moves, as in the amp.

Real amps. Start from opens the power amp of a real amp on the bench, from its factory drawing or a redraw: inverter, output tubes, transformers, rectifier and bias supply as drawn, with the supply running from the wall. B+ carries ripple and sags on a hard note as it does in the amp, so the page updates slightly slower. Only values change on it. Its switches, presence and screens stay as the amp has them. Values the drawing doesn’t show are typical, not measured. Saved to Your power amps, it keeps the amp it came from and the values you changed.

How this tool works. The output stage is the same as in the power load line tool: tube models, screen supply, bias, grid parts, output transformer and loads, so the results match. There an ideal source drives the grids. Here a real inverter drives them from its own supply, and feedback runs from the secondary to the feedback leg: under the tail on a long-tail pair, under the driver’s cathode on a cathodyne or paraphase. A tap other than the speaker’s is a perfect tap on the same winding. We solve the whole amp with the bench’s engine, every part in place, interelectrode capacitance and grid current included. Loop gain, response and output impedance are small-signal, every tube at its idle point. The sine runs 0.3 s so the caps settle, and the last few cycles are drawn. With two or three pairs, each side’s tubes are one paralleled device and the figures are per tube. Ripple, the rectifier itself (the sag is an average resistance) and the floating paraphase aren’t modeled yet.

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