Long tail pair phase inverter

Tube amps: build, repair, modify. Search this site. Navigation Home. Visit my blog. The holes left empty by the removal of the parts are used to build the new circuit. The phase inverter gets its plate voltage from a previously unused filter stage. Add the 82K and K resistors, and connect their common end to the 18K 1watt resistor at the top left of the fiber board.

In what ways does the phase inverter tube affect tone?

Add the rest of the components. Add the new wires to connect the tubes. Wire up the presence control. Mike Tolomeo, Jun 19,PM. A long-tailed pair phase inverter can supply quite a bit of gain, but requires another tube stage. If you're willing to lose the tremolo, the tremolo tube stage can be used to build a long-tailed pair phase inverter. There's the option of adding a presence control at the same time.

Gerald Weber wrote a column on doing this mod in a issue of Vintage Guitar. A note up front--I don't like the sound of this conversion in a Princeton Reverb. It gives it too much gain and it becomes real dirty at all settings, and converts an amp with several tones into a single-tone amp. John Stokes has a tweak that gets some more volume out of the amp without sacrificing tone.

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My notes on his tweak are in the Phase Inverter section. Here's my sketch of the stock Princeton Reverb amp layout.

Not all details are shown, but there's enough here to orient yourself. Click on and print the images to see them larger. It's necessary to have a schematic of the Princeton and a schematic of the long-tailed pair to really understand this.

long tail pair phase inverter

Remove the wires on the wiper and end lug of the tremolo intensity pot, solder and heat-shrink the joint. Unsolder the 2 wires that connect to the end of the 56K resistor, solder and heat-shrink them together.

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Unhook the ends of the 2 red wires that attach to the K resistor, solder and heat-shrink them. Remove all the parts in red. Unhook the wires with a red X on them.Log in or Sign up. The Gear Page. In what ways does the phase inverter tube affect tone? Nov 30, 1. Messages: Just like the title asks. Can anyone explain? Nov 30, 2. A truly fulsome answer would require an entire chapter in a good tube-amps-for-guitar-players type book, but here's a little something to think about.

The three main areas of consideration are the type of inverting circuit, its operating point s and the type of tube. What follows is a response absent of electronic theory and is not an invitation to other amp builders to or electronic engineers to 'correct' me.

I have purposefully oversimplified my explanation to make the discussion more meaningful to guitar players and not military radio technicians with an inferiority complex. No offense intended to well adjusted 'Sparkies'. While there are many inverting circuits out there, let's compare two popular ones: the split load cathodyne and the long tail pair schmitt. The split load phase inverter takes advantage of the circumstance that the signal voltage at the junction of the plate and plate load resistor is out of phase with the grid of a tube, while the signal at the cathode is in phase.

So if you take two signal paths off the same tube, one at the plate and one at the cathode, you will have two identical, out-of-phase signals to drive your push pull output stage. The long tail pair LTP is a little more complicated but a simple answer for our purpose here today would be if you share a common cathode resistor that controls the current flow in a second tube, you can have two out-of-phase signals coming off the plates of two tubes or more commonly a miniature dual triode like a 12AX7.

Notice how I didn't say identical for the long tail pair. The commonly attributed tonal characteristics of the two phase inverters are such: Split Load Much more balanced signal so less harmonic distortion compared to a LTP The load is split across a single tube, so it has a lot less output than an LTP.

It is commonly used to drive a pair of 6V6s.For this design example, we will choose a seldom-used, but good-sounding tube, the 6SL7GT octal high-mu dual triode. The 6SL7 has an internal plate resistance, Rp, of around 44K. Using the general rule of thumb for the "optimum" triode plate load, two times Rp, an 88K load would be good, so we'll choose 82K as the nearest standard value, and use this for the two plate resistors.

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However, if the second input is to be used, as a second channel input, for example, the resistors should be made equal, as it will only be balanced for one input, and the other input will be unbalanced even more. In order to plot a load line on the characteristic curves, the maximum current and maximum voltage is first determined, and a straight line is drawn to connect the two points. Since the plate voltage is at minimum when the plate current is at maximum, the a plate voltage of zero volts would mark the x-axis value of one of the end points.

If the plate voltage is zero, this means that all the plate supply voltage is dropped across the load resistor, so the maximum current can be calculated by dividing the plate voltage by the load resistor. The intersection of the plate voltage of 0V and the plate current of 3.

The other end point is simply the intersection of the plate supply voltage and the zero current line, because the plate voltage is at maximum when the plate current is zero, as no voltage is dropped across the load resistor. A straight line is then drawn between these two points. It will have a slope equal to the negative reciprocal of the load resistance, and will intersect the x axis at the plate supply voltage as shown in the picture below.

In a normal common-cathode stage, assuming a V plate supply, the 6SL7 looks like it would be okay with a bias point of around -2V, which would result in a quiescent plate current of 1.

long tail pair phase inverter

The asymmetrical output voltage swing indicates the presence of some even order harmonic distortion. It could be minimized by careful adjustment of the load line resistance and the quiescent operating point if desired. Note that the bias point is midway between the two cursors, at a point on the load line corresponding to a grid voltage of -2V and a plate current of 1. Note also that a curve corresponding to the 1W plate dissipation limit has been added to the graph.

This is the upper limit of the operating area of the tube. The load line should not cross into this area, or the tube may be damaged due to overheating the plate element. The preceding bias point is good for a common cathode stage using the 6SL7, however, since this will be used as a self-biased phase inverter, the plate-to-cathode voltage will be lower by around 80V or so, due to the required voltage drop across the "tail" resistor. Why 80V? We don't want to drop more than that, or the amount of headroom will be compromised.

This is the equivalent of running with a plate voltage of V instead of the V supply we started with. With a V supply and an 82K load, the 6SL7 would be best biased around In order to achieve this bias point, a cathode resistance of 1. Half this value because the resistor is shared between the two tubes would be around ohms, so a good standard value would be ohms. The 80V is the cathode voltage, and the V is the quiescent plate voltage relative to the cathode as determined from the plate curves.

This new load line is shown below, with the cursors indicating the maximum and minimum undistorted plate voltage swings. The quiescent bias point is midway between the two cursors, at the point on the load line corresponding to a grid voltage of The 2. Since the effective input impedance will be around 2Meg, the input coupling cap should be no more than around 0. Too high a cap can lead to unwanted "blocking" distortion, but you can probably go up to 0.

Following is a schematic of the completed phase inverter design. May not be reproduced in any form without written approval from Aiken Amplification.Log in or Sign up. The Gear Page.

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Phase inverter tone : Sweeter clean or sweeter distortion? Nov 15, 1. Messages: Comments about the phase inverters in amps often say a split load cathodyne circuit as "sweeter" and more musical sounding than a long tail circuit. Are players referring to the clean tone or the distortion tone?

Is there any difference in the clean tone between these two phase inverter designsor is the difference mainly when driving the amp to distortion?

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Nov 15, 2. Messages: 3, Nov 15, 3. Messages: 1, Clean tone or distortion tone? It is kind of both. To my ears a 5C3 with its paraphase inverter is warm, woody, has lower headroom and is lower powered than a blackface with its long tailed pair. The BF has more power, more headroom and those glassy cleans that Blackfaces are known for. Nov 15, 4.

Messages: 2, It is part of the power section in a push-pull configuration; all of the signal goes through it, so it has to impact tone in some way, whether the tone is clean or distorted.

I'd save my money for the best V1 tube I could find and not bother with a particular kind of PI tube or a perfectly balanced one - if that is your question. Nov 15, 5.

IME, most phase inverters sound very similar until overdriven, at which point their individual character comes out. Diagrammatiks is right, but if you have a non-master volume amp, or an amp with a post phase inverter master, you're definitely gonna hear the PI when you crank it up. Science AmpsNov 15, Nov 15, 6.

#193: Back to Basics: the differential amplifier, aka long-tailed pair, diff-pair

Nov 15, 7. Original poster checkin' in againI play a Gibson through a blackface Deluxe Reverb amp which was the amp recommended to me a long time ago by several experienced guitarists as best for that guitar. I play mainly clean and with only a lightly overdriven tone.

Reading these forums I have seen comments about the cathodyne split load PI as being "sweeter and more musical" than amps with the long tail PI. Deluxe Reverb is long tail, Princeton is cathodyneimplying Princeton is a sweeter more musical sounding amp.

I got to wondering if the forum comments referring to PI tone were about the clean or the crunch, and why the blackface DR rather than the blackface Princeton was so much more widely suggested to me for the Gibson Nov 15, 8. To me the cathodyne Pi breaks up quicker and gets kinda nasty when driven too hard. Some don't like the "raspy" breakup,but I do.

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Nov 15, 9. Messages: 5, Good discussion. The PIs breaking up defines what kind of distortion you will hear.I just wanted to tell you how much I like the OSD. Absolutely the best amp I've ever played.

The kit was fantastic, went together without a single hitch and has me staying up late unable to stop playing. It has one foot-switched channel — Clean and Overdrive. Since most on-stage set-ups are miked, watts power is almost never required and 22 watts is both sufficient and surprisingly loud even for medium gigs, especially in this design. This lower wattage helps keep volume levels manageable and makes it very useful in a studio.

On the back of the amp there is a control to configure Overdrive channel Trim. There is also a passive effects loop, like in the originals. This is very effective if, for example, you want to inject some reverb. We have used a Holy Grail Reverb pedal here and it works very well. There is an option for a buffered effects loop. This preamp stays nice and clean and allows you to crank the power amp to achieve a highly percussive tone that taps into the compression and sustain of the power amp tubes.

The magic of the OSD comes from the Overdrive channel sound. The Overdrive channel provides a very smooth, sweet sustaining distorted tone. Activating the Overdrive circuit adds two additional gain stages and the result is an Overdrive that ranges from clean and harmonically rich to smooth and thick with ample saturation.

No FET input. The first tube triode is followed by a passive tone stack with controls for Treble, Middle, Bass and Volume. The Treble, Mid, Bass tone stack is followed by another triode stage. The resulting signal is either fed straight into the power amplifier Clean channel or into the Overdrive channel circuitry.

The output section uses a standard long tail pair phase inverter utilizing negative feedback. Since there is negative feedback, the phase inverter section of the amp also features a Presence control to add sparkle and clarity to your tone.

There is a Master Volume control prior to the phase inverter which is very effective at setting the overall volume of the amp. Both the channel switching and PAB switching is done by two relays inside the amp. The PAB accentuates midrange frequencies and attenuates low frequencies, making it great for cutting through during a solo. Engaged, you will also notice an increase in gain and distortion characteristics.

This is useful for pushing the power section, lead boost and getting a bit of extra dirt from the amp. The Overdrive channel consists of two cascaded triodes being fed from the clean channel signal. Gain is attenuated before and after each of the two triode stages in the Overdrive channel.

This limiting of gain keeps the amp smooth and prevents over the top harsh distortion tones. On the back of the amp there is a Trim control that controls the amount of signal sent into the first stage of the Overdrive circuit.

long tail pair phase inverter

Parts selection, lead dress, voltages and tube selection are critical in getting a great sounding amp. The actual build went very smoothly.

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I did have one small issue on power-up that had me really scratching my head for about an hour but once I finally identified the problem I got it fully operational quickly. This was probably the second most-complicated build I have done out of a dozen or so so I was very happy with that.Email: billy guitars. Of all the circuits in a tube amplifier, the Phase Inverter, also known is the Phase Splitter, is the most difficult to understand by even some experienced techs.

Its function is relatively simple: take a signal input, and create two outputs, one that is identical e. Single-ended power amp, like those contained in the Fender Champ, which sport only one power tube, do not require this additional step, and need only a driver before the power tube to boost the preamp signal to a level usable by the lone power tube.

So, why is the Push-Pull method of power amplification used when it is inherently more complex and costly? Several reasons.

First of all, it enables us to use a more efficient amplifier class called Class AB. No signal in, no power use, therefore the tubes remain relatively cool until pushed.

The second reason for using Push-Pull is that unwanted sonic artifacts, such as hum and odd-harmonic distortion this is the nasty, raspy kindare naturally cancelled in the Output Transformer. Even-order harmonic distortion the kind that sounds coolremains relatively untouched. Back to the Phase Inverter. There are two designs that dominate guitar amps. The Split-Load is the simplest arrangement. The way this works is that signal input to the tube at the grid causes a variation in current flow from the cathode to the plate, producing a voltage swing on the plate, out-of-phase with the input signal.

As long as the plate and cathode resistors are the same value, the amplitude of the two outputs will be similar, except for the flipped-phase. This is a very important concept to grasp for later. The main drawback of this circuit is that it offers no signal voltage gain. What you put in is what you get out, except one side is flipped-phase. The Driver provides the gain, the Cathodyne inverter provides the necessary phase-flip, and they both live as a happy family.

But wait…. Well, you can. It DOES deliver current gain, which is good for circuits like tone stacks that tend to hog current. Save this thought for later. The signal enters the first stage Common-Cathode in the usual way, through the grid. This produces a voltage swing on the plate AND the cathode of this stage, as described earlier. Well, the cathode is still up for grabs. This is where it gets mighty ingenious. If you tie the cathode of the first stage to the cathode of the second stage, the variation in current of the first stage will be superimposed on the cathodes of the second stage.

Here is the circuit again, redrawn and simplified in Fig The cathode circuit, which is essentially a cathode follower, we already know has NO voltage gain, so the second stage Common-Grid amp provides the gain. This is the key element missing in the Cathodyne Phase Inverter Fig. After voltage gain is applied, the signal then travels to the power tubes as well. I stated earlier that the Common-Cathode amp has higher voltage gain than that of a Common-Grid amp.

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In an attempt to balance this out, the plate resistor of the Common-Cathode circuit is slightly reduced, reducing the gain of that stage.Following is a schematic diagram of a typical phase inverter found in some guitar amplifiers:.

The basic circuit is commonly known as a "differential amplifier", which means that it amplifies the voltage difference between the two grid inputs. It should be noted that there are actually three inputs used in this type of phase splitter. The first input is the obvious one, the left side of C1.

The third input is not so obvious; it is the lower end of R6. If a signal is input at this point, the phase splitter will produce an output signal on each output that is in phase with the other, rather than degrees out of phase, and also in phase with the signal input at the lower end of R6.

This means that if a signal of equal phase is applied to the first input C1 and the third input R6it will subtract from the out of phase output R1 and add to the in phase output R2. Likewise, if an equal phase signal is applied to the second input C2and the third input R6it will subtract from the in phase output and add to the out of phase output this is because the out of phase output is actually in phase for the signal applied to the second input, C2, and the in phase output is out of phase.

This third input is useful for balancing the feedback signal by subtracting from the in phase output and adding to the out of phase output in order to compensate the unequal gains to each output from the feedback input. The gain is much less than the gain into the first and second inputs. The two outputs provide nearly identical signals, except for a degree phase difference between them. This is exactly the type of signal needed to drive a push-pull amplifier, so this circuit is commonly seen in higher-power guitar amplifiers.

The value of these resistors is not critical, but they should be a moderately large value, somewhere around K - 1Meg. This is why it is not a good idea to use too large a value of coupling capacitors going into the phase inverter input. This increase in effective input impedance is known as "bootstrapping". It is similar to the effect you get when you have a self-biased cathode follower. There is an AC signal present at the junction of the grid resistor R3 and the "tail" resistor R6since there is current feedback due to the unbypassed tail resistance.

Since this signal is in phase with the input signal, the effective current through the grid resistor is lowered. The signal at the top and the bottom of the grid resistor is subtracted, and that voltage divided by the grid resistance gives the input current drawn by the stage.

If you divide the input voltage by the input current, you get the effective input impedance. For example, if you apply a 1V AC signal and the signal at the tail node is 0. If the tail resistor is large enough to be considered a constant current source, and there is no global negative feedback, the input impedance will be twice the value of the grid resistor.

If there is global negative feedback, the signal applied to the second input will be in phase with the signal applied to the first input this results in a reduction in the output voltage, which means the feedback is negative.

This signal will add to the cathode voltage because it is in phase. Assuming matched tubes with equal mu's, this means the source and load impedances are equal at the cathode, so the voltage is divided exactly in half.

This means that the input impedance is dependent upon the amount of negative feedback applied, and can get very large for large amounts of negative feedback. For example, if 1V is applied to the first input, and 0. These grid resistors have little or no effect on gain, for normal values. If they are too low in value, they will attenuate the input signal. They do have an effect on the frequency response. Higher values will result in greater low frequency response for a given input coupling capacitor, but this effect is diminished somewhat due to the local negative feedback.

The input coupling capacitors. These capacitors also determine the lower -3dB point of the frequency response of the phase inverter. Too large a coupling capacitor will increase the tendency for the phase inverter input to generate "blocking" distortion. If C1 is made small less than. An interesting thing can happen, though, when the phase inverter hits clipping. This very high input impedance suddenly drops, and can severely clip the input waveform by "clamping" the top to the cathode voltage level and raise the lower -3dB point.

For this reason, when tapping off the phase inverter input to go to another tube, say, for instance, an effects loop or reverb amplifier, a large value k or so series resistor should be included in front of the grid of the PI, and the signal should be tapped off before this resistor to preserve the original signal.

This resistor can also help smooth out the tone of the PI when it clips.

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