23.11.2020

Category: Negative feedback with cathodyne phase inverter

Negative feedback with cathodyne phase inverter

Log in or Sign up. The Gear Page. Aug 29, 1. Messages: I have a Peavey Classic 50 that I'm rolling tubes in at the moment. What old production tubes would be good for the PI? Thanks, Ken. Aug 29, 2. Messages: 30, The PI isn't as much of a tone generator as V1.

Blue StratAug 29, Aug 29, 3. Messages: 7, I would rather use a new one that has been tested for equal gain.

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I get JJs from Eurotubes. CharlyGAug 29, Aug 29, 4. Messages: 2, A good PI tube is just as important as any other tube in the chain. Aug 29, 5. You won't find much of an upgrade going to NOS on this. V1 will most likely have more of an impact on your tone. Aug 29, 6. Aug 30, 7.Think of them as two sine waves with one turned upside down. Because the voltage signals at the grids of the output tubes travel in opposite directions, so do the anodes. While in single-ended amplifiers one end of the output transformer is fixed in AC terms, in push-pull amplifiers both ends of the output transformer swing simultaneously, creating potentially larger voltage differences and moar powar.

So, how do we get these equal but opposite phases? You can do so with transformers see the Bad Hombre headphone amplifierbut that gets expensive, especially as the signals get large or if asking the interstage transformer to handle DC current. The most common option is to use tubes to split or invert the signal.

The Circuit: A guide to Fender’s glorious Fifties amp models

There are several approaches, all having relative advantages and disadvantages. A grounded cathode amplifier inverts the signal it sees. A cathode follower does not.

Modified Fender 5E3 Cathodyne Phase Inverter

You see, when a follower and an amplifier love each other very much, they do special hugs. Then nine months later, the tube stork brings them a cathodyne, which is a combination of both. The cathodyne has two outputs and one of them inverts while the other does not.

Although the unbalanced output impedance of a cathode follower is much lower than the unbalanced output impedance of a grounded cathode amplifier, if both the anode and cathode resistor of a cathodyne are equal, the balanced output impedance will also be equal. A cathodyne is a very simple way of splitting phases, requiring only one tube and equal resistors by the way, it also works with MOSFETs. Relatively limited headroom and slightly less than unity gain are the trade offs for this simplicity.

The paraphase looks a little crazy until you remember that grounded cathode amplifiers invert the signal. One output tube sees the same signal phase as the grid of the inverter, while the other output tube sees the inverted signal from the output of the inverter the anode of the grounded cathode amplifier. The trick here is that the grounded cathode amplifier will have some gain, unbalancing the amplitude of the two phases creating distortion.

To overcome this, the input to the inverter is divided down by resistors R7 and R8 in the diagram. The paraphase preserves more headroom than the cathodyne and only requires an extra resistor or two.

However, the output impedance and gain of the two phases will be difficult to match. This makes it rare in hifi circuits, but an interesting alternative for guitar amplification. This splitter configuration is complicated to direct couple and so it may limit negative feedback. The long tail pair LTP has become a mainstay in push pull hifi applications.

It preserves headroom, provides voltage gain, and has equal output impedance with one caveat: the tail must be long for good performance. The first triode in the circuit can be viewed as a grounded cathode amplifier, inverting the signal it sees at its input. The second triode in this arrangement, with its grid grounded, gets its signal from the cathode, meaning it does not invert the output at its anode.

Generally, the higher the impedance of the tail, the better matched the gain and output impedance of the LTP splitter. Although we often use a CCS today, a large resistor and negative voltage rail can also work well.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.

Generic Tube Amplifier Modifications

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.

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It is commonly used to drive a pair of 6V6s. If you are driving a big output section it's typical to follow a split load with separate driving tubes. It overdrives a lot like a regular preamp gain stage, which can be fairly euphonic in the early stages of overdrive [There's your "So what?

negative feedback with cathodyne phase inverter

Long Tail Pair Compared to a split load the signal is less balanced from side to side, so you get some even order harmonic distortion Because the signal is coming off the plates of two tubes, it is capable of significant amplification as well as phase splitting duties.If you have not been trained to work with high voltage then have an amp technician service your amp.

Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. See more tube amplifier safety info here. The following mods are not specific to any particular amp model and apply to many tube guitar amplifiers.

Change Tone Cap Value. Baxandall or James Tone Stack. Get overdrive at lower volume and control power tube overdrive. I love this thing.

Switched Cascaded Channels for High Gain. Add a 6" Practice Speaker to your amp head. This easy mod can tune the tone. The 'No Feedback' position makes the amp break up early kind of like a 5E3 Deluxe. The extra feedback of the Heavy position makes the cleans cleaner and tightens up the boundary between clean and overdrive which can make it easier to control breakup with finger technique. It also makes the amp's overdrive tone cleaner, tighter and more Marshallesque. When changing the feedback source from one output transformer secondary to another you change the feedback resistance by a factor of 1.

In blackface and silverface amps I recommend removing the original resistor and connecting the switch to the two now empty resistor eyelets. Heavy is 2. To add adjustable bias to fixed bias amps with no bias pot you simply replace the bias circuit's second, larger resistor connected to ground with a minik linear pot or trim pot and a resistor of about half the value of the original resistor.

The two bias resistors in a typical bias circuit form a voltage divider to reduce the voltage coming out of the bias rectifier diode.The Deluxe is one of Fender's iconic amps. It has had many changes over the years and had many variations.

To the casual observer they may look similar but all the models bring something different to the table.

negative feedback with cathodyne phase inverter

The Deluxe has usually been 1X12 Combo amp ranging from 12 to 22 Watts making ideal for smaller gigs,practice as home, and recording. In this series of articles I will briefly discuss different models of the Deluxe and their sonic and circuit differences in brief. I hope this article is useful to interested players who want information on the Fender Deluxe. The early years of the Deluxe models were some really good years.

The main characteristic of these amps is great overdrive and compression. They can have really nice cleans too but most players get them for their dynamic and harmonically rich overdrive. All models have two channels; either labeled bright and normal or in earlier models Microphone and Instrument.

All model had three inputs expect for the 5E3 which had four. All models use a 5Y3 tube rectifier. Over the years many amp builders and player have experimented with other rectifier tubes.

The GZ or 5V4 offer a bit more headroom, a little less compression, and tighter bass. In most Tweed deluxe models they can be swapped out with no bias adjustment so it's good thing to try. All models used a pair of cathode biased 6V6 tubes for the power section.

The cathode bias give the amp singing quality when cranked up and a more compressed feel. All Tweed Versions had finger joined pine cab that resonated. In addition the baffle is the thin "floating" kind. This allows the baffle to act almost as if it is part of the speaker. These features made the cab sound woody and gave them unique punch.

This was Fender's very first amp! It got really unique looks with a hardwood cab. They came in variety of colors and woods. The Woody Deluxe usually came with Alnico magnet Jensen P12R likely a smooth cone version but some people report amps with a field coil speaker.

The circuit is differs more than any of the versions that came after it. It used a 6SC7 pre-amp tube and 6N7 phase inverter. The first gain stage is cathode biased like the 5E3 and 5D3 Deluxes. The circuit is pretty much a straight copy of many P.For a look at the cathodyne from a hi-fi persepctive, see my web exclusive book chapter here.

The cathodyne phase inverter is a cross between a gain stage and a cathode follower, because the total load resistance is divided into two parts and shared between the anode and cathode. It has been used in many popular guitar amps including the Fender push-pull Princeton, most Orange amps and several Ampegs. It is also known as the 'split load' or 'concertina inverter'. However, it has received criticism from some quarters, though not always fairly.

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It is true that without due care and attention the cathodyne can produce some fairly ugly overdrive tones, but this is avoidable with a simple ValveWizard trick. When the input signal swings negative the valve will reduce its conduction, so current through it falls.

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The voltage drops across the two load resistors therefore also fall, meaning the cathode voltage must fall while the anode voltage rises. When the input signal swings positive the opposite happens. The output from the anode is inverted, while from the cathode it is not.

Since the same current flows through both loads, the signals generated across them will be identical but degrees out of phase assuming we use equal loads. This might be seen as a disadvantage, but remember, we only need one triode, unlike a long-tailed pair.

This usually leaves you with an extra triode which can be used as a gain stage -the combination then provides about twice the gain of a long-tailed pair using the same two triodes. The cathodyne usually provides better balance too.

negative feedback with cathodyne phase inverter

Load Lines Any triode could make a good cathodyne. Valves with small internal resistance like the ECC82 can acheive greater swing into heavier loads, and are also easier to DC couple to the previous stage. When choosing a load we are really only concerned with output signal swing.

If you need to overdrive bigger bottles like EL34s then you might want to push this closer to k. For example, the blue load line in the image represents a total load of 94k with a V HT. In other words, the anode and cathode resistors are both 47k. The green line is an AC load line and represnts the effect of choosing a 1. The valve sees two of these, or 64k. You can see from the green load line that the total output signal swing is about Vpp.

However, this is shared between the anode and cathode, so we will actually get a maximum of 75pp from each output.Fender defined state-of-the-art guitar tone for many years, starting in the late Forties.

From sparkly Tele-laden country to roots rock and humbucker-driven blues, Fender amps set the stage.

negative feedback with cathodyne phase inverter

A comprehensive Fender amp collection should include woodies and tweeds as well as blonde, brown, and blackface models. The tweed amps, in particular, are prized for their tone. Early tweed amps used octal tubes, and while octals sound good, it is generally tough to find metal octals, such as the 6SC7 tube, that are not microphonic. Fortunately, glass bottle octal tubes, like the 6SN7, tend to be fine. Fender made 10 different tweed models, and each went through various changes during its lifespan.

For example, the tweed Super has at least eight different schematics. The two models are basically the same, with Champs sporting a lone volume control and Princetons having volume and tone controls. The Champ started out with a six-inch speaker but later graduated to an eight-inch, while Princetons have eight-inch speakers.

Both are great for recording, due to their harmonically rich tone and low-volume crunch, which has been heard on countless records. The Harvard is a great amp, with 10 watts of output due to the use of two 6V6s in a push-pull configuration, and a inch speaker. Add tremolo and you have the Vibrolux.

The Harvard and Vibrolux are two favorites of mine. They are great living-room amps and sound warm and satisfying with overdrive pedals. The Tremolux is basically a watt Vibrolux with a inch speaker, although the phase-inverter circuits are different. Fender amps feature three different types of phase inverters: paraphrase, cathodyne, and long tail.

These circuit differences have an effect on the tone and feel. The cathodyne phase inverter is a little more forgiving and distorts easier. The long-tail phase inverter has a touch more clean headroom and is used in every brown, blonde, and blackface amp. You will also find fixed- and cathode-biased tweed amps as well as those with and without negative feedback. The channels on these amps are interactive and produce quite a variety of tones.


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