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Fender Cathodyne Phase Inverters

Discussion in 'Amp Tech Center' started by Bendyha, Aug 3, 2017.

  1. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Your right rob, that the fixed bias voltage isn't really over the 1M pot, it is referenced through it, with both ends of the the pot, as well as the grid, all sitting at the same potential.

    That 38% of the input signal would still be available, not being shorted out by the 1µ cap to ground, seems rather unlikly to me; that's one hell of a bassy tone cutting control.
    None-the-less, I don't think the circuit would give satisfactory results.
     
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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yea, I was just saying that with the master volume full down you'd still get 38% of the signal through the amp.
     
  3. SSL9000J

    SSL9000J Tele-Meister

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    Outstanding write-up! Particularly the emphasis on practice, rather than theory. (And what's the difference between theory and practice? Absolutely nothing, in theory.) Plus I couldn't help but giggle every time I read "nipple distortion."
     
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  4. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Having finally managed to find a recently made available source of RCA Aplication Notes at the archive section from the good people at one-electron.com RCA-AppNotes
    I was pleased to have found AN-63, which is titled "A HIGH-GAIN SINGLE-TUBE PHASE INVERTER", and is, as far as I can tell, the first -July 1936 - description widely available of this circuit in America.

    Interesting is the comment about keeping the bias resistors bypass cap physicaly distant from any grounded components, such as the chassis....and that they refer to it as high-gain, because it can drive their new 6L6 tube to the limits of its Class A1 setting.

    this is the diagram from the AN-63;

    upload_2019-9-19_15-48-21.png


    Right at the end they mention the phase inverter it could replace, as shown in AN-62, which is the paraphase shown here;

    upload_2019-9-19_15-47-2.png
     

    Attached Files:

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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'm thinking the cathodyne's cathode bypass cap fell out of favor because it isn't needed. Without it we get a gain of almost unity so what purpose could it serve?
     
  6. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Quote from; The naval ship engineering - Electronics Installation and Maintenance Book - 1965 - 09670000120

    When used in audio, video, or radio-frequency circuits, the cathode resistor must always be adequately bypassed to prevent a constant change of bias with signal. If the cathode is not bypassed, any change in plate current will produce a corresponding change in cathode bias voltage. The change in cathode voltage will be in such a direction as to oppose the effects of the input signal, and therefore will have the same effect as degenerative feedback. While a controlled amount of degenerative feedback can be beneficial in extending the over-all frequency response and in reducing distortion, a large amount of degenerative feedback will result in a serious loss of amplification. When the cathode bias resistor is adequately bypassed, the fluctuating a-c plate current caused by the input signal is effectively shunted around the cathode bias resistor, through a much lower reactance path offered by the bypass capacitor. Thus only the d-c component of plate current flows through the bias resistor, and the total cathode current remains constant or the initial static (d-c) value of no-signal current, and is unchanged by the varying input signal. For satisfactory bypassing, the bypass reactance should be about 10% of the d-c bias resistance at the lowest frequencies used. In some applications, such as high fidelity audio amplifiers, a relatively small value of unbypassed cathode resistance may be used. The degenerative action of this resistance increases the fidelity (frequency response) of the stage and reduces the possibility of overloading from strong signals.
     
    Last edited: Sep 23, 2019
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  7. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Why not have the whole text from the Navy....they have a seldom seen Failure Analysis;



    SINGLE-TUBE PARAPHASE INVERTER.

    APPLICATION. The single-tube paraphase inverter supplies a push-pull output from a single-ended input. It is used mainly to drive audio push-pull power amplifiers in public address systems, or modulators, and in receiver audio-stages.


    CHARACTERISTICS.


    Self-bias is usually used although fixed bias may be used, if desired.

    Two out-of-phase outputs are provided, one from the plate circuit, and one from the cathode circuit.

    Frequency response is relatively uniform from about 100 to 15,900 cps.

    Either triodes or pentodes may be used (pentode provides slightly higher output with improved high frequency response).

    Provides less gain than is possible with transformer coupling (output is always less than the input).


    CIRCUIT ANALYSIS.

    General. The single stage paraphase inverter, also known as a phase splitter utilizes the phase inverting property between the grid and plate of the electron tube to supply a 180 degree out-of-phase output. The plate output together with an in-phase output, which is taken from the cathode, provides the desired push-pull output. Since balanced signals are desired, the amplification is limited to that which can be obtained from the cathode, which is always less than unity for a cathode follower. Thus, when plate and cathode outputs are made equal, the output is always less than the input signal. This circuit, however, is more economical to produce and has a better overall response than the transformer coupled circuit. Hence it is usually used in lower priced equipment. For best results two-tube paraphase circuits are preferred, since the overall amplification and response may be arranged to provide better performance with a larger output than either the transformer coupled, or the single-tube stage.
    Circuit Operation.
    The schematic of a typical single-tube paraphase inverter is shown in the accompanying illustration. upload_2019-9-23_21-26-43.png



    The input signal is RC coupled through Cl and R1 to the grid of triode Vi. Cathode bias is supplied by R3 by- passed by capacitor C4. The outputs are developed across plate load resistor R2, and cathode load resistor R4, and are capacitively coupled to the push-pull driver or output stage by C2 and C3.

    With no signal applied, V1 rests in the quiescent condition with Class A bias supplied by cathode current flow through cathode resistor R3. Since grid coupling resistor R1 is returned to the ground side of R3, any voltage developed across R4 by cathode flow has no effect on the bias between the grid and cathode of V1. Furthermore, since the voltage developed

    across R4 in the quiescent condition is steady (DC) no output appears from coupling capacitor C3. Likewise, any plate voltage drop developed across plate resistor R2 is also a steady DC and no output appears from C2.

    Assume a sine-wave audio input signal is applied to the input terminals. During the positive excursion, the grid of V1 is driven in a positive direction in the conventional manner and an increasing plate current flows. When plate current increases, electrons flow from ground, through R4, and C4 (which bypasses R3), within the electron tube from cathode to plate, and through plate resistor R2 to the voltage supply, creating the polarities shown on the schematic.

    Note that the cathode voltage is positive, is in phase with and follows the input signal, while the plate voltage drop is negative and out-of-phase with the input signal. Since these two voltages are constantly varying at an audio frequency they appear as outputs across C2 and C3, and ground. The values of R2 and R4 are made approximately equal so that equal amplitude output signals are produced,

    When the input signal reaches its peak positive excursion and swings in a negative direction, the plate and cathode current through V1 is reduced, and the output voltage is reduced, likewise. As the input signal reaches the zero level and swings down into the. negative region, the polarities across R2 and R4 are reversed. That of R2 rises towards the plate supply source and becomes positive going, while that of R4 continues towards zero, drops below the quiescent level and is effectively negative-going because of the reduced cathode current flow. Again, two oppositely polarized (phased) and equal output signals are produced from the single input signal. Thus as the input varies at audio frequency, the cathode and plate outputs do likewise, but oppositely. Since R3 is bypassed for audio frequencies by C4 the bias remains unaffected by the signal current variations (see explanation of cathode bias given below).

    When used in audio, video, or radio-frequency circuits, the cathode resistor must always be adequately bypassed to prevent a constant change of bias with signal. If the cathode is not bypassed, any change in plate current will produce a corresponding change in cathode bias voltage. The change in cathode voltage will be in such a direction as to oppose the effects of the input signal, and therefore will have the same effect as degenerative feedback. While a controlled amount of degenerative feedback can be beneficial in extending the over-all frequency response and in reducing distortion, a large amount of degenerative feedback will result in a serious loss of amplification. When the cathode bias resistor is adequately bypassed, the fluctuating a-c plate current caused by the input signal is effectively shunted around the cathode bias resistor, through a much lower reactance path offered by the bypass capacitor. Thus only the d-c component of plate current flows through the bias resistor, and the total cathode current remains constant or the initial static (d-c) value of no-signal current, and is unchanged by the varying input signal. For satisfactory bypassing, the bypass reactance should be about 10% of the d-c bias resistance at the lowest frequencies used. In some applications, such as high fidelity audio amplifiers, a relatively small value of unbypassed cathode resistance may be used. The degenerative action of this resistance increases the fidelity (frequency response) of the stage and reduces the possibility of overloading from strong signals.

    As long as R2 and R4 are equal, and C2 and C4 together with their coupling (load) resistors (RL) are equal, the frequency response of both circuits is almost identical,

    At frequencies above 20kc the plate output of V1 tends to drop off because of the effect of the appreciable triode grid-plate capacitance which is usually larger than the grid-cathode inter electrode capacitance. Therefore, where higher audio frequencies are desired, the pentode tube is used instead of the triode so that its reduced inter electrode capacitance minimizes this effect.



    FAILURE ANALYSIS.

    No Output. Open input or output circuits, a defective tube, improper bias, or lack of plate voltage can result in loss of output. Check the bias and plate voltages with a voltmeter. Use an oscilloscope to observe the input waveform and follow it through the circuit from grid to cathode to plate, and then across the outputs. If an input appears across the input terminals but no signal appears on the grid, coupling capacitor Cl is open. If the grid of V1 reads positive Cl is shorted or leaky. When checking the bias across R3, measure from grid to cathode using the proper polarity, and then from R4 to cathode. If the grid to cathode reading is zero, grid return resistor R1 is open. If an output appears across R4 but does not appear on the plate of V1, either the tube is defective or R2 is open.

    With plate voltage present from R2 to ground, V1 is defective (if the voltage is equal to the supply on both sides of R2, the resistor is shorted). When an output appears on the plate of V1 but not at the output load, coupling capacitor C2 is open. If Cl is shorted the plate voltage of the preceding stage will drive V1 into saturation, a constant high voltage will appear across R4, and a constant low voltage across R2 (on the plate of V1) and no output will be obtained. When the tube is suspected, replace it with one known to be in good condition before making any further checks.

    Low Output. Insufficient bias on V1 due to R3 changing to a lower value (or if C4 is shorted) will cause a low plate voltage and a high cathode voltage, and reduce both outputs. If Cl is leaky the grid of V1 will show a positive voltage to ground. If R1 is open the grid of V1 will tend to block or build up a higher than normal bias, operating at or near cutoff with reduced output. If V1 is leaky or gassy, a positive voltage cause by grid current flow will appear between grid and ground. Should normal bias and plate voltage be indicated by a voltmeter but low output still exists, tube V1 may be low in emission and produce a much weaker than normal signal. When operating properly, the cathode and plate outputs will be equal and just slightly less than the input signal amplitude, because of cathode follower action reducing the gain to less than unity.

    Distorted Output. Normally, the output signal will be of the same shape and of only slightly less amplitude than the input signal. Use an oscilloscope to observe the input and output wave forms, with a constant sine wave input signal applied. Flat-topping or rounding off of the positive peaks of the output signal indicate distortion caused by low emission or reduced plate voltage on V1.

    If the plate voltage is normal and flat topping occurs the tube is defective. If the tube is operated with too high a bias (near cutoff) the peaks will also be clipped when the tube is driven to cutoff. If cathode bypass capacitor C4 is open, the bias on V1 will change with the signal, and amplification will not be linear, some amplitude distortion will occur and degeneration will cause a drop in output at the signal peaks. For small input signals little or no distortion will be observed. However, on large signals the bias may be driven into the cutoff region causing bursts of distortion. If C4 is open, the cathode bias will vary instantaneously with the input signal variations and show on a voltmeter as a constantly varying (instead of a steady) voltage. With normal plate and bias voltages, distortion can also be caused by overdrive (too large an input signal). When the distortion observed on the oscilloscope at the outputs disappears as the input signal amplitude is reduced overdrive (or improper biasing) is the cause. Since output coupling capacitors C2 and C3,together with the associated load resistance RL, are in parallel across the plate and cathode resistors, any change in these components or in the load can create some distortion.

    Such a condition will usually create an imbalance end result in different output amplitudes. Leaky coupling capacitors will show a positive voltage on the load side as well as on the plate or cathode sides and are easily detected by a high resistance voltmeter.
     
    Last edited: Sep 24, 2019
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  8. KellyWalrus

    KellyWalrus TDPRI Member

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    Very interesting read! Thank you.
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    With a bypass cap around the cathodyne cathode resistor you get amplification from the cathodyne phase inverter but you are limited to the typical voltage swing the grid can handle. Without the bypass cap the cathodyne can pass a much larger signal without distortion but we end up with unity gain. I have never even seen a schematic with a bypassed cathodyne phase inverter.
     
  10. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I do seem to recall having seen it done in a few home-brew Hi-Fi amp schematics, but not in a guitar amp.

    Strange perhaps, in that I could show you literally dozen of cases in books and articles covering the cathodyne where the bias resistor bypass cap is refered too:confused:, an anomaly.

    Maybe it was often discused in books because the original design back in 1932 used one.

    upload_2019-9-28_23-20-58.png altough it does differ in other ways....
     
    Last edited: Sep 28, 2019
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  11. AndyPanda

    AndyPanda Tele-Holic

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    I've really been enjoying reading this thread - I'll probably have to read thru it a dozen more times as I understand a little more each time:D

    I have been working on an older (non-fender) amp from the 40's with a cathodyne PI using a 6j5 tube driving a pair of 6v6 (it may have run early 6L6 originally-no schematics available so anyone's guess - but it had a 15 watt replacement OT so I'm running 6v6). This thread has been really helpful as the PI signal was starting to distort before the output tubes and reading thru this thread has given me a lot of ideas. The original circuit had a 1.2k bias resistor on the PI and only about 130v on the plate. I kicked the plate voltage up closer to 200v but was still seeing the PI distort before the 6v6 outputs. I found that increasing the 1.2k bias to 2.2k did the trick and the waveform from the PI doesn't start to squash until after output tubes have started squashing the peaks. This is how I have the circuit running right now and it sounds pretty good to me but I'm open to any suggestions to improve on this:
    AmpowerCathodyne.jpg
     
  12. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Looking good, but I notice on the 6J5 Data sheet, that the recomended maximum "Grid Circuit Resistance" is 1M. That figure is refering to how much is between grid and cathode, and at the moment you have ca.50% more....so it might be a good idea to move the 1M resistor to the grid side of the 470K resistor. This won't effect the performance of the P.I, but it will reduce any problems that might otherwise occur.

    From RCA note - D.G.Koch - 1952;

    GRID RESISTANCE
    In the present stage of the tube-manufacturing art, grid currents of the magnitude of one microampere may be present in receiving-type tubes. The designer, however, can frequently compensate for this grid current by using the smallest practicable value of grid resistance. This precaution is necessary because excessive grid resistance may produce "run-away" tubes. Whatever negative grid current exists flows throughthe grid resistor, causing a shift in bias that is proportional to the value of resistance. For instance, a negative grid current of one microampere through a grid resistance of one megohm decreases then egative bias by one volt. The bias shift tends to increase plate current.
    A larger plate current increases ionization collisions and thus ion current to the grid, which, therefore, swings even less negative. If grid resistance is too large,this effect may be cumulative, so that plate current reaches destructive values. Negative grid currents may result from ionization of residual gases, grid emission, or leakage across insulation insidethe tube or externally across the tube base. An additional reason for reducing grid resistance is that high grid resistance increases the susceptibility of the circuit to the pick-up of undesired voltages. Transformer coupling between stages of amplifiers is often advantageous for obtaining low d-c grid resistance. Usually, the grid resistance should be kept under 200,000 ohms for power-output types, and under 2 megohms for all other types. Cathode-resistor bias, by virtue of its effective degeneration, may be used to minimize the harmful effects of larger grid resistances.
     
    Last edited: Oct 2, 2019
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  13. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Reminds me of this direct-coupled 6J5 amp....
    upload_2019-10-2_22-0-6.png
     
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  14. AndyPanda

    AndyPanda Tele-Holic

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    Thanks for the heads up on the max grid resistance, I've changed it now as you suggested.

    Just for fun, here are some pictures. The yellow trace is the output from OT across an 8 ohm load and blue trace is one leg of the PI. The tops start to squash before the bottoms. Acceptable? Or should I try to balance it?
    CathodyneDistortion.jpg
    Push it harder and you get a perky nipple distortion (or droopy, depending on which way you tilt your head) :lol:
    NippleDistortion.jpg
     
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  15. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Udderly Phasinating ;)

    Does the Yellow Trace's clipping shift to the bottom if you swap your output tubes over ?

    You could try upping the value of those 2k2 grid resistors to lessen the nipple distortion. 22k should solve it, and not change the output discernibly.

    As to whether the asymetrical output clipping is acceptable...that all depends on what you like the sound of.
     
    Last edited: Oct 2, 2019
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  16. AndyPanda

    AndyPanda Tele-Holic

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    No it doesn't - and I've also tried other pairs of 6v6 and 6L6 and it doesn't shift. I'll try increasing the grid resistors and see what happens.

    At this point in time (just practicing at home) I'm not playing anywhere near the volume required for it to get asymmetrical. It's hard to believe that it's only putting out 12 watts (starts clipping around 9.8v RMS) because it's very loud if I crank the volume!
     
  17. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    An option you could use to get the output tubes to drive and clip more symmetrically. Would be to adjust the signal strength coming from cathodyne. There are a few ways one could go about this. (The 1k bias resistor might want a bit of individual adjusting after doing any modifications.)

    ...............EDIT
    : First thing to check, of course, is wether the plate and cathode resistors are matched.

    Option 1;
    With an adjustable potentiometer, a control to dial in different sounds. Not particularly elegant, but it should work well enough. The value of the * resistor could be chosen to get the output “spot on” symmetrical when the pot is set to maximum.

    upload_2019-10-3_17-25-51.png

    Option 2;
    A trim-pot to set and forget the symmetry.

    upload_2019-10-3_17-26-15.png

    Option 3;
    Peavey's “Texture Control”......I don't know if this sufficiently original and innovative enough to merit being eligible for a Patent, - similar circuitry has been used on many amps for a long time, it's not a new idea.
    One could combine a trim-pot as in Option 2. above, by replacing the R163 100k, with a 50K trimmer and 56k tail resistor.

    upload_2019-10-3_17-26-39.png

    Here the Peavey marketing blurb;

    TEXTURE
    This new, patent-pending feature available only from Peavey is used to fine-tune the power sensitivity, response and “break-up” of the power amp section of your ValveKing™ amplifier. Normal, full-power, Class A/B operation results when the TEXTURE control is set at its fully clockwise (wide open) position and should be used as a starting point when setting this control. As the TEXTURE control is rotated counterclockwise, the effect of one half of the 6L6GC power tubes is progressively subtracted from the circuit, while the gain of the driver tube is slowly increased. The driver's low-frequency response is also altered along with the gain, resulting in more even-ordered harmonic distortion from your power amp, even at lower-than-stage-volume settings. Finally, with the TEXTURE knob in the fully counterclockwise position, the result is a real single-ended power amp section that operates and responds exactly like a true Class A power amp, driven by a real single-ended high-gain tube stage. This setting still allows the unused power tube(s) to draw idle current, thus retaining the efficiency of the standard Class A/B topology. In this mode, power output is also reduced by as much as 60% versus maximum rated power.
     
    Last edited: Oct 3, 2019
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  18. FenderLover

    FenderLover Poster Extraordinaire

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    Do you have the 470K grid stopper before the PI? Make sure it is not 47K or 470.
     
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  19. AndyPanda

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    Yes I do, and it's definitely 470K
     
  20. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Just a little up-date for those designing there own cathodyne P.I.'s. Here is a handy tool to help you.

    I Just came across this relatively new website DC solvers for triodes - A collection of predefined small signal circuits based on triodes ,

    which has some handy calculators for working out, or at least checking the suitability of the parameters, of among other things, the circuits for a self-biasing cathodyne phase-inverter, and also for a fixed-bias cathodyne phase-inverter. I did a few quick trial checks, and the things seem to work as one might hope.

    Have fun!
     
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