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Thread: Cathode Follower in Single Ended Amp?

  1. #1
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    Cathode Follower in Single Ended Amp?

    Hey all,

    I was hoping for some advice on adding a cathode follower circuit to act as a faux phase inverter stage in a single ended amp.
    I have a single ended project that I build years ago, but would like to try to convert it to a TW Express style preamp.

    I've put together a schematic I cobbled together from existing circuits on the web, would my placement of the cathode follower work, or is there anything else I'm missing?

    cathodefollowertest.png

    Thanks!

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    The CF placement looks good but it needs a dc blocking cap after it to prevent putting >100Vdc on to the power tube g1.
    The g1 would also need a grid leak resistor, eg 470k.
    Making it a pot, to act as a gain control (aka master vol type 4), may turn out handy.

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    I did the same thing with a pair of 6L6WG's in single ended,parallel.It was a long time ago,but I think I used a .047 cap,I used a few and really forget where I settled.I def used a 220 grid leak.I have a couple of TW schems,dont recall which one I used.

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    Thanks for your responses! Iíll make sure to include the grid leak resistor and coupling cap and update the schematic.

    Also, would it be a good idea to connect the prescense pot (donít think itís in this schematic) with negative feedback to the cathode follower, or before?

    Is there any reason a cathode follower doesnít require a plate resistor? I just havenít ever incorporated one of these in a design and still learning the theory.
    Last edited by bw1985; 01-09-2018 at 12:48 AM. Reason: Additional question.

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    The "Cold Clipper" third stage will have a high plate Voltage. With a 250V B+, plate Voltage might be 220V. (there are two different Voltages indicated for B+2) The high plate Voltage makes direct coupling to a cathode follower impractical unless a higher B+ can be used for the follower. The grid to plate Voltage for a 12AX7 should be at least 100V.

    The Cold Clipper doesn't output much on the positive side, but big negative swings on the bottom of it's output. IMHO you don't need a cathode follower.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Quote Originally Posted by loudthud View Post
    The "Cold Clipper" third stage will have a high plate Voltage. With a 250V B+, plate Voltage might be 220V. (there are two different Voltages indicated for B+2) The high plate Voltage makes direct coupling to a cathode follower impractical unless a higher B+ can be used for the follower. The grid to plate Voltage for a 12AX7 should be at least 100V.

    The Cold Clipper doesn't output much on the positive side, but big negative swings on the bottom of it's output. IMHO you don't need a cathode follower.
    I noticed some pretty high voltages on V2...I believe they were exceeding the 250v+, I'll need to double check my numbers again when I'm in the shop tomorrow. I believe I may have fried the preamp tubes because now the amp has barely any volume and sounds super fuzzy. First order of business will be to test the tubes.

    However, the first time I fired the amp up...it sounded incredible! So, is there any way of using this circuit but modifying values to bring the voltage down?

  7. #7
    Lifetime Member Enzo's Avatar
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    The cathode follower does not need a plate resistor because, well, it is a cathode follower.

    250v is not going to hurt a 12AX7. The only potential trouble is that 100 volts on the cathode. Tubes have a maximum heater to cathode voltage.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by bw1985 View Post
    I noticed some pretty high voltages on V2...I believe they were exceeding the 250v+, I'll need to double check my numbers again when I'm in the shop tomorrow. I believe I may have fried the preamp tubes because now the amp has barely any volume and sounds super fuzzy. First order of business will be to test the tubes.

    However, the first time I fired the amp up...it sounded incredible! So, is there any way of using this circuit but modifying values to bring the voltage down?
    A Marshall Super Lead may put ~400V on its CF, with a consequent cathode voltage of ~200V, which can damage the tube as Enzo describes.
    But 250V on its plate should be no problem.
    Are you sure that the circuit isn't oscillating?

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    It depends on the 12ax7 indeed.

    Sovteks do not go over 100 Vhk, while EI, Tung-sol and many others are rated around 180 Vhk like old datasheets show.
    Raising the heaters by 40-50V can't be bad for noise reasons too, so why not?

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    Quote Originally Posted by Roberto View Post
    It depends on the 12ax7 indeed.

    Sovteks do not go over 100 Vhk, while EI, Tung-sol and many others are rated around 180 Vhk like old datasheets show.
    Raising the heaters by 40-50V can't be bad for noise reasons too, so why not?
    Well, I've checked the tubes and they are fine! I found one of the pins is acting funny and if I wiggle it around I get my signal back.

    I'm noticing something very strange, seeing 40vdc on the heater windings and hear a weird buzzing in the output. I'm assuming this is related...

    I've updated my schematic with the additional components.

    trainwreckse-updated.png

    Here is a list of my voltages:

    V1
    1 - 195v
    2 - 0v
    3 - 2.38v
    4/5 - 6.6vac
    6 - 171.6v
    7 - 0v —-> 7vac??
    8 - 1.65v
    9 - 0v ——> 40vdc


    V2
    1 - 336v
    2 - 0v
    3 - 3.71v
    4 & 5 - 6.51vac
    6 - 377v
    7 - 0v
    8 - 4.7v
    9 - 0v ——> 40vdc?

    V3
    1 - 0v
    2 - 42vdc / 6.5ac
    3 - 378v
    4 - 394v
    5 - 0v
    6 - N/C
    7 - 43vdc
    8- 43vdc

  11. #11
    g1
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    Your tube numbering on your schematic is funny and I don't think it matches your measurements.
    I think the measurements you call v2 are for the 3rd and 4th triodes shown. One of them should have a higher voltage at the cathode where the 100k cathode resistor is.
    Perhaps you have the cathode to heater breakdown that was mentioned in post #7 & 8. That may cause the buzz and extra DC on the heaters (beyond the voltage from point 'A')
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    Quote Originally Posted by g1 View Post
    Your tube numbering on your schematic is funny and I don't think it matches your measurements.
    I think the measurements you call v2 are for the 3rd and 4th triodes shown. One of them should have a higher voltage at the cathode where the 100k cathode resistor is.
    Perhaps you have the cathode to heater breakdown that was mentioned in post #7 & 8. That may cause the buzz and extra DC on the heaters (beyond the voltage from point 'A')
    Yes apologies, the pin outs donít match the schematic. They are reversed for the 2nd tube.

    So I measured the cold clipper and am seeing +300v across the grid and anode... so should I just chop the cathode follower out of the mix?

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    The CF stage grid needs a dc reference.
    It came from the preceding plate in the previous schematic version (direct coupled cathode follower, see valve wizard).
    Chuck H and bw1985 like this.

  14. #14
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by bw1985 View Post
    I've put together a schematic I cobbled together from existing circuits on the web, would my placement of the cathode follower work, or is there anything else I'm missing?
    Boy, the original schematic you used of the preamp section sure looks familiar... Yikes, it was mine!

    Back then I was using TurboCAD 10.5 with a custom library of electronic symbols I drew up myself. That was like 6 computers ago and I'm sure I backed up the library somewhere but I have no idea which hard drive I put it on...

    One question for the electronic experts here... is there a reason that the "bottom" noninverting phase of a cathodyne PI should be used instead of the "top" phase which is inverting?

    I put a cathodyne PI in my untested Trainwreck Express "PR" Version A5i drawing which was posted strictly for discussion as it compared the Trainwreck design to that of a Princeton Reverb with the reverb circuit bypassed with a jumper. It was inspired by the report that Ken Fischer's initial pre-Trainwreck prototype was a modded Princeton Reverb. Although the specific values differed as shown in red I thought that there were a lot of similarities in the overall architecture.

    In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other? Of course the push-pull power amp section would have to be modified for a single output tube. In particular you would want to use a different cathode resistor. A Vox AC15 uses a 270R for two EL84's; without doing the math I'd try a 270R resistor for starters, but that is assuming that you bring down the B+ a bit. I was told that EL84s are "happiest" with a B+ around 320v although they can handle more than that. You can insert a string of zeners in the CT of your Hi voltage PT windings going to ground (or in the circuit to ground from a rectifier bridge) to bring the B+ down.

    My Trainwreck Express "PR" Version A5i drawing...





    Steve A.

    P.S. I see that your schematic does not show a center tap for the heater windings. You can create a "virtual" center tap by wiring the two heater secondaries through a 100R resistor to ground... or better yet to point A in your drawing which will "float" the 6VAC at whatever DC voltage is present at the cathode of your EL84 and reduce some noise in the preamp tubes as mentioned by Roberto in Post #9. Not as effective as a DC heater voltage to the preamp tubes but so much easier to implement!

    P.P.S. So what software program are you using to create and edit your drawings? They look pretty slick!

    EDIT: I redid the schematic upload because it wasn't working right.


    Attachment 46485trainwreck-pr-a5i.jpg
    Last edited by Steve A.; 01-12-2018 at 09:01 PM.
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    Quote Originally Posted by Steve A. View Post
    Boy, the original schematic you used of the preamp section sure looks familiar... Yikes, it was mine!

    Back then I was using TurboCAD 10.5 with a custom library of electronic symbols I drew up myself. That was like 6 computers ago and I'm sure I backed up the library somewhere but I have no idea which hard drive I put it on...

    One question for the electronic experts here... is there a reason that the "bottom" noninverting phase of a cathodyne PI should be used instead of the "top" phase which is inverting?

    I put a cathodyne PI in my untested Trainwreck Express "PR" Version A5i drawing which was posted strictly for discussion as it compared the Trainwreck design to that of a Princeton Reverb with the reverb circuit bypassed with a jumper. It was inspired by the report that Ken Fischer's initial pre-Trainwreck prototype was a modded Princeton Reverb. Although the specific values differed as shown in red I thought that there were a lot of similarities in the overall architecture.

    In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other? Of course the push-pull power amp section would have to be modified for a single output tube. In particular you would want to use a different cathode resistor. A Vox AC15 uses a 270R for two EL84's; without doing the math I'd try a 270R resistor for starters, but that is assuming that you bring down the B+ a bit. I was told that EL84s are "happiest" with a B+ around 320v although they can handle more than that. You can insert a string of zeners in the CT of your Hi voltage PT windings going to ground (or in the circuit to ground from a rectifier bridge) to bring the B+ down.

    My Trainwreck Express "PR" Version A5i drawing...




    Steve A.

    P.S. I see that your schematic does not show a center tap for the heater windings. You can create a "virtual" center tap by wiring the two heater secondaries through a 100R resistor to ground... or better yet to point A in your drawing which will "float" the 6VAC at whatever DC voltage is present at the cathode of your EL84 and reduce some noise in the preamp tubes as mentioned by Roberto in Post #9. Not as effective as a DC heater voltage to the preamp tubes but so much easier to implement!

    P.P.S. So what software program are you using to create and edit your drawings? They look pretty slick!


    .Attachment 46485
    Hi Steve,

    Thanks for replying, and sorry for adapting your schematic without credit, I actually found it through google image search! But it has been very helpful in working on this project so thanks for creating it. I'm honestly just hacking these schematics together in Paint for now. I'd like to redraw everything once I have it finalized.

    Also, I realized that the 40Vdc that I'm seeing is actually the positive bias voltage coming in on Pin 8 which is also tied to pin 7. I'm still trying to understand the point of this, I'm still quite the amateur amp builder but slowly improving haha.

    It's funny you mentioned the virtual center tap, I was thinking of adding this to the amp. I think it will fix the issue as the sound I'm getting sounds like a ground buzz (I haven't found any other grounding issues in the amp yet).

    I also need to remove the coupling cap before the cathode follower to get that DC reference back.

  16. #16
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by bw1985 View Post
    Hi Steve,

    Thanks for replying, and sorry for adapting your schematic without credit, I actually found it through google image search! But it has been very helpful in working on this project so thanks for creating it. I'm honestly just hacking these schematics together in Paint for now. I'd like to redraw everything once I have it finalized.

    Also, I realized that the 40Vdc that I'm seeing is actually the positive bias voltage coming in on Pin 8 which is also tied to pin 7. I'm still trying to understand the point of this, I'm still quite the amateur amp builder but slowly improving haha.

    It's funny you mentioned the virtual center tap, I was thinking of adding this to the amp. I think it will fix the issue as the sound I'm getting sounds like a ground buzz (I haven't found any other grounding issues in the amp yet).

    I also need to remove the coupling cap before the cathode follower to get that DC reference back.
    After all of these years I am tickled pink that so many people are still using the basic format of my TW and ODS drawings and editing them in Paint or whatever to reflect different values that people have found or used. After Dumble himself sent a fax to a supplier about me being a "thieving pirate" I would just as soon not get credit for those drawings...

    Steve A.
    Last edited by Steve A.; 01-12-2018 at 08:29 AM.

  17. #17
    Old Timer J M Fahey's Avatar
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    In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other?
    Well, there is a reason to use none of them.
    Take signal from the stage driving it and you save a tube, space, and a few parts.
    And some heater current.
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    Juan Manuel Fahey

  18. #18
    ric
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    As far as using the plate or cathode of a concertina to drive a SE amp: since one inverts and the other doesn't, how about making it switched after the coupling caps. Then you would have that switch as a phase switch if you wanted to run it in tandem with other amps.

  19. #19
    Supporting Member eschertron's Avatar
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    The traditional CF design in Marshall (& their ilk) is DC coupled following a HOT biased gain stage. This helps with the voltage on the grid and helps set up the kind of distortion that you may be expecting to hear from a CF. I'm using Merlin's page for reference HERE. A cold clipper in front may not have the effect you desire. Try it and see.
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.
    Quote Originally Posted by Justin Thomas View Post
    We need more chaos in music, in art... I'm here to make it.

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    I've been reading that page, but will dig into it further.

    I've tried a couple things today, fixed my grounding setup and isolated the B+ filter cap grounds from the input section.

    I also tried 2 versions of the artificial center tap, the floated ground and the resistors tied to ground from the heater supply and I'm still experiencing this buzz!

    I know it was mentioned that it could be the cathode to heater breakdown. Are there any ways to deal with this? Once again, not something I'm familiar with yet.

  21. #21
    Supporting Member eschertron's Avatar
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    Cathode-heater breakdown is irreversible, buzzing (from heater current making its way to the cathode relatively unimpeded?) is the symptom. Put a different tube in there. If it's immediately quieter, a re-design may be in order (and pull the fresh tube to save it from the horror of that fate!).
    To track down the buzzing, ID which part of the circuit it's coming from. Short grids to ground, or pull tubes to see where the buzz stops. If you pull the ground reference to the heaters completely (no CT or virtual CT at all) you should hear a fairly loud buzz. Make that your benchmark. [Ask me how I know! ]

    Re: the valve wizard. I have Merlin's preamp book, it goes into more detail about CF. Good read (no, He doesn't buy me a pint for saying this!)
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.
    Quote Originally Posted by Justin Thomas View Post
    We need more chaos in music, in art... I'm here to make it.

  22. #22
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by eschertron View Post
    The traditional CF design in Marshall (& their ilk) is DC coupled following a HOT biased gain stage. This helps with the voltage on the grid and helps set up the kind of distortion that you may be expecting to hear from a CF. I'm using Merlin's page for reference HERE. A cold clipper in front may not have the effect you desire. Try it and see.
    Quote Originally Posted by bw1985 View Post
    I've been reading that page, but will dig into it further.

    I've tried a couple things today, fixed my grounding setup and isolated the B+ filter cap grounds from the input section.

    I also tried 2 versions of the artificial center tap, the floated ground and the resistors tied to ground from the heater supply and I'm still experiencing this buzz!

    I know it was mentioned that it could be the cathode to heater breakdown. Are there any ways to deal with this? Once again, not something I'm familiar with yet.
    I think that most of us here probably had problems with hums or buzzes in our first builds which were usually using tried and tested designs like the tweed champ or deluxe. Building something that has never been built before complicates matters as you don't know if the problem is with the design or the build.

    For starters you can assume that the preamp design is okay and look at the PI, the output section and power supply schematic.

    The power supply for the heaters is unusual. There were many amps from the 50s that did not use an actual or virtual center tap which made the preamp tubes noisier. Connecting the virtual center tap to the cathode(s) of SE or cathode biased output tubes is a trick I learned from long time contributor Bruce Collins of Mission Amps. But you do not connect the cathode directly to the heater taps, it is connected to the two 100R resistors coming from those taps.

    As for driving the output tube from a cathode follower I would have tried wiring the last two stages as a Marshall or tweed bassman DC coupled cathode follower only driving the output tube rather than the tone stack. But only after running my design through a forum like this for feedback on how to wire the EL84 to the cathode follower.

    BTW I had thought that |A| on the cathode of the EL84 was a test point and not connected to the heater supply.

    Good luck!

    Steve A.
    Last edited by Steve A.; 01-15-2018 at 10:31 AM.

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    Quote Originally Posted by Steve A. View Post
    I think that most of us here probably had problems with hums or buzzes in our first builds which were usually using tried and tested designs like the tweed champ or deluxe. Building something that has never been built before complicates matters as you don't know if the problem is with the design or the build.

    For starters you can assume that the preamp design is okay and look at the PI, the output section and power supply schematic.

    The power supply for the heaters is unusual. There were many amps from the 50s that did not use an actual or virtual center tap which made the preamp tubes noisier. Connecting the virtual center tap to the cathode(s) of SE or cathode biased output tubes is a trick I learned from long time contributor Bruce Collins of Mission Amps. But you do not connect the cathode directly to the heater taps, it is connected to the two 100R resistors coming from those taps.

    As for driving the output tube from a cathode follower I would have tried wiring the last two stages as a Marshall or tweed bassman DC coupled cathode follower only driving the output tube rather than the tone stack. But only after running my design through a forum like this for feedback on how to wire the EL84 to the cathode follower.

    BTW I had thought that A on the cathode of the EL84 was a test point and not connected to the heater supply.

    Good luck!

    Steve A.
    I'm going to reconnect the 100R resistors and see if it helps further kill the buzz. I've added some additional shielded wire in the preamp section and it seems to have improved...but only slightly.

    Looking at the AX84 SEL, they are using a cathode follower in an SE amp (my amp was originally an AX84 P1 extreme), but there are a few additional resistors and a diode before the CF...would those make a difference in my design and should I implement them?

    http://www.ax84.com/static/sel/AX84_SEL_101004.pdf

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    Supporting Member eschertron's Avatar
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    If you're talking about R13 and D3, I think you'll find them mentioned, along with the design rationale - on the valve wizard page linked in post #19. Plus, there's a jungle of shielded connections called out in the drawing you linked. Lead dress is important in a high gain design like this. There's easily 70-80dB of gain in the preamp. Any amount of noise introduced early will be too much.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.
    Quote Originally Posted by Justin Thomas View Post
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    Supporting Member Steve A.'s Avatar
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    The advantages of the DC-coupled cathode follower in this design...

    Although I had earlier recommended a traditional design basically like half of a cathodyne phase inverter I imagine that using a DC-coupled cathode follower would push the output tube a lot harder. Is that the case here?

    That sounds pretty cool because simple SE amps often lack the gain and distortion of their larger brothers. One exception to that is the JCA PicoValve as designed by Andy Marshall of THD.

    In any case for a different take on using a DC-coupled cathode follower in a 3 tube SE amp here is Andy's design which unlike the modded Trainwreck SE design discussed here puts the tone stack after the CF like a Marshall rather than after the initial gain stage like a BF/SF Fender. The PDF file has a more detailed drawing than the JPG.



    jetcity%20pico%20valve_1.pdf
    picovalve-1-.jpg

    Steve A.

    P.S. I modded my PicoValve to be more like a TW Rocket which was Ken Fischer's take on the AC-30 which the tone stack driven by the CF. If I can find the schematic I drew up I will post it.
    Attached Thumbnails Attached Thumbnails picovalve-1-.jpg  
    Last edited by Steve A.; 01-15-2018 at 10:35 AM. Reason: Edited title of post to reflect my attitude adjustment on this issue.

  26. #26
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by J M Fahey View Post
    In any case since the PI in my drawing is from the tried and true Princeton Reverb design I would suggest that either the inverted or non-inverted phase be used for a SE output stage. Is there any reason to use one or the other?
    Well, there is a reason to use none of them.
    Take signal from the stage driving it and you save a tube, space, and a few parts.
    And some heater current.
    My bad! Looking at the revised drawing from the OP what I saw looked like half of a cathode phase inverter. As you suggest why waste a tube stage for a phase inverter for a single-ended amp.
    My most recent post points out the error of my ways. I repent, Lord Jesus... forgive me for my sins!


    Steve A.

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    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by ric View Post
    As far as using the plate or cathode of a concertina to drive a SE amp: since one inverts and the other doesn't, how about making it switched after the coupling caps. Then you would have that switch as a phase switch if you wanted to run it in tandem with other amps.
    As Juan pointed out my suggestion would be a waste of a gain stage. Besides it would be much simpler to add a DPDT switch between the OT and the speaker jack(s).

    Steve A.

  28. #28
    ric
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    Quote Originally Posted by Steve A. View Post
    As Juan pointed out my suggestion would be a waste of a gain stage. Besides it would be much simpler to add a DPDT switch between the OT and the speaker jack(s).

    Steve A.
    Oh, I don't think a gain stage is ever wasted!
    $-)

    It also dawns on me that set up with the anti-pop resistors could be used as a boost.

    I agree with Juan that there's no need. I was offering what-if.

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    What would be the reason for adding a phase inverter to a singled ended amp?

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