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Doodling a Hybrid Amp

Discussion in 'Shock Brother's DIY Amps' started by printer2, Oct 2, 2017.

  1. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    With all the fun we had with the AC10 amp, whether it was a hybrid or not because it had some SS devices in the signal chain I thought I thought, what would I do with a two tube limit in a hybrid amp? I picked two tubes as I wanted to keep it relatively compact, almost a large pedal format. I have a PWM amplifier board to use along with a DSP chip to do the reverb. So it is the preamp section I really had to work on.

    I thought of whether to use a pentode in the circuit, to use a Marshall (or 5F6 Bassman) preamp circuit, in the end I thought I could get the most mileage (yummy distortion) using the second stage of the Marshall/Bassman amp, gain stage and cathode follower, along with the phase inverter section. Some amps with a master volume have it after the phase inverter just to get its contribution to the sound. And since different phase inverters also have different sounds by the way they are biased I added a switch to change between some standard resistor values.

    Another thing that I did was to add a switch that changed the signal level on one side of the phase inverter output to give an unbalanced signal to the summing IC. The other side of the two pole switch will switch a bypass capacitor on the input stage, without it there will be some bass rolloff. That is when I breadboard the circuit and find the right values. Might be a little while yet, I used to think up circuits when I did not feel up to actually building something, I have a ukulele on the bench now that needs to be completed. But I did want to build a pocket sized amp this winter so this is not just a paper exercise.

    I thought of a lead switch and using a triode as a stage of gain but did not want to give up one from the other stages. In the end I thought I could just drop down some of the signal from the normal pot and bypass it with a lead pot. The inline cap will roll off some of the highs. I wasn't sure if I should go with a standard volume control arrangement or as a variable resistor, decided to go different. The schematic shows a switch but eventually it could be a relay or some FET's to control the switching. So here it is, anything but a finished product.

    [​IMG]
     
    Teleterr likes this.

  2. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Forgot to mention as I rushed to post last night, if anyone has some suggestions or ideas on what they think would be interesting to see I would appreciate the help.
     

  3. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Still building guitars, with the coming of winter may have to huddle around some tubes instead. Now if you didn't think the above was different enough got another one for you. I have a SE OT that may be able to put out 10W. But the catch is it has around a 1.1k primary impedance. So what to do? Three 12L6's should be able to drive it. So how about a 10W Champ, I have a C12R that should go good with it. Now how about have it switchable between a Tweed tone control or Blackface treble and bass controls? Instead of futzing around with the circuit values to get close to either circuit I thought to use two sets of controls, Volume and Tone or Volume Treble and Bass. Seems simple enough, can even change the NFB to each amp's circuit. Heck, add a master volume for bedroom playing.

    But then I got thinking, how about adding a pentode channel? Move the volume control so that it can cover the BF triode circuit or take care of the pentode while the switch was in the Tweed position for the Champ? That done I realized that moving things around you could get the Tweed on the one volume control with the pentode going through the tone stack and then to the other volume control. The only thing I was not sure about is the mixing resistors in before the second triode, but what the heck, if it is good enough for the Bassman it is good enough for the champ. I was going to use a 12AU6 for the pentode but they are a little gainy, then I thought of a 12V pentode/triode I have that runs off of 12V.

    Well that opens things up. Maybe a pentode with cathode follower? Or a triode slamming the pentode, or at least warming things up a little? In the end I decided to draw out the cathode follower circuit. Later I thought, just for the heck of it as I don't want two four pole switches in the amp, I wonder if you can switch between a pentode with cathode follower and a triode feeding the pentode? Well it wasn't easy but it seems to be possible. That is the companion schematic on the lower left. While I was on a roll I thought what if you wanted to go from a pentode with cathode follower into the triode working as a gain stage? This one took less time but I was unhappy I could not use a three pole switch.

    So just a few circuits that are food for thought. The part values would have to be adjusted but on paper workable circuits. I almost can't wait for the snow. (That is a lie, woke up yesterday to a blanket of the white stuff outside. I was going to do my leaves, drew up a schematic instead.)

    [​IMG]
     

  4. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Hung out at the Marshal amplifier forum and was inspired to do a schematic that goes from a plexi styled preamp to a higher gain 2204 using a four pole switch. Then I was thinking about having a Plexi channel and a lead channel that was 2204. eventually I came up with this.

    [​IMG]

    And with the talk of a battery powered Princeton and the 2204 preamp into a Class D amp thread I decided to jump in, I tried to find a case to build it into and the best I found at home is the following box. The top is an anodized aluminum plate from a old disc drive. I bent up and drilled some aluminum to hold the tube sockets, still have to bend the tab to be riveted to the top plate. The well worn practice of using two drawer handles as protection for the tubes. The first row of controls next to the tubes will be Gain and Volume of the Normal (low gain or warm biased Plexi) channel and the Gain and Volume for the Lead (high gain or 2204 inspired) channel.

    The next row will have Treble, Mid, Bass and Reverb controls. I have a Spin chip somewhere and it can be configured as a reverb. I think I will have a send and return loop. Then the Class D module. I might do some clipping circuit before the module but that is a secondary concern. Getting the tube section working first. The high voltage module has a large heatsink on it that might get chopped since I am not drawing many watts. Two foot switches, one to bring in the gain channel and the other for the reverb. The top plate will get drilled out for the tubes and moved up to fit on the box.

    [​IMG]
     
    bftfender likes this.

  5. bftfender

    bftfender Tele-Afflicted

    Dec 21, 2017
    York PA
    not so good at this from a tech point, interested in your plexi circuit if you go that way, i have a HW ptp jtm45 with el 34,bassman on one side and plexi on other, it is an incredible amp, with a master it can do jcm800 gain but clean up to pristine or dirty clean like a bassman. wish i could build em and understand. not my strength
     

  6. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    The Bassmam or Plexi channel is the first triode bypassing the second triode, including the second triode gets you into 2203/4 territory. The foot switch S1B changes the cathode bias from the 2203/4 values to a bassman, well a little hotter, more the normal Plexi channel. The footswitch also brings in the second triode with S1A and the volume is also switched between the volume controls for either channel with S1C.
     

  7. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.
    Very cool...I m a novice amp building wise, but I built a Nuvistor 30V preamp for my guitar using a book that explained tube circuits including calculations for bias and plate loading. I did a six stage cathode bias resistor switch to hear how the sound and distortion changed. I remember having to change the plate resistor for each change of the cathode R. Your switch on the first schematic doesn t bother. Is this because the plate load is within safe current draw for either ?
     

  8. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Yes it is safe, standard values for a Marshall amp. Easier than a six position switch is a pot. If you want to have fun get a pentode and vary the cathode, plate and screen resistors, three dimensions rather than two. Using pots and your ears you can find some sweet spots. Then for even more fun changing the power supply voltage.
     
    Teleterr likes this.

  9. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.
    Excellent ! Thanks for the tip.
     

  10. SSL9000J

    SSL9000J TDPRI Member

    Age:
    46
    96
    Dec 18, 2016
    North of Atlanta, GA
    Battery powered Princeton? Intriguing... I've wondered about the feasibility of designing a tube amp that could be powered by some sort of off-the-shelf 24v rechargeable battery one could find at Home Depot.
     
    shortcircuit likes this.

  11. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Easy enough, the problem is battery life. Just swap out the laptop supply I am using for a battery.
     

  12. elpico

    elpico Tele-Holic

    644
    Sep 14, 2011
    Vancouver BC
    I mean, the reason we call the power supply B+ is because all tube amps were powered by batteries once (it's short for "battery positive terminal") so you're talking about well travelled territory here.
     

  13. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    No although you are right. The 12V6GT was produced to be used in car radios, later the 12AB5 which is a 6V6 equivalent placed into a 9 pin package to reduce the size. Car radios had the voltage boosted by use of a vibrator circuit, an electromechanical device similar to the coil and points that provided high voltage for the spark plugs. The modern day vibrator circuit is the switching regulator feeding an inductor, a switching power supply like the one in the laptop PS. Or the little circuit board in the last picture. It will do 380V. With the three 12AX7's heaters in series placed across the 19V supply and the 19V supply feeding the switcher you have the heater and high voltage dc ready to use in a matter of minutes.

    For a Princeton amp I would use a 12V switching power supply, although I have a 13V and 16V, drop a couple of volts down with the 16V for 12.6V for the 6V6's in series. Or better yet, buy some 12V6's and place them in parallel with the 12AX7's. The good thing about this is you can have NOS 12V6's for about $10 a piece, but keep it under your hat as they will get bought up otherwise. The switcher can put out the current to drive the HV the tubes need, easily do what the wimpy PT of the Princeton/Champ can do. No tube rectification though.
     

  14. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.
    Yeah, I ve seen a lot of NOS equivalent to standard tubes that have a unusual heater V for cheap. W lower V power supplies cheap and plentiful, they might start to be used more , ssshhh.
     

  15. elpico

    elpico Tele-Holic

    644
    Sep 14, 2011
    Vancouver BC
    If you wanted to do a battery powered amp you wouldn't want to waste power running the heaters off the switching supply. The reason the heaters want 6 or 12 volts in the first place is because those are standard battery voltages and like I said, in the beginning all tube amps were powered from batteries. The standard stuck around all the way through. The heaters would love to be run straight from the clean, smooth dc supplied by battery, so the ideal way to do a battery powered Princeton would be to choose a battery that can run the heaters directly and only use a switching supply for the B+.
     
    Last edited: May 30, 2018

  16. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Of course you can run the heaters on the battery voltage and switch the HV. That is why the 12V suggestion, tubes are there and lead acid batteries cheap. But that is not the only voltage to run the heaters. On the other forum they are talking Lithium batteries and a higher voltage. Mind you I think they dumped the tubes and gone all SS. But plenty of ways to skin the cat. They have modules to bust or buck the voltage to give you whatever you want as a heater voltage and efficiency is in the 90's %.


    Anyway I stopped in on the Rob Bassman One Watt Marshall comparison and had the thought (oh this is always dangerous) of sending out the heater, HV, and phase inverter signal to a pair of tubes and transformer on a separate box to give a low watt tube amp. I will have to debate it, not like I need that kind of animal. Mind you, it is not like I need any of these projects.
     
    Teleterr likes this.

  17. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.
    If anyone is worried about DC heater problems where the one end wears out... I ran the DC thru 2 toggles. Both up is on , both down is on , mixed one up one down is off. One toggle is the ON switch, one is the OFF. It flips the heater ends each time its used for even wear.
     

  18. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Why would the heaters ends dissipate different amounts due to the direction of current?
     

  19. Teleterr

    Teleterr Friend of Leo's

    Feb 7, 2011
    Lewes De.
    I don t know off-hand, but i ve seen that discussed as a caution against DC heaters. Not nessesarily here.There were even active circuits to address the alleged problem, so I figured better safe than sorry and the switches seemed easier than a SS chip circuit.
     

  20. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada

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