Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

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.

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  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.)

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