12BK5 Push Pull Amplifier

Discussion in 'Shock Brother's DIY Amps' started by printer2, Jan 1, 2019.

  1. printer2

    printer2 Poster Extraordinaire

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    I haven't built an amp in a long time and it has been something that seems to bother me as I am not short on ideas but rather time. To scratch that itch I was thinking of doing a relatively simple build using a tube I have many of, the 12BK5. It is an orphan tube, developed to be used for service as in TV sets, in SE it only needs -5V bias so +/- 5V to drive it to full output. Could use an opamp to do that. Maybe a 12AX7 through a tone stack will be able to drive the outputs to a reasonable level.

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  2. printer2

    printer2 Poster Extraordinaire

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    Nothing fancy, may give it a shot of paint later. To sit in a Tweed type of cabinet, maybe with an 8-10" speaker.

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  3. sds1

    sds1 Tele-Holic

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    Have you done the opamp PI before? That's got my attention, I'm real interested in the design behind that. What's the projected B+? I notice the drain and sink resistors are identical.
     
  4. printer2

    printer2 Poster Extraordinaire

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    in SE it only needs -5V bias so +/- 5V to drive it to full output

    Might have been my fault but I was feeling lazy and wrote single ended in short hand, no phase inverter, I was implying that you could drive a tube biased up in single ended operation with an opamp as you only need a swing of 10V and it is easy to have an op amp powered with +/- 15V. But since you mention it, it would be easy enough to drive a push-pull pair using an opamp. You use a dual opamp and take the signal from the output of the first one to drive the one side and use an inverting opamp configuration to give the 180 degree out of phase signal to drive the other output tube. This is the simple way of doing things and has been done with tubes in the Paraphase PI before the Cathodyne and the Long tail Pair took over. But where the Paraphase had errors in operation the IC version has all the gain of the IC and the 100% NFB making things right. That is the simple answer.

    Now it gets a little more complicated. The grid of the tube, let's keep with the single ended version here, gets biased half way on and the signal drives the tube down to cutoff or up to saturation (for the above tube per datasheet, -10V to 0V with a -5V idle). We could do this with our opamp driving a capacitor with the tube biased up with a cathode resistor or by fixed bias. With an opamp on a +/- 15V supply the opamp could overdrive the grid roughly by three times before the opamp clips. And the opamp would recover from clipping before the tube comes out of clipping. You don't even need a +/- supply as with a single ended supply where the opamp has a capacitor on the output removing the dc offset at the opamp's output.

    But why stop there, the opamp can drive the above tube in the negative direction, why not the positive? It can not with a capacitor in the way because when we drive a grid positive the impedance drops and discharges the coupling capacitor but the opamp can if we run off a dual supply since no capacitor is in the way. So say we have our above inverter driving a pair of tubes the opamp can supply current into the grid when it goes positive. The impedance of the grid drops from a high value to roughly 1K ohm when driven above 0V. And many IC's will drive a 1k load. So rather than the tube conducting its maximum current at 0V, we can push more current through it by pushing the grid positive.

    Getting late, let's speed things up. Say we took the above inverter and a pair of voltage followers behind each output but we give the followers a dc reference that is where we want our tubes bias point to be we can have them biased up at -8V (rather than at -5V) for Class AB operation rather than Class A when biased at -5V. But since the followers can drive the grids positive we can operate the tubes in Class AB2, pumping more current through them on the positive voltage peak above 0V. So we can bias them up in Class AB which gives us more power than Class A but also drive the peaks positive giving us even more power. (That is if the tub's grid can take the current we are pumping into it. The 6V6 or the 6L6 were designed for this.) Just grabbed a drawing I could find, we would have a capacitor on the input at the resistor junction fed from the inverter. In our case we would have the +/- supply voltage on the ends of the resistors and we would have a pot to make the output voltage adjustable (idle voltage, fixed bias amp).


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    So when I saw your question earlier I thought, not a quick answer. I hope the explanation makes sense, look up class AB2 if needed. There are some finer details to get things to work for a guitar amp like finally limiting the power through the tube so it does not burn itself up. I will do a class AB2 amp yet, not this project though.

    The drai, sink resistors are the same as the stage is used as an inverter as a Cathodyne inverter is. The stage is fixed bias rather than cathode biased as in a Cathodyne.
     
  5. printer2

    printer2 Poster Extraordinaire

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    Here is one example.

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    Just sorting through my parts seeing what I have to work with. I am going to use some caps I scrounged, resistor values are changing a little to reflect what I have.
     
  6. printer2

    printer2 Poster Extraordinaire

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    With some part changes and no NFB for now but mostly the schematic.

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  7. printer2

    printer2 Poster Extraordinaire

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    Finally found the board again, looked for it all over the house. Here is the board without the messy wires going places. Mounted the sockets and added the screen and grid resistors.

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  8. printer2

    printer2 Poster Extraordinaire

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    Also picked this radio up for $20 today. Is a battery powered radio, so 8.4V and 67V. A big 0.25W from the 3V6 output tube. some interesting pentodes that run off 1.4V with 0.05A heater power. Thought of maybe using it but not enough interest. It is a five tube radio so five 7 pin sockets available. I could put in a decent output transformer, looks like enough room for a 6x9.
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  9. printer2

    printer2 Poster Extraordinaire

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    Seems someone else had the same idea with the 6x9, they also beefed up the output transformer, still a radio though. A little better shape.

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  10. printer2

    printer2 Poster Extraordinaire

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    So much for working on the 12BK5 amp, cleaned up the radio for future consideration.

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    Going to see if I have any available seven pin tubes. I never expected to do a whole amp with them so I didn't stock up. As an idea.

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    I expect the volume and master volume will go in the volume and tuning positions and a input jack as well as treble and bass controls in the tuning window on the plate.
     
  11. printer2

    printer2 Poster Extraordinaire

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    I like aluminum myself, don't come across a lot of it free though. Speaking of cheap, found a radio today for $20 that was built in Vancouver in the 1950's. I was not going to bother as the picture of it showed the grill area with a height for an 8". I thought the last thing I need is another five tube line powered amp with crappy tubes. But I took a chance on it because it was five minutes away and it is twenty degrees warmer than it was two days ago.

    Painted top, who cares, didn't even look at the turntable. Looked at the back and I saw the 10" speaker and that is all I needed to decide to get it. Saw it had octal tubes, another plus, also a power transformer although I have enough of them. When I got it home found out it is suppose to have a 6K6 output tube, had a 6W6 in place. So I got a decent speaker and a output transformer for another SE amp, I have about 20 6K6GT's.

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  12. bigguy12321

    bigguy12321 Tele-Meister

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    These are fun!

    I like to gut the radios (only one AM station near me and it's all talk all the time) and build them with an input jack for guitar and phone/MP3 player. That way the wife lets them stay around a little longer...

    By the way, I just copied another of your schematics. You have some good ones. Thanks

    a
     
  13. printer2

    printer2 Poster Extraordinaire

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    No I am not a horder, I could not help myself on this one it looks so cute. A small floor standing radio, I actually works and I payed $75 as well as drove an hour for it. Other than recaping the power supply will leave it stock. It does have a phono input and switch on the chassis which I can hook up a guitar preamp to. The person that had it refinished it and it looks pretty good.

    http://pacifictv.ca/schematics/viking51-45data.pdf

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    Somehow lost the image in the above post.


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    I have wired up some more of the 12BK5 amp but the trip to get the radio and changing the arrangement of my workshop is taking up my time.
     
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