Class AB2 Power Amp: Bringing 2nd Harmonics Back to Push-Pull

andrewRneumann

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Fascinating stuff mate! When I did my electronics apprenticeship in the Royal Navy many years ago, it was all thermionic emission. I loved building amps and using a scope in the workshops Tube theory is fascinating. Glad to know that guys like you are out there! Mind you, if an amp distorted, that was an exam fail!

Right! Love the photographs. Hopefully those are de-classified. :lol:
 

Phrygian77

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I noticed you didn't mention anything about a separate screen and cathode follower supply. I would think you could use a sperate transformer if you really wanted to avoid the mosfet.
 

Gi Tar

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Hello All,

I’ve been slowly working on a design of my own based on some discussions I had when I finished my last amp, the MC10. I became intrigued with Class 2 operation and the possibilities opened by grid current. I wanted to design a Class AB2 power amp that utilized 6V6s and had output similar to a Deluxe Reverb. I also wanted to avoid the use of transistors if at all possible. I dug around in old books and magazines and found that there were Class AB2 designs before the advent of solid state devices. Could I figure out how to adapt them to the guitar world? The answer is YES!

The general specifications for this amp are:
2x 6V6S in Push-Pull
1x 12BH7 driver
1x 12AT7 long-tailed pair phase inverter

B+ 425V
Maximum Clean Output: 23.8W

What’s the big deal? That’s about the same as a Deluxe Reverb. What is gained by pushing into Class AB2? Because grid current is flowing to get those 24 watts, we can adjust the series resistance of the grid circuit and control the amount grid current. I accomplished this by a single 25KL pot in series with each 6V6 control grid. When the pots are at 0, the tube is permitted to run up into Class AB2 as far as it can. (I estimate that I can maintain voltage on the grid all the way up to Vgk +20V! That’s PLUS 20V.) When the pots are set at 25K, the control grid loses voltage at 0Vgk and operation is limited to Class AB1. The best part is that the player gets to choose the output of each power tube individually with a pot. Crank one up and one down for plenty of second harmonics. Crank both down for limited headroom and lots of third harmonics. Or, choose anything in between. The player is in harmonic control of the power amp. In full Class AB2, the amp puts out about 23.8W of clean power. Crank both “class” controls down and the output power is limited to only 4.8W of clean power. Yes, that’s an extra 19W of power achieved by driving into Class AB2.

How is this all achieved? I’m attaching the schematic as built, so you can take a look. Some differences from the standard push-pull configuration:
  • There is a 12BH7 cathode follower between the LTP and the power tube. These 12BH7s provide the grid current necessary to drive into Class AB2. The 12BH7s are direct-coupled to the 6V6s—providing another benefit of no bias shifts and blocking distortion in that stage.
  • Individual bias controls for each side of the power amp. The bias controls control the driver current which in turn control the bias of the power tube. Essentially the driver tube and power tube become one unit with one bias control.
  • 50V bias winding used for 12BH7s but also for setting up the correct cathode voltage on the 12BH7s.
  • MOSFET voltage follower used to achieve lower than normal screen voltage and cathode follower plate voltage. I tried to find a way to avoid solid state devices in this design, but there was no feasible way to drop enough voltage, avoid excessive sag, and keep from wasting tons of power into heat. The screens and cathode follower plates run at about 225V. (B+ is 425V, so that’s 200V of drop that needs to happen.) At least the signal chain stays completely tube.
  • Type-II PPIMV
Here are some scope images.
First image, maximum clean power into an 8-Ohm dummy load. 13.8Vrms = 23.8W.
View attachment 1016647

Second image, one power tube limited to Class AB1. Other tube still in Class AB2.
View attachment 1016650

Third image, both power tubes limited to Class AB1.
View attachment 1016661

Forth image, maximum clean power limited to Class AB1. 6.2Vrms = 4.8W.
View attachment 1016667

Finally, some images of the different wave-forms achievable by the player. These were all created with the same input signal and gain levels. The only thing I'm changing in these are the "class" controls which are just adjusting the control grid resistance on the power tubes.
View attachment 1016664 View attachment 1016668
View attachment 1016669 View attachment 1016670

This thing is a real screamer and bone shaker when hooked up to my Screaming Poseidon pre-amp. But with 24W of clean power available, beautiful, rich, loud clean tones also available. Just add a touch of uneven grid resistance with the “class” pots and the complexity just deepens. I really like this amp. I will attach some audio once I get a descent mic up and can shut down the A/C in my house for awhile.

So what’s next? This amp is built on an open test rig. It needs to be married up permanently with a pre-amp and put into a spacious head cabinet. I’ve documented everything as built, so it may be time to dismantle the test subject and start layout on a real chassis. In the mean time, that will free up my test bed for the 6N7 Class A2 amp that I will be building and testing. It follows the same general principles, but the power supply will be quite a bit different.
What is the cost involved in building this amp?
 

andrewRneumann

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I noticed you didn't mention anything about a separate screen and cathode follower supply. I would think you could use a sperate transformer if you really wanted to avoid the mosfet.

I mentioned it! The CF and screen supply come off the B+ just like any other amp. But instead of dropping resistors, I’m using an adjustable MOSFET voltage follower to set the voltage. The thread on that is here https://www.tdpri.com/threads/high-voltage-regulators-reducers.1101049/

An extra transformer is one way it could be done, but kinda expensive and heavy. Another way would be a custom PT with a set of lower 160V taps.

The MOSFET follower is cheaper and has one huge advantage—I can adjust the voltage anywhere from 0V to nearly B+ and get the same performance. A separate transformer would be pretty much fixed.

What is the cost involved in building this amp?

I can’t say for sure. This is more of a test subject, and just a power amp. Take any push-pull amp with a 50V bias winding and you could probably implement this for an extra $50. The biggest expense would be the additional 12BH7 tube and socket. (Assuming the PT could also handle the extra heater current for another tube…)
 

peteb

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Then you never drove it into AB2.

i never said I drove it into class AB2.

I drove a champ into class A2. my impression was that nothing really happened when the signal surpassed the bias, except it had to be really cranked and the pick attack is over the top.

and very distorted.


that’s why I question whether Andrew is in class AB2.

I expect some drive.


when class AB1 is at low volume, it is really class A1 until the clipping starts, in the form of cut off.

Andrew‘s recording is low volume and clean. it sounds like either class AB1 or class A1.


until we know the voltages, the sound is all we have to go by.
 
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printer2

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i never said I drove it into class AB2.
Sorry, Class A2. So how did you drive it into A2 with a capacitor between the output tube and the preceding stage? A capacitor coupled stage can not supply grid current for A2 (or AB2) operation.
 
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peteb

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The easiest way I know how to tell when a push pull amp goes from operating in class A to class AB is by watching the current draw at the wall.

at low volumes, a push pull amp will draw constant current from the wall. This is a class AB amp operating in class A.

the volume needs to get about halfway up, and some chording, will drive the tubes into cut off during part of the cycle. When this happens, the current draw at the wall increases. This is the onset of class AB1 operation.

in class AB2, I expect some volume and distortion.
 

printer2

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you do see I wrote class A2, not class AB2?


be careful. These small errors could sink a ship.
Yes, sorry. I edited my post as you answered so I added more to it that you could not reply to.

The question I had is how you could get into Class A2 with a capacitor coupled stage? If you think you were in A2, your ship was sunk.
 

_Steve

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Great thread! I've been experimenting (in my head) for a while now with AB2 designs, but my theory is nowhere near as good as your's (yet). Looking forward to seeing how it progresses!
 

2L man

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I am going to test cheap (~$10) 10VA / 2 x 115V / 2 x 24V power transformer as interstage transformer to be able to bias and drive PP power amp to AB2. Perhaps use 6N7GT double triode first time?

I plan to connect coils so that interstage primary biases and secondary power tube biases operate opposite each other so that interstage transformer core would not saturate. That should be possible when there are two separate pairs of primary and secondary coils.

Perhaps a very simple power transformer (which do not have a cap between E- and I-iron plates) which has only one primary and one secondary, operate as SE interstage transformer if its both coils biases are set to compensate each others?
 

andrewRneumann

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For your listening enjoyment:

Here I am trying to play an 8 bar progression as evenly as possible. The first pass is with both tubes in AB2, right on the verge of clipping. The second pass is one AB1 and the other still AB2. The third and final pass is both AB1.

I did not make any gain adjustments, only changing the grid resistance of the power tubes to control the clipping.

So… AB2/2, AB1/2, and AB1/1.

 

andrewRneumann

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Great thread! I've been experimenting (in my head) for a while now with AB2 designs, but my theory is nowhere near as good as your's (yet). Looking forward to seeing how it progresses!

Thank you, but it’s more than a theory, it’s a working amplifier! Yet there are some leftover questions—
What is the longevity of power tubes?
What are the weak points of the circuit —what will fail first?
Could the circuit be simplified and still work reliably? (For instance the bias circuit.)
What is over-engineered? Maybe a 12BH7 driver isn’t required?

I’m not sure I will ever come to conclusive answers for any of these. There comes a time to say good enough is good enough—buy a chassis and cabinet and build it for good.
 

andrewRneumann

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I am going to test cheap (~$10) 10VA / 2 x 115V / 2 x 24V power transformer as interstage transformer to be able to bias and drive PP power amp to AB2. Perhaps use 6N7GT double triode first time?

I plan to connect coils so that interstage primary biases and secondary power tube biases operate opposite each other so that interstage transformer core would not saturate. That should be possible when there are two separate pairs of primary and secondary coils.

Perhaps a very simple power transformer (which do not have a cap between E- and I-iron plates) which has only one primary and one secondary, operate as SE interstage transformer if its both coils biases are set to compensate each others?

Although I can’t comment on coupling transformers because I didn’t go down that route and I know very little about them, I would say go for it. What sonic flavors will you alone get to taste?
 

Phrygian77

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I've been trying to think if it would be possible to do this with a single toroid, like a 100VA 300V Antek. Have you measured the total ripple current at full tilt? I know you estimated the maximum current for B+2 at 50mA, but have you checked to see what the total power is with regulation? I'm also not sure that a capacitively coupled bias supply would work.
 

dcoyle

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Thanks for posting this. I like your test set up with the many-hole rack for pots.

I wonder if you would find a different arrangement on the series grid pots more useful in practice. (I assume you're using linear pots.) Rather than have an individual pot for each tube, were you to have one pot a double section 25k, it would set the overall gain level for both tubes at once, and a second series double pot of, say, 10k, wired so one section goes up as the other goes down, to unbalance them.
 




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