Class A Push Pull - output transformer specs

[merlin]

I just want to see what PP Class A sounds like in person.

FWIW, at low volume a Class-AB amp is running in Class-A...

 

[Matador]

Here is a set of load lines around your pentode operating point (300V supply voltage, 275 screen voltage, and -12V bias point, idling at 25mA). You can see that so long as the input signal is fairly small, it follows the class A load line (the lower line). Not until the plate voltage of the tube that is rising with input signal and swinging below 200V does that side transition into class B (the kink formed by the intersection of the two load lines).



Now keeping everything else the same, you can extend the lower portion of the load line by biasing the tube hotter. Raising it up all the way to 70mA results in this composite load line (changing the bias to about -8V):


This tube would be screaming in pain: it's biased at nearly double the recommended dissipation (over 20W) and would not survive long (the dashed line is the 12W maximum plate dissipation curve). However the class A operating portion of this tube extended all the way to the knee of the curves.

You can drop the B+ down to around 250V (like Merlin recommended) and reduce the idling current to around 50mA to restore operation under the dissipation limits. Note output power is down about 60%:

 

[abbey road d enfer]

A good example of amps adevertized as class A is the VOX AC series.
Actually they are their own type of class AB. For a good part of their operation they are genuinely class A, with almost constant power draw, but above a certain level, the total current increases significantly, resulting in sliding bias.
As drive increases, teh cathode voltage also increases, which tends to reduce the idle current, so at high power, the amp is running nearly class B, which explains why the tubes survive.
Some attribute the "chiming" tone to class A.

 

[emrr]

A thing you don't see any more but was most common in cathode biased class A PP broadcast monitor amps before say, 1955, is screen voltage higher than plate voltage. I recall higher screen helps long term stability, attracts more stray electrons out of the vacuum. I may remember wrongly, but I recall PRR mentioning some mechanism. It's such an old forgotten standard that you find endless freakouts about Fender 5E1 Champs with that circuit being slightly higher voltage on the screen.

The only thing I can find, and it's a tangent, is RDH4 - partial triode operation of pentodes (ultra-linear) - "when the screen and plate of a pentode are operated on the same voltage, pentode operation occurs when the screen is connected to the B+ end of the output transformer primary, while triode operation is obtained with the screen connected to the plate end of the primary. Any desired intermediate condition can be obtained by connecting the screen to a suitable tap on the primary". Yes, I'm talking about pentode operation but with a barely higher voltage on the screen, which just isn't done later on, with separate bypassed screen feeds.

Fully PP amps like Langevin 117 or Collins 6N, 6X, etc are cancelling most of the 2nd harmonic, so they are the sound of 3rd harmonic dominance. Transformers make 3rd. 6V6 is strongly predominately 2nd harmonic, goes nicely with PP. See RDH3 pages 14-15 for comments on distortion.

 

[SoundBlaster]

I would like to point out that the requirements for class A are LESS stringent re: the transformer. As both valves are always conducting, there is no handover from one to the other when they transition to class B. As a result, the glitch that happens at that transition doesn't "trigger" the leakage inductance between the primary and secondary. If you have a transformer with poor coupling, Class A can "fix" it.. of course there'll be less power. And a lot of the glitch can be solved by feedback.. AC30 is an example.. while not totally class A, It carries class A pretty deep into that 30 watts. When it does crossover into "B" mode, it's so loud, you're probably looking for the distortion....

 

[SoundBlaster]

McIntosh amps are notorious for their tight coupling. Necessary for their class B operation.

 

[Wavelength]

Most class A amps I have tested in guitar world are not really Class A..... and if you look at it your 50W amp is probably running class A up to some wattage.
Heyboer has some of the Peerless 20-20 and 20-20+ designs that Dumble and other people used for high end guitar and high end Audio. That would be a place to start.
I make a bunch of single ended A and A/A2 amps (A2 is grid forward). But after doing this for 40+ years I would say most pentode single ended are really only Class A up to like 70% then they kind of go away from A because of the increase in screen current and bias no matter how you bake it.
Remember the core area of a transformer is a square property so going north in current is going to make the damn thing real large quickly.

 

[SoundBlaster]

Most class A amps I have tested in guitar world are not really Class A..... and if you look at it your 50W amp is probably running class A up to some wattage.
Heyboer has some of the Peerless 20-20 and 20-20+ designs that Dumble and other people used for high end guitar and high end Audio. That would be a place to start.
I make a bunch of single ended A and A/A2 amps (A2 is grid forward). But after doing this for 40+ years I would say most pentode single ended are really only Class A up to like 70% then they kind of go away from A because of the increase in screen current and bias no matter how you bake it.
Remember the core area of a transformer is a square property so going north in current is going to make the damn thing real large quickly.

Single ended by its very definition is class A. Period. To leave class A is to cut off the tube so that it stops conducting. (Class C) This can happen if the driver applies enough voltage to completely overcome the bias. This is akin to blocking distortion in Class A/B push pull amps.. Where the diode effect of the cathode pulls the coupling capacitor charge to empty and it has to "refill".. it's why smaller coupling caps are better at overload.. Yes you lose bass response but they catch up after the "block" even it over quicker. Now if you can drive the grid positive, you're in the A2 (Single ended) or B2 (push-pull) category.

 

[Wavelength]

Single ended by its very definition is class A. Period. To leave class A is to cut off the tube so that it stops conducting. (Class C) This can happen if the driver applies enough voltage to completely overcome the bias. This is akin to blocking distortion in Class A/B push pull amps.. Where the diode effect of the cathode pulls the coupling capacitor charge to empty and it has to "refill".. it's why smaller coupling caps are better at overload.. Yes you lose bass response but they catch up after the "block" even it over quicker. Now if you can drive the grid positive, you're in the A2 (Single ended) or B2 (push-pull) category.

A Class A like DHT 300B SET amp is true class A. Pentode on the other hand is different because it has more than one thing drawing and giving current. I use a reactor follower and direct couple the cathode of the driver (usually a 6SN7 or smaller 12AU7) to drive pentodes into A2. What I was saying is not text book, but the way it works. Even a 300B when reaching 60-70% of it's bias will start to draw grid current. That current will excite the resistance to ground from the grid and rebias the tube. One of the reasons we use grid chokes as they have high AC Z and low dcr (less than 4K for a 4000H high nickel unit). Look at any pentode and you can see the plate vs screen current as the incoming signal reaches the bias point in SE. The amp on the left is a 5686 (small EL84) and in the protos I tried using a LM317L to set the bias current and of course that current screen + plate will change and as the screen draws more the plate would draw less. The same thing happens with self bias R/C. Now I could fix bias the amp and if nothing is in the cathode then the power supply current would still go north as the input signal reaches the bias point.

 

[SoundBlaster]

That absolutely makes sense.. Yes. It outside of the typical tube amp topology. That 60-70% threshold is where the 2nd harmonic really starts to creep in and create the typical single ended distortion profile. But if you can do as you do and use a low Z driver and supply current to the grid then a single ended amp extends it's range quite profoundly.. but with that cost of a a grid choke. I like where you're going.. except for that 3-legged fuse of a LM317.