300b lovers

Great response, Ralph, answering questions I have long wondered about.  My Sachs monos put out a fair amount of heat so I mounted a couple of noiseless computer fans at the back of my rack to dissipate the heat to a degree.  

The Statement ... or Blackbird ... or whatever it’s called, is pretty sensitive. Don told me 1.5 Vrms to full clipping, and my gain calculations indicated a bit more sensitive than that. Since there is no RC loading anywhere, and all the cathodes are bypassed, all tubes run at full gain.

For a 6SN7 that’s a mu=20, and for a triode-connected 6V6 that’s a mu=8. Since the 300B grids take 80 volts to reach zero bias, and can go positive to +20 above that, that’s 100 volts of swing for each side, about 3x what a pentode requires. Since this is a PP amplifier, full power requires +100 volts on one grid, while the other receives -100 volts. The input+driver provide 20 * 8 = 160 gain, excluding transformer losses, which are typically 5% or less.

Imagine those 200 volt peaks at 30 kHz, with no local or global feedback, at less than 1% distortion, with another 3 dB of headroom above that. This is why driver design for DHTs is difficult, and not in the Radiotron Designers Handbook.

Let’s talk a bit about distortion, particularly in a DHT driver. It defeats the purpose of using a very low distortion 45, 2A3, 300B, or 845 if the driver has more distortion than the DHT itself. You might as well use a much cheaper power pentode or beam tetrode and save yourself a lot of trouble. Not only that, but pentode and ultralinear-connected output sections requires feedback across the whole amplifier, which cleans up the mess from the input and driver sections. This is why the driver section is an afterthought in Golden Age PP pentode amplifiers. In the Dynaco circuit, there isn’t even a driver ... the power tubes are driven from the split-load inverter, or "concertina" stage.

But ... if the goal is the most linear possible amplifying stage, with zero feedback, a completely different approach is required. True, push-pull substantially reduces distortion, but in reality it only reduces even-order distortion ... 2nd harmonic, 4th harmonic, 6th harmonic. etc. It has no effect on odd-order distortion ... 3rd harmonic, 5th harmonic, 7th harmonic, etc. Most tubes have dominant 2nd harmonic distortion. But not all. The 6DJ8/6922, an RF tube, has dominant 3rd harmonic. Tubes designed for video amps can also have dominant 3rd harmonics, since distortion in a video tube has almost no effect on the picture (even 5% distortion would be barely visible).

Aside from the subjective sonics of dominant 3rd harmonic, this has consequences for a PP amplifier. The PP circuit will have no effect on 3rd harmonic; it will not reduce it even 1 dB. So tubes with dominant 3rd harmonic should be avoided.

What about more "normal" tubes with dominant 2nd harmonic? Well, one of the requirements for PP cancellation is the magnitudes and phase of the cancelled harmonics line up with each other. The magnitudes of the (even) harmonics from each tube should be within 1 dB of each other, and the phase of the (even) harmonics should be within 45 degrees of each other. Normally this would be of no concern with reasonably matched tube sections, but it does matter if the capacitive loads are not the same. The odd harmonics, of course, do not cancel.

The notion of harmonics having phase might seem a little odd, but keep in mind that square waves and triangle waves have identical magnitudes of harmonics; the only difference between the two signals is the phase of the harmonics. So the phase of the harmonics is not insignificant, and actually reflects a different transfer curve. In graphical terms, you want complementary transfer curves; if kinks appear in the curve of one tube but not the other, that kink will not cancel.

The full scope of the driver requirements for DHTs is probably now coming into view.

* Three times as much swing is required, compared to pentode or beam tetrode.

* Linearity should be better than the DHT itself.

* The load is mostly capacitive, consisting of the 60 to 80 pF Miller capacitance of the DHT grid. Reactive loads increase distortion at high frequencies, where it is most audible, and also reflects the power back to the plates of the driver tube.

* If the driver is PP, 2nd harmonic should be at least 10 to 20 dB greater than 3rd harmonic, and the two loads should be symmetric to keep the phase of the driver harmonics the same.

* There should be at least 3 dB of driver headroom so the amplifier doesn’t all clip at once. More headroom is desirable.

* Momentary sags, or program-correlated noise, in the power tube B+ supply should not interact with the driver tube B+ supply.

Since rolling out of bed in the morning, your amps have been filling my room with amazing sounds, and I appreciate greatly the design thought that went into them, the technical derivations of which is way over my head.

The amps, now with the excellent new Linlai WE300b tubes also run in, create a sheer wall of sound, effortless in its presentation, heft in the LF, delicate highs, and a mid-range to that sounds exactly right.   Even compared to Don's excellent Kootenai KT88, these monos are in a whole different realm.  The sound with my Spatial Audio speakers just hangs effortlessly the room.  

This thread, with both the participation both you and Don, has been most informative, and I am an early beneficiary of your design which as been to elevate my listening enjoyment to degree to a level that I never anticipated possible.  With your monos, there is no amp at all... just joyous music, heard in a way I never thought possible with no sonic signature at all... just pure clean vivid sounds.  Perhaps that ought be the design goal for all... create front end gear that sounds like nothing at all...  You gents have succeeded in that respect. 

@whitestix 

I am very happy for you. What you wrote expresses genuine joy and satisfaction listening to music. Isn’t this the ultimate objective? You are fortunate to have received this early opportunity. I know you are very grateful.👍

It’s been so much fun and interesting reading about the development of this new 300b push-pull amplifier and what it took to get here.

Charles