300b lovers

I should note my description of feedback circuits is a grossly oversimplified, non-mathematical overview of a complex subject. For the curious, read about how op-amps are stabilized, and the concepts of loop gain, excess gain, dominant-pole compensation, and phase margin. Once you get a reasonably firm grasp of how it works, then read about slewing distortion and settling time. I tend to use settling time as a figure-of-merit when looking at op-amps, or more complex discrete circuits.

It all comes together at the summing node, which is simply an analog comparator between input and output. In an op-amp, which has extremely high forward gain, the high gain of the op-amp forces the differences between the two nodes to zero. This is fine until the op-amps clips or slews, which creates very large error voltages at the comparator input. The large error voltage can force the comparator itself into nonlinearity, and feedback theory relies on a distortionless comparator.

In addition, if the comparator is saturated, or if the power supply sags or is discharged, then recovery time can be quite long (tens or hundreds of milliseconds), much longer than the original clipping or slewing event.

During this settling time, amplifier distortion can be quite high, since feedback is only partially effective. This will not appear in FFT harmonic distortion or multitone IM distortion measurements, which are taken over several seconds and then averaged.

This is the gap in existing measurement techniques. Harmonic and IM distortion are averaged over several seconds, and do not sense events happening in microseconds or milliseconds. High-speed scope measurements are insensitive to distortion unless it is very high, such as 10% or more, where it becomes visible. Transient distortions, in the microsecond to millisecond range, are not seen.

The key principle of non-feedback amplifiers is they are insensitive to transient upsets or interactions with the load. Steady-state distortion is higher, but there are no issues with phase margin or settling time.

It doesn't mean you cannot build a nice amp that uses NFB, but that "air" and sense of "realism" that you treasure is hindered by NFB.

That depends on how the feedback is implemented!! If the feedback is sent to a non-linear point in the input of the amplifier which is used as a feedback node (such as the cathode of an input tube) then you can expect it to be problematic, as Crowhurst pointed out 60 years ago, and Baxandall 'rediscovered' 15 years later.

In other areas of electronic design, feedback is known as 'control theory' and is very well understood. But in audio, it seems to get misapplied (and so gets a bad name) on a regular basis, then everyone points at feedback being the problem when its really just design flaws.

In addition, if the comparator is saturated, or if the power supply sags or is discharged, then recovery time can be quite long (tens or hundreds of milliseconds), much longer than the original clipping or slewing event.

During this settling time, amplifier distortion can be quite high, since feedback is only partially effective. This will not appear in FFT harmonic distortion or multitone IM distortion measurements, which are taken over several seconds and then averaged.

The settling time referred to above is a process of many amplifiers with feedback, but not so much opamps (unless overloading, which is easily avoided). In a nutshell, the reason you run into the problem described above is that part of the amplifier circuit is not in the feedback loop. So it can behave as described and as pointed out, lots of test equipment ignores this phenomena, although it can be measured if you have advanced gear. There is more at this link:

https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf

If you don't want to read the whole thing, start at page 11, where the math is a bit lighter- but stay with it till the end of the article- its all relevant to this conversation.

Following Lynn and Don recommendations, I added a separate filament transformer for 300B. It made the sound clearer. Then I added a separate transformer and rectifier for driver and input tubes. I use a Hexfred bridge (Ralph's recommendation) and C-L-C-R-C filters. With 30H chokes for each channel. Input tubes B+ is connected to the capacitor after the choke with R-C. I’m going to connect it to the driver B+ capacitor later when I move from RC to IT coupling between input and driver tubes. The 300B output tubes are fed by an old transformer with the 5u4g Linlai rectifier with CLC and each channel has separate 15H choke and B+ capacitor. This upgrade doesn’t break in yet. Two weeks and around 30 hour is not enough (despite all capacitors being previously used). But what I can hear is that the bass control, speed and rhythm accuracy are significantly, radically better than they were before. I can’t hear improvement in the midrange yet. I also can hear the tone of instruments loose a little bit "tube magic". I understand I need more break in to make a more accurate conclusion.

Areas for improvement: The 5U4G rectifier is not ideal. I’d use HEXFREDs, high-voltage Schottky rectifiers (Don’s choice), or damper diodes for the 300B plate supply. Any of the three will have more dynamics and more vivid tone colors. The improvement should be immediately audible, two weeks will not be needed, you should hear it right away.

RC coupling will sound more dull and compressed compared to dynamic loads, LC coupling, or IT coupling. What RC has going for it is resistor coloration is less potentially noticeable than the other three methods, which each demand very careful component selection.

The final capacitor coloration of the filter sections will be audible, although less so than the cathode bypass caps, which are extremely sensitive to cap coloration.

I wouldn't try mixing and matching Ralph's approach with ours. Ralph has his way of doing things, and his own unique taste in sonics, and we have ours. Most designers in this biz have a distinct "house sound" that they aim for, which results from design approaches and parts selection.

I should mention SE tuning is not the same as balanced-amp tuning. The dominant coloration with SE are the tubes themselves, and it requires artful selection to avoid heavy additive coloration. The fad for 2-stage SET amps makes this worse, since high-transconductance tubes are not designed for audio, and distortion can be all over the place using tubes designed for RF use. Selecting designed-for-audio tubes in a 3-stage amp makes things simpler and more manageable.

Unfortunately, common design practice in Golden Age push-pull pentode amps is not helpful in designing a balanced non-feedback amp. All of the 1950’s and 1960’s Golden Age amplifiers use feedback as a required part of the design, and the "balanced" part of the circuit (the output section) is typically running in Class AB. You have to reach back to the 1930’s to find useful non-feedback Class A designs.

Once pentodes and beam tetrodes took over, feedback came along with them, and that changed the overall emphasis of the contemporary designs. The search back then was for more power, more efficiency with B+ supplies of 500 volts or less, low measured distortion, and cost and weight reduction.

That search reached an end when high-powered transistor amps replaced tube amps around 1966~1968. Transistors dominated the broad consumer market with the exception of guitar amps, which kept the tube factories going. The decades-long Japanese fascination with triodes finally came West in the early Nineties, where it created a niche market in the high-end sector. (Helped along by Joe Robert’s "Sound Practices" magazine.)

It really helps that tubes are now so popular in the true high-end sector of the market. Transistors ruled the market in the Seventies and Eighties (with the exception of Japan), and the Nineties were an era of transition and growing acceptance of tube electronics. The home theater and 500-watt crowd are still all-transistor because they need the efficiency, and Class D will give them even more efficiency.