300b lovers

Lastly, I have said this before in this thread.  It is about your design goals.  We could make a low powered amp that uses 45 tubes in push pull and it would produce maybe 5-10 watts and stay in class A.  I am sure it would sound incredible.  If you had 94+ dB speakers it would probably be plenty of power.  But 45 tubes cost a LOT, and many folks have 88-90 dB speakers and maybe a larger room.  So our amp is 25 watts, well really about 27 watts, and has easily driven 88 dB speakers to screaming levels.  Would the 45 amp sound better on more efficient speakers?  I don't know.  It would certainly sound a bit different, and possibly better.  We tried to make an amp that had a much wider appeal and used modern production tubes that didn't cost a fortune.   So it depends on your design criteria.   The Citation II amp will drive darn near any rational speaker.  It has three nested feedback loops and is very stable.  It is one of the few tube amps you could let idle away for an hour on a bench with no speaker load attached and it would not oscillate.   That said, it doesn't have the clarity and just spaciousness of the zero feedback DHT circuit.   It makes wonderful music and is non-fatiguing to listen to, but it doesn't have "the piano is in the room" sound of the 300b project.   I know this because I rebuilt about 80 of them, and lived with one for a few years.  I could rebuild one in my sleep:) 

Ultimately,  it depends on what you want the amp to do.   I want an amp that will drive a pretty large number of speakers and have the clarity of a flea watt DHT, with the drive and authority of a push pull amp.   So that is what this project is about.  Trying to get a sweet spot that will make a lot of people happy and not cost a fortune to re-tube, and to run the tubes at really sane operating points so they last a long time, while pushing the state of the art sonically.   There is no single design that can make everyone happy and drive all speakers and retain all the sonic characteristics we desire.  There are always compromises.  Tubes vs SS, SE vs PP, etc...  Within SS and tube worlds there are many topologies, devices, and tube types.  So there are many choices.  For me to say our way is the best is ridiculous.  So our comments in this thread just point out why we have made the design choices we have.

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.