[MrFixr55]

Some of the Williamson amps made by Heathkit and Dynaco advertise very low harmonic distortion.  Attached is the Heathkit WA-6M manual.  Though this amp is capable of very high output, the distortion at 20 W from about 50Hz -10KHz is under 0.7%. I believe that some of the lowest distortion comes from preamps that use "Op-Amp" ICs such as the NKM2068 and the TI RC4580 have THD ratings as low as 0.0005 - 0.001%

Re guitar amps with tube rectifiers such as Fender, Ampeg, etc., Guitarists expect that the characteristics of the particular tube rectifier such as voltage drop vs current contribute to the "sound" of the amp.  Many of these amps also have a "Standby / On" switch that cuts B+ to the tubes in Standby mode.

[radiorich]

Hello Peter,
thank you for sharing your knowledge and experience. I’m sure you remember that some home solid-state amplifiers used MOSFET transistors to produce a warmer sound. I have a 1970s Sony amplifier with MOSFETs, and my Hafler power amp also uses them.
There are few companies like Mark Levinson
Sincerely Richard

[RadioSvit]

Well, McIntosh is, after all, AB pushpull.

It uses some techniques to eliminate odd harmonics. Magnetic coupling / bias for crossover elimination, unity gain and pretty deep feedback rarely used in tube amps.

Well, McIntosh is, after all, AB pushpull.

It uses some techniques to eliminate odd harmonics. Magnetic coupling / bias for crossover elimination, unity gain and pretty deep feedback rarely used in tube amps.

Michael, you probably know very well that McIntosh achieved high sound quality precisely with its phenomenal output stages, contrary to the laws of sound physics . Because in general, a push-pull output stage mutually cancels out even harmonics, but doubles the amplitude of odd harmonics, - therefore it is an ideal output stage when using output triodes ( greetings to you from my beloved Philco 37-116 ), in which odd harmonics end at most at the third harmonic number. But how McIntosh managed to achieve this on the output 6L6G beam tetrodes is a big mystery to me. Well, I understand - a common negative feedback (which always kills the dynamics), cross-connection of the anode of one radio tube and the screen of the second radio tube in the grid pushpool to one half of the output transformer winding (how, how does it work?!), class AB instead of class A (why?!), a few more tricky circuits, but in the end - a masterpiece and an icon for audiophiles!
Here are some spectrograms of triode and pentode small-signal resistive cascades from my database of radio tube characteristics measurements for comparison, to better understand what a huge difference in the harmonic spectrum between triodes and pentodes.
The first screenshot is a spectrogram of a single cascade with a resistor anode load on a 6SN7 triode with a 10-volt amplitude signal at the cascade output. Without any feedback, but according to the operating modes recommended by the manufacturers of radio tubes.
As we can see - the second harmonic is at the level of minus 43 dB. This is approximately 0.8%. Other harmonics are at the level of radio tube noise, about 90 dB.
   

The second screenshot is a spectrogram of a single stage with a resistor load in the anode on a 6J7G pentode, also with a swing of 10 volts. As we can see - the value of the second harmonic has increased to 32 dB (about 3%), but the third harmonic has already appeared -60 dB (0.1 %) and the fourth -80 dB (0.01%).
In my opinion, this is very indicative.
   

[morzh]

McIntosh had very specific transformer design also.
The cayjodes drive a separate primary and so magnetically couple with Anode primary introducing feedback.
Also, the Anode primary of one tube is wound bi-filar with the Cathode winding of the opposite tube. There goes leakage inductance.
There are a few tricks.

[RadioSvit]

Michael, I also noticed this, but I honestly admit that I don't understand how it works. And I haven't found any information on the Internet explaining the principle of operation of such a scheme.

[RadioSvit]

Hello Richard. Yes, I am familiar with amplifiers on MOSFET transistors, but I have only encountered them when repairing powerful stage amplifiers with a capacity of more than 300 ... 500 watts, but I have never had to repair household audiophile amplifiers on such transistors. Of course, due to the much more linear volt-ampere characteristics, they have a much better and more natural sound, more similar to the sound of tube amplifiers.

[morzh]

Well...

1. They put two windings of the same number of turns, both in the plate and in the cathode of every output tube. This makes the tube a partial cathode follower (катодньiй повторитель).
This cuts the output impedance 4 times.
Here's why: because the two windings are in parallel AC-wise (they contribute to the same part of the magnetic flux), they double the current. Meaning they only need 1/2 the voltage.
1/2 the voltage means 1/2 Number of turns.
1/2 number of turns means 1/4 of the impedance.

2. The windings are wound bi-filar , and it is for both tubes (they actually call it tri-filar due to the third one for Screen Grids). This eliminate leakage inductance and stray capacitance. They turned a stray (leakage) inductance of about 10mH into 0.5uH.
Also the third winding that moves the SG together with Cathode "cheats" the tube into thinking that the SG vs Cathode is not moving.

3. The windings in P and K introduce 100% feedback, hence the gain of "1."

4. Shared magnetic coupling between the two tubes results in the tubes never "stop conducting", so the "notch" (crossover, "ступенька") do not occur.
To be more specific, the tubes DO stop conducting, but because they share the magnetic flux, which is in this transformer never stop / falls to zero, so when one tube is fully open, the flux simply "jumps" to the other tube, so it is never interrupted, as it would be in a regular push-pull transformer.
Also the fact that all windings share the magnetic flux (because the plate winding from end-to-end (P-P is one winding,, after all)) by being wound bi-filar, this what helps this crossover elimination.



https://www.youtube.com/watch?v=W58Rqx_4u2w

This is Gordon Gow explain his Unity Coupled Transformer.

And, again, the 100% feedback smooths what otherwise would be a hard tube shutdown into a soft transition.

[Arran]

It used to be quite common to use FETs in the audio amps of some stereo receivers, ostensibly for that "warmer sound", cheaper stuff used germaniums for the power output stage, well into the 1970s, whether this was for a warmer sound or just to cut costs alone I don't know.
Regards
Arran

[rfeenstra]

Wow! This R2R thread has turned into something way over my head!!! I just like to see the reels go round and round...and sound good to my aging ears!

[morzh]

OK, Everybody:

- Reels on AKAI go round and round, round ad round, round and roun.
- Reels on AKAI go round and round, moving the tape.

[radiorich]

Hello Peter,
Yes, Commerical Amps my Halfer is a commerical Amp .
Arran , I have Harmon kardon model 520 Fm receiver from the late 60s that uses
germaniums for the audio outputs two per side .

Mike is that your new lullaby song?

Sincerely Richard

[rfeenstra]

That, while watching the reels go round and round put me to sleep!

[morzh]

- Reels on AKAI go round and round, round ad round, round and roun.
- Reels on AKAI go round and round, put me to sleep.

PS. There are no moving things around but I am being put put to sleep with something. Need some matchsticks to keep my eyes open.