300b lovers

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.

Hi Lynn,

In my SET amplifier is IT coupling between driver and 300b and RC between an input and a driver stages.

I think I understand why the tone issue appeared. I played with driver tube B+ voltage and current. The cathode resistors 680 Ohms weren’t changed. The driver is 6f6 in triod mode. In begging, the idle current was 20mA. Then I increase it to 23mA and then to 26mA. So, in the middle setting 23mA, the sound character was faster and rough with flatter musical instruments tone. With 20 and 26mA, the amplifier sounds more like SET, the tone is more reach, and speed is more realistic and natural.

Has anyone that is using Linlai E-6SN7 tubes in 300B amps found them to be noisy? Kind of like a ground hum but not very loud. The system was quiet with the previous RCA VT231 tubes. Sorry if I'm off topic a little. They were favorably mentioned here so I gave them a try. Thanks.