Here is an affordable single-ended stereo amp project that can use various nominally priced Beam Tetrode or Pentode output tubes. The design is not an original, but inspired by a Meng Yue Mini Single-Ended EL34B amp that I picked up in Hong Kong on a business trip several years ago.
The performance of the stock Mini EL34B was not great; noisey (hiss and hum) and high distortion at low volumes. The input tube was a 6N2 capacitively coupled to the EL34B.
Surprisingly, the output transformers must have been wound with some care and consideration because the frequency response of the modified amp is out to 70kHz, -3dB @ 1 Watt/8 Ohm. The low end goes down to 25Hz, -3dB @ 1 Watt. This is quite extraordinary for an inexpensive Chi-Fi amp.
The main modifications to the stock Mini EL34B are the power supply, input tube circuitry and various grounding issues. The original power supply was a full wave voltage doubler CRC design with marginal capacitance which was the reason the hum was audible on my Klipsch LaScala's. I changed the power supply design to a CLC with more capacitance to reduce/eliminate the hum. With my head inside the mouth of the LaScala horn, nothing is audible now. The power transformer shown on the schematic is for a redesigned power supply using a full wave topology instead of a voltage doubler.
Since I always use a preamp to drive my amps, I did not need high gain in the Mini EL34B input circuitry. 0.5VRMS will drive the amp to 1 Watt/8 Ohms and a little over 1VRMS will give you full power. The input tube is now a 12AV7, which is inexpensive and sounds great.
The real beauty of this amp is that you can use multiple types of output tubes in the amp depending on what you like or can afford. All tube prices are going up, but if you shop around, there are still great bargains to be had on USA NOS or tubes from current manufacturers.
The performance of the stock Mini EL34B was not great; noisey (hiss and hum) and high distortion at low volumes. The input tube was a 6N2 capacitively coupled to the EL34B.
Surprisingly, the output transformers must have been wound with some care and consideration because the frequency response of the modified amp is out to 70kHz, -3dB @ 1 Watt/8 Ohm. The low end goes down to 25Hz, -3dB @ 1 Watt. This is quite extraordinary for an inexpensive Chi-Fi amp.
The main modifications to the stock Mini EL34B are the power supply, input tube circuitry and various grounding issues. The original power supply was a full wave voltage doubler CRC design with marginal capacitance which was the reason the hum was audible on my Klipsch LaScala's. I changed the power supply design to a CLC with more capacitance to reduce/eliminate the hum. With my head inside the mouth of the LaScala horn, nothing is audible now. The power transformer shown on the schematic is for a redesigned power supply using a full wave topology instead of a voltage doubler.
Since I always use a preamp to drive my amps, I did not need high gain in the Mini EL34B input circuitry. 0.5VRMS will drive the amp to 1 Watt/8 Ohms and a little over 1VRMS will give you full power. The input tube is now a 12AV7, which is inexpensive and sounds great.
The real beauty of this amp is that you can use multiple types of output tubes in the amp depending on what you like or can afford. All tube prices are going up, but if you shop around, there are still great bargains to be had on USA NOS or tubes from current manufacturers.
