
Janusz Krzemień (January 8, 2018)
Here's a description of a stereo amplifier using QQE03/12 tubes in the output stage. These tubes were designed for use in broadcasting equipment, primarily in the output stages of FM radios at high frequencies of 200MHz. They were not an easy design element – they created various surprises in the form of circuit excitation and oscillations. The amplifier operates in pentode mode in a push-pull configuration with two QQE03/12 tubes in class AB per channel. Initially, I planned to use output transformers in a "Quad" configuration, but implementation encountered some difficulties.

Here is the final result in all its glory.


Let's start with the schematics. Below is the initial schematic of the input circuit, phase inverter, and gain stage.

During construction work it was slightly modified:

With permission from HAIKU-AUDIO, I used the output stage circuit with a modification consisting of removing the "Quad" circuit. Here's the output stage used in its original version...

... and after using additional rectifier diodes in the grids of the output tubes:

Finally, of course, the power supply:

And now a set of photos illustrating the amplifier's design details. The individual modules have been mechanically separated.

Power stage module.

Modules with screwed-on printed circuit boards.

I placed the modules in a "recycled" metal enclosure. It came from a damaged receiver. It's made of solid, rigid sheet steel.



The entire unit is covered with a perforated cover. It's made of relatively thin sheet metal. However, this isn't particularly significant, as it's common knowledge that, for air circulation reasons, nothing should be placed on top of the amplifier.

Time for some photographic documentation of the amplifier modules. Here's the power stage module ready for assembly.

The following photos show details of the component assembly on dedicated printed circuit boards. The module support elements are made of 1.5mm thick aluminum sheet, to which 1.5mm thick laminate "mounting strips" are screwed.
Power stage module (two channels).




Input stage modules (individual channels on separate boards) and power supply filter.

Power supply board.

And some more photos for curious website visitors.


View of the modules after inserting the electron tubes.



And here are the speaker transformers...

...and their installation method. The speaker transformers were made by HAIKU AUDIO from Krakow. They are designed for push-pull operation and Raa=5kΩ.

Now for the mains transformer. I set it up for the photo. Ultimately, it's screwed in horizontally...

...as shown in the photo below.

Details and parameters of the mains transformer can be found on the label. The transformer is designed in an "audio" version and has a safe power reserve.

Signal level meters - old-style analog, of course - and the amplifier circuit that directly controls them.

The use of a digital-to-analog converter (DAC) extends the functionality of the amplifier.

ALPS 2 x 50KΩ potentiometer and switches.


Input board with relays.


The following photos show the final assembly of the amplifier. The power module is mounted centrally, with the preamplifier modules on the sides. The mains transformer is located at the rear. Next to the transformer is a common power supply board for both channels, with 2x1150μF filtering capacitance.



On the left side of the mains transformer (looking from the back) there is a board mounted with support systems on both sides: soft start, a system delaying the activation of anode voltages by 90 seconds and a +12V power supply.



The front panel was made "from scratch" from 1.5mm thick steel sheet.

Rear panel of the amplifier.

Measurement results
Measured amplifier frequency response

And now, a comprehensive set of photos of the measurement system I used during the measurements.
A square-wave signal with a frequency of 1 kHz and an output power of 10 W.

The lower cut-off frequency with a 3dB bandwidth drop is 12Hz for an output power of 1W (taking a photo was not easy).

The lower cut-off frequency with a 3dB bandwidth reduction is 15Hz for an output power of 5W.

The lower cut-off frequency with a 3dB bandwidth reduction is 18Hz for an output power of 10W.

The upper cut-off frequency with a 3dB bandwidth reduction is 70kHz for an output power of 5W.

The upper cut-off frequency with a 3dB bandwidth reduction is 69kHz for an output power of 10W.

250Hz square wave signal with 10W output power.

2500Hz square wave signal with 10W output power.

5000Hz square wave signal with 10W output power.

10000Hz square wave signal with 10W output power.

15000Hz square wave signal with 10W output power.

20000Hz square wave signal with 10W output power.

1000Hz sinusoidal signal with an output power of 10W.

1000Hz sinusoidal signal with an output power of 16W.

1000Hz sinusoidal signal with an output power of 20W.

1000Hz sinusoidal signal with an output power of 24W.

Finally, I invite all interested parties to follow the thread dedicated to this amplifier on the TRIODA Forum at: [Wzmacniacz Push-Pull na lampach QQE03/12 w stopniu mocy].
Prepared by: Janusz Krzemień
Polski (PL)
English (United Kingdom)