Omak Jewel Three Brochure

Measurements of nonlinearity of output and driver stages of OMAK J3 amplifier serial No 040.

The input tube was removed and the signal from very linear analogue LF oscillator was fed
directly to the grid of phase splitter (driver stage), which is a long tail pair utilizing 6N23P-EV
(Sovtek 6922) tube. This driver stage provides a 3 dB headroom in respect to full-power driver
voltage necessary for the output tubes. The latter are two parallel push-pull pairs of 6P45S
(Svetlana EL509) pentodes in cathode feedback circuit with 12.5% of cathode winding. The
plate-to-plate loads for each PP pair, taking into account the copper losses in the output
transformer (0.2 Ohms as referred to secondary), are the following:

RL = 8 Ohm Rpp = 6560 Ohm

RL = 4 Ohm Rpp = 3360 Ohm

RL = 2 Ohm Rpp = 1760 Ohm

The global NFB (3 dB in OMAK J3) in this case was disconnected together with the input stage.

The measured output resistance appeared to be 1.3 Ohm, therefore the calculated amount of local
NFB introduced by means of CFB winding is:

For 8 Ohm load: 17.4 dB (7.45 times)

For 4 Ohm load: 11.6 dB (3.81 times)

For 2 Ohm load: 5.6 dB (1.9 times)

The figures for 4 and 2 Ohm loads are close to the amount of internal NFB in a triode for typical
class A SE applications.

The measurements were carried out at f = 1 kHz.

THD at 8 Ohm at corresponding levels in respect to full output:

-3 dB: THD = 0.32% (2.38% calculated THD without CFB)

Here and on other pictures: upper trace – output signal, lower trace – output of distortion
analyzer (fundamental suppressed)

-6dB: 0.23% (1.71% no CFB)

2.83Vrms output (1 Watt) 0.08% (0.596% no CFB)

As one can see, the distortion product is pure 2
at 8 Ohm load in pure class A.

4 Ohm -3dB 0.43% (1.64% no CFB)

nd

harmonic. This amplifier delivers its full output