Zeus 6C33C Triode SE Preamp & PP MOSFET Amp System Test

Test Configuration

Zeus System Test with 2:1 Output Transformer: 1 kHz THD at 4.1 Vac RMS into 8 ohms = 0.189%

Note: Although the overall THD is a lot higher than the mosfet version this is due to the 2nd harmonic. The third harmonic is quite close to the previous tests and the difference is because of the change in output transformer (EI 2-2/16" laminations plus a 0.15mm gap).

The setup is open on the bench and as is shown in the mute test there is a lot of pickup from the open power supplies and the computer equipment. Mounted properly on a screened chassis the majority of this will disappear.

The sound is nice and crisp with good detail. Overall the sound appears a little fuller to my ear than the mosfet version. However the sound differences are very small between the two versions and the warm triode glow may also have a subjective influence.

Test bench with hardware - with genuine warm triode glow.





PC + EMu 1212M
(Balanced Output)


Zeus SE 6C33C Triode Pre-amp Line Driver

8 ohm

PC + EMu 1212M
(balanced input)

1M point FFT

+ HP8903B

  • 6C33C-B Triode
  • B+ 37 volt supply
  • Rterm: 220K

  • Bias: 300 mA
  • Input Transformer: TX-102 +6dB
  • Output TX: 1:1; 25 ohms imp.
  • 2 x STW34NB20 mosfet
  • V+ 34 volt supply
  • Rterm: 120K
  • Bias: 750 mA per mosfet
  • Input TX: 1:10; 600 ohms
  • Output TX: 4:1 or 2:1

Test Configuration.

2:1 PP Gaped Output Transformer

4.2 watts = 5.82 volts into 8 ohms - 1 kHz FFT (-3dB) - 0.1893% THD

2.1 watts = 4.08 volts into 8 ohms - 1 kHz FFT (-6dB) - 0.1306% THD

1.1 watts = 2.88 volts into 8 ohms - 19 + 20 kHz FFT (-12dB peaks) - 0.1812% IMD

MUTE = 0 volts into 8 ohms - noise, 50 Hz and harmonics, plus computer / monitor PSU >30 kHz pickup - FFT

(All tests 24 bit, 192 kHz sampling, 1 million point transforms.)

I have just updated to a new release of WINaudioMLS, hence slight differences in graph format and the change in line colour.

( PRO EX Version + 64-bit ultra high precision FFT; 192kHz/24 Bit ASIO support.)

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Design by: Susan Parker, MIEE.

The information contained here may be used to construct one set of amplifiers specifically for personal NON commercial use only.

N.B. Personal liability disclaimer applies.

This page last modified on: 30th November 2005

All information, drawings and images Copyright 1995 - 2005 Susan Parker unless otherwise credited.