This page details my first trials with the 6C33C triode in the
PP Zeus follower configuration which I made in May 1995.
I will be rebuilding this shortly using the same transformers
and auto biasing as per the VMOS line driver, and will update this page
accordingly.
Tube first switch on.
Fig 1. General 6C33C Line Driver Schematic.
Breadboard Tube Line Drive Amplifier
600 ohm balanced input from E-Mu 1212M soundcard, balanced XLR
output to power amplifier.

First Test: 1 kHz THD+N at 4 V rms into 600 ohms < 0.08%
Matched Pair - 200 mA Bias per Tube (I = 400 mA Supply)
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B+ 64.5 volt supply: 2x Bench PSUs in series (500 mA
limiting).
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Vg bias of -13.84 volts (for 400 mA supply current):
onboard bias PSU.
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Heater supply 12+12 Vac 100 VA toroid underneath
breadboard.
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T1 Input Transformer: Gardners 1 : 2+2 with Rterm: 10K
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T2 Output Transformer: 1+1+1+1 Quad-filar wound.
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Q1 = 279 mV, Q2 = 291 mV on primaries.
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Output configuration to drive: 600 ohm series, 150 ohm
parallel.
-
Output impedance: 60 ohms series, 15 ohms parallel.
-
Bandwidth: 25 Hz to 70 kHz.
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Noise floor: c. -150 dB.
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Heater power on to B+ power: MINIMUM 120 seconds.
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Power on to stable operating temperature: c. 10 minutes.
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THD v Frequency at 4 Vac RMS into 600 ohms (HP 8903B - No
HF low-pass filter).
% Distortion and Vac Output Level Graph - Bandwidth is 25
Hz to 70 kHz (63 kHz shown).
Supply at 250 mA and 400 mA curves, plus
mosfet version of PP line amplifier for
comparison (using same transformers).

Harmonic Amplitude v. Signal Level (E-Mu 1212M + FFT).
Note: measurements below -120 dB are limited by the PC
/ sound card performance, see loopback test at bottom of page.
Harmonic amplitude v. signal level at 1 kHz.

Harmonic amplitude v. signal level at 10 kHz.

Note: Distortion levels are mostly dependant on bias current, not on B+
voltage.
Operating at a higher B+ voltage, e.g. +90 volts, does not make any
substantive improvement in performance,
but does increase the power dissipation and also affects the thermal
stability of the tubes when run at higher currents.
Tests for B+ Supply Voltage / Current and Vg Bias
Harmonic Amplitude v. B+ Supply Volts at 400 mA B+
Current (E-Mu 1212M + FFT).
WITH
Vg Bias v. B+ Supply Volts to Obtain 400 mA B+ Current.

Harmonic Amplitude v. B+ Supply Current at B+ 65 Volts
(E-Mu 1212M + FFT).
WITH
Vg Bias Voltage v. B+ Supply Current at B+ 65 Volts.

Distortion v Load
Harmonic Amplitude v. Load (at B+ 65 Volts, I = 400 mA).
Constant input amplitude - note small variation in
fundament level.
N.B. Measurements at or below -120 dB vary during testing.

Percentage THD v. Load (at B+ 65 Volts, I = 400 mA).

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Q1, Q2: 6C33C-B Triodes "out of the box".
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B+ 60 volt supply: Bench PSU (300 mA limiting).
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Vg bias of -19 volts (for 250 mA supply current): Bench
PSU.
-
Heater supply 12+12 Vac 100 VA toroid underneath
breadboard.
-
T1 Input Transformer: Gardners 1 : 2+2 with Rterm: 10K
-
T2 Output Transformer: 1+1+1+1 Quad-filar wound.
-
Output configuration to drive: 600 ohm series, 150 ohm
parallel.
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Bandwidth 25 Hz to 70 kHz.
-
Noise floor at -150 dB.
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Test pictures are 1061 pixels wide to show detail.
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WINaudioMLS 192 kHz, 1 million point FFTs
( Note intermodulation products on 1 kHz fundament and it's
harmonics.)
1 kHz input at -6db 0.071% -62.96db THD, Display range
45hz-96khz.
1 kHz input at -6db 0.071% -62.96db THD, Display 850hz-20khz;
close-up showing intermodulation.
Input mute, peak signal 50Hz 64.6db display
range 45Hz-96kHz

Distortion and Output Graph - Bandwidth is 25 Hz to 70 kHz
(63 kHz shown).

E-Mu 1212M Loopback Comparison
E-Mu 1212M loopback, 1 kHz input at -6db 0.0017% THD, Display range 45 Hz
to 96 kHz
Note HF rise above 50 kHz

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.
ALWAYS take extra care when prototyping and
testing any items connected to mains line voltages or that use high voltages
or currents in the circuit.
ALWAYS fuse power supplies appropriately.
6C33C Power Tubes / Valves run
HOT
- Please be careful.
N.B. My prototype breadboard is for testing
purposes only and is not a suitable method of construction for general use.