|
|
 |
15W SE Tube Power Amp and Power Supply |
|
 |
 |
|
Introduction: The 6C33C-B Triode
|
Originally designed for use as a series element in voltage regulators for stationary and mobile equipment, the 6C33C-B is a large tube. Its largest diameter is 65mm and it is 130mm tall. It has two filaments and two cathodes. The two 6.3 Volt/3.3 Amp filaments can be connected in parallel or in series, consuming a total of 42 Watt! Maximum anode dissipation is 45 Watt with one cathode operating and 60 Watt with both cathodes operating. Maximum anode voltage is 250 Volt, when the anode dissipation is more than 30 Watt and 450 Volt, when the dissipation is less than 30 Watt. Due to the high filament and anode dissipation the tube runs very hot (260 degrees Centigrade), and it can only be used with ceramic sockets. It is recommended to drill holes around the socket for increased ventilation.
The 6C33C-B has a transconductance of 39 mA/V, a mu of less than 4 and a very low plate resistance of around 100 Ohm. Due to its unique characteristics it is very well suited for audio use, however, you need to use transformers with primary impedances, which are much lower than conventional ones. In single-ended mode the primary impedance of the transformer is about 600 Ohm, in push-pull amplifiers it is 1-1.2 kOhm.
|
|
15W SE Power Amplifier with Two-Stage Driver.
|
15 Watt SE Power Amplifier with Two-stage driver.
Fig. 1 shows the EB-396/225 SE 6C33C-B amplifier with fixed bias and a two-stage driver. The two-stage driver can also be used with cathode-bias output. The input stage is a series connected 6SN7GT/6H8C with very good linearity and low output impedance. The second stage is a grounded cathode amplifier with a 6J5GT/6C2C triode and with distortion cancellation built into it (1). The adjustment requires a distortion analyzer and is done with P1. If you don’t have one, replace the trimpot with a jumper. Distortion will not be as low as with the adjustment, but certainly close enough to work well.
|
|
|
|
|
 |  | | The EB-396/225 SE Power Amplifier with 6C33C-B |
|
|
|
The distortion of the amplifier in fig. 1 is shown in table 1. The distortion has been minimized at 4 Volt RMS output. Again, the cancellation is not 100%, but now we have enough open loop gain in the complete amplifier to apply a small amount of feedback around it and reduce the distortion further.
|
|
The maximum output of the amplifier with 10% THD is over 20 Watt. The open loop gain is 23.1 times (27.3 dB) and it needs 0.476 Volt RMS to drive it to 15 Watt. The feedback is going from the 8 Ohm output to the cathode of the input stage. As we have an extra 180-degree phase shift in the chain, we have to reverse the primary taps of the output transformer to get the right phase for the feedback. The feedback reduces the gain to about 7.3 times (17.3 dB), so the feedback is approx. 10 dB. The input voltage is 1.5 Volt RMS for 15-Watt output. The feedback has reduced the distortion further, as indicated in table 1. The output impedance has been reduced to 0.6 Ohm, with a damping factor of 13.3 times. This is enough to work with most commercial speakers on the market. |
|
|
|
|
Table 1.
Output voltage across 8 Ohm Output power into 8 Ohm Distortion with built-in Cancellation Distortion with optional feedback
1 V RMS 0.125 W . 0.06% 0.02%
2 V RMS 0.5 W 0.13% 0.043%
4 V RMS 2 W 0.38% 0.125%
8 V RMS 8 W 1.4% 0.48%
10 V RMS 12.5 W 2.2% 0.78%
11 V RMS 15.125 W 2.8% 1%
12 V RMS 18 W 3.3% 1.6%
13.5 V RMS 22.7 W 10% 10%
Rise time for the feedback amplifier is 4 microsecond at 10 Volt peak-to-peak output voltage. Frequency response without feedback is -3 dB at 50 kHz. With feedback the frequency response is increased to -1 dB at 100 kHz. The amplifier’s power bandwidth with the EB-696/224 transformer is 40Hz to 20 kHz without feedback. Because of the wide bandwidth of the output transformer it is easy to apply a small amount of feedback around the amp without instability problems. Experimentation is very much encouraged, both in terms of driver circuits and ways of reducing the inherent distortion in the SE output stage. Naturally, if you want to preserve the inherent distortion of the SE amplifiers, you can remove the distortion cancellation circuit and the feedback. |
|
The power supply for this amplifier can be conceived in many different ways. Tube rectifiers for the output stage would probably cause too much voltage drop and are not recommended. BORBELY AUDIO offers a power supply board: EB-596/228, which provides two high-voltage rectifier sections, one for the output tube and one for the drivers, see fig. 2. All diodes are fast/soft recovery diodes. The output tube is fed from a C-L-C filter, the drivers from a C-R-C filter. Since the 6C33C-B takes about 2 minutes to warm up, I have indicated a switch between the transformer and the supply, so that you can switch on the high voltage after 2 minutes. It is very important to have a very well filtered supply for the drivers to avoid hum. Consequently, if you have a small choke in your jolly box, it is worth replacing the resistor in the driver power supply.
The driver tubes are heated with DC. A 3 Amp bridge and a 10000-microfarad cap off the 6.3 Volt will do it. Check the DC voltage on the tubes, if it is more than 6.3 Volt; connect a small resistor in series with the filter capacitor. The filaments are biased at approx. +100V on the driver board to avoid cathode-filament breakdown and to reduce hum. Alternatively, you can use one of the filament regulators offered by BORBELY AUDIO.
|
|
|
|
|
|
Bias compensation.
The fixed bias voltage should be present before you switch on the anode voltage. This can be done with a diode bridge and a capacitor. However, you need to note that an increase in mains voltage causes a significant increase in the anode current if you keep the bias constant. To compensate for this you need to increase the bias faster - percentage wise - than the mains voltage is increasing. Ideally, an increase in mains voltage should not cause any increase in the anode current. Practically speaking, the increase should be kept to -say- less than 20 mA over a mains voltage increase of 20 Volt. This can be done with a Zener diode in series with a resistor chain (1). This circuit ensures that the anode current doesn’t “run” away at an increase of the mains voltage.
Mono blocks.
It is recommended to build the 6C33C-B SE amplifier as a mono block. This ensures short cables between the amp and the speaker and lets you optimize the power supply and the internal wiring. The wiring of the amplifier is partially point-to-point, in that the 6C33C-B has a chassis-mount socket and the grid and the cathode resistors have to be wired directly to the socket. The rest of the amplifier, including the power supplies can also be hand wired, or, as with the BORBELY AUDIO kits, can be made on PCBs.
The 6C33C-B SE amplifier design with the EB-396/225 driver board and the EB-596/228 power supply board is the property of Erno Borbely/BORBELY AUDIO. Commercial use and duplication in any form is not authorized without a license.
1. The concepts of the “Distortion cancellation” and the “Bias compensation” are the intellectual property of Johan Bernstrom, Sentec AB, Sweden. Courtesy of Johan Bernstrom, private communication.
2. For a detailed description of the EB-396/225 SE power amplifier see the article “15W SE Power Amplifier with 6C33C-B” on the homepage under “Special Articles” or Glass Audio issue 5/96. |
|
|
|
|
