No.52
Hi-Fi Audio
6D22S

Although most current builders of tube amplifiers commonly use either solid-state rectifiers or octal-base rectifier tubes in their plate supplies, there are better alternatives available. One of the best is the so-called "TV damper diode". Although not originally intended for audio-amplifier use, damper diodes possess very high performance and ruggedness compared to more typical octal-base tubes such as the 5AR4.

The only damper diode still being manufactured today is the Svetlana 6D22S. It possesses a magnoval 9-pin base, and its cathode is connected to the tube's top cap - a very unusual feature which helps increase the voltage-handling capability of the tube. The cap connector (PC509) and socket (SK509) to fit the 6D22S are available from Svetlana distributors. If OEM use of a damper tube is intended, the 6D22S has the major advantage of continuing manufacture.

Damper diodes were designed to clip the negative "ringing" pulse appearing across a TV set's horizontal deflection coil during the retrace interval. Dampers must be able to handle relatively high peak currents and high reverse voltages without arc-over. Since the cathode of such tubes is usually floating above ground and sees very high-voltage peaks, running the damper on the same filament supply as the rest of the tubes in a TV set requires that the damper have its heater very well-insulated from the cathode. This characteristic makes it possible to use the 6D22S as a plate-supply rectifier in all kinds of audio applications, while running its heater from the same supply as the other tubes in the amplifier.

This heater construction also causes the 6D22S to warm up much more slowly than conventional octal rectifiers. A warmup delay is a boon in tube equipment, as it allows the other tubes to warm up to operating temperature before the plate voltage reaches its full value.

We tested a 6D22S against an original Mullard 5AR4/GZ34 and a generic Russian-made 5V4GT in the circuit shown in Figure 2. Clearly, the 6D22Ss warmed up much more slowly than even the 5AR4, which was specifically designed for slow warmup. Thus, with 6D22Ss in the supply, there would be no need for an electronic delay relay or other turn-on delay device.

WARMUP DELAY TESTED IN FIGURE 2 POWER SUPPLY CIRCUIT
6D22S heaters operated on 6.3v heater winding, the other tubes operated on the 5.0v winding.

All tubes were started cold. No load was applied during the test.

Russian 5V4GT 12 seconds
Mullard 5AR4/GZ34 29 seconds
Svetlana 6D22S pair 36 seconds

Another test involved operation of 6D22S heaters on a 5.0-volt heater supply. Even though it is rated for 6.3v operation, a load test showed that the 6D22S heater can be operated on 5.0 volts if that is the only supply voltage available. This shows that the 6D22S is a very conservative rectifier tube. A filament winding rated 3.0 amps at 5.0 volts AC appears to be adequate for a pair of 6D22Ss.

LOAD TEST OF 6D22S PAIR, FULL-WAVE RECTIFICATION WITH HEATERS OPERATED AT 5.0 VOLTS AC, 3 AMPS TOTAL:

Attached are three circuit suggestions using the 6D22S. All have been extensively bench-tested and include load test curves, showing expected voltage drops from the supplies. All these supplies deliver about 16% or better regulation within their rated current capability. If more filtering or voltage stiffness is required, more capacitance may be added after the filter choke. With all these circuits, a protective shield over the 6D22S cathode caps is strongly recommended to prevent electrical hazard to the user.

Figure 1 may be used to power any of the circuits in our 6BM8 Technical Bulletin No. 10; Figure 1 or 3 in the EF86 Technical Bulletin No. 23; the mu follower circuit in Technical Bulletin No. 27; or any other preamp or small power amplifier. A separate heater supply for the audio tubes is recommended.

Figure 2 will operate the Technical Bulletin No. 9 circuit Figure 3, Technical Bulletin No. 27's circuit, or any commonly-used EL34, SV6L6GC or SV6550C push-pull amplifier circuit using two tubes. Note that the 5-volt winding on the transformer may be used to run the 6D22Ss to free up the 6.3v winding for use with other tubes. This has been tested as above, and seems to result in virtually no performance penalty.

Figure 3 is for high-power amplifier applications. It is suitable for use with Technical Bulletin No. 9's Figure 1, or Figure 2 if a separate -150v supply is provided for the cathode follower. It is also suitable for running a pair of Svetlana 3CX300A1 tubes up to full power dissipation. On the bench, this supply has delivered up to 500 mA.

Figure 1- Small supply is suitable for preamps or for 6BM8 power amps
Figure 2 - 450-volt supply can power simple push-pull tube amplifiers or preamps.
Figure 3 - High-voltage supply is suitable for use with SV572 or 3CX300A1 triodes

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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