by Eric Barbour
The Svetlana 3CX300A1 is derived from a Russian power triode used in military equipment, and is designed for audio applications. It is an external-anode type, mechanically similar to the 4CX250B, but with a low mu and low plate resistance. Mounting and cooling a 3CX300A1 is more involved than for a conventional glass audio tube. It requires use of a fan or blower, a sealed chassis plenum, openings around the socket, and a chimney which fits around the anode and directs the moving air through the anode fins.
The socket used with the 3CX300A1 is the Svetlana SK2A. This tube also fits any standard loktal tube socket, or specialized sockets for the 4CX250B, such as the Eimac SK640. Ceramic (such as the SK2A) or high-temperature thermoset plastic construction is recommended. The low-cost loktal sockets seen in antique radios, made of phenolic plate material, are not recommended as they will not tolerate the higher operating temperatures of the 3CX300A1. Note that some Eimac-type sockets have screen bypass capacitors permanently attached between pins 1 and 8; these sockets should NOT be used with the 3CX300A1. Many surplus sockets have these capacitors, and should be avoided (or modified if possible).
Figure 1 shows a suitable mounting scheme for the SK2A on a chassis. There should be openings around the socket body, to pass cooling air. These can be achieved by making the opening 1/4" (6 mm) larger in radius than the socket body. Air can then pass the socket and flow upwards to the anode fins. Best airflow is obtained when the opening around the socket is roughly equal to the open area of the anode fins, to minimize back pressure. This area is about 754 square millimeters, or about 1.1 square inches.
Since a chimney is required around the tube, the socket is shown mounted below the chassis in Figure 1. Standoffs should be 5/16" long (7.5 mm). The chimney will then slip over the tube and rest on the chassis, thus directing the air past the lower body of the tube and through the anode fins. The standard Eimac SK606 chimney is suitable. Note that some chimneys are not perfectly round; if the tube anode does not fit the small ID of the chimney, you can slightly increase the ID with coarse sandpaper.
Alternatively, a chimney can be machined or molded to fit the 3CX300A1. It should have a lower ID of 1 13/16" (46 mm) or larger, and an upper ID to fit around the 3CX300A1; 1 11/16" (42.5 mm) will fit all production 3CX300A1s. Suitable materials include any non-conductive ceramic, or a non-conductive plastic with a high melting point and which does not outgas substantially. This includes Teflon PTFE, Delrin, Ultem, PEEK, Bakelite, PFA, and others. Polyethylene, PVC, polypropylene, polyurethane or fiberboard are not recommended unless they are specifically rated to operate continuously at a temperature of 250C and do not contain plasticizers.
The Svetlana AC-1 is the recommended anode cap connector. Note that it is simply a large fuse-type clip, and is attached to the anode wire. Alternatively, the Svetlana PC-1 can be used; it requires some force to fit over the anode cap, and it should be pre-loaded by forcing over the tube cap before installation. The PC-1 blocks some airflow through the anode fins. If it must be used for appearance reasons, derate the tube dissipation at least 25%.
The 3CX300A1 should be used in an enclosure or cage which is difficult for the user to remove. As the entire anode is at the anode voltage, a tube operated without a protective shield represents a hazard.
A small cooling fan may be mounted under each tube socket. 115v AC fans that are suitable include the Mouser 432-81552 or 5912-8800N, Digi-Key CR093-ND, or similar quiet fans producing about 20 cfm. These fans are 'muffin' types, and would need to be mounted under each socket and shrouded to direct air upwards to the socket opening. The chassis should be partially open to let the fans take in outside air. Note: this method of cooling is best used only if the tubes will not be used at full plate dissipation, due to the limited capacity of such fans. Operate the 3CX300A1 at 150w maximum dissipation if small fans are used.
For full-power operation, a sealed chassis should be used, with a single large external blower forcing air in. Suitable blowers include RF Parts 87-1250-00 or Allied 598-1430. Note that fans and blowers will give lowest noise conduction to the chassis if mounted on rubber shock mounts.
Most SE or push-pull driver circuits will work with the 3CX300A1. Drivers intended for low-mu triodes in Class A1 or AB1 operation are especially suitable. Try to avoid driving the grid positive. Application of moderate negative feedback is recommended, especially in SE operation. The 3CX300A1 is well-suited to retrofit into older amplifiers to replace large power triodes that are no longer available, such as the 8045G (used in the Luxman MB3045), or other types dissipating up to 300 watts.
**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 materia