An insertion of a solid state amplifier is substituting the obsolete first stage of a full tube RF power amplifier. The amplifier is based on two tube stages. The first, equipped by a tetrode, the RS1054, was being manufactured by Thales until some years ago. Some spare parts have been ordered but not enough to guarantee smooth cyclotron operation for the next few years. It was necessary to come up with a new solution. We were basically at a crossroad: replace the first stage with another tube still in production or change the technology from tube to solid state. A study, from market research to the technology point of view was carried out and the final decision was to use a solid state stage as an innovative solution for this kind of power vs frequency range of operation. The prototype of this hybrid amplifier has been in operation with our cyclotron since January 2015. The details of these decisions, the description of the modified amplifier (solid state – tube) and the successful results of this hybrid configuration will be shown in this presentation. RF AMPLIFIERS STORICAL OVERVIEW The RF power amplifiers are made by two tetrode stages driven by solid state commercial amplifiers. The drivers are class A, wideband, 50 dB gain and a maximum out power of 200 W. The RF power amplifier can deliver a maximum power of 75 kW, the total gain between 1 and 2 stage is about 30 dB. The first stage is wideband, based on the tetrode RS1054L by Thales, in groundcathode configuration, air-forced cooling. The second stage is a narrow band stage, common grid configured, based on 4CW100000E by CPI, water cooled. Figure 1: RF amplifiers and the 1 – 2 stages view. The three RF power amplifier cabinets with the internal view of the final stage are shown in Fig. 1. The tuning system for all the frequency range (15-50 MHz) is automatic. This amplification system was made, under technical specification by our Institute, at the end of 1980s. Since the commissioning of the Superconducting Cyclotron, 1994, the amplifiers have been operating, related to the RF power stages, without significant changes or upgrade. The robust and classical electrical design of the amplifiers has ensured, more or less, an uninterruptable operation, for the last 30 years. Figure 2 shows a simple block diagram of the RF system, with drivers and final amplifiers [1]. Figure 2: Block diagram of the RF system. The total gain between drivers and final amplifiers is approximately around 80 dB. The distribution is about 50 dB for the driver and 30 dB for the final amplifier. The final is divided between 14 and 16 dB respectively for the 1 and 2 stages. Despite the cost of the two tetrodes, in terms of spare parts, has greatly increased in the last decade, we decided to continue operating with this configuration. But some expedients was adopted. For example, we optimized the main parameters of the amplifiers, to reduce the maximum output power from 75 kW to 30 kW, enough for our cyclotron performance and as result, the average life span of the tetrodes was increased. We decided to refurbish the exhausted tetrodes, instead of buying new ones, reducing the cost by a third, and the reconstructed tetrode, can be considered like new. Unfortunately, this refurbished technique can be done only for the second stage, the 4CW100000E by CPI, while for the first stage, the RS1054L by Thales, it was not available. Apparently, the type, hardware and geometry of the tetrode itself made the rebuilding of the Thales tetrode almost impossible [2, 3]. ___________________________________________ * INFN-LNS † caruso@lns.infn.it Proceedings of Cyclotrons2016, Zurich, Switzerland WEB02 Cyclotron Technology ISBN 978-3-95450-167-0 263 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s TETRODE VS SOLID STATE In any case, the main reason for modifying the amplifiers and consequently changing the first stage, was the end of production of the tetrode, RS1054E, in a few months. The strong decrease in demand on the worldwide market for this specific tetrode, was the reason Thales stopped production altogether. We were only given the opportunity to buy the last three tetrodes at the very unreasonable price of about 60 k€! In the meanwhile the cyclotron was in operation, the total RS1054L spare parts were not many, no specific planning was scheduled, in terms of funding/timing, because Thales sent us notification a very short time before ending production. Figure 3 shows the schematic of the RF amplifier and, in the dotted line, the obsolete 1 stage to be removed and replaced. Figure 3: RF schematic, part to be removed in dotted line. Obliged to change this tetrode, we had two options: use another tetrode or a new solid state technology. The first approach was more conservative, we considered more than one possible tetrode. Commercial partners such as Eimac and Thales offered very few valid alternatives and, it is not a plug and play job, some hardware modifications have to be made, like new sockets, some changes in the polarization network, redesign of the matching board between the 1 and 2 stages, and recalibration of the RF response. As a possible alternative to the originally used RS1054L the CPI tube 4CX3500A was selected. This tube is less powerful than the original one but was selected because we thought that the final power of 30kW was enough for normal cyclotron activity. The obsolete and new tetrodes are shown in Fig. 4. Figure 4: Mechanical difference between the two tetrodes. The most critical parameter is the input capacitance of the 4CX3500A as it influences the input circuit negatively. The existing input wide band circuit has to be redesigned in order to cope with the higher tube capacitance. An insertion of new crowbar circuit in the anode power supply plus retuning of anode matching circuit. The positive points of this “conservative” choice are: • 4CX3500 cost is relatively low, high efficiency, high reliability, robustness; • apparently no end of production in the near future, according to the manufacture; • CPI, ensured total assistance to rebuild the tube in case of failure (not necessary to buy a bright new tube every the time). The risk and negative points of this operation are: • The tetrode manufacturer can notify the end of the production of this new tetrode at any moment. With a very short margin in terms of time, according to our experience; • It is not possible to store a lot of spare parts, economic and vacuum tube technology; • Cost/timing of the total operation is quite high. But, the most important point, to investigate other solutions, for example the new emerging solid state technology, was the increased number of dismissed tetrodes and absence of new products in our frequency/power range. Figure 5 shows the trend of the power tube market. The area of our interest inside the dotted line is empty. Figure 5: Frequency and power range of tetrodes. The interest of the market in our frequency/power range is moving from vacuum tube to solid state devices. The miniaturization of the RF power components and the maximization of power per single device is only one reason for this new trend. The new solid state technology is going to cover the slice of market under a power of 100 kW and up to few hundred MHz of bandwidth [4]. SOLID STATE AMPLIFIER FIRST STAGE Removing the first stage based on the tetrode and installing a solid state amplifier means to adapting and matching this new device mainly in terms of impedance and amplification factor. In the original configuration of the amplification section, according to Fig. 2, the driver, a commercial amplifier, was connected to the main amplifier, through a 50Ω wide band input circuit of the first stage, based on the tetrode RS1054. The output of this stage, through a further matching section, a variable πfilter tuning system, is connected with the input of the second stage, cathode of the 4CW100000E tetrode. For 10 10