Peppe was born from my curiosity to experiment with a VT audio signal on a transceiver (my Kenwood TS-140S). After the successful design and construction of Giacomo, the solid state preamplifier used until the beginning of Spring 2013 (and still fully functional today), I dedicated myself to the making of Peppe, who started with evident and consistent operating limits, but shows an efficiency no less than that achieved by Giacomo.
Peppe's heart is an ECC83 tube, a double audio frequency amplifier triode. At first, it was Salvo IT9FST's advice: to use an ECC82 instead of this tube. The ECC82 has a gain (mu = 19, approximately) significantly lower than that of the ECC83 (mu = 100), which suggested that the ECC82 was more suitable to amplify the audio signal coming from a microphone in an adequate way to drive an RTX with linearity. After several attempts, I decided to use an ECC83, since the greater amplification of this tube gives my mid-bass voice a more consistent presence among the various tones found in my modulation. The preamplifier input has been designed to be used with:
In the first case, use pins 1 (AF signal input) and 4 (AF signal ground) of the input connector; in the second case, if you want to use an ECM cartridge, just make a bridge between pins 1 and 3 of the input connector, so that DC supply can reach the cartridge (the bridge can also be done outside the preamp).
Inside Peppe there is capacitor C1 which decouples the dc power supply to the ECM cartridge from the AF signal on pin 1 of the input connector. C1 and the R1 potentiometer realize an RC network which constitutes a first order high pass filter, whose cut-off frequency (-3 dB) is at 15.9 Hz, approximately.
The two triodes of the ECC83 work in microamp configuration, the same configuration used for the first two FETs present in Giacomo's preamplifier stage: each triode is supplied at 100 Vdc and operates with zero grid bias (grid voltage = cathode voltage). The microamp configuration has the advantage of allowing high amplification (in this case, about 100 times the AF signal at Peppe's input) with very few discrete components, and allowed me to leave the preamplifier stage (almost) unchanged compared to the circuits used in precedence.
The amplified AF signal exits between the anode of the first triode and the cathode of the second, to be applied to the gate of a BF245B type FET, used in a "source follower" configuration to work as an audio frequency source with very low output impedance, a feature that allows any type of cable to connect the preamplifier to the input of an RTX.
The use of a solid state component (the FET) next to a vacuum component (the tube) is not an audiophile circuit solution. However, a low impedance output is a necessity that has proved indispensable, and I would have used a second tube (a second triode). But the space available within Peppe's chassis, and the limitations of the power supply stage, allowed me to use a transistor and nothing more. Probably, what makes this choice acceptable is the fact that the valve and the FET work (amplify) in the same way, while the only substantial difference between them is the means used for the flow of the electric current (vacuum for the valve, a semiconductor for the FET).
The audio signal is taken from the source of Q1 through a second RC network, similar to the one found at Peppe’s input. However, the R component of this RC network consists of the series connection between two resistors, at the common point of which a part of the AF signal generated by the preamplifier is available. From here, the AF can be applied to an additional preamplifier/amplifier stage to create a monitor output that allows you to verify Peppe’s correct functioning.
The tube is mounted on a shielded socket, to prevent it from picking up AC hum or other noise: an aluminum cylinder is fixed to the socket (with a "bayonet" insertion mechanism), which covers it entirely and protects it from AF/RF electromagnetic fields that could disturb its operation. The tube shielded socket is connected to the electrical ground of the preamplifier, but it is isolated from the metal box that contains both Peppe’s stages: in turn, the box is electrically isolated from them, and is in contact with the braid of the two cables connected to Peppe's AF inlet and outlet. Therefore, the box acts as an intermediate point for the shielding conductor, which goes from the ECM cartridge to the RTX input.
Peppe's power supply stage starts with the mains transformer. The 200Vdc for the power supply of the tube are derived from the 155Vac of one of the two secondaries of the transformer, which are rectified by a W10M diode bridge and leveled/filtered by a 220uF 350V electrolytic capacitor.
The 200Vdc (approx.) measured at the electrolytic capacitor pass through two low-low filters:
All this translates into a noise-free preamplifier, whose output includes no hum caused by any AC present in the power supply voltage.
The filament voltage is obtained from the 10.5Vac available at the ends of the other secondary of the mains transformer, also rectified by a W10M diode bridge and leveled/filtered by a 4700uF 25V electrolytic capacitor. The 12.6Vdc thus obtained are applied at pins 4 and 5 of the ECC83, while pin 9 acts as a “center tap” (that is, it is connected to the ground of the anode power supply circuit).