A 250TH glows warmly inside the 40M PA cabinet.Image of a homebrewed 40M class C RF amplifier. It is operated in CW, and with cathode modulation, in AM mode.The HK54 amp uses a 2000V power supply at 80-100mA.The 250TH amp is modulated by an Altec 1570B tube amplifier running a pair of 811A's. The audio is coupled from the 32 ohm output of the Altec through a 30V to 120V power transformer, to the cathode circuit ofthe 250TH amplifier. Fine fidelity is obtained since the Altec has a low output impedance and more than enough power in reserve to compensate for any impedance mismatch in this service. Below the Altec is a microphone mixer, and below that, an equalizer for selecting the audio bandwidth for speech modulation. The mike is an old RCA FM base station type made by Sure.Here is a view of the entire modulation stack. The solid state amplifier is used to modulate the 40M push-pull HK54 RF amp. The modulators are interchangeable with respect to connection to the RF amps by means of simple plug-in connectors.In the CW room, This push-pull 808 RF amplifier operates class C and is used for CW work on 160 meters. The plate coil assembly is a BC610 low range coil, designed for 2-4 MHz. The tank coil is resonated by a dual section 180pF air variable with an added 50pF doorknob across it to center the band. The doorknob stays cool in this application. (the capacitances stated don't include the plate capacitances of the 808 tubes.) The output is easily 150 watts with no color on the classic plates.The glow of the 808'sA shot with the flash, showing the plate circuit well. To put the amp on 160M, a new grid coil (transformer) was made up. Here's the link that shows how to put such a coil together.Back in the AM shack, here is the 80M amp with a pair of HK54's. It is also cathode modulated.A shelf above the gear in the AM shack holds exciters and receivers for various bands, so that each band can be used with minimal disturbance of settings on equipment used for other bands. The large black rig in the rack is a 30M exciter.
