The photo shows a rig I put on the air in summer 2015. It is the first high level plate modulated rig I have ever put on the air and consists of a pair of Eimac 3-400s in the final modulated by Cetron 572Bs. It is currently operating on 75 meters. It was homebrewed by a faculty member at the University of South Carolina Greenville, to be the school's AM broadcast station transmitter. His wife sold it to a ham who moved it to 75 meters. That's all I know. I purchased the rig from Tim WD4TC in Lenoir N.C. and moved it up here in spring 2012. For a long time I wanted to run a converted broadcast rig, but I had to conclude that getting one into my basement shack would be very difficult. I began looking for a rack and panel rig, knowing one of that construction would be much easier to dismantle and move and it would fit into a variety of rack combinations. I was extremely fortunate to find this very well constructed and beautiful rig.
It is in a 7 foot tall rack and consists of seven chassis, from the bottom up: modulator h.v. supply, modulator, RF final h.v. supply, control panel and driver, RF final amplifier section, and meter panel. RCA modulation iron is in the bottom, a XT1690 modulation transformer and its matching reactor. There is a separate p.p. 6V6 speech amplifier at the operating desk.
The RF driver is a 6AG7 and 6146B that is excited by a Globe V10 VFO. It drives the grids of the 3-400Zs via a split coil LC network with a grid leak bias resistor at the CT of the input coil, a network identical to what Gates used in the BC1T. A coil tap on one side of CT goes to the grids; the matching tap on the other side goes to Cn coming off the final plates. The modulator h.v. supply uses a Stancor plate transformer, 115 v. primary 3500VCT secondary with FW solid state 866A rectifier sticks and choke input filter. The supply for the RF PA h.v. is a UTC CG311W with a variac controlled primary v. of 0 to 230 v. and a secondary v. of up to 2.5KV with a FWB rectifier of four 3B28s. The CG311W is rated for 350 ma at 2500 v. CCS in FWB. That power supply also has a choke input filter. The output network for the final RF amplifier is a pi network. The 3-400Zs sit in chimneys and the final PA chassis is pressurized with a Dayton blower running on 240 v. The audio comes into the rig to a UTC LS10 which feeds the grids of a pair of 6B4Gs. These drive the grids of the 572Bs via a UTC LS47 transformer. There is a feedback ladder from the plates of the 572Bs to the grids of the 6B4Gs. The control panel consists of the variac for adjusting the final plate voltage and relays that switch line service to the filament supplies and driver, the PA, and modulator supplies. The rig has some protection circuits so that the h.v. to the final will not come up if there is no drive to the final amplifier, and the modulator h.v. will not come on if the final PA is not in transmit. There is a modulator delay circuit that is adjustable so the operator can set the delay time between the carrier coming on and the modulator getting energized. The driver has a 6V6 clamp tube on the 6146B so there is no damage if the driver is keyed with the VFO off (which I do too often). The final PA has overload protection also. Metering: Line and final plate v., Driver plate current, Final grid current and plate current, Modulator plate current. Modulator driver bias test jacks and adjustment. I estimate the rig weighs in somewhere around 500 lbs.
There was no schematic for the rig so I went through as much of it as possible trying to figure out how things worked. There was a steep learning curve for me and there were a few times when I wondered what I got myself into and almost gave up. Most of it was a fairly straightforward AM rig but some, such as the driver, and control and some protection circuits remain a mystery. I still need to get into them and do wire tracing. It continues to be a work in progress that will occupy me for years to come. I wish to eventually make it a band switched rig for 160 m. as well as 75. From spring 2012 to summer 2015 I made a number of minor alterations and (in my opinion) improvements, but kept the basics unchanged so I could get it on the air with its original design.
The speech amp is a 2 x 6SC7, 6SF5, 6J5 phase splitter to 6V6 p.p. circuit. The output transformer is a Stancor A3311 60K to 500 ohm step down to balanced line to the rig. I use a Shure 737A crystal mic to a 10 meg grid resistor on the input to the speech amp. I modified the circuit to integrate an Inovonics 222 peak limiter to ride the audio gain and modulation negative peaks. I received a great deal of advice from Don K4KYV and credit him for his assistance. Without his help the rig would probably have never put RF to an antenna. For now I am running the rig and getting familiar with its behavior. They all have personalities and you have to get to know them like a new friend. I hope to work you on 75 m. with this rig.