Info on this page by W5KP. All pictures are of W5KP's T-368.
N9FOY Speech Amp/Feedback Ladder
"As-Built" Photos - W5KP T-368C Installation
Here's a photo of the feedback ladder I built to use with my modified speech amp. It is constructed on a small terminal board salvaged from an old Motorola transmitter, with all new components per the N9FOY schematic. Brackets to hold the board were fabricated to keep everything away from nearby components. Don't forget the high voltage inputs have full modulator HV on them! This photo, although taken before the ladder input and output leads were installed, will give some idea of the mounting location on the underside of the mod deck. Lots of space to work with in this particular area, so installation is easy. HV input leads coming from the mod xfmr secondary are 15 KV rated GTO HV wire, and the low voltage leads are shielded two conductor studio audio cable, double shielded by further enclosing them in 3/16" braided tubing. Ladder output leads are run through one of the chassis deck holes and up to an XLR connector, which plugs into an XLR chassis jack on the amplifier chassis. This photo shows the mod deck and speech amp from the rear, including the fan installation. The fans are fed by DC from a stereo phone jack using a 90 degree connector, so the speech amp can still be easily unplugged and removed. Stereo was used to keep the fan DC supply above chassis ground, since I was paranoid about hum. The transmitter's rear cover still fits fine, although I usually leave it off. The shielded cable in the center is the feedback, and the black cable on the right exiting the cabinet is the balanced XLR mic input cable.
Modified speech amp w/XLR and fan connections
Here's a detailed photo of the speech amp itself, in all it's holey glory. The shielded feedback lead, the mic cable, and the two Triad A-65J audio transformers are clearly shown. Be sure to tag the plate leads for the 2E26's, crossing up the feedback paths would be a bad thing. The original RG-58/U speech amp coax cables for 600 ohm and carbon mic inputs were tucked away and not used. Close-up photo of the cooling fan setup. Might be overkill, but what the heck. This photo shows the audio chain I use in this particular installation. Not expensive, works great. It's a Shure SM-58 vocal mic, feeding a small PreSonus 12AX7 tube preamp, into a Behringer VX-2000 voice processor. The VX-2000 allows me to tailor my very poor mic voice into something a bit nicer to listen to. I added the preamp because without it I was having to run the VX-2000 at near maximum gain. With the preamp, I run everything at around 50% gain in the chain, a much cleaner situation for the audio. If I had this all to do over again, I think I'd build in a single 12AX7 stage in the front end of the speech amp and do away with the preamp, but this setup is handy for controlling mic audio to the processor. All audio travels via balanced XLR cables and connectors, and I've been very lucky to have almost zero hum on the outgoing audio. Also shown is some of the monitoring gear, including an HP counter and a little BK hamfest special scope for envelope monitoring. The monitoring gear is all fed by an adjustable RF "sniffer" tap from a Bird meter with a 500W element.W5KP's Overall T-368C AM station photo
This is just a photo of the AM station. The main receiver is an R-390A I overhauled. The 599ZX filter is worth it's weight in gold during noisy band conditions. The Racal 6790/GM is used for general purpose monitoring in all modes. The modulation meter is a little Racal 9008 analog unit. The Ranger II is fired up when I don't need the horsepower of the T-368C.