Designed by Steve, N9FOY
Thank you to Jerry W5KP for providing the pictures an putting together this info in to a nice web page and for letting me copy it.. And special thanks to Steve N9FOY for the design and for sharing the information with the ham community.
This simple but extremely effective homebrew T-368 speech amp was designed several years ago by Steve Konopka, N9FOY, a veteran of the broadcast radio industry. It employs a pair of 2E26 beam power tubes, driven by a balanced mic input via a broadcast quality audio input transformer. It employs balanced feedback from the modulation transformer's secondary via a high voltage step-down "ladder" circuit utilizing close tolerance components to help maintain audio fidelity. I knew this amplifier had been built and was being successfully used by several T3 owners, but detailed information on it proved to be very difficult to find. Although this speech amp is featured on several popular AM websites (do a Google and you'll find them), all the reference and photo links I could find were invariably broken and led to internet dead ends. After much frustrating searching, I contacted Steve directly and he graciously copied his original info, mailed me the package, and gave me written permission to share the data via the internet. Per Steve, N9FOY's permission, please feel free to copy and share the information provided.
The schematic/data package includes three images (links are at the bottom of this page). The first is a complete schematic of the amp using a UTC K-9459 input transformer. A second image shows the wiring connections for an alternate input scheme using a pair of parallel-wired Triad A-65J input transformers instead of the UTC iron. This alternative was provided in case the builder had problems finding the specified UTC transformer, and appears to work equally well. The third image is a scanned list of Steve's construction notes.
Good things about this speech amp's overall design are:
1. It uses the same plug connection as the stock unit, and therefore can be easily installed and removed.
2. It is simple, relatively cheap, easy and fast to build, the parts (with the possible exception of the transformers) are easy to obtain, and the resulting audio quality will amaze you.
3. The entire mod can be "undone" quickly and easily, leaving the T3 totally stock.
In my case, I had a spare beat-up stock T3 speech amp on hand, so I stripped that entire chassis, leaving only the original filament transformer and male multi-pin chassis connector installed. I then built the new speech amp on the same chassis, re-using the filament transformer and some of the original connector's pins, which also allowed easy access to the existing mod deck's B+, 115VAC filament transformer supply, and 4-125A bias wiring. The final product looks pretty ventilated because of all the now-unused chassis holes, but it works fine. Since I had lots of room on the "new" speech amp chassis, I borrowed some 115 VAC from the filament transformer supply and built in a small DC supply for a pair of 12 VDC fans (operated in series at a quiet 2/3 speed) which help cool the 4-125A modulator tubes and the mod transformer. This may have been overkill, but it was easy to do and it can't hurt to have a little extra cooling for Old Buzzard transmissions. The fans run anytime the speech amp is powered up. The only changes required to the T3's modulator drawer are to jumper the existing speech amp B+ dropping resistor (to increase the amount of B+ availabe to the 2E26's) and to build, install and connect a simple negative feedback ladder on the underside of the modulator deck. Of course, if you don't have a spare speech amp chassis available, then you will need to dig up a matching male chassis connector, or come up with an alternate connection arrangement of your own. Using a mating male plug for the female speech amp multi-pin connection is by far the easiest option.
Clearly, the stout negative feedback is the key to this little amp's great sounding audio. As a bonus, I have had no reports of any detectable hum in the transmitted audio. In case it might be helpful, I have also included a few photos of the installation as done in my Crosley-built T-368C, with accompanying comments. If you have any comments, good, bad, or otherwise, please email me at firstname.lastname@example.org. All comments and suggestions are welcome, with the exception of those that poor-mouth my JS non-Handbook construction methods. :-)
73, Larry W9AMR
N9FOY's Speech Amp and Feedback Ladder schematic diagram
Wiring connections for alternate (Triad A-65J) audio transformers
N9FOY's Construction Notes
W5KP T-368C "As-Built" photos
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