25L6GT Power Amplifier
Without Power Transformer

The first version of my project to build a Stereo "Power Amplifier Without Power Transformer", is shown in the photos below. The amplifier uses a series heater string, and the B+ supply is taken from a rectifier operating directly off the AC power line. The chassis is grounded by a connection to the green protective ground lead in the power cord, and the audio inputs, and speaker outputs, are isolated from the AC power line by audio input, and output transformers. The amplifier uses 4 25L6GT output tubes, and 2 12SN7 push pull driver tubes, phase splitting is handled by the input transformers. Since the photos were taken, there have been some minor parts and wiring changes which have brought the amp to it's current state of refinement. The first version had a couple of sonic flaws, including "one note bass", which was not unexpected due to the high source impedance of the beam tetrode output stage. The amp also had a grainy sound. All these problems were resolved by three changes. I increased the value of the coupling capacitors, deleted the Zobel networks I had been using across the output transformer primaries, and replaced the Radio Shack Bridging transformers I was temporarily using to match the output of my preamp to the 600 Ohm transformer coupled inputs of the power amp. The Radio Shack transformers were replaced with a pair of RCA Broadcast Quality Bridging Transformers, which I think were wound by UTC, and are in the same case as the UTC HA series transformers. While these changes were only intended to eliminate the grain problem, an unexpected result of these changes was the elimination of the "one note bass". I haven't gone back to try and determine which change corrected the bass, I'm just enjoying the sweet sound of this amp every day now. I think the grain was a result of the Radio Shack Bridging transformers, it would be easy to try them again, I just haven't felt the urge to do it. The Radio Shack transformers may have also been responsible for the "one note bass". Here is the schematic of my stereo PP 25L6 amp.

If you have any suggestions, or comments on my 25L6GT amplifier design, you can contact me by email at byrnsj@sbcglobal.net. If you have built an amp using the 25L6GT, the 6W6GT six Volt heater version of the 25L6GT, or any of the other members of the 25L6GT family tree, such as the 50L6GT, I would be interested in hearing about your experiences.


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