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6B4G push-pull amp
This great amplifier is a class-A, push-pull amp using a pair of 6B4G directly-heated triodes as the output stage.
I built two versions of this amp. Version 1 uses two 6SN7's in the input stage, and delivers full output with around 300mV input. It's high input sensitivity is good for passive preamp systems.
Version 2 uses one less gain stage, and a 6SL7 phase splitter (both sections wired in parallel) at the input. It requires about 3V RMS for full output. This version is best if you have an active preamp that can deliver 3V, and it is a little quieter than version 2.
Both versions have the same driver (6SN7 diff amp) and output stage (6B4G's with cathode bias). Maximum output is about 7-89 watts
This has been a good-sounding and reliable amp for me for several years now.
I made a few quick measurements on version 1:
5% THD at 8V RMS into 8 ohms (8 watts)
0.3% THD at 1W into 8 ohms
This is an FFT of the amp's output at 1 watt. The 2nd harmonic is highest, then the 3rd, then 4th. 5th and on were in the noise. This is a very good harmonic profile for a P-P amp. Again, this is version 1; version 2 may be a little different.
This scope trace shows the output and distortion residual, again at 1W, 1kHz into 8 ohms. The distortion residual is (obviously) not to scale. You can see the 2nd and 3rd are close in amplitude.
Here is the version 1 schematic (PDF file) (33k), and the version 2 schematic (PDF file) (33k).
Also the version 1 parts list (text file) and the version 2 parts list (text file)
Here's an image of the inside of version 1. Click on the image for a full-sized view.
I did an AutoCAD layout of the parts inside. You can download a PDF, DWG, or DXF file:
Version 1: 1:1 PDF file (217k), or the AutoCAD 2000LT .DWG file (182k), or a R12 DXF file (372k)
Version 2: 1:1 PDF file (217k), or the AutoCAD 2000LT .DWG file (182k), or a R12 DXF file (374k).