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Single Ended 6BQ5/EL84 Stereo Amplifier. Another part Zenith/part my-own design based on the power amp section of an SFD2530 radio/phonograph set. The amp came with big 95-1660 output transformers with a unique output configuration, two Sylvania USA 6BQ5s and a Sylvania USA 5U4GB. I planned to use these tubes plus a Sylvania USA 12AX7. This amplifier poses some interesting challenges in design. The chassis originally contained only the power supply and output stages. There was no driver/input tube on the chassis so one had to be installed. There is not much extra room and the space available dictates where the tube has to go. The original power connector on top of the chassis has been removed, the hole enlarged and the socket for the 12AX7 has been placed there. The input/preamp connection socket will no longer be used so the tags can be used as terminal tags for making needed connections under the amp. The amp used chassis grounding with some points on the input socket being "ground" but this scheme had to be changed to accomodate the 12AX7 and allow enough extra solder points. I ran a 12 gauge ground buss wire across the amplifier and connected all grounds to the buss. This provides enough solder points to complete the amplifier. I also found that the supplied Sylvania 5U4GB provides excessive voltage to the amp and is really a waste for an amp of this size. There is little advantage to using a tube with this capacity so I elected to use an RCA 5Y3GT. It is large enough and brings B+ down to a useful level. I also placed a 5 watt 33k ohm bleeder resistor on the screen supply to assist in bringing the screen voltage down. First I listened to the amplifier without negative feedback for a week and was very pleased. The soundstage is wide and deep. The output impedance is obviously rather high with no feedback so I have now installed enough feedback to reduce the output impedance by a factor of 16. My startup and test tubes were a pair of Amperex France EL84s that test marginal but sound excellent in this amp. As a test tubes were switched to a new set of OTK 6n14n valves and they sound really superb, even better than the Amperex did. Of course the original Sylvania 6BQ5s have been tested now but, unfortunately, these tubes are a disappointment in this amp. They break up early and are not nearly as good sounding as the Russian tubes. In present form the amplifier easily delivers at least 7dBW. Here is the Schematic. This amplifier, like many other 6BQ5 amplifiers, has pleasantly surprised me at the marvelous presentation from such a small valve. This amplifier is easily large enough for my mid-range section in my multi amplified system. I do not intend to imply that this project is finished, rather that it is ongoing and this is it's present state. |
