I was inspired by a thread I read here on diyaudio back in November about current production 7591A tubes by New Sensor and JJ. I’d known about the 7591A, but I hadn’t played with it before, and I hadn’t known that it had been reintroduced by two different manufacturers. I designed a circuit board for a very simple stereo amplifier and built a prototype. It turned out better than I expected, so I thought I’d share it.
I really like this little tube!! It seems that Westinghouse was telling the truth about its linearity.
I spent a few days with a THD analyzer optimizing component values, and especially exploring the operating point for the 7591A push-pull output stage. I’m getting about 35W per channel with both channels driven, and quite respectable THD readings. The Tung Sol Reissue 7591A works well, and I have a set of JJ 7591S that I intend to try as well. The JJ 7591S in particular seems reasonably priced, at least in the US.
I had originally intended to use ultra-linear (UL) operation, but I found that the 7591A seems to work much better as a pentode (see the THD readings: "proto_meas.pdf"). And when I added a 390v screen regulator, I was very pleased to see the improvement in THD. THD at 1kHz is below 0.1% up to 25W, and only 0.15% at 35W. Note that the THD data was taken with a 5-ohm load on the 4-ohm tap of a Triode A-470 output transformer, which yields an effective 5400-ohm plate-to-plate load. Power output and distortion are almost the same with a 6500-ohm plate-to-plate load.
The amp sounds quite nice when driving my Maggies, even with its modest output power. The next step for me is to revise the circuit board design, to include the screen regulator on the PCB. And then build it into a box. The PCB is only 10” x 6”, so the final amp will be quite compact, limited by the size of the transformers.
But first, I was curious if anyone in the diyaudio community has insight into the 7591A’s strong preference for operating in pentode mode, especially with the regulated screen supply?? Looking through the schematics of classic equipment from the 1960s, I don’t find any using the 7591A in UL mode. I did see a post from a couple years back using the 7591 with cathode feedback, but without any measured data. If there’s anything I can do to improve the linearity even further, it’s best to do it now, before I redesign the PCB…….
If there is interest, I’d be happy to post the revised schematics when the new PCB is done. New amp designs using these reintroduced 7591 tubes seem relatively rare, but I think they deserve more attention.
I really like this little tube!! It seems that Westinghouse was telling the truth about its linearity.
I spent a few days with a THD analyzer optimizing component values, and especially exploring the operating point for the 7591A push-pull output stage. I’m getting about 35W per channel with both channels driven, and quite respectable THD readings. The Tung Sol Reissue 7591A works well, and I have a set of JJ 7591S that I intend to try as well. The JJ 7591S in particular seems reasonably priced, at least in the US.
I had originally intended to use ultra-linear (UL) operation, but I found that the 7591A seems to work much better as a pentode (see the THD readings: "proto_meas.pdf"). And when I added a 390v screen regulator, I was very pleased to see the improvement in THD. THD at 1kHz is below 0.1% up to 25W, and only 0.15% at 35W. Note that the THD data was taken with a 5-ohm load on the 4-ohm tap of a Triode A-470 output transformer, which yields an effective 5400-ohm plate-to-plate load. Power output and distortion are almost the same with a 6500-ohm plate-to-plate load.
The amp sounds quite nice when driving my Maggies, even with its modest output power. The next step for me is to revise the circuit board design, to include the screen regulator on the PCB. And then build it into a box. The PCB is only 10” x 6”, so the final amp will be quite compact, limited by the size of the transformers.
But first, I was curious if anyone in the diyaudio community has insight into the 7591A’s strong preference for operating in pentode mode, especially with the regulated screen supply?? Looking through the schematics of classic equipment from the 1960s, I don’t find any using the 7591A in UL mode. I did see a post from a couple years back using the 7591 with cathode feedback, but without any measured data. If there’s anything I can do to improve the linearity even further, it’s best to do it now, before I redesign the PCB…….
If there is interest, I’d be happy to post the revised schematics when the new PCB is done. New amp designs using these reintroduced 7591 tubes seem relatively rare, but I think they deserve more attention.