Hi; would appreciate someone can tell about 7868 tubes. They are used in a vintage? Fisher valve receiver (Model 400) TQ
You have caught a break. The 7591 and the 7868 are electrically equivalent. NOS 7591s are in no better supply than NOS 7868s. The "recently" introduced current production 7591 by ElectroHarmonix changes the "picture". The EH 7591 is physically large, but my understanding is that it will just fit in a Fisher 400. You need a competent technician to change the sockets from Novar to Octal and rewire for the different pin out. BTW, the reports of the sound of the EH 7591 have been favorable.
You beat me to the punch. I was going to suggest the same thing. I just finished restoring an AMI jukebox that used 7868's. I converted it over to use EH 7591's. Sounds great (for a jukebox). The owner is now happy that he has relatively inexpensive replacement tubes available. Size was no problem, plenty of room in a jukebox. I wonder why they used 7868's in the first place? Maybe because they were less likely to vibrate out of their sockets? Just wondering.
I have no idea as to why the 7868 would be preferred over the 7591. Maybe pricing affected the choice.
BTW, don't forget the 3rd electrically equivalent type, the Noval based 6GM5. NOS 6GM5s are not common, and should, IMO, be left for situations where the EH 7591 will not fit mechanically.
7868 tube, from the NJ7P Tube Database Search
Novar type Power Pentode used in output stages of high-fidelity audio amplifier.
Heater or Filament Voltage . 6.3 volts
Heater or Filament Current . 0.8 amperes
EIA Base . . . . . . . . . . 9NZ
Prefered Substitutes . . . . None
Substitutes. . . . . . . . . None
Section. . . . . . . . . Pentode
Cin. . . . . . . . . . . . 11 pf
Cout . . . . . . . . . . . 4.4 pf
Cgp. . . . . . . . . . . . 0.15 pf
Plate Dissipation. . . . . 19 watts
Screen Dissipation . . . . 3.3 watts
Maximum Plate Voltage. . . 550 volts
Maximum Screen Voltage . . 440 volts
These were used in juke boxes too, and other types of amps as well. They are also expensive, I guess because there are no easy substitutes. They easily bring $40 each on ebay. The big tube seller guys probably get more than that.
This post is made possible by the generous support of people like you and our sponsors: