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I just finished putting a new OT and electrolytic caps etc. in a 1974 Super Reverb. All six preamp tubes were 12AX7 so I replaced V3 and V6 with 12AT7 and kept the 4 strongest 12AX7's. When I fired it up and played through it for about 15 minutes I noticed the reverb was sounding weak. It's there, but nowhere near surf territory. We already have nearly $300 CDN in parts invested in this amp and I don't want to suggest to the owner that he invest in a new reverb tank unless absolutely necessary. At that point I also noticed that V3 was super hot. I've been reading on here and elsewhere that this is typical for the reverb driver 12AT7 on BF/SF amps. To boot, this particular model has the 470 ohm cathode resistor for that tube as opposed to the more common 2.2Kohm. I figured my best bet would be to go back to the 12AX7 in that position as I understood that the heat would be less of an issue and to give the weak reverb a boost. V6 was also pretty hot and I figured maybe that's why a previous owner had put 12AX7's in V3 and V6. Anyway, with the 12AX7 the reverb does sound stronger. But it's hard to tell from touch alone whether the heat factor is less than with the 12AT7. So, my question has to do with the specs for the two tube types and how that relates to the heat issue. I measured voltages and resistor values with both tube types in V3 and calculated plate dissipation of 5.3W (2.15W per triode) with the 12AT7 and 2.13W (1.07W per triode) with the 12AX7. The 12AT7 datasheet lists max plate dissipation as 2.5W per triode and the 12AX7 datasheet specifies 1.0W per triode. These numbers seem counter-intuitive to me, I would have expected a tube with higher gain to dissipate more power/energy/heat than the lower gain tube. Further complicating matters, with the 12AT7 the measured plate dissipation is less than the specified maximum while in the case of the 12AX7 the max dissipation is exceeded. Is there a simple way to look at this that might help me make sense of it? The other specific question would be, have I really killed two birds with one stone with the 12AX7, stronger reverb and less heat, or is it wrong to assume the 12AX7 is generating less heat on account of the lower plate dissipation number.
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