Re: 90V ht battery
90V batteries are not actually 90v.
If they are 60 x 1.5V Carbon Zinc cells they will vary from a bit more than 90V when fresh to about 66V when totally flat. Alkaline would be 98V "fresh" dropping to about 68V when "flat".
If you discharge the NiMH PP3 too much, one cell in one PP3 will be flat before the rest and then reverse charged and destroyed.
The normal "end point" of a single NiMH is about 0.9V. If stacked in a battery, 1V per cell is a safer end point. A PP3 with 6 x NiMH cells would be about 8.7V to 9V fresh, about 8.1V for initial discharge typically falling to 7.2V and then 5.4 to 6V when "flat".
1.5V filament Battery Valve radios designed with Nominal 45V, 67V and 90V HT batteries typically and data sheets for later Dx96 series show 67V and 90V characteristics. As batteries go flat performance degrades due to lower gain, though between 75V and 100V likely not much difference with Pentodes.
Download datasheet for LT1073. Look at 90V supply for the pulse generator.
You can plug the PP3s into each other and use one clip cut in half at each end.
10 or 11 PP3s will work. Using Alkaline I'd use 10 x PP3 and using NiMH I'd use 12 actually.
But the biggest issue with the NiMH is not over discharging them and damaging one cell in one pack.
I'm designing a battery radio with 45V on Receive and 45V + 135V nominal on TX. I was going to use loads of NiMH AA cells to avoid "modern" electronics. But I think I will make two sets of power packs, one using Alkaline cells for 1.2V, 45V, 135V and one using 8 x C or 8 x D NiMH and LT1073 based inverters, a boost for 45V, a boost with external switch for 150V and a buck for 1.2V filament. Then the pack can charge from 12V to 14V external power inc car Cigar socket.
There is no simple protection for 10 to 12 x PP3 rechargeable (or 60 to 72 AA cells) in series without additional electronics like a micro-controller with 6 analogue inputs. (You can judge 2 x PP3 in series safely).
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