[Amps] 8877 for 3-500s

[Dennis W0JX]

Don:
I will not be judgemental about the swap. I changed out the 3-1000z for an 8877 in my old BTI. It works superbly but there are some downsides to such a change.
 
First, why do you think you need to change the filament xformer? I think that both tubes require 5.0 volts. However, the greater current capacity will require a surge protection, soft-start type of circuit.
 
Second, cooling requirements are much greater with the 8877. The 3-500's can be cooled with a fan where the 8877 requires a blower. You also should have a grid current trip circuit.
And you will need to adjust the bias voltage depending on plate voltage and other factors. The input circuits required minor touch up, nothing major.
 
I made my own socket using a Johnson part, spacers, and grid clips.
 
The main advantages are additional dissipation, low drive requirement, and oodles of power (at higher voltages). I do not find the 3 minute warm up time to be a huge burden although there are times when I wish it was instant on. Regardless, the tube manufacturers recommend only one on and off cycle per day.
 
I do not regret the modification. However, I saved all of the parts in case I want to convert the old girl back to stock.
 
73, Dennis W0JX/8