recto`s

[Stevem]

those blackface amps could have been fitted with a 5AR4 recto, or a 5U4GB, both set up`s use the same power transformer.
The 5U4 recto should not pump into more the 40 uf of filter cap, the 5AR4 can handle 60 uf of filtering.
If your current recto is a 5U4 replacing it with the lower impeadance 5AR4 will get you even a tighter bottom end as you will have more B+ voltage and it can handle the greater size filter.
It also needs less current for its 5 volt filamnet and eases the load on the PT,PLUS its a soft start indirectly heated type recto which is better for overal tube life.
If you do go from a 5U4 to a 5AR4 be sure to check the outputs idle current, with these amps anything more than .022 amps ( 22 millamps) will make the amp lean to musshy side when low notes are played at high volumes.
Unless I have JJ brand 6V6`s for outputs I find that .018 amps is a good overall setting for low volume punch and some good lows when cranked up.The JJ`s being a bit of a 6L6 in a the 6V6 bottle can run hotter and still sound good!
Its too late now, but the best way to recap these amps and gett less 120 hz hum is to ditch the nulti section can and go with all sprague atom type filters and stack them up and silicone glue them down next to the power supply end of the tag board.
This would have let you have the screen filter and preamp filter grounding point seperate from the main filter ground.

[Jana]

Ah, more wisdom from the interwebs.

Saying that a setting of .018 amps is good means nothing. What is the plate voltage? If it's 350 volts, that is barely 50% plate dissipation. If it's 450 volts, it is still lower than 70%.