Thanks guys. Yeah, after screwing around with a lot of capacitors I concluded it just wasn't worth the money and effort.

The diodes do a fine job and only require a little bit of extra resistance to get the current right on the money.

The CL-90 provides a soft power up. It may look smaller than CL-90s you seen before. The effect is that it warms up and cools off faster because there is less mass. Also makes it easier to fit inside. I put it up top along with the resistors to keep heat away from the diodes.

Here's the compacted version that easily fits inside the metal housing.
https://live.staticflickr.com/65535/...69d3dee1_c.jpg https://live.staticflickr.com/65535/...8f5a07f5_c.jpg

Yes, the tube filaments don't care if they run off of AC, DC or rectified AC as long as the RMS value is correct.
There is one sort of theoretical aspect however. The rectified AC may contain high frequency harmonics of the 60Hz. High end audio and test equipment sometimes use DC on the filaments to prevent 60Hz hum effecting the operation. In the same way, the harmonics from rectified AC MAY cause a problem. I suppose if it did a small capacitor across the diodes might cut down on the problem. Early solid state (Ge, Si) rectifiers circuits had capacitors across the diodes because of this sort of concern.

Why did the use ballasts anyway? Why not just mount the resistors on a terminal strip? Did they think that placing the resistors on top of the chassis would provide better ventilation? With ballasts you have to replace all of the parts when one goes bad or way out of tolerance.

Good point, adding a small cap across the diodes is a good idea. Also, there are bypass caps and series inductors through the filament string that help suppress noise.

Heat. The ballast kicks out around 30 watts. Hot enough they put a piece of asbestos above it to shield the cabinet.

Also the ballast doesn't act quite the same as a fixed resistor. Especially, if an Amperite regulator type is used. They resist the current surge when a cold set is turned on. Sort of like a modern thermistor.

Besides better ventilation on top side of the chassis, they got hot enough that they were prone to burning open, so I suppose the thinking was to make them user replaceable, like a tube. I suspect a lot of owners would have thought they were just another tube. And everyone knows tubes go bad. :D

After a long shitty week on a cold rainy winter night, seeing a FNR of this set pop up on YouTube made it all better. Beautiful set, and a masterful restoration, as usual.