Add a CL-90 inlet surge suppressor in series with the heater string. Slow easy warmup. It looks like a "fat" ceramic cap so doesn't take up much space. It does get warm so needs to have air around it under the chassis. Mouser, DigiKey, etc.

I haven't spent a lot of time going through the chassis, but the voltages are all in spec so far. I'm about halfway done. *crosses fingers*

Played with it a little today and found out that it's not oscillating. Now i have something to go on...

Kirchoffs Law. It is a series circuit. The current is the same for all of the filaments in the string.
That is, if the current is high in one section of one filament, the current through all of the series
filaments must also be the same.

Cold filaments have less resistance and draw more current, the exposed filament leads will heat
much faster than those bundled in the cathode assembly. That "flash" isn't unusual since the
the voltage on those small exposed filament sections will be abnormally high until the filaments
bundled in the cathodes also reach normal operating temperatures. And those "flash points"
are typically where a tube will fail on turn on. A properly sized PTC device will help extend tube
life but few restorers bother using them.

Thanks Reece, I certainly will experiment with that next time I make an order.
Vin

What a marvelous suggestion! NTC discs for inrush current limiting. And the CL-90 with 120 ohms sounds just right for the job. I'm no math wiz so could you please explain how to read and understand this CL-90 current rating? 0.5≤ 1≤ 2.0

http://www.ge-mcs.com/download/temperature/920_325a.pdf

I just go by the ratings in the vendors' catalogs on line: for example, DigiKey shows the CL-90 as having a resistance of 120 ohms at room temperature, and 1.18 ohms at at full 2 amps. This size unit is good for AA5 radios and any radio drawing less than 2 amps. It's all I stock. It does give a soft start and may add 10 seconds to radio warmup but worth it, I think. With one of these installed, hook your voltmeter up to monitor total voltage across the heater string and see how voltage starts way down and gradually ramps up. Like being behind grandma's car when the stoplight turns from red to green.

I did some price comparing of the CL-90 Thermistor at Allied Electronics, Mouser, and DigiKey.

Allied ---- 1.94 each with no price break for higher quantities. They also impose a $5 handling fee for orders <$25.
DigiKey - 2.53 each or 1.64 quantity 10
Mouser - 1.75 each or 1.57 quantity 10 {plus sales tax for Ohio}