AA Electronics PCB Group

Could be several things. The 30 to 60 second delay on the current limiter getting brighter suggests that you may have one or more bad power tubes, as it takes about that long for the tubes to reach full operating temperature and the power transformer reaching full plate/screen DC voltage under load. You can test by installing pairs and seeing which pair produces the same issue (V5/V8 - outside pair, then V6/V7 - inside pair). If you do this, halve the speaker impedance: if your running an 8 ohm cab, change to 4 ohm or if 16 ohm, then 8 ohm.

Parasitic oscillation - if lead dress or power-section ground paths are slightly compromised, it can drive the power tubes into maximum current conduction without making any audible sound through the speaker. Verification is best done with a scope, but you can check lead dress around the power tube grid stoppers (pins 5) and screen resistors (pins 4).

Screen grid resistor(s) - A screen grid resistor (on pin 4) breaking down or a screen/plate wire positioned too close to a ground lug/chassis. Check screen grid resistors for burning or cracking. Measure resistance from screen grid nodes to ground with the amp off and caps drained.

Less likely, unless you noticed red plating, but a failing coupling capacitor coming off the phase inverter (V4) can leak DC voltage onto the power tube grids (Pin 5) only after warming up. Positive DC voltage on the grids overrides negative bias, causing immediate thermal runaway. Check by CAREFULLY measuring the DC voltage directly at Pin 5 of each power tube socket during the 30-to-60-second warmup. If the negative bias voltage drops toward 0V DC (or goes positive), replace the phase inverter output coupling capacitors.

Hopefully, it's just a failing power tube (easiest fix).

What taps or transformers do you have available to you? The AA 183 schematic uses a voltage-doubler cell to convert a 6 VAC secondary winding into the raw DC voltage required for a 12 VDC regulator and 12V relays. There are other ways to power the relays, some more involved than others, but as you correctly point out, the decision is driven by what AC source you have available to you.

The 3.15 volts is for each side of the tube heaters. The Hammond 290FEX doesn't have a center tap (the center tap would connect to ground, splitting your 6.5 volt heater supply into two 3.25 volt legs.) You can easily make what is called an artificial center tap. You would use two 1/2 watt, 100 ohm resistors to connect each heater tap to ground. Most commonly done where the heater wires connect to the pilot light, but since the Dumble schematic is using an LED, you would need to ensure that it was connected before (on the transformer side) the 2 watt, 330 ohm resistors that is dropping the voltage to the LED. There are a ton of different examples online. Just search for "tube amp artificial heater center tap".