• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

A2 experiments

The startup sequence in direct coupled tube amps can become quite nasty if everything isn't done right. The best case should be if the output tubes are biased to cutoff until the filaments are fully heated and the HT is up. The worst case would be a high positive voltage on the grid, but too much negative voltage would also be dangerous to the tubes.
My solution to this in the 833 amps was 2 power switches. The first paddle switch must be thrown for the second pushbutton switch to operate. First switch powers the driver stage, fixed bias and the 833 filament. After a warmup I press the HT power pushbutton which ramps up the 2.3kV HT via a soft-start circuit.
Quite an amp you have there, Magz! Respect!

My latest idea to get a controlled startup is to use 6,3V damped diodes (6AU4GTA) as HT rectifiers, heated by a transformer with a slightly higher voltage ( I ordered one 7V and one 8V heater transformer to run som practical tests) and a bit of resistance in series with the heaters to get some extra delay and a slower ramp-up. This should be slow enough to let everything else settle before the tubes see any high voltage and the slow ramp-up hopefully prevents any nasty voltage peaks into the power tube grids during startup.
A conceptual drawing of my latest idea. Ignore the lack of grid/gate stoppers, current measurement resistors, protection diodes etc:
Looks-wise, I like the warm glow of tubes right up front on the top plate. I also like to run the signal from the preamp to the front side of the amp to keep the signal path short and minimize noise pickup from the power supply.
See the pic on the previous page as an illustration.
I agree, the tubes should be as visible as possible from the front (although you have to look down into the anode system from exactly the right angle to see any glow in the 6S4S tubes). I'll probably have to buy a larger sheet of aluminium to be able to make a good layout. The power supply will be in separate, equally sized chassis which will be very crowded with transformers and chokes.