[Michael Maurice]

The way the heaters are configured on ECC81 / ECC82 ECC83 and I believer ECC88 are for 6.3V pins 4 and 5 are connected together and pin 9 is other side of the heater this will require 300mA

But you can connect it with 12.6V between pins 4 and 5 which will draw 150mA

The way it is connected in the Trio is that each valve heater is in series with V13 pin 5 connected to the cathodes, pin 4 goes to pin 5 of V12 and pin 4 of V12 is ground thus the voltage across the cathode is 25.2V and the current going through the heaters is 150mA. but note that there is a 680 ohm resistor R309 in parallel so the total cathode current is 153mA. but note this is common to BOTH channels.

One other point, you'll notice that pin 9 of V12 is used to provide 6.3V bias for the Grids of the output valves (via resistors)

The reason for this rather unorthodox arrangement is to provide DC heaters to V12 and V13 without going to the expense of a separate transformer winding, rectification and smoothing.

It also means that if there's a fault in the output stage that the outpul valves dont pass any current, then V12 and V13 wont start to warm up.

Also V12 and V13 wont start to warm up until V15, V16, V17, and V18 have warmed up.

In my book that quite ingenious.