I must preface this with the statement that I have no actual knowledge of the circuit. However, from the tubes, the switch between 45/101D and 2A3/300b isn't a voltage switch. I imagine it isn't a heater switch at all. It may have a current limiting function or a resistance function for the anode voltage or the grid bias, but the 45 and 2A3 both have 2.5 volt heaters (amperage is different) and the 300B and 101D have heaters over 4 volts. I imagine the heater voltage winding on the Lampi is 2.5 or 3 volts (I suppose it could be a constant current transformer with variable voltage, but that seems unlikely). That is why the filaments on the 6A3 and higher voltage tubes barely light up. How he gets enough electrons off the directly heated cathodes in this instance is a good question in my eyes. Frankly, I would love to understand the circuit more.
And I also should say that I have run plenty of tubes slightly out of spec on the heaters (7dj8s instead of 6dj8s, 8bq5 and 10bq5 instead 6bq5, and 12sn7 instead of 6sn7). All work. Some better than others and it is highly circuit dependent. My 6bq5 amp sounds awesome no matter what is used. However, I could not use a 25SN7 in place of a 12sn7. The cathode was too starved to function. This raises more questions than answers about operating points, indirect vs direct heat, triodes vs pentodes, etc.
I cant recall but I made a post here months ago, based on what I was told by Lukasz. The circuits is based on watts, not voltage and the 6a3 is underheated in the 101d setting, but most here prefer the sound. When I tried, both settings sounded the same to me. I also think, but am not sure, that recent builds of B7 and GG changed/optimized the operating point of the 2a3 setting to favour more the 300b specs. This could explain why you Shawn and others prefer other tubes in the GG compared to what was your favourite when you used the B7. Andy, who does not post here, has a Gg and a B7 and says that the Sophia CP 2.5v is great in his B7, but not so great in the GG, where the Elrogs and Anniversary 45 are kings. I need to ask him if he tried any primo 300Bs like the Psvane WE replica, Takatsuki, Kron or vintage WE.
Here is what Lukasz said:
The 2A3/300b setting can work up to 6.5watts, while the 45 setting is up to 4.5watts. This is voltage times amps.
As the 6A3 is 6.3v x 1amp, its should correctly be in the 2a3 setting, however, many/most report that it sounds better in the 101d setting, which means it only gets about 4.5w when it's specs demand 6.3w. This means its underheated and when I googled that,, it says underheating can shorten tube life. however, that comment was in context of its use iun POWER amps and here they are used as preamps with maybe 5% of its rated capacity, so very little stress. In addition, its one of the more economical tubes out there with price per pair varying from $80 to $150...so people are not scared to use it as they see fit.
Finally 6a3 in any setting will run cool in the Lampi as the voltage drop from the power supply is just 0.5v times the moderate amperage of 1.0 means the heat needed to be dissipate is just half a watt per tube!
