Modification of 2A3 SE Circuit

[jamesjung21]

Hey, I got another question.

Do I need to modify the circuit, if I replace a 2A3 with 6A3?

Of cource, I need to change the heater supply voltage, but what about the amp circuit itself? Isn't 2A3 directly heated?

[Eli Duttman]

Except for the filament voltage, the 2A3 and 6A3 are "indentical", including the UX4 base. The 6B4G is electrically equivalent to the 6A3, but uses an Octal base.

The "gotcha" with the 6.3 V. tubes is higher residual hum. In a PP design O/P trafo cancellation helps. In a SE design, energizing the 6.3 V. filaments with DC seems indicated.

AFAIK, 6A3s are strictly a NOS proposition. Sovtek offers current production versions of both the 2A3 and the 6B4G. Current production Chinese 2A3s are available too.

[jamesjung21]

So, no modification is really necessary, unless it is for an improvement...

And 2A3 could also be replaced by a 6B4G with some physical modifications...

OK, thanks!

JayJay

[jamesjung21]

Wait,
What about 45?
Would it perfectly replace a 2A3?

[Eli Duttman]

Wait,
What about 45?
Would it perfectly replace a 2A3?

No, it would not. Hop over to TDSL and take a look at the data sheets. IIRC, biplate 2A3s are 2X 45s in a single bottle.

[jlsem]

The "gotcha" with the 6.3 V. tubes is higher residual hum. In a PP design O/P trafo cancellation helps. In a SE design, energizing the 6.3 V. filaments with DC seems indicated.

How so? I always thought AC hum from filament heating is current related - the higher the AC current, the greater the magnetic field induced, the more the effect on the electrons moving toward the plate.

John

P.S., I believe the Russians still make a 6A3.