!2ax7 heater wiring? Series / parallel?

[glaznost45]

Hi,

I want to connect a 12ax7 in to an existing tube amp, but need some help with the heater wiring.

The transformer i am using has two 6.3v leads , but no central tap.
That is one wire goes straight to the chassis and the other to the heaters of the other tubes.

all the diagrams i've seen have pins 4 & 5 connected together connected to one heater lead and pin 9 connected to another heater lead, with a transformer that has a center tap grounded for the 6.3 v tap of the transformer.

I have a few old radiograms that have 12ax7's in them, with the same kind of transformer in them. I can't clearly see whats going on in them... but they are ordinary 12ax7( ( have swapped in and out tubes) not 12ax7A's which can be wired in series. Normal 12ax7's can only be wired in parallel?

can anyone help to shed some light ( on heater) for me ?

[arnoldc]

The 12AX7 heater can be wired for either:

a) 12.6 volts, filaments in series, 4 and 5 connected to filament transformer
b) 6.3 volts, filaments in parallel, 4 and 5 tied together, and 9 connected to filament transformer

the difference is in the current draw, as parallel connection normally draws twice the current.

for a filament transformer without a center tap, you can fake the center tap. see post #189 in this thread - Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

it also shows how to elevate the heater above ground in situations that need it.