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 ?
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 ?
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 - http://www.diyaudio.com/forums/showthread.php?s=&threadid=68267&perpage=10&pagenumber=19
it also shows how to elevate the heater above ground in situations that need it.
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 - http://www.diyaudio.com/forums/showthread.php?s=&threadid=68267&perpage=10&pagenumber=19
it also shows how to elevate the heater above ground in situations that need it.
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