Hi

I would like to use two (parallel) 12AU7 in an SE output stage of a
(small wattage) guitar amp. I was wondering how I work out the value of
the primary impedance required for the output transformer.

If I had one 12AU7 SE in the output stage then the primary impedance of
the Output transformer is x (say) and from memory, if I have two 12AU7
tubes in parallel then the primary impedance of the output transformer
is x/2. Is that right?

Any help would be appreciated.

Thanks
Gian
[remove all the x from my email address to reply to me directly.]

"Rchtop" wrote

I would like to use two (parallel) 12AU7 in an SE output stage of
a (small wattage) guitar amp. I was wondering how I work out the
value of the primary impedance required for the output
transformer.

If I had one 12AU7 SE in the output stage then the primary
impedance of the Output transformer is x (say) and from memory, if
I have two 12AU7 tubes in parallel then the primary impedance of
the output transformer is x/2. Is that right?

er, probably. Your use of the word "is" is tricky.

Two valves in parallel require half the load impedance of one,
that's true. But do you know what impedance you would require for
one?

It's usual to load output triodes around 2-3 times the anode
resistance. For guitar amps maybe 2 times is about right. Ra for a
12AU7 is about 7k, so one valve would want a 14k primary, and two in
parallel would want 7k.

cheers, Ian

"Rchtop" wrote in message
u...
Hi

I would like to use two (parallel) 12AU7 in an SE output stage of a
(small wattage) guitar amp. I was wondering how I work out the value of
the primary impedance required for the output transformer.

If I had one 12AU7 SE in the output stage then the primary impedance of
the Output transformer is x (say) and from memory, if I have two 12AU7
tubes in parallel then the primary impedance of the output transformer
is x/2. Is that right?

Any help would be appreciated.

Thanks
Gian
[remove all the x from my email address to reply to me directly.]

I think Gian's concern is that (2) output tubes in a PSE configuration does
not follow the same impedance formula as in a push pull config.
west