12AT7 in Poseidon

for Dynaco Mark II/III/IV and DIY PP monoblocks

12AT7 in Poseidon

Postby audiowize » Tue Apr 22, 2008 6:47 pm

FYI - I just changed out the 12AX7 for a 12AT7 in my monoblocks with Poseidon boards. I removed the dropping resistor between stages and placed a Triad C7-x choke (10h@270r) in place of the resistor to keep the B+ nice and high and offer some decoupling. Next, I removed the 12AX7 plate load and substituted a current source set at 4.5ma. The lower cathode resistor got swapped to 20 ohms, and the upper to 270 ohms. Oddly enough, I have a little more input sensitivity which I didn't expect (yeah, the feedback was reduced a tad, but the 12AT7 cathode is up another .5vdc...), but overall the sound is more lush, with a much more dominant low end better control. Generally speaking, I've never been enamoured with the 12AX7, so I could be very biased indeed.

On another note, switching to this tube will also allow for significant padding at the input. The 12AX7 seems to want at least 100k of grid leak resistance, whil the 12AT7 is ok with around 30k. I plan to tack in a 250k pot to each amp to help with the padding.

If anybody has a pair of Bottlehead C4S boards floating around, you want 217 ohms for R1 and 180k for R2, enjoy! If I up the feedback and drop the noise more, I'll let you all know!

-Paul
audiowize
 
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