ricardoramos wrote:Hi
I was wondering about if can the 12AU7s be replaced with 12AX7s in the ST70 modified. I saw in the datasheets that the 12AX7 has more gain and a much smaller plate current than the 12AU7s.
Surfing the web I faced several schematics (old and new ones) of UL amplifiers that used the ECC83/12AX7 as PS. Now I ask: Can I replace the 12AU7s with 12AX7s in the Dynaco ST70 amplifier ?
Sorry for such a *dumb* question, but I am a total newbie to this tube-talk ! Yellow_Light_Colorz_PDT_12
Cheers,
Ricardo
Brazil
Hi Ricardo,
This is not a dumb question at all. In fact, my circuit is a variation of the famous Mullard 5-20 circuit which indeed used a 12AX7 as the phase splitter. Mullard agrees with you!
That said, voltage drive is more than fulfilled using the 12AU7 in this application. It is more robust than the original cathodyne triode half of the 7199 (funny - just had this conversation with some friends earlier today). The flip side is that the 12AU7 will also have a lower output impedance for better matching with the power tubes.
Definitely possible, but circuit values would have to be tweaked for operation so it is not a plug and play swap.
Shannon
