Hello all. I’m new to the forum and am hoping someone can help answer a couple of questions. Many years ago I rebuilt my ST-70 using a Purist Audio Design input board modification. It uses 12AX7 tubes instead of the original 7199’s. I have no problem with the board and like it very much. I don’t remember much about the modification but I also ended up using two 5AR4’s instead of one. The amp has performed flawlessly for decades. My first question is does the second rectifier offer any advantage or is it possibly causing harm? I’m running the EL34’s in triode mode. I don’t remember what I did to wire it for triode mode. Seems like I added a jumper to some of the tube pins but I’m not sure. I would like to try the amp in ultralinear mode and may add a switch to allow me to switch back and forth. Can someone tell me if I am recalling this correctly, and if so, how to return the amp to ultralinear mode? Thank you very much for your attention.
2 posters
ST-70 Tube Rectifier Duplication
franksr- Posts : 1
Join date : 2020-08-22
peterh
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
Unfortunately the batterys in my crystal ball has gone out. I need schematics to answer the question.franksr wrote:Hello all. I’m new to the forum and am hoping someone can help answer a couple of questions. Many years ago I rebuilt my ST-70 using a Purist Audio Design input board modification. It uses 12AX7 tubes instead of the original 7199’s. I have no problem with the board and like it very much. I don’t remember much about the modification but I also ended up using two 5AR4’s instead of one. The amp has performed flawlessly for decades. My first question is does the second rectifier offer any advantage or is it possibly causing harm? I’m running the EL34’s in triode mode. I don’t remember what I did to wire it for triode mode. Seems like I added a jumper to some of the tube pins but I’m not sure. I would like to try the amp in ultralinear mode and may add a switch to allow me to switch back and forth. Can someone tell me if I am recalling this correctly, and if so, how to return the amp to ultralinear mode? Thank you very much for your attention.
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