In Reply to: 45s in paramour? posted by triodont on August 07, 2000 at 23:16:43:
Hi.This is a general statement about modding a parafeed 2A3 amp to take a 45. I haven't seen a Paramour schemo, so Paul and Doc may have to weigh in on specifics.
The Rp of the 45 is approximately 2X the 2A3, so a 4.6K to 5K primary on the OPT will likely yield the best results. This is not to say that it can't be run into a lower primary impedance such as the 2.5K typical of 2A3 amps, because it can. Assuming the secondary load to be the same, you'll get more power (good) but also more distortion (not so good).
One way to partially compensate is to select operating points for the 45 from the plate curves and loadlines that will reduce Rp. Problem here is you may not care for the sonic results. I don't have the curves in front of me, but IIRC that's going to be a pretty low plate voltage. You'll compensate somewhat by a higher Ip. Some say running triodes "low and hot" gets the best sound.
But assuming you swap out the OPT for a 5K primary, then the 45's operating points will need to be set by a change in the cathode bias resistor. The 45 runs at a lower Ip than the 2A3 - usually 32-36 mA compared to 60 or so for the 2A3, so the power supply should be able to handle it. Grid bias voltage will be a little higher on the 45, around -50 to -54 VDC or so. Can't remeber if the amp is direct coupled, if so you should still be in the ballpark PS-wise. If it's RC-coupled that may require some tweaking, and it's here I'll cede the floor to Paul or Doc.