1/23/2024 0 Comments Cathode follower preamp![]() Mark the leads somehow with tape or a sharpie or something so you know which way to reconnect them after the PT is bolted to the chassis.Ĭoncerning the RCA phono preamp circuit you posted, if you want to power both channels from the same 250V supply, then there are only two components that need to change: R4 and R9. Once you find the correct polarity, turn up the variac to 120VAC and you should get something like 13VAC reading on the meter. If the meter reads lower, the leads are connected in opposite polarity. If the meter reads higher than the first attempt, the secondaries are connected in additive polarity, which is what you want. The meter reading will be either higher or lower than the first attempt. Turn off the variac, reverse the leads on either the 5V winding or the 6.3V winding (but not both), reconnect the meter, and turn the variac back on, making sure the adjustment knob on the variac does not move from it's location in the first attempt. (Make sure all the other secondaries are taped off so as to not accidentally come in contact with anything else). Hook the variac to the primary and turn it on for about 30VAC. Take the 6.3V leads and the 5V leads and connect them in series, leaving one end of the 5V winding and the other end of the 6.3V winding to hook to your meter. There are several ways, but I find the easiest is with a variac. Tube designers thought of that and built in "controlled heater warmup" features on the filaments (like on the 6SN7GTB) to combat this sort of thing, but I don't know if any 6SL7s offer that feature or not. In the dummy loaded filament situation, you might see a filament "flash" for a fraction of a second on startup until the filament warms up and goes to correct resistance. In that case the third tube's filament will need to be dummy resistor loaded to drop the other 6.3V, or find a phono circuit that would utilize an even number of tubes. On the other hand, three 6SL7s will be kinda weird making the heaters work out with a 12V supply, since there is one tube that would sit by itself. Also, 12SL7s would work without mods in the previous calcs for heater circuit. There's enough headroom on your low voltage secondaries to support several more 12AX7s though IF using the 12V conversion idea previously presented. Yeah, I just threw out three 12AX7s total (covers the needs for both channels), only because that is kinda typical of what I've seen for a MM phono stage.
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