Foreplay III XFMR Resistance Check Problem

[Paul Joppa]

I just thought - there is a tube called the 9AU7, which is a 12AU7 that runs on 4.7 volts (or 9.4 volts but we don't care about that). It will probably require a larger resistor that 0.62 ohms, but it should be OK with that value to start with - it takes more current so it will be less than the 5.18 you measured. Worth a try if you can find some!

[Laudanum]

I wonder if using a 100V to 120V step-up transformer be worth a try.   I dont know much about them and whether you can use an inexpensive unit with good result.  Just thought it was worth a mention as atleast I know they exist.   

[Deafer_than_a_doornail]

Hello Paul

I suppose I could find a transformer as you suggested, but I would much prefer to modify the circuit to better match the heater voltages in the FPIII Extended upgrade, and would enjoy the challenge of it, too.

I would greatly appreciate it if you could draw-up a plan for it!  You are very generous indeed to offer such a thing.   

I would also need to know some things about component types for audio quality, etc., if and where it might be relevant.

Laudanum, thank you also for your suggestion about a transformer.

[Paul Joppa]

Here is a sketch of a circuit. The diodes are Schottkys, the same as in the kit. The LB317 should be in a TO-220 package and will need a hefty heat sink - a fairly common one is 2" tall, extruded aluminum with a cross section 1.0 by 1.65 inches. The 1.0uF cap is said to be unnecessary but is probably a good idea; I labeled it tantalum but an X7R ceramic should be fine as well.

[Deafer_than_a_doornail]

Thanks again, Paul!

I looked-up the parts in Google to see what they look like, etc.

It's always a big struggle for me to buy parts at the only electronics parts shop in town - my Japanese is too lousy, and its nearly impossible if I don't know what they look like. 

I plan to start on the upgrade in a couple of months. Because of my work schedule, it took me a year to make the FPIII, and I hope not to have it out of action for too long in making the upgrade. I'm hoping I can first prepare some upgrade parts to assemble before taking apart my FPIII.