Loot from NEARfest

The power supplies at the right front are Tektronix type 125 and 128. They seem to function fine. I will be using them for my tube circuit experiments since they both provide nice B+ and filament voltages. The big one was $20. The small one was $15.

The radios in the rear of the picture are functional Heathkit AR-3 receivers. They have a little bit of hum. My plan was to part them out. If they’re common on eBay, I will part them out. Since they are from kit, the sockets aren’t riveted in, they’re screwed in. They both have nice transformers. The tuning and bandspread both have 5 to 1 gear reduction. I paid $40 for both. I think the parts inside may be worth more then I paid.

The odds and ends on the blue pad are some trimmer capacitors, some tube sockets, and a couple of banana plugs. I used one to fix my Signal Corps Frequency Meter which was missing one.

Butterworth Low Pass 2mhz

I used a tutorial to transform the schematic to a net list with gnetlist. The schematic includes all of the values needed to properly import into a net list. The command I used on the finished and saved schematic was:

gnetlist -g spice -o filter.net filter.sch

Then I called ngspice, “ngspice filter.net”

and from the ngspice command line, I setup a frequency sweep from 0.1 hz to 4 mhz (I can’t figure out the magic to do mhz in ngspice, so I used 4000 khz):

> ac lin 1000 0.1 4000khz
> plot n1

The result of the (voltage) plot is:

voltage plot of low pass filter

I then attempted a plot of power, which I am probably getting wrong. I’m using the formula v(at n1) / 50 (the termination resistance) * v (at n1 again). Below is the plot:

power plot of butterworth. Clearly wrong.

I’m doing something(s) wrong, but it’s a fun learning experience. Once I sort out my mistakes, I’ll add a post with some guidance on simulating a filter from a gschem schematic through the ngspice machinations.