Built myself a retro signal tracer for tube radios

[VTVM]

Here is the homebrew signal tracer project I made. I tried to keep it retro in styling and electronically. It is 100% "hollow state" --there are nothing but tubes in the circuits right down to the 6AL5 small signal rectifier to drive the meter.

The speaker is a 5 watt, 4" unit. Fidelity is good, as it must be to find sources of distortion or excessive hum in an amplifier or radio. There is an output for an external meter or scope if specific (relative) signal voltages are needed or if the audio waveform or RF modulation envelope (demodulated) is to be observed visually. I also put in a noise test circuit with a momentary push button that puts out B+, through a high value resistor, to the audio input probe. Any roughness in this current flow caused by a cold solder joint or suspected "noisy" component is amplified by the audio amplifier. That test works similar to the old Eico 147 signal tracers.

I prefer a meter to an eye tube, so this meter was salvaged from the junk box. It is an old HP 400 series meter. The RF input probe uses a subminiature 5603WB triode as a grid leak detector to demodulate AM radio signals and amplify the corresponding audio signal, which is then sent to the audio-taper gain control potentiometer in the grid circuit of the 1st 12AX7 dual triode amplifier. The other half of this tube gives another stage of gain and feeds the grid of a 50C5 power pentode to drive the speaker though an audio output transformer. The secondary of the audio output transformer also feeds a signal to a 6AL5 dual diode tube to rectify the signal to the meter. The meter gives a relative signal level, and is responsive enough to bounce along with audio of a radio station or give a steady reading from a tone injected into the device being tested. The speaker can be muted by a single pole double throw switch that cuts out the speaker and replaces it with a 5 watt, 10 ohm load resistor. The power supply is a 35W4 half wave rectifier tube filtered by an RC pi filter.

The unit is sensitive enough to give a clear signal right from the antenna lug on an AM radio that is tuned into any reasonably local radio station, or can be turned down enough to get a signal right off the plate of a typical radio's output tube. It should be handy for audio amplifiers and phonos as well as radio work.

The faceplate and chassis are made from 6061 sheet aluminum. The cabinet is 1/4" red oak with poplar framework. The probe housing is made from 3/4" brass tubing, insulated with heatshrink.
I am still waiting for a 4 pin jack to arrive for the final hookup to plug in the active RF probe, and also have a black hinged handle ordered for the top of the cabinet.

(Note: I do not claim that this tracer meets UL or any other safety standards, but it is only for my use, and I know where and how I'm using it.)