Signal Tracers as Recording Amps

Tablebeast Modified Heathkit IT-12 Signal Tracer:

I made a promise years ago to adopt a thread from an online forum regarding a Heathkit IT-12 Signal Tracer. I modified mine and never updated with details. From what I remember I had to sell it pretty quick, so it didn't stick around for long once it was finished. It was certainly cool though. With a 12AX7 as the preamp and driver tube, it doesn't really matter what power tube you're going into they all sound great. So, it won't be hard to apply basic 12AX7 guitar circuit ideas here. Plenty of designs of low watt amps will work here.

Here is the Heathkit IT-12 Stock Schematic, downloaded from vintage-radio.info http://www.vintage-radio.info/download/download.php?dir=heathkit&file=it-12.gif

Here is my modified schematic. It looks complete despite being a quick sketch:

Start out by removing entirely the external probe, C2, C3, and R14. R1, the 1 meg potentiometer will be replaced with a 500k Audio taper pot that will be placed between the first two stages. Replace probe jack with a 1/4" switching jack. Wire ground to the switch so it shorts out when nothing is plugged in. Wire a 68k resistor from the input jack to the grid of V1A. Replace R2 with a 1Meg resistor. The plate resistors are kind of small so we will replace R3 with a 220k and R5 with 100k.  You can keep the R4 bias resistor, but add a 25uf-25v electrolytic capacitor across it from cathode to ground. C4, the coupling cap should be replaced and increased to a .02uf film type. Install the new 500k pot after this cap. Replace the on/off knob for a tone pot. This tone pot will also be a 500k Audio type. Wire the wiper to the wiper of the volume pot, a 500pf cap from the first pin of the tone pot to the first pin of the volume pot (where it interfaces the new .02uf cap), and finally wire a .005uf cap from pin 3 of the tone pot to ground (the grounded pin 3 of the volume pot works perfectly). Then run a 1k resistor from where the wipers of the volume and tone pot meet to the grid of V1B. The old speaker switch will be the new power switch by the way.


V1B has a grid leak type bias setup with the cathode tied directly to ground. It is cheaper from a design point of view and was adequate for this type of circuit. For guitar amp use and for tone it is kind of a one trick pony, once it clips it gets gnarly really quick. There is little nuance, the nastiness is either on or off. I personally don't like this sound and when I find it in PAs or in test equipment I toss it in favor of a proper resistor bias. Plus adding a resistor here gives me the ability to add a feedback loop and feedback mode switch to give the amp two distinct tonal variations. My favorite size here would be 1.5k, though you could go lower like 820R or higher like 3k depending on what value you like best. 


Now the mode switch is where it gets a bit tricky. Leave this out if you don't want to bother. Use the old noise switch to perform this function. Run a feedback loop from the positive output of the speaker transformer secondary to a 27k resistor and connect that to the cathode of V1B. Wired in parallel with the bias resistor of V1B is a 47k resistor in series with a 25uf-25v electrolytic capacitor. The 47k resistor sits between the cathode of V1B and the + end of the electrolytic cap. The - end is grounded. Wired across the 47k resistor is a SPST switch that simply shorts out the 47k resistor. Essentially what this switch does is have the amp be cleaner and more linear with the switch open and the feedback loop engaged and more driven and non-linear/exaggerated with the switch closed. Closing the switch engages the bypass cap directly and effectively erases the feedback from the output transformer's feedback resistor.


The power section and Magic Eye meter section remains stock, but I will increase C7 to 100uf-25v and replace C9 with a film type .001uf cap. 


Adding to functionality I will add a switching 1/4" external speaker jack and padded 1/4" TRS headphone/line out jack with its own volume trim. First I will run a 100 ohm resistor across the transformer output as a minimum load even if a speaker cable is plugged in and there is no speaker hooked up. Then I run the + signal to the tip of the 1/4" jack. Wire the switch on that jack to the positive terminal on the built-in speaker. From the output transformer + terminal again run a 100 ohm resistor to pin 1 of the 1k Audio taper trim control. Pin 3 is grounded and pin 2 goes to two 100 ohm resistors that terminate at the two + lugs of a headphone line out jack. When using stereo headphones it provides mono signal to both phones or when using with an unbalanced TS 1/4" cable it shorts one of the signals to ground safely while the tip becomes your line out signal. You can send this padded signal to a recording device, power amp or the input of another instrument amplifier.


Heaters and power supply stay the same, but replace the caps with 50uf types of at least 200 volts. Install a panel mount 3AG fuse block of 1 amp slo-blo type fuse. Replace two prong AC connector with a 3 prong grounded IEC connector or hardwired three prong plug. Ground the ground wire to the chassis securely and run the Line signal to the fuse which is then in series with the power switch Wire the neutral AC leg directly to the power transformer primary.


I think that covers it. If I can help clarify anything please be sure to comment.