Using a HeathKit IT-12 Signal Tracer as a Guitar Amp

[Snarl 12/8]

Hey Peeps,

Based on this GS thread, I just scored a Heathkit Signal Tracer and installed a 1/4 jack on the front. It seems like an OK little tube amp for $30 bucks. I have a few questions now about where to go from here as far as using/optimizing it.

1) That thread says to "disable the noise switch". What do you think that means? In the back the signal wire from the 1/4 jack basically goes straight into that noise switch, there's some other circuitry attached like a resistor (I think). Should I just wire a jumper across all the leads of the switch or something?

2) Also, the thing hums like a mofo. But it only seems to hum when my guitar is plugged in. Does that mean that my guitar is the problem? Or could it be a grounding issue with the amp.

2b) The amp only has a 2-prong AC cable. How do I ground this thing? Do I just hook up a 3 prong AC cable and hook the 3rd (ground) prong up to the chassis somewhere? Is it that simple?

3) How do I hook this thing up properly to an external 8 Ohm speaker cab? It's got this set of terminals on the front marked "Output Transformer" There's a black terminal marked CT and 2 red ones marked +B and P. I'm thinking I can stick some banana plugs into some combo of those and crank a speaker, but how do I figure out which ones to use for an 8 ohm load. I know tube amp traffos are picky about what they're driving.

4) Should I be thinking about changing the tubes? One's glowing orange, the other isn't. The one that's not glowing is an 12AX7, I have no idea what the other tubes are. I'm guessing the one that's put horizontally through the front of the thing to provide the "light show" is going to be pretty hard to replace.

Any thoughts about this thing would be appreciated.

Here's a clip of how it sounds. It starts with some of the hum with the pickup selector in different positions. Then there's a bit of single coil and then humbucker with the gain dimed on the amp. I mic'ed it with an N/D 468 about a centimeter from the speaker.

Here are some pictures from all angles. The closeup shot at the end is of the switch that supposedly needs to be bypassed.











Thanks,

[RefD]

yah, don't replace the magic eye.

as long as it's doing its job (signal amplitude indicator), there's no point.

[Snarl 12/8]

It's not doing much. It's green, and when I strum real hard the top 3rd of it or so gets a little dimmer.

I did some experimenting today with the speaker outs. I hooked my EV 12L up to all the different combos of the "Transformer Outputs". They all sounded OK, but pretty quiet. I didn't know what to expect so I was pretty happy. Then, I plugged into the two banana jacks at the top marked "speaker" (duh?!) Pretty powerful loud tone. A lot louder than the traffo outs. It they're hooked up in parallel to the internal speaker, so it was still going and I'm sure that dropped the impedence. I think I'm going to rig a switch that deactivates the internal speaker when I use those jacks.

Is it possible to get a new eye tube? What's that even called beyond "signal amplitude indicator"?

[Snarl 12/8]

Is it possible that this thing is getting better sounding by me playing it? I'm digging it more and more each day and I can swear it's getting warmer and punchier the more I use it. Is that possible if it sat on a shelf for a year or more before I cranked it up? Or is this the sound of the output transformer blowing up because I don't really know how to hook up an external speaker properly?

*knocks on wood*

[tubetapexfmr]

These old heathkit signal tracers make great guitar amps once properly converted. Like many old Tube amps that aren't meant for guitar use (PA, Signal Tracers, Test Equipment, etc) you can add a quarter inch jack or two to it and get some sound, but it is far from ideal this way. What you really want to do is reuse the housing and parts, but rearrange the circuit for a more guitar friendly design. It looks like you have a 12AX7 and a 6AQ5 (or similar) in there in addition to the EYE tube. If I were you I would use this chassis as a platform for experimentation. The most important part of the amp is the 12AX7 there as it is your preamp. If you just rewired that tube for a more Fender-like champ style preamp you'd be most of the way to a smokin' guitar amp. And yeah, replace that power cable with a properly grounded one. You hook up the hot and neutral leads to the power switch and fuse, then to the power transformer primaries. The ground lug should go directly to the chassis. Good luck!

[Snarl 12/8]

These old heathkit signal tracers make great guitar amps once properly converted. Like many old Tube amps that aren't meant for guitar use (PA, Signal Tracers, Test Equipment, etc) you can add a quarter inch jack or two to it and get some sound, but it is far from ideal this way. What you really want to do is reuse the housing and parts, but rearrange the circuit for a more guitar friendly design. It looks like you have a 12AX7 and a 6AQ5 (or similar) in there in addition to the EYE tube. If I were you I would use this chassis as a platform for experimentation. The most important part of the amp is the 12AX7 there as it is your preamp. If you just rewired that tube for a more Fender-like champ style preamp you'd be most of the way to a smokin' guitar amp. And yeah, replace that power cable with a properly grounded one. You hook up the hot and neutral leads to the power switch and fuse, then to the power transformer primaries. The ground lug should go directly to the chassis. Good luck!

That was actually what I was hoping to do. Use this as a way to learn about tube amps. But...

My eyes quickly glaze over looking at all that point to point work. I guess to a veteran it's easy to see what's going on, but when I see a capacitor between two lugs on a pot with a bunch of resistors hanging around I can't quite make out what it's all doing. What's the best way to suss that out? Should I try to make a schemo based on what I'm looking at? Then compare that to a champ?

[tubetapexfmr]

Cool, I just got one of these off ebay so I am officially adopting this thread and will completely redo this device for guitar/recording purposes. There are at least two others on ebay right now that I found (maybe more, they are pretty common). Let's not get feral with these and drive the market up, OK? Pretty much all tube type signal tracers and squawk boxes will work for this project with some slight tweaks. So, I got a schematic for this specific Heathkit IT-12 from here:
http://www.vintage-radio.info/download/ ... =it-12.gif
This amp is a perfect first project, but still would be interesting for the initiated. My short list for features is:

1. Fender blackface style input and preamp.

2. Marshall 18 watt style EQ.

3. Mode Switch from blues to overdrive.

4. Speakerless operation with dummy load switch.

5. Line level recording output with trim for direct use.

6. Dual-mono headphone jack.

7. Sensitivity control for eye tube.

To do this I will remove all the test equipment stuff, swap some parts and drill some holes for sure, but I will try to retain as much of the original design as possible to make this project as basic as possible and easier than a total gut and rebuild. Everyone interested in this project take a look at the schematic at the link provided and think about it, I will redraw it and get a prototype schematic up here soon, followed by revisions once I get my donor amp and can actually do the mods myself.

Stay tuned...

[tubetapexfmr]

I got mine in today and have already removed all the excess. I do all my schematics by hand so once I get something scanned I'll post my mods. I should be able to actually put it all together this week. Pictures to come.

[freddiefreeloader]

This is way cool. Please keep us updated!

[Snarl 12/8]

Resurrecting a dead thread here. Sorry. I'm getting into some guitar mods right now. Started modding my wah by swapping out the inductor for a Fasel and now I'm finding fault, I believe, with other parts of my chain. Gonna open up a guitar and clean the switches and rewire some stuff... Gonna try the wah with a "normal" guitar amp, etc.

BUT

Can anyone walk me through the input impedence mod for a heathkit signal tracer? Someone, in another thread, said something about a resistor on the input. Maybe a cap? How is that wired? What are the likely values? I was thinking about experimenting a bit outside the amp, like a little adapter thingy at the end of the cable with some alligator clips. But I don't want to blow up the signal tracer. I know no one can give me precise values, but some starting values and a description of the wiring (in series with the signal lug of the input jack (I'm assuming)? Or is the cap across the signal and ground? I can't visualize this.

Thanks so much. Viva la TOMB! The thread is dead, long live the thread.

[kickoldman]

On a typical fender tube amp, the high input would be wired with a 1meg resistor from jack to ground, and 2 68k resistors in parallel to the preamp tube.

But looking at the schematic for the signal tracer, the preamp tube is not wired up anything like a typical fender tube amp. As tubetapexfmr suggested, you might get the best results rewiring the whole preamp to look like a fender champ. If you take a close look at the champ schematic and layout, you'll see that this mostly requires removing components from the signal tracer and relocating the volume pot between the two stages of the 12ax7 tube.

In case you haven't seen this, be careful with that noise switch:
http://www.gearslutz.com/board/geekslut ... -idea.html

[Snarl 12/8]

Thanks for the explanation. I guess I'll try that with the preamp stage although I'm worried I won't like it as much "fenderized." I kinda like what a gnarly little beast this thing is. I was basically hoping to keep its character, but maybe make it a bit more predictable when interacting with various pedals, guitars, etc. immediately before it in the chain.

Thanks for the warning. I disengaged the "noise" switch, just because it was musically useless. But now I gotta go in and suss out that 150VDC to the case issue. That sounds problematic.

Thanks again.

[tubegeek]

I came across an EICO 147A signal tracer that is very, very similar. The schematic is here for comparison:

http://bama.edebris.com/download/eico/147a/147a.pdf

So far there are several things I did to mine and it actually sounds really nice for guitar. I haven't even rigged up any tone controls yet!

SAFETY:

1. 3-wire power cord. Removed "death cap."
2. Repurposed the fuse to protect the unit and not the wattmeter circuit, which I . . .
3. Removed the wattmeter circuit, and rewired the front panel AC jack in parallel with the new AC power plug coming in.
4. Removed all the other connections that carry high voltage to the front panel banana jacks
5. Removed the "noise" injection circuit.

CIRCUIT:

1. The EICO circuit used grid-leak bias on the first 12AX7 section (the cathode is grounded) and I biased this triode with a red LED instead.
2. I reduced the resistors in the power supply pi filter to 680 ohms, 22K and 22K. Result: Higher supply voltages to all three stages.
3. Removed the WATTMETER circuit's current-sensing transformer.

PARTS:

1. Removed coupling cap at the input jack (with LED bias it is no longer needed)
2. Replaced ceramic disc coupling caps with polyester film 'n foils.
3. Replaced front panel RF connector with 1/4" jack into shielded wire.
4. Replaced the original flimsy speaker with one I had on hand that matches it for front-panel hole pattern but has a much, much bigger magnet structure. In reality I'm not sure I like the new speaker better than the pathetic old paper-cone with a magnet the size of an acorn!
5. Replaced the front panel "audio in" and "ground" banana jacks with phono jacks into a 1K ohm "Y" connection (summing these two L+R inputs to mono) and shielded wire - so I can play my iPod through this bad boy! In MONO!

[This seems to be one big difference between the Heathkit and the Eico - mine has a switch with 3 positions for RF In/Audio In/Noise - the Audio In goes to the coupling cap for the 2nd 12AX7 stage and bypasses the first gain stage. This is where I connected the phono jacks. Note also, the EICO has the volume control between the first two stages.]

My list of things to do includes to try an LC tone stack using the now-useless WATTMETER transformer as an inductor (measures 4.7 H) but the only problem is, I had to pull out the transformer because the new speaker has such a large magnet and the transformer was in the way. It sounds great even without any tone controls, I may just leave it alone. Also I might try a switchable cathode bypass cap on the second 12AX7 stage - is this what you meant by "Blues/Overdrive", tubetapexfmr?

Another possibility will be to try the WATTMETER transformer as a power supply choke replacing the 680 ohms that replaced the 1K 5W in the original schematic.

Snarl 12/8 said:

Is that possible if it sat on a shelf for a year or more before I cranked it up?

I am quite sure it will have sat unused for at least TWENTY years. I doubt mine was used much at all - it has an EICO quality control tag on the power cord.

I totally agree - this is a great project for newbie or old hand alike.

[tubetapexfmr]

Also I might try a switchable cathode bypass cap on the second 12AX7 stage - is this what you meant by "Blues/Overdrive", tubetapexfmr?

That is exactly what I meant. When you run a feedback loop from the speaker out to the cathode of the second 12AX7 stage, then the switchable cathode bypass cap effectively erases the feedback, BOOM instant overdrive and non-linear response! You can use a simple toggle switch and/or add a shorting type footswitch.