Signal Tracer Questions

[Gibsonman63]

I built a signal tracer with a chopstick, a 600V .047uF capacitor and a battery powered amp. I cut the end off of an instrument cable and wired the signal conductor to my capacitor and wired the shield to an alligator clip that I ground to the amp chassis. I am troubleshooting a Hot Rod Deluxe at the moment that loses signal somewhere in the preamp section. (You can plug a working pre-amp out into the power amp in and the power section works fine.) I am using my iPod to inject a signal. I can get audio to the grid of the first tube, but get nothing on the anode. I was thinking that maybe the signal was just too hot, so I built a little attenuator out of a couple of 1/4" jacks, a 1 Mohm pot and a plastic box that a pickup came in. That works well and I can run the volume up and down, but still can't get a signal from the anode of the first stage. Out of frustration, I pulled the chassis from a working amp, a SF vibro Champ. With the VC, if you stick the signal tracer anywhere near the wiring inside the chassis, you will pick up a signal. Interesting, but it does not help solve my current problem. At this point I am not 100% sure that my signal tracer is working as it should. Any suggestions?

[martin manning]

Maybe you isolated the problem? Check the voltages around that stage.

[husky]

I built a signal tracer with a chopstick, a 600V .047uF capacitor and a battery powered amp. I cut the end off of an instrument cable and wired the signal conductor to my capacitor and wired the shield to an alligator clip that I ground to the amp chassis. I am troubleshooting a Hot Rod Deluxe at the moment that loses signal somewhere in the preamp section. (You can plug a working pre-amp out into the power amp in and the power section works fine.) I am using my iPod to inject a signal. I can get audio to the grid of the first tube, but get nothing on the anode. I was thinking that maybe the signal was just too hot, so I built a little attenuator out of a couple of 1/4" jacks, a 1 Mohm pot and a plastic box that a pickup came in. That works well and I can run the volume up and down, but still can't get a signal from the anode of the first stage. Out of frustration, I pulled the chassis from a working amp, a SF vibro Champ. With the VC, if you stick the signal tracer anywhere near the wiring inside the chassis, you will pick up a signal. Interesting, but it does not help solve my current problem. At this point I am not 100% sure that my signal tracer is working as it should. Any suggestions?

One major issue with those amps is the supply for the switching is derived from a dropping resistor which unsolders itself from the PC Board, just a thought. The switching becomes intermittent.

[Gibsonman63]

Thanks for the suggestions. The switching seems to work fine. I can change channels and engage the boost and the LED seems to indicate correctly.

I have heater voltage to the tube and my anode voltages look reasonable, but I can't pick up a signal past the first stage even when I measure right on the tube socket lead. I have tried another tube that I know works.

I have to either have a problem with my testing or I am missing something very simple.

[husky]

Thanks for the suggestions. The switching seems to work fine. I can change channels and engage the boost and the LED seems to indicate correctly.

I have heater voltage to the tube and my anode voltages look reasonable, but I can't pick up a signal past the first stage even when I measure right on the tube socket lead. I have tried another tube that I know works.

I have to either have a problem with my testing or I am missing something very simple.

A Signal Generator and scope would make it easy as pie

[husky]

xx

[Gibsonman63]

I am rapidly heading that way. I guess if these things are going to keep showing up on my workbench, I need to invest in a couple more tools.

It is much easier to troubleshoot an amp that actually makes noise.

[martin manning]

If you can detect your test signal at the grid of the first stage that says your probe is working, no?

[Gibsonman63]

True, but there is no high voltage DC to decouple there.

[martin manning]

As long as the cap in the probe is not shorted you are good- it passes AC, and blocks DC. If it were shorted, I'd think the input to the battery powered amp would be toast, and of course you can still blow it out if your attenuator is turned up too high (assuming it's a SS amp with a transistor or op-amp input stage). I'll admit it's baffling as to why you can't get anything from the plate of the tube, though.

[dorrisant]

Couldn't you use the ipod volume control as you're attenuator? Just asking...

Did you check continuity from the tube socket to the termination at the preamp? Could be a bad solder joint or even a defective socket. Probably not a bad solder joint, as you said you tested at the socket lead with no result.

Tony

[sliberty]

A Signal Generator and scope would make it easy as pie

Actually, in this case, I doubt a signal generator and a scope would add any value at all. You'd simply see signal at the grid, and no signal at the plate. He already knows that is the case.

Assuming that you are correctly touching the plate when youa re testing, this preamp stage is just not working as it should. Check all cathode connections. Check for any shorts, like dripped solder. Look for a missing ground. Check for any carbon traces on the socket (unlikely). Look for a damaged copper trace on the board.

By the way, if you are going to use an iPod to inject signals, it can be convenient to store a 10 minute 1K tone saved as an MP3. You can generate such tones online - Google signal generators.

[Gibsonman63]

Thanks for the suggestions, gents. I have been using the volume control on the iPod as well. The fact that I haven't fried my little battery powered amp tells me that the capacitor is blocking DC as it should.

I'll do a little more investigation tonight. I did meter the cathode resistor and cap in the circuit, but I guess you can't really tell much from that unless you have a direct short. It might be worth pulling the boards again so I can desolder them and check those componants.

I reflowed the solder on the tube sockets before I built the signal tracer, but didn't really see anything suspect there.

I borrowed an old function generator, but it is really hard to get it set for 4.0 mV like the schematic calls for. The output range is 0-10V. I have tried running this through my re-amper box and my little attenuator box in various configurations, but it just doesn't want to produce a stable 4.0mV 1Khz sine wave. It gets frustrating when you don't know if you are fighting amp problems or test equipment problems.

[katopan]

If you were to disconnect the cathode cap, you should be able to measure a signal with your probe at the cathode as well as the grid. That would prove the valve is actually seeing and responding to the input signal on its grid. You've said the plate voltage is OK, so that would assume the valve is drawing current and so both preamp supply and cathode resistor ground are OK. Are you getting the expected voltage drop across the plate resistor and is the cathode bias sitting where it should?

[Gibsonman63]

If you were to disconnect the cathode cap, you should be able to measure a signal with your probe at the cathode as well as the grid. That would prove the valve is actually seeing and responding to the input signal on its grid. You've said the plate voltage is OK, so that would assume the valve is drawing current and so both preamp supply and cathode resistor ground are OK. Are you getting the expected voltage drop across the plate resistor and is the cathode bias sitting where it should?

I'll remove the cathode cap and check there this evening.

I have to admit that I am not sure what my expected voltage drop across the plate resistor should be or how to check the cathode bias. I did check the cathode cap and resistor last night out of the circuit. The cap fell apart getting it out, but I have replaced it with a good one and the resistor is well within tolerance.

I am using a speaker for a load. I can faintly hear 4kHz coming out of it, but I can't measure it on the plate of the first tube, so maybe it is being radiated and picked up in a later stage. It is very faint. I get absolutly nothing when I plug a guitar in.

Another odd thing. My VOM picks up about 50mV AC of ambient noise with the leads laying out on the bench not touching the chassis or anything.

I picked up an inexpensive scope yesterday afternoon, so I lost some time figuring out what I used to know 25 years ago, but it is slowly coming back to me.