Signal Generator works but no sound through speaker

3

391Hz

New Member
Hello Everyone, Im a new registered User. Im a musician, and Tube Amplifier Tech (in training) I have a particular problem driving me Nutz! That I hope someone might be able to help with.

I have a Function Generator (Iwatsu SG-4101) running a signal out of the 600ohm output to my Fender Tube amp, 1963 fender bassman. (or any other fender amp that I own.) and I can signal trace the signal using an Oscilloscope, through the gain stages right to the Output of an 8 ohm dummy load, all works just fine. However when I attach the amp to the 8ohm speaker cabinet, I get no audible signal. none, just static. If I connect my Guitar to the input of the amp, I can hear it just fine. Im not sure but I should be able to hear the amplifier signal from the signal generator throug the speaker of the amp?

Also if this is not the correct Forum for this thread please let me know where it should go, as this is my first post. Thanks very much for your help!
 
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391Hz said:
I can signal trace the signal using an Oscilloscope, through the gain stages right to the Output of an 8 ohm dummy load, all works just fine. However when I attach the amp to the 8ohm speaker cabinet, I get no audible signal.
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Welcome to AK! When the 8Ω load is in use, are you connecting the scope directly across the resistor? Are you sure the cable to the speaker cab is good? This sounds like a connection problem.

Jack
 
TriodeLuvr said:
Welcome to AK! When the 8Ω load is in use, are you connecting the scope directly across the resistor? Are you sure the cable to the speaker cab is good? This sounds like a connection problem.

Jack
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thanks for your response, Yes, I am connecting directly across the resistor of the 8 ohm dummy load. (if this is your question?) Important to note: If I disconnect the signal generator from the input of the amp, and plug my guitar in. I get audible signal through my speaker, which tells us the cable going from amp to speaker is good.
 
Also of worthy note, I also have the same problem going through another Fender amplifier, same thing, the signal increases in amplitude throughout the gain stages, but I get no audio through the speaker. Plug in my guitar, I get audio through the speaker.
 
try flipping the leads on the scope. I wonder if the ground side of the scope isn't going to speaker positive and shorting the output through the ground side of the signal generator.

Or just unhook the scope entirely and see if that does it.
 
I do not have the scope in the loop at all, Im just running the signal generator to the input of my amplifier.

However. I might have finally figured this out. I made a cable from what I thought was a guitar cable. Which is: on one ende of the cable soldered 2 male banana connectors 1 to pos, 1 to negative, out to a 1/4" jack on the other end, however I think it might be a speaker cable I used, and not a guitar cable! Im going to purchase a guitar cable in the next couple days, and create a new cable and try that. Stay tuned.... thanks for the help!
 
gadget73 said:
there is a difference between speaker cables and guitar cables when both have 1/4" plugs on the ends ?
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There definitely is!

Speaker cables are not shielded- not coax cable. Guitar cables ARE generally coax cables. And never the twain should meet!

I've seen some sketchy pro audio amps, that actually would oscillate (to the point of blowing up) when a coaxial guitar cable- with the inherent capacitance (impedance) of coax- plugged into the speaker jack. It's rare, but I have SEEN it actually happening.

In addition- a speaker cable plugged into the input of a guitar amp, is an ANTENNA. You never heard buzz/squealing/howling, as you will, if you plug an unshielded cable into many high-gain guitar amp inputs! It's absolutely as bad as an un-shielded phono input cable on a home audio amp. That, also, can send a marginally-stable amp into oscillation. Probably more likely to blow up the speaker than the amp, but it's not good!

Regards,
Gordon.
 
ok shielding I get but electrically would that matter to connect a sig gen to an amp? Its 2 wires, ground and signal. If its showing signal gain through the amp, the signal is getting there. I don't see why that would have anything to do with why sound doesn't come out of the speaker.
 
Oscillation so loud, that it's saturating the input tube of the guitar amp?

Yeah, I know, that's a stretch-I mean, it's ridiculously improbable- but not patently impossible.

Regards,
Gordon.
 
Problem Solved, my hunch was correct! It was indeed the cable. I built my own cable, out of 22awg, and it worked. The "bad" cable was too low impedance being a speaker cable, which uses 16 guage wire. A guitar cable which is very high impedance uses very thin 22awg wire. All is well and I now pickup the signal through the speaker. I hope this thread will be beneficial to those who are having the same problem down the line.

The cable I used to build my test cable, is not shielded so its noisy, Im going to purchase a guitar cable and build a "nicer" cable to run from the signal generator to the input of my amp, to keep the noise floor down.

Thanks for all the help, and wishing everyone a Happy New Year.
 
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