Signal generator settings question

[deci belle]

One of the dials on my Heathkit 5218 is the Amplitude - Volts (RMS) - DB. The outer dial is for volts (.003~10) and the inner dial is for DB (-50~+20). There are dials for both sine and square wave generation.

I'd never done this before and I caused my amp's mains fuse to blow (too much amplitude for too long)!

For a guitar amp, how many volts and db is enough?

When should I use sine waves and when should I use square waves?

[Enzo]

I almost never have any use for square waves. Hifi guys have more use for them, you can feed one into an amp and the resulting output tells you about freq response at the extremes. But that is useless information for a guitar amp.

A sine wave will more readily show distortion on a scope. A missing half of a waveform is pretty obvious to the eye.

I would not even worry about the decibels. In audio work some measurements are done using them, but not really in guitar amp work. A decibel is a ratio, and it is comparing something against a particular standard. So 0db might be one thing on a 600 ohm balanced line and still another elsewhere. WHen set up with 0db as for example 1 volt of signal, you can reduce the signal level by so many db and see what the gain might be through an amp.

A guitar might put out something in the 1/10 to 1/2 volt region. Sure a hot pickup and a hard strum might make a larger peak, but in general that is a reasonable range. On the other hand, I routinely plug a line level input into guitar amps on my bench. The music is a reasonable test signal, and I always play a guitar through it once I am ready. Line level is about a volt.

Usually if you hit the input of a guitar amp with too much signal, all it does is clip it.

So I guess the answer would be don;t usually run it up over about a volt, unless you have a particular reason.

[jrfrond]

I set my signal generator for a maximum of -30dB (which is at the top of the variable control) for guitar amps, and -50dB for mixers. Line level inputs are set of 0dB. I do use square waves sometimes, usually to check tone controls, as their action is very easy to see, and it's easy to find the "flat spot". You can also roughly gauge the frequency response of the amp if you know how to read the square wave.

[deci belle]

I liked the part about -30db, John— cuz blowing the fuse scared me. Any time I dial the volume past 3 on this old Showman, it's starting to clip, and past 4, it's already around 30 volts and I don't wanna stay there for long… at 10 it hits 37v at 8Ω.

On the other hand, I routinely plug a line level input into guitar amps on my bench. The music is a reasonable test signal, and I always play a guitar through it once I am ready. Line level is about a volt.

I'll try that with the music too, just to do it, Enzo.

The sigGen has a switch under a "600Ω Load" label that says INT. or EXT. I have it set to EXT. Is that correct? What is the INT/EXT switch doing?

[nashvillebill]

I have a hard time understanding how overloading the amp's input signal blew the amp's main fuse.

[mac dillard]

From Fender's service diagram for the 57 Deluxe reissue: "All DC voltages measured with no input signal" "All AC voltages measured with a 100 mVAC 1kHz sinewave." Hope that is some help.

[deci belle]

Hi Bill~ I guess it was about driving the amp too hard for too long at too high an amplitude?

[deci belle]

I'm not worried about the amp. I'm just learning about the test equipment and settings right now.

The sigGen has a switch under a "600Ω Load" label that says INT. or EXT. I have it set to EXT. Is that correct? What is the INT/EXT switch doing?

[Tom Phillips]

...Any time I dial the volume past 3 on this old Showman, it's starting to clip, and past 4, it's already around 30 volts and I don't wanna stay there for long… at 10 it hits 37v at 8Ω.

It is common for this amp to start to clip at 3 or 4 due to the way Fender set up the gain structure. The output voltages you are measuring seem high but none of what you are doing should cause the fuse to blow. My input would be that something else in wrong caused fuse to blow.

Your voltage readings may be due to equipment calibration errors or set up. The output voltage is also affected by the accuracy of your 8Ω load.

The sigGen has a switch under a "600Ω Load" label that says INT. or EXT. I have it set to EXT. Is that correct? What is the INT/EXT switch doing?

That switch is connecting an internal 600Ω resistance across the output of the gig gen. It is not active for all output ranges. It is used when you are driving a 600Ω input but is not active for all output ranges. This simulates actual conditions when testing 600Ω systems and makes the dB readings on your Heathkit 5218 meter accurate. (If the 5218 is operating properly and is not too far out of calibration). There is more to it also but for your guitar amp testing you just need to be sure that you know the actual voltage being applied to the input. You can’t be sure unless you actually measure that small voltage with a meter that you trust.

From your questions I'm guessing that you don't have a copy of the 5218 manual.
I have attached a copy to this post. Hope you find it useful.

Cheers,
Tom

[g1]

One of the dials on my Heathkit 5218 is the Amplitude - Volts (RMS) - DB. The outer dial is for volts (.003~10) and the inner dial is for DB (-50~+20).

The outer dial is for coarse setting, the inner for fine. For each setting you are given values in volts and db, lets just forget the db for now (like Enzo suggested). The outer dial sets the max. voltage for that range and tells you which meter scale to read. The inner dial lets you go from 0 to the max. of the range selected.
For example, on the 1V coarse setting, you would look at the meter scale that goes from zero to 10, 10 would be 1 volt. On the .3 volt range, you would look at the meter scale that shows 0 to 3 and 3 would indicate .3 volts. The inner dial is like the volume control for the range selected.
As far as the 600 ohm load, I would set it to int. as that is more likely to give accurate readings on the generators meter.

[deci belle]

That's great!!

The INT position puts a 600Ω load across the sine wave; the EXT position doesn't.

There was good information about my question on page 48. Thanks, Tom!!

for your guitar amp testing you just need to be sure that you know the actual voltage being applied to the input. You can’t be sure unless you actually measure that small voltage with a meter that you trust.

OK.

[deci belle]

For example, on the 1V coarse setting, you would look at the meter scale that goes from zero to 10, 10 would be 1 volt. On the .3 volt range, you would look at the meter scale that shows 0 to 3 and 3 would indicate .3 volts. The inner dial is like the volume control for the range selected.
As far as the 600 ohm load, I would set it to int. as that is more likely to give accurate readings on the generators meter.

OK! That was just starting to gel for me, g-one, about the selector dial, the ranges and the meter. Now I get it! Cool, I'll set it to INT.

[Tom Phillips]

...There was good information about my question on page 48...

You will also find information in the manual about how to do internal adjustments and calibration. It's not critical for guitar amp work but I personally like to know that my test equipment is giving me proper signals and readings. I have found test equipment that is within cal after 40 years and I have found kit built items that were never set up correctly by the original builder.
Have Fun,
Tom