Simple method to measure an amp's frequency response ?

Yes and no. You can download a signal generator to get the necessary signal into the amp
http://www.world-voices.com/software/nchtone.html
and then use a digital multi meter to read and chart the output voltage at various frequencies at the output via a second speaker cable. What you'll find with a Dual Showman, or just about any Fender, is anything but flat response, but that's mostly due to the pre-amp. As I recall from testing one 40 odd years ago it works OK down to at least 30Hz or so. It certainly goes a lot lower than most speaker cabinets will.

 

You should be measuring the AC voltage.

 

Click on my sig and download my little program, written for exactly this purpose.

 

The only thing to watch out for is that he is trying to measure the frequency response of the whole amplifier (not the just the preamp or the speakers as far as I can tell). As such he should not be plugging the speaker outputs of the amp into the inputs on his sound card.

 

Very good point.

 

Hi OldDogNewTrick!

Are you planning to measure the output signal with the amp feeding a loudspeaker, or a dummy load? Each method will produce different results.

A good dummy load will give you a plot of the amp's "naked" response, because the load's impedance will remain nearly constant throughout the audio frequency range.

A loudspeaker is a reactive load. Consequently it has an impedance "curve" - i.e. its impedance changes according to frequency. Every type of loudspeaker has its own unique impedance curve, and this curve is further affected by what sort of cabinet it might be loaded into. So, measuring your amp with a speaker attached will give you information about the amplifier's frequency response only when working into that specific speaker and cabinet arrangement.

Both sorts of information are useful!

 

I'm not sure what the wattage of the Dual Showman is. My researches seem to suggest that it must have changed over the years - I've seen it quoted as anything from 60 to 135 !!

For safety's sake, why not use two 8-Ohm 25 Watt resistors in parallel - or even four 16 Ohm ones (if you can get them) ?

Anyhow, just make sure you don't let the beast run at or near full output for more than a few minutes, or those resistors will get very hot indeed.

Have fun!

 

The Silverface 70s 'Dual showman Reverb' was 100 watts, finaly, as with 70s the Bassman 100 it was re-rated to 135 watts, I was told that they just increased the voltage somewhere, maybe someone can explain how.

 

Given the frequency dependent impedance of a speaker, you might need to measure the response curve under the actual load. Wear earplugs. Perhaps, borrow a direct box that has a speaker level input, and run the output to your PC sound card. It is still prudent to start out at a very low signal level. In any event, the direct box might not be a waste of money if you are using an amp that doesn't have a DI output.

 

DOD use to make a 31 band frequency analyzer with a build in noise generator. This is how I do it. Quick simple, and somewhat accurate.