QUESTION: How to Diagnose a defective speaker?

[Mango Moon]

Hello All,

Quick question . . . I own several vintage (twin) combo amps dating back to the 50s and 60s. Often times, one or more of them will crackle and pop at idle, even with the volume off or at low settings. Others have suggested tubes or tube sockets, still others suggest power supply or instrument cables, but its left me wondering about the health of the original speakers.

You can bench test vacuum tubes and power supplies, and you can re-tension tube sockets, but if you suspect that a 50-year old, 12", 8-ohm speaker is blown or cracked or compromised, what would you hear or how would you go about determining whether it was truly bad?

Thanks everyone!

[NorCalTuna]

Although all of the above could cause crackles, I would be more immediately inclined to ask, given their age, How are the filter caps? You can eliminate the speaker easily by attaching a known good speaker (anything will do) of the right impedance and see if it misbehaves.

Bad filter caps will do all that, and more- including possibly smoking very expensive parts of the amp and shocking you. Replacing old caps with new ones doesn't lower the value of vintage amps as much as assure any potential others that your amps are "tip top". Also, three prong cords are safer, allow the elimination of the gnd. switch, and don't diminish value.



good luck-

[Enzo]

First, speakers do not create sound on their own, they reproduce sound from the electrical signal running through them. So a speaker will never sit there on its own making noises. ANy sounds it makes like that are coming from the amplifier.

Now when it is being driven to make sound, ut can be defective in various ways that affect the sound. They can certainly also stop making sound, either momentarily or altogether. But if they continue to function, they can distort the sound in a number of ways.

SO your rackle and pop at idle situation has nothing to do with speakers. But it is certainly important that the speakers do work well for their part of the sound.

A lot can be determined by simply playing through them. If an amp sounds crappy, connect it to different speakers. If it still sounds crappy, then the amp probably has issues. But if the amp sounds good through other speakers, then the original ones are probably bad.

You can test a speaker just to see if it makes sound with a 9 volt battery. Briefly touch a 9 volt battery terminals to the speaker terminals. DOn't leave it there, it will drain the battery. Every time you touch, the speaker should make a pop or thump sound, and you can see the cone move a little. You can also measure the resistance of a speaker with a meter. Always disconnect it from the amp or the readings will be meaningless. A working speaker will have a few ohms resistance - generally a little less than the rated impedance.

That tells you it works, but not how well. Working speakers can sound crappy.

If the voice coil is warped, started to melt, is coming apart, then it can rub inside when it moves. This adds an unpleasant buzz to the sound. Typically more n oticable at lower levels, when loud the sound overpowers the rubbing.

A hole or tear in the cone can buzz or flap or add other unwanted sounds.

The outer edge of the cone is glued to the frame. If the glue fails, then the cone will flap or buzz.

If the frame is warped or bent, the voice coil can rub, or the cone will be distorted and so not move properly.

The windings on the voice coil and scrape off their insulation and intermittantly short to frame causing noises - but only while moving.

The voice coil windings can overheat, the insulation start tp break down and turns can short together. It might still gunction, but it will fight itself resulting in weak output and poor sound.

But the ultimate test is to run sound through it and listen.

[Mango Moon]

Enzo,

Many-many thanks for this detailed and highly informative reply. I greatly appreciate it. From what you've said here, it sounds like my speakers are probably just fine, because they do indeed sound perfectly fine at volume. Thus, the problem must lie within the amp itself (i.e., caps, tubes, tube sockets, PT, etc.). And that's cool! Onward and upward, as they say! You've helped me narrow it down with confidence. So thanks again.