Now what?!?

[bgjazzman]

Hi everyone, I asked a question a few weeks ago and I was excited to try some of the answers. My problem was with a D style combo amp a local builder built. I loaded it with a 12" Altec and certain notes generate this "growling"/wolfy sound as the note decays. One of the suggestions was to swap the speaker. I have two other cabinets and I ran a cable from the main output to the external cabinet (2x12 Naylor) and no growl! Next I had a second Altec in a 1x12 cabinet and I tried that. No growl! So I was convinced the Altec loaded in the combo was bad. I just switched the Altec speakers. The one that was in the external cab is now in the combo. And I have the growl!!! I have to admit I was ready to throw it out the window (but it's too heavy). Is there some weird combo ghost that won't be there if I use a head/combo unit? Any suggestions will be tried. Thanks in advance.

[km6xz]

There is a good chance the problem is mechanical feedback to a gain stage, or even an output. With the gain up fairly hight listen while tapping the tubes with a wooden handle of a screw driver. Use your hand as support behind the tube so it does not move away from vertical alignment in the socket. One of the tubes is likely highly microphonic. Other parts may also be modulated in the same way so if none of the tubes are obviously microphonic, it is probably a passive component under the chassis. If you are not comfortable with the high voltages present, have a tech look at it. being able to repeat the symptoms so easily and reliably will mean the cause will be easy to track down.

[Gibsonman63]

You may pull the head out of the cabinet to see if the woofiness is still there. That will tell you whether it is a mechanical rattle problem with the cabinet or vibration affecting your chassis. If you don't have them installed, tube shields may help as well as tube retainers on your power tubes.