Fender Champ Silverface - bass resonance

[saint gillis]

Dear all,
I've just restored the electronics in an old little Fender Champ silverface , here's the schematic : http://www.prowessamplifiers.com/schematics/fender/champ_aa764_schem.pdf
Only difference is a 330p capacitor between power tube anode and grid.

The little 8" speaker has been repaired (reconed I guess) few years ago.

Problem : when I play a low G on guitar there is an annoying resonance.

I did some frequency response measures :
- With a 4Ohm power resistance instead of the speaker : labelled "cap"
- With a 4Ohm power resistance instead of the speaker and removing the 330p feedback capacitor : labelled "no cap"
- With the speaker connected (the frequency response is measured with the voltages going into the speaker, not with a microphone !) : labelled "speaker"
- Withe the speaker connected, and I added 4 little metal weights tapped at the back of the speaker : labelled "speaker weight"


Then I played guitar with the weights on the speaker and it seemed ok.

Does the frequency response of the amplifier with the 4Ohms resistor seem normal to you guys?
Do you think I should purchase a replacement speaker? If yes is there a risk to get the same kind of behavior again?
Should I check things in the amplifier circuit?

Thanks for reading !
(Frequency curves attached)

 

[saint gillis]

and you can see that the low frequency peak with the speaker connected is ~98Hz (G note)

 

[radardoug]

Its probably the speaker. And did you not notice the rest of the eq curve?

 

[Tubetec]

100hz would be right around resonance for an 8 inch speaker so you'll see more movement of the cone at that frequency , the voice coil may be rubbing on the pole piece , usually if you move the cone by hand thats easy to detect . It could also be something in the cabinet or chassis vibrating , check all fasteners fixings and screws .

 

[saint gillis]

And did you not notice the rest of the eq curve?

What do you mean?

 

[saint gillis]

100hz would be right around resonance for an 8 inch speaker so you'll see more movement of the cone at that frequency , the voice coil may be rubbing on the pole piece , usually if you move the cone by hand thats easy to detect . It could also be something in the cabinet or chassis vibrating , check all fasteners fixings and screws .

The voice coil is relax and free, all the mechanical parts are firmly in place.

 

[Tubetec]

I guess try a different speaker cab seperate from the amp just to make sure .

 

[Tubetec]

I have seen speakers that sound scratchy on a certain low note , can be easy to miss at times.

 

[saint gillis]

I guess try a different speaker cab seperate from the amp just to make sure .

I tried, a bit rumbling at low frequencies but nothing compared with the rattling of the 8" speaker on low G notes

 

[saint gillis]

Maybe I can just try to glue some things at the back of the membrane to add some weight ?

 

[Tubetec]

If it was re-coned and the coil not centered properly your probably fighting a loosing battle, maybe weights might pull it enough not to make contact , but upset the movement of the cone or cause other spurious noises at another frequency . Sometimes you can also get a loose turn on the voice coil again not much you can do about that .

 

[CJ]

that amp requires a 4 ohm speaker IIRC,

measure DCR, a 4 ohm speak usually reads about 3.2 ohms on a meter,

you can tune the speaker to the cabinet with the mounting screws, most amps like a light torque on the hardware, this limits the mechanical coupling to the baffle board, kind of like negative feedback.

but do try the speaker removed from the cabinet,

 

[saint gillis]

I tried the speaker removed from the cab and it did overreact on low G notes too.
Isn't it here a too soft damping factor of the speaker reflected back to the amplifier creating a peak at its resonance frequency ?

 

[saint gillis]

The speaker seems correctly centered

 

[Disco Volante]

Old speakers often suffer from sagging cones, from having been "sitting up" in the same position for ages. Often simply removing it, rotating it 180 degrees and remounting cures the problem. Some models with heavier cones and soft suspension like the (legendary?) Leak 2060 needs rotating yearly of both bass and mid...
If your driver has been re-edged or reconed, a bodged job might show similar results.

 

[Disco Volante]

I'd avoid gluing anything to the (back of the) membrane, for risk of spurious break-ups, resonances and ripping the membrane if it falls off.
In a pinch and if the driver is considered scrap anyway, I might try adjusting the basket with a mallet and a piece of wood...