Fender Bassman Cab Insulation

[admin]

I have a 1967 Fender Bassman cab. I have insulation in this cabinet but have no idea whether it is original or not.

Does anyone know what sort of insulation was standard for these cabinets when they left the factory?

[johnallg]

Mine I had back in 71/72 (used, so it was no doubt a late 60s cab) had the yellow batts of fiberglass.

[fatcat]

Both of my cabs (small and large) have the yellow soft batts lining the back and sides. I just took a peek thru the port on the silverface one to check, and noticed that the sheet on the back panel has loosened and fallen down.That's probably why it doesn't sound so good lately.It used to be that pulling the back would be a chore, but a cordless drill will make short work of that.
The cabinets would have left the factory with some insulation to prevent standing waves causing peaky response.

[admin]

Thanks. I have the yellow batts in mine as well. What I am uncertain about is the arrangement of these batts for best effect. Removing the back, even with modern technology is still a chore! Please explain what you mean by "peaky response."

[fatcat]

Unwanted volume variations at different frequencies.Some notes sound louder than others,due to reflections off hard surfaces of the back or sides. .As well as fundamental note variation, it can affect overall tone by affecting harmonics.
Insulation minimizes those reflections.

[admin]

Thanks Don. I love the tone of the bassman cabinet, however, there is an artifact of either it or my amplifier that I find annoying. Attacking tones more forcefully in a single coil instrument particularly over a pickup produces an annoying sound particularly at higher volumes. It is a difficult sound to describe but it sounds like a thumping sound as if the speakers might be fighting pressure within the cabinet. I am not sure if all mid to late 1960s cabinets were made in the same way. My cabinet has a divider panel between the upper and lower speakers and athe only vent is in the lower section. The same guitars do not produce this sound in a Marshall 4 X 12 cabinet. Your thoughts are sought in regard to this observation.

[beatlefreak]

Unwanted volume variations at different frequencies.

Otherwise known as resonances.

[admin]

There is also the issue of cone excursion as well. I wondered whether this insulation would be necessary at all for guitar, however, some comments made by experts in the area suggest that speakers could be damaged without it.

[rictified]

Is this with a guitar Peter? The insulation which usually was all around the sides in Fenders, It evens response as the others have said, absorbs tones and evens out peaks. Almost all speaker cabs have it, my SVT's also have it, B-15's have it. A bassman does not have any ports in it, at least none of mine ever did, they were sealed. One of the reasons the insulation is especially important in a Fender is because although they are sealed the cabs are not made of very strong material and vibrate all over the place and the insulation absorbs vibration.

[admin]

Bob: It is indeed with a guitar. There is a port also in the bottom chamber of the cabinet. I am not sure if the sound I hear may be produced by the difference in sound pressure between the lower and upper chambers. I will continue to experiment.

[admin]

Ok, here is some more information for those who may be able to assist me. I plugged a Bassman 100 head into the cabinet that was giving me the trouble and guess what, the problem with the cabinet has disappeared. This would suggest that the amplifier is the culprit. I then tried to replicate the initial problem by plugging my Bassman head (50 watts) into the 4 Ohm setting on a Marshall 4 X 12 cabinet. The annoying thumping sound returned.

This points the finger at the head once again. So any thoughts that you may have would be most appreciated. Again the speakers are Weber 12F150 8 Ohm speakers with a 25 watt rating that are wired in parallel. They sounded very sweet with the Bassman 100. The Bassman 50 has been serviced recently and has new tubes as well.

It is possible that this sound may be a distortion of some sort but it does not appear to be in the same league. Perhaps it is briefly cutting out but this aberration is not easy to describe.

[fatcat]

That's fortunate that it sounds good with speakers that it wasn't designed for.The port was sized and located with the original speakers in mind.Installing something else could have had disappointing results.
But I'll bet that your Weber speakers would sound good in nearly any enclosure.
And if you like the bassman 100, you should try a Showman.

[rictified]

4 ohms may be too low for that amp with a single cab. Tube amps don't like to have improper impedance (if that's the case). Only thing I can come up with.

[admin]

Don: Your comments are appreciated. The speakers I chose are, as near as I can tell, in keeping with the Jensen's originally stock with the cabinet. That the speakers and cab sound outstanding with the Fender Bassman 100 may argue against a mismatch of cabinet and speakers. Both heads are rated for 4 Ohm cabinets. With this observation in mind, your additional thoughts would be appreciated.

Bob: The original cabinet has two 8 Ohm speakers wired in parallel for a 4 Ohm cabinet rating. Unless I have misunderstood the point you are making in your last post, the match should be a good one. I changed the preamp tubes with the same problem noted.

[fatcat]

Does it make a difference whether the bassman head is on the cabinet, or not?