Fender 59 Bassman 410 Cabinet resonances

[nevetslab]

I have a new Fender 59 Bassman 410 combo amp on the bench, which was sent over under the guise of having tube rattles. I removed the upper rear panel, unscrewed the grounding wire between the chassis and the foil on that rear panel, and had a look mechanically. Then, fetched my flexible shaft 1/4" socket driver & 11/32" deep dish socket to check/tighten the four speaker's mounting screws. Of course, with the chassis installed, you can't get at the top screws on the upper pair of 10's. All were solid/tight.



I connected my Sine-Warble Generator (B & K 1023) via long speaker cable to clip onto the four speaker's wiring, and engaged the output to sweep the cabinet. Resonance city....CABINET resonances....NOT Tube rattles, as I expected. I replaced the 11/32" socket with a 5/16" socket, and with a #2 Philips screwdriver, tightened the five baffle mounting screws/nuts I could get at. The top middle screw is not accessible due to the Fender Emblem, and the nut is only accessible by removing the amplifier assembly. Tightening those up with the oscillator tuned to the various resonances, I nulled those out. The biggest resonances were from the amplifier mounting screws...which are two #10-32 Truss Head screws and hex keps nuts on the inside top of the chassis at both sides. Plus a third flange beyond the power xfmr. Tightening those got the majority of the resonances out.



I changed from driving the speakers directly to plugging the signal source into the amp input and driving it directly. Many of the resonances occurring above 150Hz disappeared, leaving those below 82Hz with no direct solution. Pressing on the middle of the speaker baffle would cut that out, though it is level dependent. I thought about maybe installing cleats on the left and right sides of the cabinet thru which I could tighten the baffle up to those. But, the baffle is removed from the inside of an empty cabinet, so those cleats would have to be removable.

I also thought about making a wooden frame, attached to the side walls, centered about the middle of the two vertical pairs of speakers, then with a dado'd cross brace on the baffle between the four speakers, tying that to the frame to dampen the baffle from the middle. I looked around the absence of a wood shop in my lab, and chuckled, Fat chance that's gonna happen.

I did tighten up ALL of the chassis hardware....those of the four RCA speaker connectors, the PCB mounting screws, further tightening of the chassis mtg screws...all while the amp/cabinet was being driven at various resonances. I got it to where the remaining resonances are below 82 Hz (Low E on the Guitar). Turning up the warble-sine wave signal will still excite the cabinet to resonate, but it's a far cry better than what I had to start with. Amp looks brand new. These resonance issues are typical of the Hot Rod Series, the Blues Deluxe, and a few others built in this fashion. The upper rear panel, mounting to the rear cabinet side cleats use the traditional Oval head chrome #6 wood screws with the FH cup washers. The upper left corner screw just kept stripping and the corner of the panel was forced out from being fully seated. I changed to #6 x 1-1/4" Truss Head SS screws, leaving the cup washers in place, drove that in which pulled the panel flush. Replaced the other three so they all looked the same, and had further anti-resonance mounting.

So much for stock Fender amps working as expected, without the amp 'singing' out of tune with your playing.

[Helmholtz]

This is a long text, which makes it hard for me to follow.
So forgive me if I missed something, but could you identify something specific which makes this amp/construction more susceptible to cabinet resonances than a 50s Bassman, a brown Concert or a BF Super Reverb?

[nevetslab]

The 50's bassman is much like this in general construction. The Brown Concert or BF Super Reverb are much like the Fender Twin Reverb, Deluxe Reverb and that style, which have more amp chassis mounting hardware to lock the chassis firmly to the cabinet. I rarely have resonance problems on those amps, but have had loose baffles or loose speaker mounting hardware.

The baffle on this amp, and those like it (Hot Rod Series, Blues Deluxe, etc), three screws across the bottom of the baffle, as well as across the top. Screw head is on the front flanges, one hidden by the Product Emblem. That hardware over time tends to loose up, so the baffle assembly will rattle on different notes. There's only two screws that do the main chassis mounting to the cabinet, and those tend to loosen. And, of course, any of the speakers can loosen. Speakers in this amp were all tight to the baffle. The chassis mounting hardware was loose, so that yielded lots of resonance. The baffle mounting hardware was all loose, so additional resonances to add to that of the amp chassis resonances. I can't say if the loose RCA mounting hardware (two per jack, eight total for the four jacks) contributed, but found them loose none the less.

For a brand new amp to yield this much resonance.....I have to wonder how it passed final inspection at the factory. Then, you have our Guitar Techs specify all this as Tube Rattles. Those were all nice and tight, thankfully. On our older 59 Bassman that's been in our rental inventory, THOSE had been loose until I replaced the bear trap clamps with newer ones.

[Helmholtz]

Very interesting. Thanks a lot!

[J M Fahey]

Canīt you add thin EVA foam strips between mating surfaces?

[nevetslab]

Canīt you add thin EVA foam strips between mating surfaces?

I hadn't thought about installing foam between the Amp Chassis and the cabinet walls. This particular 59 Bassman amp has small Oval Head speaker baffle mounting screws, rather than the Truss Head screws common to the Hot Rod Series. Looked like #6-32 without checking. I did think about replacing them with the #8 Truss Head screws, but left it to just tightening them up. I've sent the amp back to the Guitar Dept to put it back into play. If it comes back with similar complaints, I'll see about the next steps. Good idea.