Bass Ampuum - Conversion of a Vintage Vacuum Cleaner Into a Bass Amplifier

Yeah, you read the title right. This page describes how (and possibly more important, why) I converted a vintage vacuum cleaner into a bass amplifier. This is not really a step-by-step description, but there are probably enough pictures and enough text here to give anyone that wants to build such a cool thing enough information to at least get started.

Last updated: September 11, 2018

Why?

A number of months back I got an email message from my pedal steel guitar playing friend Jon. The message contained nothing more than a link to a website and the following text: “Oh God, I want one!” Following the link brought me to the home page of Hottie Amplifiers [note: apparently now defunct], which sells small electric guitar amplifiers fitted into vintage toasters (I'm not kidding). Jon thought this was probably the coolest amp he had ever seen, and I of course agreed – who would not?

Now, I'm (nominally) a bass player, and looking at the toaster amp got me to thinking about a toaster bass amp. But you need a relatively large enclosure to support the low frequencies with any efficiency and so the likelihood of a decent sounding bass amp made out of a toaster is pretty low. The logical next step, making a bass amp out of a toaster oven just didn't seem to have the coolness factor that the toaster guitar amp has, so I didn't pursue that one. But I did mull over what other appliances might serve as appropriate housings for bass amplifiers. It didn't take long to mentally scroll down to what I remembered as the coolest domestic appliance I had ever seen in my life. This was the 50's vintage Filter Queen vacuum cleaner that my parents had when I was young. It was a really cool R2D2-looking vacuum cleaner on wheels. And I thought, if I were to convert an appliance into a bass amp, I'd like to start with one of those cool looking Filter Queens.

So that night at supper I tell my wife (like me, an engineer) about the toaster amp and my thoughts about converting a vacuum cleaner into a bass amp. She asks me if I am going to do this, and I of course tell her that, no, this is just one of my wacko musings, not worthy of even more thought, much less actual implementation. But being married to me for a while she knows that I'm not above whack projects. Plus, she can't figure out what to get me for Christmas. The upshot is, Christmas morning I open up a big box that contains an early 1960's vintage Filter Queen vacuum cleaner that she tracked down on (where else?) eBay.

The Design Process (Aborted)

So now I'm committed to the project and I start considering the design of the amplifier. The first step was to remove some parts of the vacuum, just to see what I had to work with. The vacuum motor and the filter holder were removed, as was the inlet baffle. Looking at what I had left, it was pretty obvious that the best approach would be to use the lower “can” portion of the vacuum as the speaker enclosure, and use a down-firing driver (that is, the speaker points at the floor). The inlet could serve as the port for the speaker enclosure, but may have to be resized.

The general approach to speaker enclosure design is to specify the frequency response you want out of the thing, chose the driver (speaker) you want, and plug the parameter values for that driver (called the Thiele-Small parameters) into a speaker enclosure design formula, which will give you information about how big the enclosure and the port should be. But here I'd be kind of working backwards. I know what frequency response characteristics I want, and I have an enclosure of known volume and a port of known dimensions. From this I'd need to figure out an appropriate driver. But back of the envelope calculations let me know all of this is doable, so I pass on the details for the time being and start thinking about the amplifier and power supply circuits.

Since the enclosed volume of the vacuum cleaner is pretty small I know it will not be possible to design a particularly efficient speaker enclosure. So I think I will need a lot of power out of the amplifier and its power supply. There's not much space in the “head” of the vacuum cleaner, and so I start to think along the lines of designing a small switching power supply and a class D amplifier, the obvious ways to get a lot of power out of a small package.

But these are all the pipe dreams of an engineer, and eventually it dawns on me that, given the severe limitations I have in terms of time and the difficulties I have these days in doing just this sort of work, that maybe it would be a good idea to scale back the scope of this project. After all, ultimately the thing I'm doing here is converting an old vacuum cleaner into a bass amp – there's really no need to reinvent something with really fine specifications, like the Acoustic Image amp, just to shove it all into a vacuum cleaner.

So I finally come to my senses, at least as far as the scope of the design is concerned. I buy a cheap used Peavey Micro Bass practice amp and take it apart with the intent of stuffing it into my Filter Queen. The Micro Bass is probably the best sounding of the pure practice bass amps to begin with. If it sounds at least as good as it did before I transfer its guts into the vacuum cleaner then I'll be happy.

Assembly, More or Less

A hole is cut into the floor of the vacuum's bottom for the speaker. I use a ring of the original baffle from the Micro Bass to hold the grill cloth, also from the Micro Bass. The original caster set from the vacuum is reattached to the bottom after the speaker is installed.

I intend to use the vacuum's original speed control switch as the on/off switch (the original vacuum on/off switch was mounted on the vacuum motor, and I don't want to have to build a mount just for that switch). The vacuum has a small outlet on the front that was used to power the beater motor. I mount the power LED in this. And I mount the input jack right next to that LED.

The volume and tone controls have to be moved off of the amp's PC board. Rather than mount them around the circumference of the “head” of the vacuum, I decide to mount them on a plate under the original air outlet. On the Filter Queen, there is a twist lock cap on the top of the machine, right under the carrying handle. Removing the cap reveals the air outlet. In my conversion the volume and tone controls will go under that cap.

Probably the only part that needed to be fashioned was a shelf to separate the “head” of the vacuum from the bottom. I fashioned this out of thin plywood and MDF. It fits in exactly the same spot as the original filter holder did, and even uses the filter holder's gasket. The top of the shelf has three wooden feet which supports the plate on which are mounted the volume and tone controls.

The amp PC board and power supply transformer are mounted on the bottom of the shelf. Wires run from the “head” section from the line cord, the power switch, the input jack, the power LED, and the volume and tone controls. All the controls and the jack being located so far from the PC board's ground plane create a grounding nightmare. I work at it methodically and it sounds fine, but doubt I'll ever get around to making it as hum free as it was originally.

The Bottom Line

The tone of the finished amp is noticeably better than that of the original, even though no attempt was made to tune the enclosure. I suspect the combination of a slightly bigger enclosure, small port, and down-firing driver make for better bass response. It is no power house, but is perfectly suited for practice and quiet rehearsal. And you can't discount the coolness factor of having an amplifier built into a vintage vacuum cleaner.

The Next Project

One of my brothers just gave me a vintage '60s Sputnik-looking Hoover canister vacuum – looks like I have a reputation as an aficionado of antique household cleaning devices. What can I make out of this one?

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