DIY subwoofer build for old tube hifi

Hello this is my first post. Also my first Hi-fi!

I have an old motorola goldenvoice hifi tube amp project. It came to me free in a pile of "junk." I'm sourcing speakers and trying to plan a re-build. I have lots of experience building and working with tube amps for guitars, but don't know anything about cabinet design or speakers/drivers/horns.

So the amp has 3 channels: L and R are two independent class A amps each driven by single 6bq5 tube, 8ohm out impedance. Bass channel is class A-B push pull driven by 6v6 pair, also 8ohm out. I'm guessing the bass channel is around 20-30 watts power.

I'm using vintage alnico speakers salvaged from another hifi for left and right satellites. Each sports a 10" low mid speaker, passive capacitor x-over to two tweeters.

So my question is: How should I build the sub cabinet, what kind of driver should I use? I would rather it go low than be loud, and be responsive and clear rather than boomy.

Also budget minded but inexpensive doesn't have to mean "cheap"

Thanks in advance!
 
Bass channel is class A-B push pull driven by 6v6 pair, also 8ohm out. I'm guessing the bass channel is around 20-30 watts power.

One can't extract more than half of that from a pair of 6V6's without considerable distortions. So you need to aim for high speaker efficiency. Next, the quailty of the output transformer is most important - the bigger the better in a dedicated bass channel. Do you know the LF roll-off of your device? For instance, there's not much use for a subwoofer to handle 30 Hz if the amplifier only does 50 Hz.
Best regards!
 
For such a project, I would think about a 38cm7 15" woofer in an open box. The Qtc of the chassis, for such a "box with one side missing", should be 0.7-0.9 with a resonance frequency of about 40 Hz. The sub channel should have some kind of x-over, but it may also rely only on the impedance of the woofer as a limiter for the high frequency.
With these parameters you will find moderately priced (cheap) speakers, usually with a steel basket, magnet on the small side and light cone. The typical low power instrument speaker.
The low Qtc will give a slight bump above resonance frequency and help the weak amp. Because of the very small power output, you have to use a very large speaker, as the satellites are large too, with 10" diameter. You can use a simulation program, that will show you what to expect from such a speaker, but it will depend on the place you put the sub, too. There are to many factors to explain them here, as this type of box will be some try and error. If you are common with guitar amps, you will know the open cabinet box principle. Bass will primarily depend on the distance front of cone/rear of cone. Second, when you move the box into the room, instead of placing it at the wall, the frequency response will change for the listening position. If you do not have the room or do not want to experiment, maybe get a usual sub woofer and power it with a transistor (D-Class?) amp.
The open baffle speaker can give very interesting results, but you have to use it in another way like Linkwitz for example, as you do not have 500 Watt to power them with brutal force. Your amp may have an early roll off at lower frequency, so you will not even move the cone in the region where the acoustic shortcut happens. You should find that sweet spot, where amp, speaker and cabinet match best.
 
Good points. I come from guitar amp world where distortion is desirable and 2x6v6=20 watts. I do intend to use this as a hi-fi though.

So lets say I go for a 15" sub speaker... I can probably get a way with the $80 and under ones at parts express since I don't need much power handling.

How can I calculate the roll off of the output transformer? Is it just like any other single pole LC filter?

Sub channel output Xformer primary is 281 ohms to center tap (blue to B+) and 311 ohms to center tap(brown to B+). Coupling caps are .1uF followed by 220k to ground with 100ohm resistors on each grid.
 
So my question is: How should I build the sub cabinet, what kind of driver should I use? I would rather it go low than be loud, and be responsive and clear rather than boomy.

Also budget minded but inexpensive doesn't have to mean "cheap"

Thanks in advance!

Since it's tube, recommend using the OEM driver and the bass response is what it is ;): Vintage Motorola Golden Voice Jensen P15 RJ 15” Alnico Speaker 8ohm | eBay


Box size isn't too critical since it's designed for open back [pegboard] or bottom, though the bigger its baffle, the more mid-bass, lower mids 'punch' it will have.

GM