A blog of my tube amp design and modification work. Primarily my own builds, but occasionally I feature work I've done on others' amps (with their permission.)

Monday, February 23, 2015

WinISD Plots of My Bass Cab Projects

So! Here are some of the plots I've been making decisions with. The yellow plots are the Eden Nemesis 212 with the MCM 55-2982 drivers, and the white plots are the Peavey TNT130 that I'm converting into a cab with the Dayton Audio PA380-8 (Parts Express's house brand). 

First, expected SPL:
SPL from my amp
The white trace is lower since, as an 8 ohm cab, it's only going to get ~125W from my amp, while the 4 ohm 212 gets the full 250W. That peak around 80 Hz makes the 212 sound really fat, and somewhat boomy, though I'm hoping that adding bracing will reduce that somewhat.

Maximum Power
Power ratings of speakers in bass cabs are largely useless; most speakers are excursion-limited rather than thermally limited. You can increase the safe power handling by raising the port tuning, though that comes with tonal consequences. I did that somewhat already with the TNT cab, but 50Hz is a pretty common cab tuning in commercial cabs. I'm guessing this is partially why. As you can see on the SPL graph, it doesn't hurt the low end (only down 3dB by 50Hz).
Cone Excursion. Yellow = 7mm Xmax, White = 5mm Xmax
Also on the "safety" front (or just designing speakers that don't explode from regular use), here's a plot of the cones excursions at various frequencies. The MCM drivers have a specified Xmax of 7mm, which is huge (probably meant to be a subwoofer) and the Dayton has an Xmax of 5mm (not showing up on the graph for some reason.)

Clearly neither speaker is going to take much (any) 30-40Hz content, so a high-pass filter is a good idea. Fortunately the Trace Elliot head I have does have the ability to remove these two bands. I might still add one to deal with any extraneous subsonic noise that gets through.

Port Velocities. Good enough, Eden.
 And the last plot is of the port velocities per frequency. There are various rules of thumb to avoid port chuffing, but the one I was following was 19 meters per second. Eden does fine with the single 6" port, and my gigantic shelf clearly is going to be fine.

So yeah, I like music enough to get into math, physics, and now woodwork. Good times. 

Sunday, February 15, 2015

Peavey cab bracing

Busy day! I got 2 of the 3 dimensions braced, and got the cutout for the jack cup done.
The braces are going to be screwed and glued. I cut little Xs in the tolex before I drilled pilot holes, and I'm going to counter-sink the screws a bit so I cab glue the tolex back over the screw head. It won't be seamless, but it'll look better than screw heads sticking out.
Still trying to figure out how I want to mount the shelf port, and I cut the back panel 1/16" too tall, so I'll shave that down.
Gluing day is going to be a real treat; I've got to place the braces in a certain order or they block each other from fitting, and some of them are so tight they have to be hammered into place. Fortunately there's some working time with PL premium.
Can't wait to hear this thing!

Saturday, February 14, 2015

Peavey cab update

I got the big panels taken out of the divider (why did they use OSB for the baffle and back but ply for the divider?!) and "Swiss cheesed" the material I couldn't rout out.
I don't think I'll drill holes through the other braces unless I get really bored. To be clear, this does have a purpose: not only am I removing weight from the cab, but I'm also increasing the volume of air available inside the cabinet. The front-to-back internal dimension is actually only 9 5/16", so without any bracing or the speaker I only have a maximum of 2.9 cu. ft. to work with. Cutting big holes is obviously great, but cutting 3/8" jokes in 3/4" square braces? Probably not so much.
Also, not going to lie, six year old me was really happy doing this.
I have most of the braces cut already, so the next step is screwing and gluing them in place. Not really looking forward to trying to tune the shelf port. It's so big that the length calculations are fairly insensitive to minor variations, so hopefully it won't be too bad.

Sunday, February 8, 2015

Peavey Bass Cab Conversion

This is a Peavey tnt Combo, a bass combo amp that put out 50W. The amp part is long gone, but I got the cab for a whopping $15. It has separate chambers for the amp and the "ported" speaker enclosure.

The existing speaker enclosure part is only about 1.8 cu. ft. thanks in part to the shallow construction (10" internally, front-to-back) but also to the section reserved for the amp (4" of height!). The overall maximum internal volume is roughly 3 cu. ft., which is a great starting point. I'm going to brace the hell out of this cab and use the existing front slot for a shelf port. I'm assuming/hoping for roughly 2.5 cu. ft. after installing the speaker, all the bracing, the volume taken up by the port.

The baffle is actually OSB (chipboard) so it might make sense to just replace it, but I'm going to try to avoid that for now. It's dadoed into place, but actually rattling so hopefully a bead of adhesive and possibly some cleats will fix that.

I've been having fun in WinISD trying to find a decent (and cheap) driver for this box. I've finally settled on the Dayton Audio PA380-8, which is (thermally) rated to 500W. I'm going to tune the cab to 50 Hz, which looks life an optimal point for balancing low-end extension and excursion. I'll add the plots from WinISD when I get a chance.

50 Hz is on the high end of bass guitar cab tuning, but a lot of commercial cabs are actually tuned about here. While a low E string is around 40Hz (and the B below that roughly 30Hz), not much of that fundamental is picked up by, well, the pickups. By far, the more important frequency to worry about is the second harmonic (first octave), which will be 80 (or 60) Hz. If you'd like to see evidence of this, search the TalkBass forums for "waterfall plots", which show frequency spectrum content over time in a lovely 3D format.

Anyway, even with the cab tuned to 50 Hz, the driver I've chosen hits -3dB around 50Hz, so even the low B should be quite loud. As much as I wish I were making a subwoofer, I'm not, so I don't need to worry about covering that range.

Plus my other bass cab is tuned to 45 Hz. That one's a real treat, an Eden Nemesis 212 that I fitted with MCM drivers. That cab requires a great deal of bracing as it can shake roughly 100 pounds of other amps and cabs to the floor. Not that I've let it, but objects that size and mass being turned into that old electronic vibrating football game, well, that's just a massive waste of energy.

But back to this Peavey: I've got to seal the back where the amp used to be, cut out the internal "top" that separated the amp from the cab, brace the living daylights out of it, verify the tuning, and then line the inside.

Total cost so far:
Cab- $15
Baltic birch- $35
Driver- $75
Jackplate- $5

Still need to buy: adhesive, lining, screws, paint, etc.

Tuesday, February 3, 2015

How it starts...

Nothing to it.

These are some quick plans for a portable record player I picked up a while back. It's got a pair of 50C5 tubes for about 5W in push-pull, one 12AX7, and a hot chassis.

I'm going to add a second preamp tube and an isolation transformer, and use the extra gain stage for bias-mod tremolo. I ripped that shamelessly from Fender; might add NFB too, make it a tiny Princeton.

You'll probably notice I write ohm's law and the equation for finding knee frequencies over and over again. I blame chemistry. It's not a bad habit, but it does look silly looking back at your notes.

Hopefully this posts; I apparently can't comment on my own posts or format things the way I want on my phone or computer, but I blog so infrequently that it's probably not worth migrating to a new blog host. So, sorry! I'm not ignoring you!

I'm surprised it's let me type this long; I shouldn't push it any further...

Wednesday, January 21, 2015

More Rockette Tweaks

Finally got around to putting the bright cap on a switch. Also added a 500pF cap option. Eh, you got some center-off DPDT switches lying around, you stay using 'em.

It's okay, nothing to write home about.

I'm also planning on making the bridged channels selectable. Using both sides of the LTP wins up costing you some gain, and why not get that back?

Also, I might as well add a sag resistor, put that on a switch too. I'm pretty proud of how rock-sold the power supply is, and the modest plate voltage, but

So a total of three switches that are pretty superfluous.

I don't know; what the hell. It's my amp.

Friday, January 9, 2015

Trace Elliot Love

I scooped up this Trace Elliot AH500X about a month ago. It is... Fantastic. It was the top of the Trace line back in the 80s, and it boasts a pair of 250W into 4 ohm power amps, for a bowel-shaking 500W at 2 ohms.
There's talk of "Trace watts" because these seem louder than similarly-rated amps, but perhaps the better description is "honest watts" as from what I've read, they do spec as advertised. Maybe it's just that they require you to use more cabs to get the full power output.
Starting at the beginning: active and passive inputs. Sure. The preamp gain control has three lights: a yellow "OK" light in the center, a green arrow telling you to turn up, and a red arrow warning you of clipping. Really nice touch.
Then it's got a 12-band graphic EQ, which is defeatable. Very nice to have 30 and 40 Hz to cut. It also has an EQ preset, which gives a nice mid scoop sound. Both of these are foot switchable, and while a mid scoop isn't the best idea for a regular tone, I can see it being useful for occasional accents. There's also a defeatable noise gate which I haven't really tested thoroughly but is certainly unintrusive.
Then there's an effects loop, a transformer-isolated DI out, and a master volume. As they mention in the owner's manual, Trace Elliot expects the user to set the preamp for maximum gain and use the master for overall volume level.
In addition to the effects loop, there's an additional (parallel) power amp in and out.

The back panel is where things get nuts.

So two power amps, each has its own volume control. It seems silly, having volume controls after the master, but it's actually very useful.
Each power amp also has a line level effects loop, which can, of course, also be used as line out and line in. The power amps also can be configured for dual mono, stereo, or biamping with a crossover that's adjustable from 250
Hz to 1 kHz.
Which is pretty crazy as far as versatility goes.

Plus there's a freaking blacklight!
It weighs in the ballpark of 60 pounds, apparently 30 of those pounds are 25 years of dust, and I really wish
I could've afforded those 1818X cabs.

I'm still learning how to tweak this thing, but biamping with the highs sent to a guitar cab is fantastic. I've also been researching bass cab design, which is another whole ball of wax, albeit fascinating.
If my winisd skills are any good at all,
I'll be posting about a couple cabs
I'm fitting with mcm drivers, bracing, and

I may even have to put the Matange project on hold, in favor of more live gear.
Still flipping the coin on that one.