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.)

Sunday, April 12, 2015

5W Head to Preamp

I've been wondering what to do with this Kustom Defender 5H that was my introduction to tube amp building, as I don't really have much use for it lately. Somehow the idea popped into my head to use it as a bass preamp.

At first I tried the Garnet Herzog method - use the existing OT, hook up a dummy resistor, and tap the voltage off of it for the line out. It works okay, but the bandwidth is restricted/shaped on the OT. Not terribly useful for bass, as the stock Kustom OT is weak sauce. This worked fairly well for guitar though, and was simple.

Then I considered wiring up the EL84 as a cathode follower, but the required heater elevation and loss of a gain stage steered me away from that. If I had a couple more tubes, it might be worth it.

Eventually I settled on this:

The real key here is that Triad line matching transformer. It boasts 20Hz-20kHz response and only costs $6. I used a 5k pot to keep the output impedance low, and so I could balance the dropping resistor. The EL84, like any pentode, has an insane ra so the Zo is dominated by the Ra, in this case 2k5. The 220k resistor ensures that the pentode isn't excessively loaded and lowers the output voltage to something more reasonable for whatever power amp this gets connected to. After a pair of 12AX7 gain stages and an EL84, the output voltage could feasibly have been in the 200VAC range.

Oh yeah, and the load and cathode biasing resistor were picked off the load line. My Kustom is running around 350V B+ so I just roughly plotted the steepest line I could without nudging the max dissipation curve and guesstimated the plate and screen currents to pick a cathode resistor. It idles around 9W but clipping is symmetrical so there's no particular benefit to biasing it any warmer. Wattages could've been picked more carefully, but components were grabbed out of the parts bins.

Rk is bypassed; wish I hadn't missed that.

Capacitors are obviously non-ideal too, but this is just to publish the general idea.

As a bass preamp it sounds good, but it still needs a lot of tweaking and voicing before I'll post the schematic. The Triad transformer is good to its word and passes even a 5-string bass without attenuation. Plenty of bass and none of the shrill treble typical post-power amp line outs suffer from.

Also the Triad transformer can be wired for balanced operation, of course. The only other note I have is that this transformer is incredibly sensitive to radiated fields, so keep it away from the PT.

Clips and full schematic once I've got it really dialed in. I've got a Matamp-style preamp in there now and it's nice and versatile but not fantastic with heavy overdrive. Probably understandable considering there are only three gain stages.

I'll probably wind up dropping the screen voltage, but the headroom is so low on the EL84 already. Hmm.

Sunday, March 15, 2015

Adventures in PL Premium

So, I have the bulk of the gluing done. I have a bead around all the inside edges, and I've glued in all the primary braces. There are four front-to-back, two side-to-side, and two top-to-back. The top panel is going to be partially braced by the shelf port assembly.
Hopefully this amount of bracing will be sufficient to prevent resonating panels. We'll find out after the glue cures, and I wouldn't be surprised to find I need a couple more on the bottom, top, and shelf panels.
PL Premium is the go-to adhesive for speaker cabinets, as it expands while it cures to form air tight seals. It is fairly nasty stuff though; the tube says it's okay for indoor use, but the offgassing is fairly odorous so I can't imagine it being used in anything besides original construction. All the braces are screwed also, so as long add it does a reasonable job, this thing'll be rock solid.

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...