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.)

Saturday, December 19, 2015

Vari-Mu Compression for Creative Distortion and Whatnot

So! Here's a new thing that I've been playing with the idea of. My dad gave me a bunch of tubes including a few remote-cutoff pentodes. I've been studying a lot of vintage schematics and come up with some ideas I'd like to try. Here's the first and simplest, primarily targeted to make interesting distortion sounds. Who knows if it'll work! I might add a feed-forward option. I have a bunch of ideas for these, but I'm not going to be able to get the parts for a while (saving up for a different toy) so in the meantime I'll be posting schematics as I manage to get them down.

Wednesday, November 18, 2015

Adventures in Reconing

Okay, I don't really recommend doing this. I have to point out though that the posts by J M Fahey on MEF and TGP were invaluable to the process.

My Twin came with a pair of Fender-labeled Utah speakers with torn (and moldy) cones. That sort of thing tends to happen when you ship a "135W" amp with a pair of 50W speakers, and then store it in a basement that's swamp-adjacent for thirty years.

Anyway, they were useless, so I got the bright idea that I could try reconing them and if it worked I'd have useful speakers, and if it didn't, I'd only be out $50.

Getting to this point was the hardest part.
So I cut the cone and surround out of the frame, measure them, and go looking for parts. I decided to go with the parts sold by Weber, as this was a guitar speaker and they sold guitar speaker parts. There are a couple other vendors, but not many as there's some weird shadowy cabal of speaker reconers and unless you get into it and start a business the vendors won't sell to you. It's basically crazy.

I decided to go nuts and order Kurt Meuller cones, in essence making the end result similar to Weber's 12F150B. The spiders Weber sells are cup-type while the original was flat, but this didn't turn out to matter too much. 

Alright, so cleaning the old gunk off of the basket was a huge pain. I used a heat gun, razor blades, I tried a couple solvents.. eventually I got it all off. Then it was just a matter of cleaning the voice coil gap, shimming the voice coil in the gap so that the coil is centered on the top plate, put the spider over the voice coil, and put the cone over that. Boom, first dry fit. 

This is actually after gluing, but you get the idea.

Now, ideally - and if you're using the correct parts - the spider and the throat of the cone will meet perfectly, so you can attach them both to the voice coil with one bead of adhesive. If you're a jerk like me and buy the wrong parts, they need a little massaging. In this case, I had to cut back the throat of the cone a fair amount, but I got it wrong from both sides - the spider was too tall and the cone was too deep. 

On the first speaker I reconed, I didn't do quite a good enough job of this and as a result the spider is pushed in at rest - not great for efficiency nor longevity. 

Also it's upside-down, which makes it hard to hear.
I did a lot of research - well, "research" - before starting this. Like I said at the top, J M Fahey was a huge help. There were a bunch of other people with opinions, but they tended to lead me astray. Like the people who suggested gel super glue for bonding voice coil/cone/spider - this worked very poorly for me. Also it left a mess when I used it to attach the dust cap.

Epoxy works much better. Contact cement was used to attach the spider, cone, and gasket to the basket. Solder the voice coil leads to the tinsel wire, solder the tinsel wire to the terminals, and boom, done. 
The second one came out nicer.
Now here's where the real craziness sets in - what to dope the surround with? There are two kinds of dope I've seen on guitar speakers: the clear stuff and the black stuff. The black stuff seems similar to what's used to seal cloth surrounds and I'm pretty sure that's just butyl rubber in a solvent. The best and cheapest candidate I've found to try is actually windshield adhesive. I haven't bought any, because I found a good DIY alternative for the clear stuff. 

Unlike a cloth surround where doping is necessary to block airflow, doping on paper surrounds is done to prevent cone cry and overall tame the breakup modes of the cone itself. If applied lightly it can contribute to that notch at 1.5kHz, and if applied heavily it can neutralize that notch at the expense of overall sensitivity. And you can look at what Celestion's done or what Jensen's done over the years, but I don't have the test equipment to optimize this process and I'm sure there are books somewhere about it.

In any case you want the doping compound to penetrate the paper and stay flexible. Some DIYers use rubber cement (thinned even) or white glue (PVA) but supposedly these don't stay flexible enough. After a bunch of research I found an adhesive known as EVA (ethylene vinyl acetate, as opposed to polyvinyl acetate). It's like regular white glue, but the addition of ethylene means it retains much more flexibility. You've seen EVA in at least one form - hot glue sticks. Finding an easy source of an EVA emulsion glue was a little trickier, but eventually I stumbled on Helmar Super-Tac.

Here's a blob of Helmar Super-Tac that I dried so I could play with it.
This blob here actually dried to a silicone-like rubberiness. It's pretty awesome stuff. Really thick like this, the surface dries much faster than the center so the water can't escape. Yes, that's right, it's water-based, which is a huge boon for DIY'ing. Also new applications of the glue (or water) dissolve the dried glue, which is great if you're applying multiple layers. Maybe not so great for ship building. 

I ran a few experiments with dilution ratios and I eventually used a 2:1 dilution of water:Super-Tac and applied about 6 coats to the surround of each speaker. 1:1 would've probably worked just as well. A straight application right out of the bottle might work too, but one coat is probably enough. I also found it was better to let the coats dry 8-24 hours before applying the next one. I rushed the second speaker and applied a couple coats within 4 hours of each other. It looked dry, but it wound up trapping a little extra water for a milkier look. This eventually wormed its way out though.

This is in the Peavey cab that I just stained.

Anyway, this resulted in what I'd guess is a light-to-moderate level of doping. I doped the first speaker and compared it to the undoped second side-by-side. And yep, a hair less sensitive but more tamed treble and a little more in the midrange. Once I liked it, I doped the second speaker to match, and they're now resting comfortably in a Randall 412 with a pair of Eminence OEMs from the 90s. 

I've left a whole lot out because I'm sick of typing. I ran a bunch of experiments on doping using the old cone material from the Utahs, though it turns out the cones I got from Weber are way more absorbent. Liquid latex never really soaked into the Utah, but maybe the Kurt Meuller cone would be different...

Super long story really short: for DIY doping of paper surrounds, try an EVA emulsion glue like Helmar Super-Tac.

Monday, October 12, 2015

Peavey Cab Refurb

So! I've been neglecting my blog for the past few months. I've been busy though. I imagine I'd keep this more current if the posting process weren't such an immense pain.

Anyway, I picked up this old Peavey combo cabinet (used to be a Stereo Chorus) on the cheap, and it'd already been mostly converted to a combo. The tolex had already been stripped and a new back panel added. Really about all I've done to this is sand it, stain it blue, and slap a few coats of shellac on it.

Shellac is probably a terrible finish for a speaker cabinet, but this cab is already beat to hell so I'm not really worried about it getting scratched/dented. Plus I had the shellac already, as well as the blue stain.

I mean, I didn't even make the grille, though I would like to do more work on it. Still have to add a jack plate. I guess this is kind of a silly post, but I just wanted to document that I did it.

Wednesday, July 1, 2015

Projects on deck

Just a quick inventory of what's popping around in my head. Decidedly incomplete. Maybe I'll start posting these every month. I don't post much on the blog because things move slowly, but hopefully I'll start posting more of the theory/planning end instead of just the rare instances when progress actually occurs.

In progress

  • Peavey Windsor -> Matamp-y beast
  • Kustom Defender 5H -> Bass preamp
  • Portastudio -> live looper


  • Repair mandolin neck joint
  • Tube mic preamp(s) - redd47 redesign; design(s) from scratch
  • Refurb/rebuild Stromberg-Carlson AU-36B
  • Diode bridge compressor - trace V8 compressor in a pedal
  • Vari-mu compressor
  • parallel effects mixer pedal
  • active variable crossover, xformer couple for splitting to guitar, bass amps
  • "Phase Wizard"-y multi-amp buffered xformer isolated splitter
  • Bogen CHB35A redesign
  • Faceplates for rat rocket
  • Geezer 212 bass cab rebuild
  • gigantic 212 guitar cab overhaul
  • mid-range driver box with 4, 8 ohm crossovers for bass cabs
  • joystick noisemaker re-bulb, re-cap(?)
  • 5 string j bass bridge pickup rout, replace
  • 4 string p bass pickup replacement
  • bari replacement neck on tele
  • Randall 412 guitar cab load


  • Nothing, ever.

Saturday, May 30, 2015

Peavey Windsor mods

What a fun little toy.
Okay, picked this guy up cheap and I'm going to somewhat follow through on my "Matange" plans, except I'm dropping the Orange part in favor of a Matamp/JCM800 blend. Hey, its got a third preamp tube, why not?
So far all I've done cosmetically is move the back grille to the front. Not sure I'll do much more; maybe cut a new back grille out of expanded metal.
The preamp is currently fairly awful. Way too much gain, too little decoupling, lousy tone stack. I'm probably going to turn that cold clipper preamp stage into a cathode follower and the tone stack into a James with mid shift. I'll probably use the worthless "texture" control for the FAC. Then I'll use the "boost" relay to switch the second gain stage in and out.
For now I've started in on the power amp mods: 100k for the EL34 grid leaks, suppressors tied to cathodes, grid stoppers up to 47k, and 1k8 screen stoppers (didn't have any 1k5). Very noticeable improvement already!
Incidentally, Peavey seems to have paid the extra nickel for halfway-decent circuit board material. Id rather remove every component in this amp twice than remove one resistor on a Marshall made in the past decade.

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.