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.)

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.

Thursday, January 8, 2015

The Matamp NFB circuit

Oh good, my phone lost my blog post.

Well, there's an LC notch filter in the Matamp's NFB loop, which seems clever except for inductors' sensitivity to noise, adding phase shifts in a marginally stable NFB loop (that anode bypass cap isn't an accident, nor is the speed up cap parallel to the feedback resistor) - plus we have an extra stage in the loop thanks to the cathodyne.

Also I should point out that when you inject NFB into the driver before a cathodyne, any presence/resonance controls serve to partially bypass the driver's cathode for a double dose of whatever range you're trying to boost. The blackface Princeton gets around this by fully bypassing the cathode and then applying NFB between the regular Rk//Ck and ground.

So the Matamp idea is an interesting one, but it's clear why it didn't last. I prefer building stable amps to GHz oscillators, plus shielded inductors are expensive, so I'll probably not bother with implementing this. Maybe an overall NFB control. Maybe.

You know what's easier to keep stable, and lets you implement a notch filter with adjustable frequency and Q? A local feedback loop.

It's not a horribly unique idea - check out the Electro-Harmonix Tube Zipper sometime. There's also a German patent on a similar circuit. As far as active mid boosts go, though, it could be super interesting. Either you could add a gentle Matamp/Orange mid boost, or sharpen the Q and get a "cocked wah" sound, or even defeat the loop for a gain boost... If you wanted to get really clever, you could even sweep the frequency at a constant Q, but at that point just buy a Tube Zipper. Puretube (who also designed the Flanger Hoax) is one of my heroes. Give him monies.

I'll fiddle with this idea first using a Kustom Defender 5H, turning it into basically a massive pedal. It might be a huge waste of time, who knows. Plus the James stack is perfectly capable of creating a "mid boost" by itself...

But seriously, puretube is a brilliant fellow.

Tuesday, January 6, 2015

The Matange!

So, I've been planning my next project lately. This year, I'm going to build an Orange/Matamp-inspired head, starting with a cheap donor. Still not going to make the plunge into woodwork/metalwork as long as I can avoid it. Someday I might build a high-gain monstrosity, but I'm not a big fan of preamp distortion, so I don't have much motivation.

Fortunately, I have the relevant schematics on my phone!

Now, my thoughts:

Fixed bias quad of pentodes. Sure, though it pains me to turn away from my beloved beam tetrodes, but I'll give these screen-current-sucking babies a try.

Cathodyne phase inverter. Awesome; love it. It's going to need a big grid stopper, and I'll put big grid stoppers on the power tubes too. Definitely going with AC coupling; DC coupling is too big of a pain for little benefit with cathodynes. What can I say, I like em center-biased.

James tone stack. Absolutely. I'll probably add, or at least try, a mid shift control so I can move the notch around.

The "FAC" switchable coupling cap bass control. This is great; I've used it for a bass control after a pentode. The series-connected version does pop; Matchless uses a parallel "one at a time" version with large resistors in parallel to prevent this. Since I've tried the series version, I'll probably try Matchless's take on the Matamp original.

The Matamp drive control. This kind of seems silly. Switchable bright cap, eh, why not. Gotta keep some British in there! I don't see the point in padding down the gain when there are only three gain stages. Also that Orange inductor-based boost control... I'm skeptical. I'm also worried about noise. It's a tricky situation. I might try variable cathode bypassing instead, possibly tied to a treble cut.

As for the order of the tone controls, I think I prefer having the FAC after the first gain stage and the James after the second. Always good to cut bass early and treble late, though there are so few stages...

It should be an interesting project. If there's space for a third preamp tube, I may use a 6GH8 triode/pentode to add reverb. I'll use the pentode to drive the tank and half of a 12AX7 for the recovery stage, and then I can use that medium-mu triode in the 6GH8 for the phase inverter! Mmmmm, sweet, sweet low-ra cathodynes...

More when I actually write up a schematic.

Sunday, October 19, 2014

Grounding, balanced cables

This is a great read. Not so useful for guitarists, perhaps, but it's good to be familiar with this info.


MPS Iron

I got a Musical Power Supplies OT40PP for the Rocket build. I have one of MPS's earlier offshore versions of this OT in Bodie and it works well in that amp.

This new model is made in the US with M6 steel, 4/8/16 ohm taps on the secondary, and 4k2 and 3k3 primaries.

It's a great chunk of iron at a great price. No idea who's doing the winding, but this guy knows how to spec his iron. I'm super happy, especially compared to the stock Crate iron.

It's kind of odd to think how similar the Fender 5F6A, Marshall JTM45 & Plexi, Vox AC30 (Trainwreck Rocket), and Sunn Model T are. Okay, the first two really aren't much of a surprise, and there are countless other examples... I guess it's just a really popular topography.

I can kind of see it, but it's just not doing much for me. Maybe with a different tone stack or some NFB. Just a little more midrange and a little less bass would go a long way.

Sunday, September 14, 2014

Silverface reverb

Okay, so during the silverface era, CBS decided to beat the living hell out of the reverb driver, lowering the cathode bias resistor to 470R. Easy fix here, just replace the cathode bias resistor with the blackface value of 2k2 bypassed with a 25uF cap.

Another nifty trick is to replace the reverb driver's grid leak with a pot, aka a dwell control. That's how it was labeled on the original reverb units, though it's a little disingenuous as it doesn't control how long it takes for the reverb to decay (I mean, how could it, right?) but instead it controls the level of signal sent to the driver, with the existing reverb pot controlling the level of signal coming off the recovery gain stage. With a dwell pot, you can dial in a much more subtle reverb sound that's more dynamic.

I just put the dwell pot in the place of the humdinger which had fried ages ago due to a heater-to-plate short. While I'd like a proper humdinger, a pair of 100R resistors off the pilot light is better than a blown pot!