Tech: Sunn Model T (reissue) schematics and discussion

EDIT 3/2011: This post should be considered outdated. See these posts on Electronic States for newer information.

I've had a broken Sunn Model T reissue head since 2007. I got it from the (then) bass player of Battlefields in Fargo, ND. It has a hole the size of a quarter burnt through the main PCB near where the power transformer primaries are connected. I took it to a shop about a year ago, hoping to have some sort of point-to-point workaround, but the amp has been sitting there for all that time with no work done.

So I'm taking it back home. Thinking that I might now have the chops to repair it myself, I've pulled out my schematic and started looking at the area in question. Turns out, it might actually be simple.

When I initially examined the amp, I noticed that no fuses were blown. The burnt area of the PCB is on the primary side of the PT --- it's like a little 'patch bay' with terminals (CP2-CP9) for converting the amp to run on 240, 230, 120, or 100V mains. The problem may have simply been caused by a piece of dust or other contaminant flashing over between adjacent terminals. While I understand the utility of using this little corner of the PCB this way, it seems silly to put a bunch of spade connectors carrying the full mains supply only millimeters away from each other. If you're going to do this, at least put the whole thing on its own PCB and spread things out. Anyway, if the PT is ok (the tech in the shop says it is), then I can just lose the conversion flexibility (and the liability), splice the wires together in the 120V configuration, and go to town. It probably isn't actually that simple, but I'm hopeful...



The schematic for this amp floating around the internet is only one page, so I mail-ordered one from Fender. I've posted both pages below, and as high-res PDFs here.

For the record, I know that this amp is absolutely nothing like the 60s-70s Model T. I happen to think that both are excellent amps in their own ways. It's a shame that Fender discontinued the Sunn reissues line, especially given all the mediocre solid-state stuff they make now.

I'll post more as the project continues; it promises to be pretty interesting...



Update 11/30/09:
FIXED. It really was that simple. I hooked up the power transformer primary leads with wire nuts and taped them up, replaced TH-1 on a terminal strip, and moved P31 and P32 away from the charring, to C46. C46's replacement got moved to a terminal strip near the standby switch. Everything works! No other repairs required.

I do think I'd like to try KT66 power tubes in it. I hear that they have a greater midrange presence than 6L6s, and that's something I like. Tung Sol preamp tubes are doing the trick nicely right now.

Update 12/4/09:
I tried some JJ KT66s in here and they sound fantastic. I don't know how "authentic" a KT66 sound this really is, but it's vastly better than the Groove Tubes 6L6Ss that were in there before. They might have been tired, or just years-old Chinese stuff that wasn't very good to begin with. They sounded compressed, fizzy, and dark, somehow all at the same time. The KT66s sound more open, midrange-y, and smooth; a little more like an EL-34. Being that most of my stuff tends toward "British" (tonally speaking), that's great. They really flatter this particular amp; it's got this awesome "spank" sound on the drive channel when you dig in with the gain around 5. Sponge-y!

I did have to dial the bias way down to get them stable at 30mA per tube. Right now there are only two tubes in the amp, running at half power. I'm also using the 1/4 power switch... which means in total I'm running the equivalent of about 12-15 watts right now. It's still way way too loud to play at home.

Update 12/14/09:
I used the amp as a slave head with Rosetta at Saturday night's show in NYC, with the 4x 6L6 back in it. It performed admirably (much nicer than my Marshall 3210) except for some cutting out at high volumes. I later traced the problem to cold/cracked solder joints on the FX loop PCB. It's good to go now. When it's used as a slave amp (power amp only), the 6L6 tubes do very nicely, especially when combined with my EL34-based Marshall master head. Playing through the Sunn's own preamp, though, I still like the KT66s better.

I couldn't resist --- in the preamp, I've added a small coupling cap between the gain control and V1B. This blocks some low bass (below 60Hz) and tightens up the gain channel with downtuned guitars. Not a lot, just a tiny bit -- too much lowcut before this triode makes the amp sound like a Marshall. I already have one and don't need another. I also added 147pf across the gain channel's volume pot to cut some of the fizz -- again, not too much. This one's a keeper!

---

Notes about the design:
Tube V2 is used exclusively by the clean channel. It sits between the volume control and the tone stack (V2B is a cathode follower that drives the tone stack). You can swap this tube out for a different type without affecting the sound of the drive channel --- one good option would be a lower-gain tube like a 12AT7 (I use a JJ ECC81 here). With a 12AX7 the channel tends to break up fast, though not at low volume, because there's no "quiet" part of the volume control's range. It goes from "off" to "loud" between 1 and 1.5. Looking at the schematic, it appears that Fender was doing their own spin on a Marshall Plexi preamp. The sound is still Fender-y when it's clean, but there's a lot of filtering going on that will keep the breakup tight if you run it all-out. Cathode bypass caps in this channel are more typical of a Marshall than a Fender. If you were to sub a 12AT7 and go lower-gain + more clean, you could also get more bass response out of the channel by increasing the value of C7. If you intend to run it crunchy at high volume, it's best to leave it as-is, to avoid farty distortion.

V3 is used exclusively by the drive channel, so this can be swapped out without affecting the clean channel. It has less of an effect on distortion than V1, though. V1A is the first gain stage for both channels, then V1B is used by the drive channel as its first distortion stage. The drive channel's gain control is in between V1A and V1B, which then feeds V3A and B, then V4A and the tone stack and channel volume.

V4B is used by both channels and provides either drive to the phase inverter --- when the FX loop is off --- or a small amount of gain to the send controls for the FX loop. V5A is the FX send buffer (an AC-coupled cathode follower), V5B the FX return buffer. V6 forms the phase inverter, a standard long-tailed pair.

In sum --- as far as preamp tubes, you'll get the most bang for your buck swapping out V1 (changes both channels), V2 (changes clean channel), and V3 (changes drive channel).

The power amp is a very standard Fender 6L6 push-pull design. It's no different from myriad other amps. I happen to think it sounds great with KT66s. One thing to watch here is that Fender recommends a bias setting of 60mV per side at the test points, or 30mA idle current per tube. For a decent 6L6GC (I like the JJs, they are TOUGH), this is a really cold setting --- less than 50% of rated plate dissipation. You usually want to be at 60-70% for a good middle range. I'm sure Fender did it to be on the safe side, but a more reasonable value for 6L6GCs at this plate voltage (489V) would be 35-42 mA per tube. 37mA gets you 60% of rated dissipation, 43mA is 70% (pretty warm for class AB operation). So try the bias at 75 or 80mV per side instead of 60, I found it sounded better.

KT66s will need to be kept right at 60mV per side, though. They are rated for 25W plate dissipation (not 30), so running them at 30mA per tube is just about right --- in this case, stick to the 60mV per side recommendation.

Remember that if you pull two power tubes to cut the power by half, you'll also need to set the bias for HALF of what it would be with 4 tubes... so 37-40mV per side for a pair of 6L6GCs, 30mV per side for a pair of KT66s. You'll also need to move the speaker impedance switch to HALF of the speaker impedance you're actually connecting to --- so for example, set it at 8 ohms if you have a 16-ohm cabinet.

---

Discussion (read: ultra nerd-out):

It's strange how the 70s Model T and the Fender-made reissue are enormously different --- and yet they're both uniquely suited to similar sludge-y/doom-y styles and tones.

In the 70s model, you have a kind of Marshall MKII amp on steroids: a Plexi preamp circuit with a bit of added bass, master volume, and a hugely overpowered 6550/KT88-based power amp which is ultralinear with a massive transformer. The result is a Marshall-ish circuit that actually stays tonefully clean at thunderous volume levels (in fact it's very much like the 200W Marshall Major). A full stack running full throttle with a nasty fuzz or distortion in front creates a truly "sinister" tone... very doomy indeed.

In the case of the reissue model that I've been working on, it's really just a Fender amp. It's not a "doom amp" made by Fender, it's actually quite literally a Fender amp. The power amp design is a very standard Fender 6L6 stage, and nearly a part-for-part replica of the power amp in the red-knob "The Twin" from the early 90s. It just runs at higher voltage. The Model T reissue's output transformer is the same as used in the red-knob Twin, the Tone Master, and the current-issue Pro Tube "Evil Twin" (though the XLR line-out tap on the secondary is sometimes not used). The Model T reissue's power transformer is used in the Evil Twin too (same part number), and an almost-identical power transformer is used in a number of Fender amps (see: Tone Master PT, Vibro King PT) --- even though the part numbers don't match, the measurements and specs are all the same, including the separate 56V bias supply winding. The Model T reissue's tube-driven FX loop and 1/4-power switch (voltage reduction in the power supply) are standard Fender designs, which are used in all the current Pro Tube series amps.

The case I'm trying to make here is that the reissue Model T is basically a souped-up Fender Twin with an additional high-gain preamp channel. It's clean channel is really clean --- some would argue here, but I think that's because the volume pot lets it get too loud too quickly. It's a typical Fender clean, nice to have. The drive channel is the real show-stealer. It's loose, throaty, fuzzy, higher-gain than Fender was ever known for... and with the amp cranked, it's doom-y as anything. At low gain it's got an apocalyptic crunch, at high gain it's stoner-fuzz-metal heaven. The amp is LOUD (though the 6L6 output section isn't a match for the old Model T's 6550s), mean-sounding, out of control. You've heard this specific amp on Isis, Khanate, Sunn 0))), Russian Circles, and Cave In recordings, just to name a few.

Weird thing is... that not-so-Fender drive channel didn't die when the Model T reissue went out of production. It too got dumped into the 2-channel Pro Tube Evil Twin. I can't really express how weird this is --- an amp known as a power tool of doom, sludge, and "post-metal" was remade in a combo format with reverb/tremolo and is selling as the new Fender Twin!! The controls are identical. The current Twin circuit is 90% the same, right down to the alternating hot/cold biased preamp triodes in the drive channel (they just took out the cathode follower that drives the tone stack... who knows why). Look at the Twin schematic and compare it to what I posted above. Sure there are some component differences (e.g. look at the 250pf cap they added across the volume pot... so they cut back the fuzziness just a tad), but the topology is all there. Weird weird weird!

If I had to guess, I'd say that the Model T reissue was essentially a test platform for Fender to develop some new, more diverse and versatile sounds to add to their next generation of amps. The Model T reissue is the missing link between the early 90s red-knob Twin and the current Pro Tube Evil Twin.

So why isn't the Pro Tube series Evil Twin the doom amp du jour? I think I might have seen Mono playing through them. But this does reinforce two points: 1. that marketing hype and prejudice have as much to do with what people choose to play as features and sound, and 2. that if there is a significant difference in sound, it has to do with speakers and cabinet design. There's a pretty big difference between two 12" Jensens in an open back enclosure and a closed 4x12 with G12Ts. Changing speakers matters a lot more than changing tubes.

I would welcome thoughts that other amp nerds and Model T Reissue fans have. Though I would love to eventually own a 70s Model T, I have always loved the reissue model. It's unique (or at least I thought it was). Like a lot of people my age, it was my first introduction to the Sunn name (right about the time the same-named band began making demo recordings). Now I'm beginning to understand why Fender ended their production with the Sunn name. There were only two amps "reissued," and both the Model T's and 300T's circuits were destined for a top-of-the-line Fender guitar and bass amps, respectively. Before now, the 300T's continuation as the Bassman 300 was obvious, but only now do I realize that the Model T was reincarnated as the Evil Twin.