Electronic States
Work blog of Atomium Amplification in Philadelphia, PA
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I get asked about these amps a lot. I have several posts detailing repairs and modifications to my own amp over the years, but here are some collected notes that specifically address the most common questions and complaints about the amp. Right now, no longer being used on tour, my T Reissue has been returned to about 90% stock, including the OT.
1970s Sunn Model T Super. This is a transitional model with the 2nd-gen preamp but without the red hardware. It needed to have the bass side of the James-Baxandall network rebuilt, since a previous repair attempt hadn’t been wired correctly. I rewired the master back to a standard configuration, without the dual-gang setup that makes dirty sounds tougher to get out of the 2nd-gen version. Both coupling caps were a little leaky and got replaced.
Now that the preamp is working properly, it’s still not producing the rated power. May just need new tubes. These amps are so simple that it usually doesn’t take long to narrow down the problem.
Yet another Model T reissue, buried in my currently very messy work space. I seem to get a lot of these, which is interesting because I hate working on them. I love the amp itself, but the PCB construction is unbelievably difficult to work on.
By the way, if you have one of these that’s trashed/broken/unrepairable, I do buy them for parts or rehabilitation.
90s Sunn Model T reissue, in need of some new tubes. This one inexplicably had a 12AY7 in the phase inverter, which went into grid current limiting before the power tubes clipped. This limited the output power to about 85W instead of the stated 100-120W. The amp really needs a 12AX7 or 12AT7 to properly drive the grids of the 6L6s (they require about 50Vp-p to reach full power).
In an earlier post I talked about installing a large 70s-spec Model T output transformer in a 90s reissue model. Because the 90s reissue uses an OT that has a separate secondary winding to create a balanced XLR lineout (it’s the Tonemaster/The Twin OT), I lost that functionality with the switch to the new OT. To get it back, I installed a small 1:1 transformer on a 30/1 voltage divider coming off of the 16-ohm tap of the OT. The secondary of the small transformer runs to the +/- pins of the XLR output, with pin 1 grounded to the amp’s chassis. This puts out 1.45V on the output, just about exactly what the previous arrangement did (+3 dBV, or 1.425V).
This mod can be done to any amp with enough space for the transformer and line out jack. It’s superior to lineout setups that use preamp signal because it includes the tone-shaping done by the power stage and output transformer, so that whatever you record from the lineout only needs to have a cabinet impulse added to it to sound fairly natural.
Sunn Model T 90s reissue with a Mercury-made output transformer intended for the original 70s Model T. It’s a huge transformer (on the right in the picture, larger than the amp’s power transformer). You can see the old one on the right side in this picture. Some extra volume, LOTS of extra bandwidth. The new OT’s 2-ohm secondary tap and ultralinear primary taps are unused in this amp, and there is no lineout winding, so the XLR output is gone. It also has a slightly lower primary impedance, at 2.2k, vs. the old one’s 2.35k. The main change is in the character of the sound at really high volumes, where there’s quite a bit more clarity.
KT66s will no longer fit because the clearance on the OT end is too small. I had all but given up on them anyway for touring applications, because they seemed to stress the heater supply and the bias wasn’t stable enough for touring applications. I’ll still use 2 of them for recording at half-power, but for touring it’s 6L6GCs. Cheaper, cooler-running, more stable, higher headroom.
I also recently realized that the presence circuit on these amps is wired to remove upper-mids and treble when rolled down, rather than to add sparkle when turned up. Flat response corresponds to about 9.5 on the control, compared to a lot of amps where the minimum setting is neutral and higher settings add treble. This circuit’s corner frequency is 650Hz (really low!) and with the presence below 9, significant extra negative feedback is added above the corner frequency to the inverting input of the phase inverter. This is quite different from the other Fender amps that use this same power amp configuration and stock OT, which typically have no presence control or one with a more center-neutral setting. I think this explains a lot of why this amp has a reputation for being “dark” even though it’s a very typical Fender circuit. Keeping the presence down will change how much perceptible headroom the amp has too; you’ll think it’s breaking up (and farting out) a lot earlier on the volume dial.
For more info on this circuit, look here.
As a follow-on to the last post, here are a couple of mods I like on my own FMIC-made Sunn Model T reissue. I should say that this is one of the vanishingly rare amps that I think is 99%-perfect as-is, and I’d be quite happy to have one bone-stock to the end of the world (disclaimer: I know the 70s Model T is radically different. I happen to also like it very much. I’m allowed to enjoy two completely different amps, right?). These mods are specific to my sonic preferences with my guitars.
Changes I wanted were:
1. Later breakup on the clean channel (make it cleaner)
2. Rounder, more prominent bass on the clean channel
3. Less fizz on the drive channel with gain way up
4. Less tubby-ness in the drive (but not “super tight”)
Clean channel:
I used a 12AT7 in V2, which is used only by the clean channel. It’s a lower-gain tube than the 12AX7. The stock bypass capacitor on V2A is .68uF, which creates a low-frequency rolloff with a corner frequency of about 280Hz. I increased C7 to 2uF, which lowered the corner frequency to 97Hz. In the tone network, I changed R13 from 68k to 36k, which has the following effect on the network’s response (all controls at 5, upper line is 36k):
You can see the full sweep plots here.
Drive channel:
I added coupling cap at 4.7nF between V1A and V1B, after the relay. This forms a highpass filter with a corner frequency around 60Hz, which is great for tightening up downtuned guitars, without losing the sludge that this amp is famous for. At the other end of the channel, I put a 100pF capacitor in parallel with the outer terminals of the volume pot, which forms a lowpass filter with a corner frequency of around 6kHz. This takes the fizz out of the top without losing the good treble. It’s similar to the basic fizz-reduction mod I’ve done on countless Marshall DSL/TSL amps, but suited to the component values in the Sunn.
EDIT, 3/2011: I ended up not being happy with the tone of the 12AT7 in V2. The conventional wisdom about it not being a good tone-generating tube is true. I think a 5751 or even a 12AY7 would be a better choice here. The 12AT7, though, is great in the phase inverter of this amp. It gives a little more current drive into the power amp for slightly more headroom. I’m using a cheap NOS JAN/military 12AT7 here at the moment, with good results.
I also moved R13 back to 68k, feeling that dropping to 34k looked good on paper but didn’t deliver in the tone department, and put C2 back to stock because it wasn’t deep, it was boxy-sounding. So…yes, all clean channel mods above ended up getting taken out. I’m looking for other solutions.
Another recent discovery is that the presence control on this amp is “neutral” at about 9.5, not all the way down like many Marshalls. It has a corner frequency of 650Hz, and with the presence at 1 you’re bleeding off about 10dB of mids and treble above this frequency. This can lead to the amp sounding “farty” at higher volumes or seeming to lack power, since the area of highest sensitivity in the human ear is attenuated. When using this amp as a slave for my Marshall, I set the presence to 8.5.
EDIT 2, 3/2011: Here’s a much better way to deal with the clean channel. It seems the main source of the over-the-top quack of the clean is the “treble bleed” network around the volume pot, which is between V1A and V2A (before the tone stack). C3 and C4 form a highpass filter before the pot, which should be left as-is. They help get rid of fartiness when the channel breaks up.
C5 and C6 bleed treble past the volume pot at lower volumes, and their effect reduces as the volume goes up. The problem is that their combined value (57nF) is so large that it’s bleeding basically everything except bass past the pot. This makes the volume jump up suddenly from nothing to very loud at about 1 on the control. Look at the range of the volume pot sweeps in the plot below (red is stock). The volume only really acts like a volume around 100-300Hz, the range of the pot is quite limited. Compounding the problem is that the channel only stays clean below 3 on the volume control, meaning that by the time the quack is gone from the clean sound, it’s distorted.
If you cut out C5 and C6, you get the green traces. Much more range on the volume control, much more predictable behavior. The problem is it sounds far too dark and boomy that way (probably why Fender put those bleed caps in to begin with). My compromise is to replace C5 and C6 with a shelving filter made of a 100pf capacitor in series with a 220k resistor. That gets you the blue trace, which still has a wider range of usable volume on the pot, plus a cleaner, less quacky tone for the channel. It sounds really natural with both single-coils and humbuckers, and is actually clean. The tone stack becomes more useful. I suspect this setup would be even better with a 12AY7 in V2.
EDIT 3, 4/30/2011: 12AY7 in V2 sounds great. These tubes are supposedly not rated to withstand the 160V heater-to-cathode voltage in a cathode follower stage like this, but appear to work fine anyway. The Fender 5E6A Bassman used a 12AY7 in just this fashion, and many install them in 5F6As in a similar position with no trouble. Beautiful rich clean sound now, that’s actually clean for a while before it breaks up. Not totally a “Fender” kind of clean, it’s more bassy. Compares favorably with big vintage Ampeg clean tones. The breakup character is a bit different than before, but really nice and usable, a great counterpart to the drive channel. It has a less Marshall-y and more Bassman drive sound.
Another Sunn Model T reissue, one of my favorite amps — to play, not work on. Servicing these things, the difference between delight and misery is whether or not you have to remove the mainboard. It has the PT primaries and secondaries running through a big hole in it, plus tiny short wires to V5-V10 sockets and ribbon cables and shielded signal wires to the back panel PCBs. It’s a nightmare to get it out and back in.
This amp has a problem with its channel-switching relays. The clean channel doesn’t work properly about 95% of the time… but that 5% of the time it does work (after a lot of switching back and forth), it’s perfect. Intermittent failures usually point to mechanical components. In this case, relay K1 stays stuck on the drive channel most of the time, so even though the amp switches to the clean channel, the input signal is still being routed to the drive channel. There’s a fizzy low volume sound that comes out when this happens, which is just the crosstalk from the still-connected drive channel. It goes up or down with the drive gain control, and none of the other controls have any effect.
It was interesting to note that this amp has had thermistor TH1 removed. That was the component that cracked its solder and caused massive damage to my own Model T reissue, before I got it.
The owner of this amp is also interested in adding reverb, most likely with a pedal inserted in the FX loop. The trouble with this is that he would like it to only affect the clean channel, which isn’t possible with a series loop with no mix control. We’ll probably address this by modifying a footswitch such that a single stomp switches channels AND turns the FX loop off in one click. That way we avoid potentially risky or devaluing changes to the amp itself.
Complete schematics here.
Installing JJ KT66s in a Sunn Model T reissue head. The bias needs to be dialed down to get 60mA per side, as these run hotter than the average 6L6GC. 30mA per tube at this amp’s plate voltage (489V) yields a nice 60% plate dissipation. Definitely thicker when cranked, but doesn’t seem any less loud than 6L6GCs, even though it’s 25W dissipation per tube, instead of 30W.
