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.