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The Second Generation
Sunn Model T


Sunn, a name famous in musical instrument and sound reinforcement amplifiers during the late part of the 1960s, was faltering during the early 1970s. Much like Fender, the company that currently owns the Sunn name, Sunn envisioned the end of the tube amplifier and took a detour into the land of solid state.

The first solid state Sunn amplifier series was known as Orion. A preamp only head was coupled with powered speaker cabinets (a design used by the Acoustic Control Corporation for their 360 model bass amplifier). The Orion was quickly pulled from the market after the sale of only a few - it is rumored that the amplifiers had a tendency to self-destruct. Sunn eventually had some degree of success with their followup solid state amplifiers - the Concert and Coliseum series, using FETs to recreate the overdriven sound of tubes.

The Model T was issued when Sunn recognized that tube amplifiers were pronounced dead somewhat prematurely. The first generation was marketed as both a guitar and bass amplifier. It featured 4-6550 tubes in an ultralinear power amplifier stage, a design that Sunn used in their earlier 1200s and 2000s series. Unlike those predecessors with power output ratings of 120 watts RMS, the Model T is rated at 150 watts RMS. Also unlike those predecessors, the rest of the Model T is a throwback to the 1959 Fender Bassman 5F6A circuit, the circuit that Marshall slightly modified and used in most of their early line of amplifiers.

Sometime during the 1975-1976 timeframe, a "new and improved" version of the Model T was issued. The only visual difference between this and the previous model was the addition of a 3 position midrange slide switch under the midrange tone control. This is the model that I refer to as "the second generation". Although similar in appearance, the sound was different and not close enough to the first generation series that guitar players appreciated. This second generation model was more popular with bass players than guitar players... but it was never really "popular".

For my Christmas of 1974 present, I purchased a Model T through mail order. Mail order was the only way to get one in my circumstances - the last Sunn dealer in El Paso, TX had went out of business 3 or 4 years earlier. It was a first generation model. I used that amplifier for the next three years, for both bass and guitar, with absolutely no problems - not even a tube change. Unfortunately, the house that "the band" was using for practice was broken into, and my Model T was one of the items stolen.

One day, several years back, I was looking through the used equipment section of one of the Phoenix musical instrument dealers. I caught a glimpse of something behind a couple of amp heads and on closer investigation discovered it was a second generation Model T. After plugging it in and listening to a few chords to verify it was working, I promptly paid the $200 asking price and took it home.

Several days later I was disenchanted. I had never played through a second generation model before, and I was quickly convinced that "New and improved" doesn't always mean better.

I gave the amp a good cleaning, replaced all of the tubes, re-biased the output tubes... but still, for some reason the amp just didn't quite have it. The crunchy overdriven sound of my first generation model wasn't there.

My favorite control settings didn't produce the same sound. The midrange control wasn't working "right" - it sounded best to my ears all the way to zero, but even at zero it was still affecting the sound, providing a slight boost effect to the selected frequency. The only decent setting with the treble and bass controls I could find was with both of them maxed out to 10.

Dialing in a clean sound, the amp did slightly better. However, it seemed like it had lost some bass response, and the overall sound lost a lot of "sparkle", even though there wasn't a lot of sparkle to begin with. The sound went from dull to duller.

In playing with different settings I turned the amp down using the master volume control, from 10 to about 8... and to my surprise the amp got louder and bassier!! Sure enough... I tried it again - up to 10 and I got less volume, less bass, and a duller sound. Put the master volume on 8, and the bass was back with additional volume and a clearer sound. WEIRD!!

Disappointed, primarily because I had never played through a second generation model before, didn't know what to expect although I was hopeful for something resembling the sound of my first generation model... I evenutally put the amp up. I left it "put up" for several years.

Then I decided I needed to do something with the Model T. I contacted Fender for a schematic for this particular model. They sent me one for the first generation model, and one for the new "Fenderized" model wearing the Sunn name. Nope, after several weeks of communication, they confirmed they could not help me with a schematic for the particular model I have.

So... I figure I am on my own. I am either going to make this amp sound good or I will trash it to make room for something else.

This is the story of how and what happens... You can select a page from the list below, or select the "continue" link at the bottom of each page. Since the story is presented sequentially, it might be good to go through it that way on first reading.



  1. Amp Features
  2. Demolition!! Removing the Chassis
  3. Inside the Chassis (a two-cent tour)
  4. Basic Approach
  5. The Master Volume Mystery
  6. The Second Preamp Tube
  7. Eliminating the Midrange Boost At Zero!
  8. Another couple of notches
  9. The Second Notch
  10. Modification Evaluation
  11. Modification Recap
  12. Postscript and Miscellaneous


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