Session 400 Upgrades Complete!

[Eric Reeves]

Brought home the old mid 70's Session 400 today from my amp tech. Started out with a bad speaker, and thought I might as well dive into the rest of the innards whilst it's there. Replaced the old shallow basket 1501 Black Widow with a much lighter Eminence Double T 15 Travis Toy signature. Did a grip of cap replacements as well that are way beyond my pay grade for understanding, but I've included a photo synopsis of the upgrades for ya'll more versed than myself to pick apart. I gotta say, this thing is KILLER and crystal clean all the way through. I hadn't expected such a turn around, but man, these amp guru's know their stuff!

I gotta thank Dale Rottacker for the Amp itself, as well as the eventual speaker upgrade that I had been jonesin for, a set of quality George L cables, a really nice bar, and some great words of encouragement.

Also wanna thank Brad Sarno for reaching out with his mod guide that I started with as a foundation https://web.archive.org/web/20180214233 ... 0page.html

Last but not least, Bruce Baumann of Brown Dog Amp Works. Bruce was incredibly fast and thorough, and was able to bring to life the vision I had, with his years of expertise. Bruce was an absolute pleasure to work with and visibly takes pride in his craftsmanship. I'll be taking in a few tube amps for a "wake up" soon.

Here's a few before and after pics with a side by side of the circuitry to pick apart and study along with a chart of upgrades we used for the build.

This is such a better amp than I deserve and I hope I'll be able to grow into it!

[Larry Dering]

Well that's a serious upgrade and some great work. Im sure you deserve it going to that much trouble and expense. Play it loud and proud.

[Jack Hanson]

I love my old Session 400 head. I usually run it into a TT-12, but it also sounds great into a BW 1501-4 (and probably many Celestions, as well).

[Jack Hanson]

No problem, Eric! I edited my snarky comment as well. That's one lovely Session you have there. Looks like it went out the back door down in Meridian only a few days ago -- not decades. Those old amps with transistors in lieu of ICs are definitely worth a little maintenance now and then. Pretty much bulletproof, like the Fullerton Fenders usually were.

[Pete Burak]

Nice job getting it fixed up!
I saw a very similar vintage Session 400 in a local shop here in Portland yesterday, and thought of your project.

[Dave Meis]

+1 on Bruce at Brown Dog. He's done my Twin and Bandmaster over the years. 👍

[Bill Terry]

You may have a ready buyer market for that Spider-Web BW if you don't want it. I and others I've talked to prefer that original BW over the later ones, it just seems smoother and less strident to me. I've got one in my Session 400 now, I think it just matches up with the amp real well.

Only down-side (other than it's obviously heavier than any Neo) is that I'm not sure you can still get replacement baskets.

[Eric Reeves]

You may have a ready buyer market for that Spider-Web BW if you don't want it.

I traded that old speaker and a Session 500 for all of the work done on this amp. Bruce seemed to think he could get a new basket for it.

[Bill Terry]

Bruce seemed to think he could get a new basket for it.

Good to know.

[Jeremy King]

I recently upgraded my late 70s Session 400 with a Double T speaker too! I really dig it. I also replaced the reverb tank and I couldn't be happier.

[Eric Reeves]

I recently upgraded my late 70s Session 400 with a Double T speaker too! I really dig it. I also replaced the reverb tank and I couldn't be happier.

That's all I have left to do is replace the reverb tank and the bushings. It's pretty loose and crashin when bumped. What'd you end up going with? I think a like replacement is about $30 or so.

[Jeremy King]

I bought one of these. It has a 3rd spring that allows for a smoother taper from what I can tell. It was super easy to replace and seems to be pretty tough. I highly recommend it.

https://reverb.com/item/27734633-reverb ... y-3-spring

[Jim Pitman]

I have a 70's Session 400 with original shallow basket BW 1504. The cone tore in a couple places. I couldn't find a replaceable basket for it. I glued it and it seems to behave just fine. I absolutely love the sound of this amp.
I refurbished mine about six years ago. I redid the caps per Sarno, touched up the tolex, replaced the spring reverb with a medium decay three spring type, and replaced the grill cloth with Fender type since the Peavey type wasn't available.
Hint - the Tolex can be repaired using Mars Black acrylic paint. It's quite thick and one can imbed a pattern in it that makes it look like tolex plus the color/gloss is a perfect match.
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[Eric Reeves]

That's two recommendations for the medium decay three spring type reverb tank. I'm likely gonna be doing that here soon. How do you feel this has changed or upgraded the reverb or did you just need to replace the old worn one and landed on that?

[Jeremy King]

It made the reverb smoother on my amp. I like it because a little goes a long way with the 3 spring and this amp. With the original tank I would usually keep it on 5 or 6 but with this one I can run it at 3 or 4 and achieve the same amount. You can oversaturate it in reverb too which I like to do occasionally for some atmospheric stuff.

[Dale Foreman]

Gotta love these old amps. Here’s mine!

[Steve Spitz]

The MOD reverb tank is a great upgrade. Got one in a fox intense mod NV400. Super lush reverb.

[Jim Pitman]

I have a musical associate friend from the east coast that stays at my house in VT on occasion and we do gigs in the local ski areas. (pre-covid) He's a singer songwriter guitarist. He's been playing with a reggae band in his area and absolutely loves my session 400. He states the reverb gives it the "reggae sound" for whatever that's worth.
The original reverb seemed a little lifeless so I intentionally replaced it with a three spring. I've forgotten now if the original was a one or two spring unit.
Gotta love those Session 400 heads too. I'm jealous.

[Bill Hatcher]

The 8 rectangular components that look like wirewound resistors......what function do they perform? I only have experience working on tube amps.

[Jim Pitman]

Bill, most of the power transistors are in parallel. The low value square ceramic resistors ensure the current is shared among the parallel transistors.
The topology of the session 400 output is called Qausi-complimentary. There' a set of transistors sourcing current from the positive rail of the power supply into the speaker during the positive half of the cycle and another set sinking current into the negative rail during the negative half cycle. The other end of the speaker is connected to ground. The power supply is +/- 55 volts. There is no output transformer like a tube amp has nor an output coupling capacitor like a class A single ended SS amp bas. It's essentially class AB, but the it's called quasi complimentary because all the transistors are NPN types rather than half being NPN and the other half being PNP.