Interesting Fender AB165 Bassman -> Express Mod

[tlpruitt]

Anybody compared the AB165 Bassman schematic to one of the Trainwreck Express schematics floating around? A few years ago it occured to me that once you blackface the phase inverter and output stages in the AB165 you are remarkably close to an Express. Just a few component changes and you are there and it is completely reversible.

I finally got around to doing the mod yesterday and I am stunned at the tone. What a rocking amp this thing is now.

I thought it sounded really good after I blackfaced the phase inverter and output stage 15 years ago. But after the Express mods I did yesterday it sounds amazing.

-Tim

[AL30]

Looks like a combination of AB165 and AA864 circuit. The NFB resistor may play a large part in that too.

AL

[tlpruitt]

The AB165 is really ideal for an Express type mod. The similarities are remarkable. All the gain stages are right where they need to be. It's mostly a matter of changing some component values. It even has an extra filter cap in the B+ of the preamp that the 864 does not. The Express has that extra cap too.

I performed the mods in about 5 steps and listened after each step. Each listen test was better than the last so I kept going.

There are a few things about the AB165 that can not be easily overcome though when turning it into an Express type circuit;
-The overall B+ on the AB165 is a little higher than what I have seen reported on some Express schematics. Probably too high for a 6V6.
-You probably have to stick with a 6L6 because the AB165 cabinet is too short to get an EL34 in and out of without pulling the chassis.

Just thought I would share this with any curious AB165 owners out there.

-Tim

[AL30]

I thought you were just talking about the PI and power section. After a second glance - yep the gain stages are close.

So, what was the tonal/sound difference between the Fender and the Express? I've got an extra Bassman lying around (not working at the moment).

AL

[doctord02]

Bear in mind that the B+ on a Bassman is higher than it is on an Express. This will give it a decidely harder edged sound. Plus modern line voltages will jack it even higher. An Express running EL34's really needs apx 395-405 VDC at B+1...

But yeah, it aint a long leap to the Express circuit is it?

:-)

[jzucker]

To really get an "express" amp you'd have to replace the OT with something that has a very high primary impedance of around 5.5k or so.

[WesKuhnley]

Just goes to show you how closely related 99% of amps really are.

[darkbluemurder]

Very interesting. I have more than once thought about converting my converted Bassman to a T-Wreck. I didn't do it so far because I fear that this thing will be way too loud to use anywhere.

[tlpruitt]

The B+ is higher than a real Express, and the chassis is steel instead of aluminum, and the trannies are different, and the output tubes are 6L6, etc, etc.

But, it does sound pretty impressive and it was a relatively easy mod. It is loud, but in a good way. It has that old school Marshall type "grind" and midrange when you push it.

So far I have been playing it at home though a single 1-12 cab with an 8 ohm V30. Not my favorite speaker for Fenders but with this circuit it sounds pretty good and the load mismatch (8 ohm speaker and 4 ohm OT) makes it a little less LOUD.

I have not gigged with it yet so the jury is still out. If the loudness is a problem I may do the "power scaling for cheapskates" mod.

[AL30]

Thanks for the tip. Which circuit did you use? There are about 1,000 of them out there.

AL

[tlpruitt]

Here is the link to the schematic I based my mods on:
http://www.blueguitar.org/new/schem/..._schematic.gif

I have seen it called both "Kelly" and "Franchesca".

It seems to match the layout that Ceriatone uses.