Considering an Express-based vintage PA conversion...Is it worth the hassle?

[carlsoti]

I recently picked up an old Bogen/Challenger CHA33 tube PA amp. This is a later model with a single 5U4GB instead of the pair of 5y3s shown in schematics. Up front it has a 12AX7 for the two inputs, mixed by a 6AV6 (one half of a 12ax7, basically) before being sent through a 2 knob tone stack to another 12AX7 used for the phase inverter, which feeds a pair of 6L6GBs driving a multi-tapped OT.

The 3 available gain stages up front had me do a 180 from the "vintage tweed" direction I was originally thinking.

Deep dives into TW lore reveal that, if you want to see what the TWE is all about, you have to have ALL the right components, with the proper layout, including very careful tuning of the lead-dress to get it "just right."

How far off the mark can someone get before it sounds like something other than a trainwreck? And what is the typical outcome? Something vaguely Marshall-y? FWIW, I have no issue tweaking the circuit for whatever PI/output tubes I end up going with to get the "sequential-saturation" necessary for the touch sensitivity these amps are lauded for.

Beyond that, what exactly is the Trainwreck Express experience?

Are the differences between a "good" and "bad" clone similar to the difference between between Heliotrope and Phlox to a painter or graphic designer, where a "layman" might not ever notice the difference?

[xtian]

Go for it. I've built a couple non-traditional Express amps. Follow good lead dress, and you'll minimize possible oscillations. You may have to add some grid stops if there's trouble. I felt the need to dump a bunch of gain in the early stages, because, DAMN there's a lot of gain. And then, plan to play into an attenuator--this amp is made for LOUD.

[Vince-]

You probably know, but there is an express build guide 2.0 pdf that outlines how to build a clone and is very helpful. I followed it on 2
Express builds, one was a hammond ao29 and was was a more proper express build.
It's worth finding and printing out.
You'll love it when you're done. These are great amps.