73 100W MV Quad Reverb - hatchet job 'restoration' into a head cab

[tubeswell]

A couple of months ago I chanced upon this chassis that had long been removed from the original cab. The chassis and trannies date to late-ish October 1973, and the internals have been well and truly butchered by past owners. There are bits of assorted off-board jumping (to avoid the pitfalls of the SF waxed board), someone added a bias bucking transfomer at some stage, the lamp was disconnected, and there was evidence of a tube blowout in one of the output tube sockets (charred 1R bias measuring resistor) - yet the filter caps were still original (go-figure!?). Someone put a PPIMV mod in at some stage. Someone added a funky bias pot. So I acquired it (I must say for very little money) for the purpose of fixing it up - in case I run out of things to do with the rest of my life.

The 100W MV Quad Reverb, Dual Showman Reverb, Twin Reverb, Bassman Reverb, Vibrasonic Reverb and Super Six Reverb from this era are basically identical amps except for some variations in the output transformers and speaker configurations (but the dual showman, twin and quad reverbs from this era even have identical OTs).

Seeing as how its been extensively butchered already, and as its effectively beyond my abilities to locate an original 4 x 12" cab complete with speakers in this neck of the woods, my plan has been to get it fitted into a head cab to go with my mid-70s Dual Showman Reverb cab.

With pre-amp tubes installed, the PT secondaries deliver 360-0-360VAC and 6.4VAC, and the (modified) bias supply range is -27 to -59VDC (I will eventually change the funky bias pot setup back to something more appropriate, that has less adjustment range). The OT tests good for Pr:Sec VAC ratio and delivers an impedance ratio of 631:1 (2k5 with 4R speakers). I may yet have to replace the PT - time will tell (once I get around to shoving some 6L6s in).

So far I've done all the e-caps, replaced the output tube sockets (while trying to keep all the old wiring), replaced the lamp holder and reconnected it, cleaned out around the power amp end of the chassis with isopropyl, and tolexed up a little head cab for it. I'm still hunting for about 6 witchhats (as some previous owners have lost these along the way).

Seeing as how there's extensive evidence of previous patch-up jobs to get around the waxy-board syndrome, I'm inclined to eventually remove the main board completely and 'blackface' that part of the amp.

Thought I'd leave a progress blog here.

[Chuck H]

This might sound snarky... But my first thought was "I wonder what that face plate is worth?". I don't put a lot of mojo into the collectibility of the Quad Reverb and Super Six amps, but some Fender nuts do. Someone wanting to make an authentic reproduction from a Twin chassis could use that face plate and the price might fund your whole project. Just thinking out loud.

[tubeswell]

This might sound snarky... But my first thought was "I wonder what that face plate is worth?". I don't put a lot of mojo into the collectibility of the Quad Reverb and Super Six amps, but some Fender nuts do. Someone wanting to make an authentic reproduction from a Twin chassis could use that face plate and the price might fund your whole project. Just thinking out loud.

Yes, its probably the most intact original part of the amp LoL

[Chuck H]

Just a passing thought. I think it's a good, practical effort at any level for that amp. I also think the idea of blackfacing it could be skipped. You seem to be practicing better lead dress than I've seen in other SF amps and that alone is 98% of the difference tonally. Well, and the grid to ground caps on the power tubes, maybe. Otherwise it's apple to apples. Combination bias? So what, if it's done well it's good. Hum/bias balance pots? What do they have to do with the signal path? My point is, since you're not really building a classic collectible you really only need to be concerned about function and tone

P.S. Thanks for the new sig

[bob p]

how tight is the fit of the reverb tank in a short height head cab like that? the dual showman reverbs were a lot taller.

[tubeswell]

how tight is the fit of the reverb tank in a short height head cab like that? the dual showman reverbs were a lot taller.

It fits okay - snug but no squeeze - slimline look.

[Chuck H]

I've had remarkable success fitting reverb tanks into heads without the obvious hum probability. Sometimes you have to get creative though. Like mounting a "horizontal" tank sideways on the rear baffle (yeah, I did that once). But there are holes in the EMF from the PT such that if you move a tank around in situ you can usually find a quiet spot.

[tubeswell]

I've never had a problem as long as the pan's output transducer is at the other end of the cab from the PT (and the pan's mounting plane is appropriate for the type of transducer)

[Chuck H]

I've definitely had major hum when doing that. Prior to scootching the tank around anyway. I've done it three times with heads and it always amazes me how quite it can be in the exact right spot compared to how much hum there can be only one inch off. I'm not doubting (and certainly not sorry) that your experience has been easier.

[tubeswell]

Yes, as long as the output transducer is away from any EMF coupling it's okay. The input transducer doesn't get affected in the same way - the current is working in the opposite direction (toward the spring instead of toward the cable) and the amount of current it takes to drive the pan spring (from the reverb transformer) is huge by comparison to the EMF from the PT, so the input transducer can quite happily sit right next to the PT without any noise problem.

[Chuck H]

Maybe that's been my difficulty.?. I've always tried to keep the input furthest from the PT, as would seem "chapter and verse". I may have done it differently if I found that is what works though. It's been so long now I can't remember.