Modifying layout for a Hammond chassis

[Larpy]

I've built both a stock FPIII and an Extended FPIII modified to use 6SN7s.  The latter is my everyday preamp, and when I hook up the stock FPIII (every once in a blue moon), I always hear much I prefer my Extended FP.  So I've decided to upgrade my stock FPIII to Extended status, so I'll have an Extended FP with 6SN7s and one with 12AU7s.  But as I upgrade the stock FP I also want to transplant the circuit into a Hammond chassis that measures 16" x 8" x 3".  I like the idea of having extra room to make experimenting with caps and such easier.

So here's my question:

Am I asking for trouble by modifying the stock layout of the FPIII circuit so that the tube half of the circuit and the transformer/electrolytic caps half of the circuit are each rotated 90 degrees? In the only thread on modifying layout I could find, Paul Joppa cautioned that "layout is complex enough that it borders on a black art," so I want to run my plan by more knowledgeable folks before I take my saw to the Hammond chassis.  What I have in mind is to line up the tubes left to right on the right half of the larger chassis (long side facing frontwards) and line up the transformer/caps section similarly on the left side of the new chassis.  Take a look at the crude drawing in the attachment (pardon my lack of computer software):

Drawing's not quite to scale (and the layout is how the circuit will look from the bottom, underneath the top chassis plate), but I hope you get the gist of it.  Not a radical mod, but might it cause problems?  Am I better off replicating the stock layout more exactly?

[John Roman]

Hello Larpy,
This is something I have also considered. My EFP3 outputs are 6SN7 also. That said I would think if you maintain a similar topology there would be no issues. Upon suggesting that I must say that I'd be curious what other BH's think. Perhaps they will weigh in.
John

[Paul Joppa]

The only risk I see immediately is that some tubes are closer to the power transformer than others. The power transformer magnetic field is a possible source of hum. If it were me, I'd move it to the far left with the power supply between it and the circuitry. I seriously doubt the current arrangement will be a problem, but that's the only thing I can think of offhand.

It's not obvious, but the original chassis layout is in four sections, front to back - controls, tubes, power supply, and connections. I always thought it could be cut up and the pieces assembled on edge inside some sort of cage. But nobody wants to hide the tubes!

[Doc B.]

I think you will probably be OK too. But it's really hard to make any promises. Basically you have to do what we do - build it, test it, if it doesn't work as planned move stuff around to optimize it, test it again, repeat until you are happy with it. Distance from the power trans is important, but also the orientation of the tubes and cabling with respect to the magnetic field of the transformer.

[Larpy]

Thanks for all the quick feedback.  The Hammond chassis and the Extended FP kit are ordered but haven't arrived yet, but once they do, I'll go ahead with my plan (though I'll move the tranny further left as Paul suggests) and report back on how it turns out.

[Larpy]

Update:

The Extended kit hasn't shipped yet, but here's the progress I've made so far.  I made a mahogany base, drilled and painted the chassis, and installed all the parts for the Extended circuit save the C4S boards and the dual channel power supplies.

As you'll see from the photos, I decided not to reorient the circuit other than to move the controls and the jacks as far away from the power supply as possible.  I also decided to stagger the position of the shunt regulator tube to make more room for the big oil caps I favor.

I'm looking forward to finishing this and hearing how it sounds.  I'm hoping the Extended kit ships soon.

[Laudanum]

I like the look.   I did something very similiar to that about 10 years ago with a ST 35 "clone" using a Hammond chassis dropped into a wood base.  Not nearly as pretty as yours but similiar idea.  Very cool.  Hope it works out great.

[Grainger49]

You know I like the big honking capacitors.  Maybe add some to the power supply? 

More Capacitors!  (Sounding like More Power by Tim Allen)

[BNAL]

Larpy,

I did the same thing with my Extented Foreplay III. The Foreplay uses 6CG7s. The power supply is on the left and the tubes are on the right on their sides with the tops pointing in the direction of the right side panel. I used this terminal board that has 3 tubes sockets http://www.partsconnexion.com/product3167.html. I also installed a power filter and remote step attenuator.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm4.static.flickr.com%2F3059%2F2935693879_f27aa5b0e3_b.jpg&hash=0c4cb4b25b6069def4803f4f349da0ce81819fd0)