Two units in one chassis

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the pitfalls are as you can imagine (PT flux inducing hum in inductors or ins/outs transformers).  A very careful layout will reduce the hum to a pro audio level.  Perhaps there is a custom PT that can supply all the voltages required. 
The LA610 (among so many other designs) employes a single PT. B+, heaters, phantom pwr, single supply or bipolar (forgot, it's been a while since I mapped it out) for an opamp.
I would strive for a single PT, however with careful orientation and distancing sensitive stages from noisy stages etc. multiple pwr supplies can be used.

My LA2A has a separate PT for two LED meter circuits, I was careful to test for noise interference and rotate/position accordingly and it's crazy quiet.

Shielded ins/outs, and possibly using a toroidal PT are a good idea.

I'm not "against" DC heaters, cause that would be silly, but I am a firm believer that AC heaters are 100% adequate. But what the heck, if you have the room, go DC heaters


Andy

There is a small benefit from a common chassis ground. You need good signal discipline passing signals around between subsections (use differentials).

JR

Hi Adam. can you better elaborate on what systems you want to run into what?
maybe a rough diagram or schematic?  I believe in a post you touched on a junction with an impedance match to consider, as well.
transistor thing with OT -> tube thing input?
In regards to your heater question. It will be a matter of cost and space. fwbr heater circuit with large mfd filtering stage or regulator, or dropping resistors and filtering.
or leave AC and create an artificial centertap, or "humdinger" (a low ohm 1-2W pot with the wiper to ground).

andyfromdenver said:

the pitfalls are as you can imagine (PT flux inducing hum in inductors ). 

Click to expand...

Sometimes having two PT's is better, if you can arrange them in humbucking.

abbey road d enfer said:

Sometimes having two PT's is better, if you can arrange them in humbucking.

Click to expand...

oh wow!  Thank you, I didn't know/ consider that.

80hinhiding said:

From the primary side of the 269JX power transformer, grey wire to the white wire, I get 7.4VAC from the secondary low voltage winding.  With the primary side connected Black to White it's closer to 7VAC.  I think I should use Black to White on the primary, since the secondary is then closer to the 6.3V that the heater needs.  Is 0.7V, +- a small margin, going to drop under load without another resistor in series?

Adam

Click to expand...

When I get a chance, I will look at Ian's scheme, and comment on that.
I love the 125V primary option on Hammond's PTs. Under load, both will come down, but I  use the 125V option (the black wire) exclusively for U.S. wall outlet levels.  If you are only very lightly loading it (say one tube for example) even the 125V option might be uncomfortably over, possibly facilitating the choice for DC heaters. 
tbc in this reply! lol.