Moody --- over the last 50 years or so all sorts of mods have been done to the original ST-70.
One mod was to add a 2nd Rectifier tube in parallel to the original GZ34. They would re-cut the hole for the Quad Cap and mount the 2nd GZ34 in its place. This was done so they could use a power tube that required more current than a single GZ34 could provide, like the 8417 tube which is no longer manufactured or available.
The quad cap would then be mounted underneath the chassis (space permitting) or individual caps making up 30-20-20-20 or they would mount a capacitance board in the space under one of the output transformers.
See Triode USA Electronics ..... http://www.triodeelectronics.com --- under Dynaco AMPS and Kits for their cap board for the ST-70 to see what I explained.
Here is their description ----
Triode Electronics ST70 Capacitor Board:This Power Supply board replaces the original can capacitor in Dynaco ST70. (You can leave the old one there for looks if you like) Bolt on standoffs under the output transformer, no cutting or drilling required to install, just bolt in, hook up the wires and go! NEWLY DESIGNED! Upgrade Cap Board: New Updated Version!!! This is a new, updated, version of the classic, SDS Labs, ST-70 Capacitor board. We have upped the capacitance to over 500uF @ 500V (Increases capacitance from original 30-20-20-20 to 40-195-135-135) while at the same time shrinking the board to make it fit into the chassis more comfortably. The board still replaces all bias supply components and we have also added 0.1uF Wima MKP series metallized polypropylene bypass capacitors onto the board. There are no more surface mount resistors as used on the old boards, instead we have standard axial metal film resistors that are easy to mount and solder in place. There is also a cutout on the back of the board to make clearance for using binding posts in the amplifier rather than the original screw terminal strips. The board is a high quality .062 FR4 2oz Copper with plated through holes which allow you to solder/resolder connections with the board already mounted in place. Now with clearer and more detailed instructions.
(Note that the maximum capacitor size is 25mm long by 30mm diameter, board requires 8 pieces rated 250V or more)
Board with instructions, all capacitors, parts & amp; hardware.
I dont use a Quad Cap on my St-70, but I use one of the Triode Cap Boards.
Hope I answered your question about the Quad-Tube Socket issue. If you need any more info, just ask.