I know this is a bit late but the amp also has 2 more filter caps per side of the preamp tube (Those 4 that extend out and feed the individual side of the Driver tube @ 220UF Capacitance which is a bit more optimal for the tube I guess). Those can also affect hum I guess but unlikely to be honest since if the first three caps didn’t fix the issue in filtering, it’s unlikely those would do any better seeing how they’d receive a pretty well filtered DC at that point.
For the Hum issue I see alot of people say to use Bypass Caps (Search Hum-Fitz Mod) on the Cathodes in order to fix this, but a better solution would be to remove the transformer and order a correctly made one for US voltages (I did this, don’t use Cathode bypass caps and my amp is dead quiet with 99% of tubes).
I’m going to assume you’re in the U.S., in which case you have 120V in your Outlets, since the transformer has a primary of 110V instead of 120, all voltages going out the secondary will be a bit larger proportionately, giving you about 7V on the heaters instead of ~6.3V and at a current of .6 Amps either way (correct me if I’m wrong though since my understanding is that the heaters determine the current and you simply have to give it the voltage it wants, so be it at 4 or 8 volts, the current amount wouldn’t really change unless the voltage dips lower than the current rating, and I guess same thing with the Plates but I suppose that’s why CCS or Plate resistors are used as well, well that and to lower voltages to the plates too proportionately for a specific operating point, but I guess max current available would change with a resistor vs CCS, idk getting off topic).
I recommend getting the transformer ordered from the original manufacturer here:
They have an online chat service and tends to be quick enough on the shipping and manufacturing (~10 days). True this is the more extreme option but it’s probably going to be the most effective method. Other than that you can do the previously mentioned Cathode Bypass cap trick (Hum-Fitz Mod) to reduce noise overall, albeit you get a bit of gain due to this and the capacitor quality is rather important here, maybe go with an ELNA capacitor.
Also forgot to give you the Transformer Specs just in case:
120 VA (Just a hair over the originals Maximum Rating for a bit less heat when using heavier consumption tubes)
Secondary 135V - 0.7 Amp
Secondary 6.3V - 3.6 Amp (This way you can also use .9 Amp Preamp tubes safely, ergo why the VA was increased a touch)
Also Ask for a Center-tap grounding since it helps with noise a touch although it doesn’t really matter much other wise unless you get confused as to why the transformer would suddenly be missing a green/yellow wire. (Honestly I remember using a multi-meter to test what the green/yellow wire does but it doesn’t seem to be connected to any winding from what I could tell, so I gotta assume it’s connected to the Core itself or something, again, idk, although online sources say that wire color is reserved for grounding center-taps, again, don’t quote me on this).
If you’re worried though you can always ask them for a Darkvoice 336SE Transformer @ 120V primary and they’ll get you the exact specs of a normal Dark Voice transformer but for 120V primary.
You can also make the heaters DC but that’s a bit more complex (More of a space issue although it can be done, also you need to drop voltage, you know, RMS vs DC, DC tends to be Peak Voltages so it goes up a bit, so use resistors if you do this, but then again, it’s be easier to just use resistors in general to drop the heater rail voltages since DC doesn’t really affect indirectly heated Cathodes.
OK so to sum up (TL;DR):
Hum-Fitz Mod: Easiest Choice, Get Good Capacitors (16V-24V @ 110 uF/220 uF <— capacitance number affects gain, higher means more). This won’t actually decrease Heater Voltages and that can lead to a lower expected lifespan for the tubes due to overheat/over-specification.
Transformer Swap: A Bit harder and a bit more expensive, but getting experimental with Transformer Values can lead to a larger variety of tube possibilities due to Min/Maxing Certain Values/ Increasing VA and increasing Current limit of the Heater Rails for more tube options. If not you can always order the original Transformer with 120V Primary Specified.
DC Rails: DC will remove any hum since DC is at 0Hz, but you’d need to build a rectification circuit as well as a CRC Smoothing Section in order to clean the DC and make sure the Resistor lowers the DC to 6.3V again. After that DC has no other noticeable benefits since the tubes are Indirectly heated (Would be great in Tubes like 2A3 and 300B Triodes of yore but the Darkvoice can’t use them, since the Transformer Theoretically couldn’t give out the Plate Voltages, well maybe it can but then you’d need 2 Tubes in the amp or another Darkvoice, which while sounds fun, it’d be better to build one from scratch using 2A3/300B). Downside to DC rectification however is a new point for RF interference as well as the quality/speed of the rectification determined by the quality of the Diodes. This sucks especially in such a tight enclosure.
Resistors On AC: This also helps and is WAY easier than DC rectification but make sure the resistances are right (You need to know how much voltage the secondary is actually outputting, so either use a multi meter or maybe a bit of math using transformer widings and stuff) as well as heat rating (Watts), also you need 2 resistors on each rail end.
Oh and if you’re already inside the Amp, maybe take the time and Twist the Heater Wires together, get some 16 or 18 gauge wire and make twisted heaters. If somehow you do all of this (Especially the Transformer Swap) and the Amp STILL makes Hums/Noise, then the tubes are probably micro-phonic (Especially NOS tubes that haven’t been maintained in a good state of use or mistreated)