persistent 60 cycle hum only on tube gear

@yesiam_a_pirate 

I suggest you watch this You Tube video on the Puritan Audio Laboratories GroundMaster.

Note: He says, Do not defeat the AC mains safety equipment ground.

Note: His mention of installing a Supplementary Ground Rod. This Is Not an Isolated Ground Rod. A Supplementary Ground Rod must be connected directly to the branch circuit EGC (Equipment Grounding Conductor) per NEC code for electrical safety. He should of mentioned that in the video.

For the Ground Master you can connect an additional ground wire from the ground rod to the Ground Master. One ground wire to the AC mains EGC connection in the wall outlet box. One for the Ground Master. 

https://www.youtube.com/watch?v=2G8-Z3BMDcw

FYI, "Supplementary" Ground Rod, is an old name used years ago replaced by,  "Auxiliary" Grounding Electrodes. Therein Auxiliary ground rod.

(NEC, 250.54)

If you must, use a ground cheater on the power cord(s) of the equipment to lift the ground from the chassis. (Not recommended for electrical safety though.) 

Hopefully @atmasphere will look at the schematic wiring diagrams I provided in my above post for your ARC equipment. 

@atmasphere said:

Does the amp make a buzz or hum without the preamp connected?

If yes, Is it both channels or just one?

If neither channel, The amp is OK.

You need to do this test. If you still have a buzz or hum with just the amp, (ICs not connected to inputs), 

You may need to use shorting plugs on the R & L inputs.

Still have buzz, hum? Try moving all the digital streaming equipment away from the amp and unplug it from the AC main oulet for the test.  

Read @bruce19 above post.

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Note: It appears the circuit signal ground is connected solidly, directly, to the EGC grounded chassis. (Not good). Therein, signal ground is not connected, (for example), to a 10 ohm resistor then to the grounded chassis. 

@jea48 The audio input is grounded to circuit ground, not chassis. Normally in a balanced circuit the chassis and audio ground can be the same thing, but since this amp and a lot of high end preamplifiers (such as ARC preamps) do not support the balanced line standards, the practice of separating the two is a good idea. 

At any rate, until we hear back from the OP, the rest is speculation.

FWIW many claim to eliminate any system hum after introducing the Vinshine/Kinky Studios TaiHang power conditioner.

@yesiam_a_pirate 

Its probably not power supply caps are any other grounding issue the posters here are going round and round about. Your equipment is three to four decades old and has probably been well loved all that time and its simply the old, leaking coupling caps between all the tubes in the circuit. You should find a qualified technician, have the equipment gone through and brought back up to spec. Goodbye hum!

@atmasphere

 Thank you for responding to my post. I have learned a lot over the years reading your posts here on Audigon, Audio Asylum, and Steve Hoffman, Forums. 

By chance did you look at the schematic for the ARC SP15 preamp

https://www.arcdb.ws/Database/SP15/ARC_SP15_schematic_and_parts_list.pdf  

From my above post:

Note the capacitors on the AC mains line input. One from Hot to EGC ground. One from the Neutral to EGC ground.  Could those be leaking AC current? Especially the one connected to the Hot conductor?

Hopefully the OP will do the power amp isolated testing for 60Hz hum/buzz you suggested in your post above. 

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@atmasphere said:

The audio input is grounded to circuit ground, not chassis.

Yeah, I wondered why the ground symbol is so small on the schematic. The small ground symbol indicates the audio signal circuit ground is connected directly to the DC power supply B - circuit. 

Because B - of the DC power supply is connected to a 10 ohm resistor then to the EGC grounded chassis of the amp, the circuit signal ground has 10 ohms resistance above the AC mains EGC ground. 

Here is the schematic diagram for an ARC VT100 amp.

 https://www.arcdb.ws/Database/VT100/ARC_VT100_schematic_and_parts_list.pdf

Note:

B - of DC power supply is connected to a 10 ohm resistor (R80) to Chassis/AC mains EGC. (Same as VT130 amp)

On the VT100 design a 10 ohm resistor (R81) is connected between the signal ground circuit and B - rail of the DC power supply. This design would put the B - rail of the power supply 10 ohms above the EGC grounded amp chassis, and the audio signal ground circuit  20 ohms above the EGC grounded chassis. I imagine also above noise that may be on the EGC ground chassis.

I assume the above circuit grounding design is to help prevent ground loop hum problems, and possible noise that may be on the AC mains EGC connected to the chassis. 

In my mind the grounding circuit design should work providing all other associated equipment that will be connected to the power amp by wire interconnects follows a similar grounding circuit design. (Therein does not connect the audio circuit signal ground directly to the EGC grounded chassis.) Or any piece of equipment having an internal problem,fault causing a small AC 60Hz leakage current to the signal outputs of the equipment.   

It appears to me if a piece of associated audio equipment that uses an AC mains EGC and the circuit signal ground is connected directly to the EGC grounded chassis, that piece of equipment through the now AC mains grounded signal ground conductor of its’ ICs will connect the circuit signal ground of the amp directly to the AC mains EGC, as well all other associated connected audio equipment ICs.

.

Food for thought for others reading this post. Do you see the problem with using a ground cheater on a power amplifier and thinking the preamp, that uses the AC mains EGC, will ground the chassis of the amp to the preamp EGC safety ground through the signal ground wire of the ICs?

If the preamp circuit grounding is properly designed the audio signal ground circuit is above the EGC grounded chassis of the preamp as well as the power amp.