persistent 60 cycle hum only on tube gear

@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.

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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. 

 

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.

@jea48 Yes, the resistor is there to prevent ground loops but is easily defeated by equipment upstream if that equipment has a grounded power cord with chassis and audio ground being the same thing. 

The chassis should always be grounded so a cheater is a Bad Idea IMO. If a manufacturer screwed the grounding bit up, IMO it needs to be repaired. 

BTW in the VT130 schematic there are two kinds of ground symbols used. One has a parallel line with a shorter parallel line beneath it and an even shorter one below that. This simply implies ground. The other is what is called the 'earth ground' and looks a bit like an antenna upside down, so its horizontal line with four lines dangling from it at an angle. in the VT130 schematic, this symbol is by the AC power input and is marked 'chassis'.   

@yesiam_a_pirate 

Some points I need to add to my post on 01-21-2026 at 12:02pm   

I should of mentioned why the 8ft ground rod has to be connected to the AC mains wall outlet branch circuit wiring EGC (Equipment Grounding Conductor). Connecting the two separate Earth connections grounds together places them in the same ground potential plane. Therein, zero resistance and zero voltage between the two connected together grounds. 

You can easily test for a difference of potential, voltage, from the ground rod to the AC mains branch circuit wirng EGC if you have a multi meter for what you have now.

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You do not have any audio equipment that uses EGCs that is directly connected to the isolated outdoors ground rod, as I understand reading your posted message. So all the 3 wire EGC power cords are lifted from the AC mains safety equipment ground. The ground pins of the power cords are still all connected together by what ever you are using to plug them into.  There is continuity from chassis to chassis equipment that uses the EGC ground.  You just don’t have a connection to the AC mains EGC. 

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From what I understand, reading your posted message, the ground wire from the ground rod is only connected to the Grand Master. I am not sure from what on your audio equipment, (signal grounds?), is connected to the Ground Master. Point is the Ground Master is not designed to handle ground fault current. Which is a good thing, IMO. It is designed to direct noise to an Earthed ground rod. (Again, an isolated ground rod should not be used, per the Puritan Ground Master video. (I supplied in my post above).

/   /   /

FWIW, you don’t read many threads or posts today from audiophiles that use an isolated ground rod for the safety equipment ground for there audio equipment.

/ / / 

Worth noting, Mother Earth does not possess some magical mystical power that sucks nasties from an audio system.  A connection to the Earth is a two way street. Noise will travel either direction in the wire. A higher voltage potential will move to a lower voltage potential. If a difference of potential, voltage is present, current will flow if a closed circuit is provided.

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Soil resistivity: 

It can be measured using special soil resistance testing equipment. Without actually testing the ground rod to soil the actual resistance is unknown.  For an electrical service electrode, NEC code says it shall be 25 ohms or less. That’s if tested. If it measures higher than 25 ohms NEC says it shall be augmented by one additional ground rod. That’s it... No further testing required. So the electrician just installs two rods. Code is satisfied

So you have no idea what the ground to soil resistance is. We must assume though there is a resistance...  5 ohms or less would be great, but very unlikely. 10 ohms? Possible. Really depends on the soil.  25 ohms or higher???

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There has to be a closed circuit for current to flow. It’s the current in the circuit that causes the circuit breaker in the panel to trip open if overloaded by a continuous overload.

If the soil is used as a ground conductor for fault current the path is from the source, through the earth soil back to the source. The source in this case is the Utility Power Transformer. 

Paths provided through the earth back to the source? Current is non discriminating. It will take any path provided. 

It will be your electrical service’s earth grounding electrode, Your neighbors, the Utility power transformer’s Neutral  to earth ground rod connection. Any provide closed circuit current path from the source back to the source. 

OHMS LAW

I = E/R

I, current, in amperes.

E, Voltage.

R, Resistance in ohms.

Voltage is 120Vac (nominal)

R? Lets use 10 ohms.  

120V / 10 ohms = 12 amps (1440 watts)

12 amps. The total continuous load of all your audio equipment added together might only be 8 amps,driving your power amp hard.

12A + 8A = 20A. The 20A circuit breaker in the electrical panel won’t trip. It should pass a 20A continuous load all day long. You don’t have your audio system playing 24/7. The 12A ground fault current will be though. 

How about the chassis of the ground faulted piece of equipment? It would be hot wouldn’t it? Along with all the other metal enclosures. (Remember all the ground pins on the power cords are connected together) 

Would you know it’s HOT? Only if you touched a piece of audio equipment and some other part of your body was in contact with a grounded item that is connect to the electrical service equipment grounding system.  Then you will be connected to 120V. Current will flow. 

When a ground fault event, like the example above, when an outdoors isolated ground rod is used for a safety equipment ground there is something else that is happening when there is a hot to chassis fault. In case I was not clear the ground rod outdoors is HOT 120V with respect to ground. A pet, or human or a child walking in their bare feet, near the ground rod could be electrocuted just walking on the earth’s soil near the 120V HOT ground rod. The risk is even higher if the soil is wet from rain of dew. 

Step potential:

https://www.youtube.com/watch?v=GpX2hgrzVAo

https://www.youtube.com/watch?v=gHQE5L6hbgs

He did not do a very good job of explaining Step Potential Voltages in the video. He should of tested two different points, for example, around 1ft to 2ft apart at least 3ft or 4ft from the 120V HOT ground rod. 

The level of  AC voltage, and current traveling through the body will kill. Just a guess a 4 legged pet would be less because it will pass through the heart, I would imagine. Small child playing???

 What Levels of Voltage are Lethal?

https://www.creativesafetysupply.com/qa/electrical-safety/what-levels-of-voltage-are-lethal?srsltid=AfmBOoqMWqGQH_caJXVoON2qf-DCsyOZXdAJ0fbreb95GH3baT0h7gIX

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Supplemental ground rods must be bonded together with at least 6 gauge copper wire.  Running a wire from a branch circuit is not ok.