I am building a tube amp with a separate power supply in a second enclosure - connected to the amp via an umbilical power cord.

What is the most elegant way to keep the power supply off if someone inadvertently turned on the power supply without the umbilical cord being plugged into the amp?

I have heard of using a relay to do this by running an additional ground wire in the umbilical cord, but haven't seen a schematic or any details of how this would actually be done.

This is a hi-fi amp with a Edcor xpwr054-120 TRANSFORMER, so I will use it for the heater power @24 VDC and the main power @330 VDC (power supply takes the 240 VDC and ramps it to needed 330).

I guess I don't have a way to run an additional low voltage signal via a relay with this setup, is that true? Any suggestions?

  • 1
    \$\begingroup\$ Is this a Hi-Fi amp, Guitar amp or ? Is it a one-off project? What are your input voltages? \$\endgroup\$
    – Oli Glaser
    Sep 23, 2012 at 14:08
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    \$\begingroup\$ I would be interested in hearing exactly why you want to put the PS in a separate enclosure? What advantage could outweigh the complication, safety issues, extra expense, etc? Not saying you don't have a good reason, just that it isn't obvious to me. \$\endgroup\$ Sep 25, 2012 at 3:16

3 Answers 3


There are even safer ways, but the method that upu suggested would work well enough [tm] in most cases.

The lead contains 3 wires. Let's call them live, ground, and go.

Live conveys high voltage.

Ground conveys ground.

How are we doing so far?

Go is connected to ground at the amplifier end.

A say 12V relay which is located in the power supply has 12V connected to one side of its coil and the other side is connected to the power supplky "go" terminal.

When the amplifier is connected there is a circuit from +12 via the relay coil via psu go terminal, via power lead to amplifier go terminal. Here the go terminal is grounded so the circuit returns via the ground lead and the relay operates. The operating relay operates the power supply.

The terminally curious can activate the psu output without a cable by grounding the psu "go" terminal.

If the psu output connector is a finger and standard probe safe socket such one would need to be very clever or exceptionally stupid and unlucky to get a shock if it was powered up (which is how it should be) then this extra layer of protection would make the system very safe indeed.

  • \$\begingroup\$ Note that this approach can be applied to multiple connectors if desired - running what @Russel calls the "go" pin in a daisy chain fashion through all connectors such that all need to be in place to complete the circuit and energize the relay. This approach is commonly used in high voltage distribution connectors/PDUs. In these applications the NC contacts on the relay are used to short the HV link through a discharge resistor to more quickly render the pins safe to touch. \$\endgroup\$
    – HikeOnPast
    Sep 22, 2012 at 3:22
  • \$\begingroup\$ Thank you so much. Would you suggest a solid state relay or electro-mechanical? I can use a +5vdc off off the power supply as the coil trigger. \$\endgroup\$
    – Ray O.
    Sep 22, 2012 at 14:06
  • \$\begingroup\$ I was thinking of using this one: jameco.com/webapp/wcs/stores/servlet/… \$\endgroup\$
    – Ray O.
    Sep 22, 2012 at 15:48
  • \$\begingroup\$ That relay appears to be AC output. jameco.com/Jameco/Products/ProdDS/162369.pdf - You want DC output capable, Voltage higher than max used, current ditto, activate vo0ltage = available. \$\endgroup\$
    – Russell McMahon
    Sep 22, 2012 at 16:12

Russell has already given you a good direct answer to your question, so here are some other things to consider:

  1. If safety is your concern, and since you are designing this all from scratch anyway, put the power supply in the same chassis as the amp itself. That completely avoids the issue, and is how it was traditionally done back when these things were traditionally done.

  2. Let the power supply stay on. If safety is the issue, use a connector arrangement that makes it difficult to get at the high voltage pins when disconnected. This usually means the high voltage comes out of a female connector with the conductive parts recessed so you can't touch them with your fingers if you wanted to. This is what ordinarly wall power outlets do too, for the same reason.

  3. If you are worried about the power supply itself getting damaged under no load, fix that. This is easy to do. Tube amp power supplies were usually just a transformer secondary, a full wave bridge, and a capacitor. Make sure the components are rated for the highest possible no load voltage and all will be fine. This is easy to do.

  4. Tube amps need multiple supplies. At the least you will need something like 6 V or 12 V for the fillaments, and around 200 V for the anodes. Depending on which tubes you end up using for the input stages and the power stages, you may want two different anode supplies. For example, the 12AU7-A (common signal amp tube) has typical operating voltage shown as 100-250 V, with 330 V maximum. The 6L6 (common for power output stage) has the typical operating voltage shown from 250-350 V. There is some overlap when you look at the specs carefully, but two supplies might still be what you want in the end. Sometimes the lower preamp supply was made by extra filtering of the power amp supply. That was OK since the preamp current draw was quite small.


The most elegant way is to design a power supply that doesn't have a problem with being energized while the load is open.


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