for a few weeks I've been trying to remove the input transformer from my little 60's Magnatone amp. The schematic indicates a 1:3 step up input transformer, but from reading elsewhere on the internet it seems that it is in fact a 1:1. Whenever the amp is run at the same time as other certain devices in my home e.g. laptop charger, washing machine, it picks up some severe hum (particularly with the washing machine). It sounds a bit like an arc welder. This makes the amp practically unplayable when these appliances are running. I'm certain this hum is being picked up by the input transformer.

Here is the schematic for the amp. The transformer in question is T2. http://www.magnatoneamps.com/schematics/magnatone_401_412.pdf

I have tried bypassing the transformer, connecting a wire from the 10k resistor on the low gain input to the volume pot (and disconnecting from the transformer, which results in greatly increased hum (louder than the guitar) and reduced guitar volume. Connecting a jumper around the 220k resistor and .047 capacitor, to the volume pot ground lug results in a short which shuts off the house's circuit breaker. I cannot see why this occurs as there is no AC or DC voltage between these.

Interestingly, I discovered the heater of the 20EZ7 is wired in reverse compared to the schematic (pin 2 connects to V2 heater, while pin 1 is used as the common return for nearby components e.g. V1 cathode, volume pot ground, 220k resistor and .047 capacitor. I'm not sure if this is relevant to the problem.

Any help here would be much appreciated. I'm completely stumped and out of ideas here. If you need any further information or clarification, just ask. I can also provide photos of the wiring if necessary I would like to think I'm not completely inept in the workings of tube amps, as I'm currently building a 5F6a Bassman.

Thankyou in advance

EDIT: My amp is run through an isolated 240-110v step down transformer, so the transformer is not needed for isolation. Also, I've added a 3 prong power cord and grounded the chassis

Further information: I replaced the 50uf filter caps a few weeks ago. Another thing, the hum reduces and cuts off when I turn off the volume pot, so the source of the hum must be before the volume pot

  • 2
    \$\begingroup\$ Are you absolutely certain that the 240V - 120V transformer is isolated? Most of these voltage-conversion transformers are made with an auto-transformer. You mention that you tripped the house circuit breaker when you shorted the RFI suppression network formed by the 220k & 0.047uF capacitor. This strongly suggests that the incoming AC power is NOT isolated! \$\endgroup\$ Commented Apr 1, 2017 at 5:12
  • 1
    \$\begingroup\$ A common mode 3 pin AC line filter would help . Is Neutral hot? A CM choke on Amp input too. 50uF caps worn out? \$\endgroup\$ Commented Apr 1, 2017 at 5:12
  • \$\begingroup\$ Neutral is neutral. Just replaced the filter caps. I'm really looking for a way to remove this before spending any money \$\endgroup\$ Commented Apr 1, 2017 at 5:22
  • 1
    \$\begingroup\$ Does hum stop with input pulled out? This will determine injection source of noise \$\endgroup\$ Commented Apr 1, 2017 at 5:30
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    \$\begingroup\$ DO NOT remove the input transformer you could kill yourself or others. The horrors I have seen where guitarist/bass players have modified their amp. to remove hum is appalling. When I tested them for safety they often fail and I would then remove the plug and add a large "Do Not Use" label to it. Can you provide details of your "240 to 110 Volt" transformer. \$\endgroup\$
    – user135867
    Commented Apr 1, 2017 at 7:17

4 Answers 4


The input transformer is required for electrical isolation. Do NOT remove it!

This circuit is similar to the old tube-type AM radios from years gone by. The circuit operates directly from the incoming AC power. If you look closely, you will see that one side of the incoming AC power cord is tied directly to the main circuit ground.

Although the AC plug is marked with (-) & (+), there is NO guarantee that you won't wind up with the incoming hot (Line) conductor connected to the circuit ground. This is a potentially lethal shock hazard should anyone touch both circuit ground and an Earth ground.

The input isolation transformer simply couples the desired input signal to the volume control while providing galvanic isolation.

Note: the metal shell on your 1/4" Phone Plugs on the guitar cord as well as the metal faceplate on your guitar are all exposed and can be touched. Do NOT remove or bypass that input transformer!

  • \$\begingroup\$ I live in Australia so the amp is run through a 240-110v isolated step down transformer. Additionally, I've added a 3 prong plug and grounded the chassis \$\endgroup\$ Commented Apr 1, 2017 at 4:58
  • 3
    \$\begingroup\$ Irrelevant, it must be made safe for use as manufactured without requiring external isolation. You might remember now but nobody else will know and one day in the future even you may forget. Wire a permanent isolating transformer inside the case or restore it to original. \$\endgroup\$
    – Finbarr
    Commented Apr 1, 2017 at 7:34

This amp derives its ground from the Neutral of the AC line. Just like the old TV sets in the 40's and into early 60's. With the isolation transformer you have no effective ground. And an electrocution hazard as well.

  • \$\begingroup\$ To clarify: the internal circuit ground is connected to the AC line as shown in the schematics. The audio input ground is isolated from this "ground" and connected to "chassis". The scary bit is that there is no sign of a mains earth connection to the chassis. \$\endgroup\$
    – Transistor
    Commented Oct 23, 2017 at 21:42

What you might need is better shielding, the wires from your guitar pick up more noise than the transformer would. Even the pickup would provide more noise since it is basically an inductor with an open core.

Although it is quite likely that it is the capacitor in the power supply, if it is very old it may have degraded and reduced in capacitance resulting in much higher ripple passing through from your mains line. If you replace it it is ok to use a much larger one. If you are able to check capacitance you may find others that should be replaced.

  • \$\begingroup\$ Oh and if you have an RCD in your fuse box that is likely what was tripping, I can't think of anything else that would trip from such a small fault. And heaters are usually symmetrical, and even when they are not wiring them backwards usually has no effect. \$\endgroup\$
    – TWiz
    Commented Apr 1, 2017 at 11:36

You wrote: "the hum reduces and cuts off when I turn off the volume pot, so the source of the hum must be before the volume pot".

I would check several things.

1) Start the washing machine and physically move the amplifier around. Move it to different locations in the room and to other rooms. If you get to a location where the noise is loudest, rotate the amplifier in all possible orientations. Does the noise change as the amplifier is moved and/or rotated?

Do the same thing in a location where the noise is minimal. Does moving or rotating the amplifier cause the noise to change?

Report back to us if it does; don't bother with items (2) and (3) below.

2) Is your voltage-conversion transformer an isolating-style or not? Connect one lead of a multi-meter set to AC Volts to an external Earth ground. Check the voltage between Earth ground and circuit ground. There is a good chance that circuit ground is about 120 Vac above Earth ground.

If this is the case, exchange wires as needed to ensure that circuit ground is as close to Earth ground as possible. That is: you want circuit ground to be connected to incoming Neutral IF the power transformer is an auto-transformer.

Start up the washing machine and listen for noise.

3) The noise that you are picking up when the washing machine runs sounds suspiciously like Variable Frequency Drive noise. Try disconnecting the secondary of the input transformer from the top side of the volume control and just let the input of the pot float. Listen for noise.

If there isn't any noise present even if the volume control is wide open, try the following test next. Note: this can be very dangerous. This is for testing purposes ONLY!

Tape up one end of your guitar cord so that it is completely insulated. You won't be able to plug it into your guitar but that's okay.

Now, carefully use alligator clips to connect your guitar cord to the circuit. The shield of the Phone plug goes to circuit ground, the tip of the phone plug goes to the top end of the volume control. Start the washing machine and listen for noise.

Note that if the circuit ground is connected directly to the Hot (Line) terminal of your AC supply, the circuit ground is LIVE and so is the guitar cord. That's why you are being extremely careful and also why you have taped up the free end of the guitar cord.

Get back to us with the results of those tests and let us know what you find.

  • \$\begingroup\$ Ok so I temporarily removed the input transformer from the circuit. This required isolating the input jacks from ground, as for some reason the common return was shorting with the grounded chassis and setting off the RCD in my home. Not sure what caused that, as I measured just a fraction of a volt between them (measured between pin 1 of 20EZ7 and the chassis). I also removed the 220k resistor and soldered the .047 capacitor to chassis ground to remove RF interference. This greatly reduced the hum, I believe due to lifting the jacks from ground and not due to the isolation transformer \$\endgroup\$ Commented Apr 5, 2017 at 2:39
  • \$\begingroup\$ The hum is only present when the volume is in between off and full. As soon as the pot turns all the way off or on, only a slight, low frequency hum is present, which I do not find irritating. In relation to the step down transformer, on investigation I'm now convinced it's just an autotransformer. I'm not sure how to rig an external earth ground, however I measured the AC volts between the transformer's output and a wall outlet in my home. It indicated 0V between the grounds and neutrals of the transformer and the outlet, and 110 between the transformer active and the wall outlet ground \$\endgroup\$ Commented Apr 5, 2017 at 2:50

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