Tube Guitar amp Input Transformer Removal

Question

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

Answer 1

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!

Answer 2

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.

Answer 3

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.

Answer 4

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.