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
Best Answer
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!