Fender Red Knob Twin increases hum connecting preamp out and/or poweramp in

[markla]

My Fender red knob twin increases hum excessively, when I connect anything to the preamp out or poweramp in plugs, e.g. using the preamp to drive a second amp or connecting an effect 19'. Actually, the second device does not even have to be switched on. It is sufficient to be pushed into the wall plug. Removing the wall plug of the second device removes the additional hum.

Using an isolating transformer in between the two devices reduces hum also significantly.

I wonder if anything is wrong with my twin or if this sort of grounding problem is normal, i.e., you have to use a transformer separation between the devices in general by design.

As I am a bit out of ideas, there are only a few things I tried already:
- Looking at the respective PCB and testing the three plugs I could not find anything obviously wrong.
- Removing ground connection from the board with the three plugs increased noise in general (as expected) but hum did not reduce.
- Checking if all tubes are properly seated in their sockets, doing the chop stick test.

Sorry, that is not too much. I would be really thankful if anyone could give me a hint, where to search further.

Thank you very much!
Markus


Schematics of "The Twin"
The_Twin_schematic.pdf

[52 Bill]

Welcome to the place.

Actually, the second device does not even have to be switched on. It is sufficient to be pushed into the wall plug. Removing the wall plug of the second device removes the additional hum.

I don't think that there is anything wrong with your amp. This is typical of a ground loop caused by the signal grounds and the ac grounds being connected together. Older U.S. model Twins had a ground switch that could be set to an off or "lift" position. Some FX units will also have a switch that will separate the grounds.

Sometimes connecting the two units using an audio cable with the shield unconnected at one end will help fix this as well. In any case these loops can sometimes be very difficult to deal with.

[markla]

Thank you very much for your hints. I am glad to hear that nothing seems to be broken with my amp.

I did some further experimenting and ended up with the following solution, which reduces additional hum using preamp out/poweramp in to a (for me) tolerable amount:
Between the power supply board GRND and the grounding connection to the chassis I put some kind of auto ground lift (cf. schematics attached). This works fine for me even without any additional galvanic separation of the devices.

Thanks again and best regards,
Markus

auto_ground_lift.pdf

[olddawg]

Have you simply tried using a $2 ground lift plug on the rack device? You can reverse the mains and connect or disconnect the ground with one. Also, you might want to buffer/isolate it with a $25 Art preamp set at unity.

[markla]

Sorry for the delayed reply.

As I am located in Germany, it is quite difficult to get ground lift plugs. Actually I am not sure if these plugs are available at all. But that is a solution I will definitely have to check.


Thanks for your hints!

[Steve Conner]

Ground lift plugs are an American thing. I don't think they are even legal in Europe.

The "auto ground lift" circuit is an acceptable solution. Quite a few musical instrument and hi-fi amps have it from the factory. You must be careful to only lift the ground rail of the audio circuitry, the metal chassis should stay connected directly to ground.