• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Buzz in new amp...

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I have just built a new little 6BM8 push pull amp to this schematic http://www.geocities.com/asl_wave8/schem/wave8stock.pdf

Slight modification to the original in that I wired it for ultralinear rather than pentode, used valve rectification rather than SS diodes and changed the concertina cathode resistor from 39K to 33K to match the load resistor (this was done after imperical measurement of the outputs of the phase splitter). This amp was a small project to build my skill with point to point wiring, specifically grounding techniques before moving on to more difficult projects. The side benefit was to be able to have a nice new valve amp in my study to power some little Fostex FE 127e speakers from my Mac computer.

After building the amp, I tested it out in my living room with my main speakers and pre-amp. Dead quiet, no hum or buzz. I have just moved it to my study and hooked up to my Macbook Pro computer. Trouble is I am getting an irritating hash/buzz sound through the speakers. The hash increases when moving the bluetooth mouse and has a random "digital" sound. When I disconnect the monitor and all USB connections, the sound goes away. This made me believe that it was a ground loop problem through the peripheral devices and the power supply of my amp. To test this I disconnected the link from the amp ground bus to the amp chassis. The amp is now dead quiet.

My question: is it OK to leave the connection between earth bus and chassis broken? I still have a connection between the power cable earth and the chassis for safety, it is just that there is no link from amp power and signal ground to earth.

Sorry for the long winded post... Any input appreciated.



A few shots of the amp for those interested:


Been there.

I suppose you could leave the circuit common lifted from chassis ground, but I used the standard back-to-back diodes, with resistor and capacitor in parallel. This network ties the circuit common to chassis. This "lift" was enough to keep the nasties out.

There's a bunch of threads on this subject. Maybe search for back to back or yin yan.
Thanks Sal. Still not as neat as some posters on this forum, but better than my first point to point effort.

As for the power supply, the computer is a Macbook Pro laptop computer. It uses an external "brick" power supply that I assume is probably switchmode. I was pretty happy with myself that it was hum free on my main system, then also pretty pleased that I was able to diagnose and (temporarily) fix the buzz problem in just a few minutes.

I have not implemented the recommended fix just yet - busy couple of days...

The fix listed above uses back to back diodes, a resistor and a 100nF cap to shunt unwanted RF to ground. As I have placed a 100nF cap from the isolated input sockets ground to chassis, and also 100nF caps from the tube socket heater pins to chassis, do I need the 100nF in the ground lift/isolation fix? I am happy to put it in there, but the whole purpose of this build was to learn about such issues and how to rectify them.

Thanks again guys,

Try it and see... The cap provides an RF ground, the resistor a small-signal DC ground, and the diodes provide the safety ground path. I use a 35A bridge rectifier for the diodes... one screw to mount, and you can use all four diodes if you want (cross wire the two AC terminal and two +/- terminals together)... should serve as a solid safety ground path if it's ever needed.

Safety standards for newly manufactured equipment require that any exposed metal be grounded, and that the ground can carry twice the supply rating or 25A for one minute with less than 0.1 Ohm resistance (2.5V drop). This SHOULD comply.
Thanks Tom, I will place it there as I have plenty on hand. Was just wanting to learn on the way and your post has helped with that.

Rob, yes it is there! in the photo, it is running vertically to the left of the components on the tag strips and just to the right of the negative feedback and signal input coax. From the centre of that vertical run is a horizontal that runs back to the power supply caps (making a little curve out of the way of the input valve decoupling cap).

Maybe on the next one I can get the wiring as neat as yours ;)

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