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zoi 27th June 2012 06:40 PM

Hum in homebuilt amp
Hi all,
First-off, I apologize if this is an inappropriate question. I am new to building amps (working on my first build).

I set out to build this amp and have just finished wiring it. It works but there is a ton of 60Hz hum. The hum increases when i turn up either of the two potentiometers, but if the preamp one is all the way down and the other is all the way up there is no hum. Does this mean the bias voltage is clean?

Also, I have tried adding another parallel capacitor to the first in the filter and it doesn't change the hum. This is another indication to me that the filtering is not the problem.

I used good wire and was very careful to check all of the solder joints and connections.

I don't have an oscilloscope, so I am wondering if there is anything I can do to try to diagnose the problem.

Any help would be much appreciated, as I don't even know where to begin.

Thanks a ton

Zibi 27th June 2012 07:07 PM

Excessive Hum

Frank Berry 27th June 2012 07:18 PM

One end of the output transformer secondary should be grounded. It's not. Without the ground connection, there is no negative feedback.
As for the hum, you'll need to check the grounding within the amplifier. It may be a ground loop.

zoi 27th June 2012 08:02 PM

Thanks so much for your replies.

To eliminate the possibility of heater noise, I disconnected the AC supply and powered the heaters with a 6V battery pack. The hum didn't change.

I grounded one side of the output transformer secondary (tried both sides) and that didn't change it either.

The wiring matches the diagram, and all ground points are at the same potential. Could there be another ground missing somewhere?

DF96 27th June 2012 08:05 PM

C3 is shorting the feedback above about 100Hz so should be removed, unless you want a very bass-light sound.

Are the grounds wired exactly as shown in the diagram?

How are the heaters wired?

What is the purpose of VR2? It is within a feedback network (or it would be if C3 is removed) so will have little effect.

Is this some sort of guitar amp?

zoi 27th June 2012 08:32 PM

I grounded one side of the output secondary (tried each side) and the hum didn't change.

The heaters are in parallel across the 6v supply from the power supply transformer. I eliminated the possibly of heater noise by disconnecting the AC heater feed and replacing it with a battery pack. No change.

The amp is designed to be a guitar amp. I assumed VR2 would be the volume control, with VR1 left in an optimal position or replaced with resistors, but maybe that should be the other way around (using VR1 as volume)?

The bass response is good as is, so I am hesitant to remove C3 but if there is a chance it would eliminate the noise I will do so.

The grounds are wired exactly as shown. None of the wire is shielded but all the runs are short and heavy gauge.

DF96 27th June 2012 09:13 PM

I assume you used twisted wiring for the heaters, spaced well away from the signal circuitry?

Signal wiring should be very short or shielded, including wires to the volume pots. By 'short' I mean about an inch or couple of cm. Ground wires are not shielded, as this would be meaningless!

Removing C3 will allow the feedback to work, if you also ground the correct side of the OPT secondary. This is the side which does not have the 33K feedback resistor. Alternatively, just remove the 33K as at present it does nothing. Up to you.

Bone 27th June 2012 09:23 PM

Short the input jack. If hum disappears then the hum is pickup into the first stage.
A shorting jack should be fitted.. Does it hum if an instrument is plugged in?

zoi 27th June 2012 10:01 PM

I used twisted wires for the heaters, but I don't think it is heater noise, since if i disconnect the heaters while the amp is running the hum continues (for 10 seconds or so until the tubes cool and there is no output)

Shorting the input jack doesn't change the hum.

Also, the schematic doesn't reflect this but I replaced the diodes with a single 6x4 tube as the power supply. Is anything additional needed to properly make that substitution? As of now, the circuit is the same aside from the diodes replaced.

DF96 27th June 2012 10:55 PM

How is the transformer secondary CT connected to ground and the negative end of C9 and then the other smoothing caps? The details here are important, as it is easy to inject charging pulses into the ground if you get it wrong.

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