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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Any potential problems when using a combo potentiometer / power switch on a K-12 kit?

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Maxwell Smart

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#1
I'm in the process of building the S5 K-12 tube amplifier kit, and I see a lot of people with separate power switches in their builds but none with a combo power switch / potentiometer. I was wondering if there's any special potentiometers that would have to be used, and if I'm overlooking anything in terms of signal attenuation or other potential issues. I ask because all of the tube radios in my collection use this type of switching mechanism and it seems like a very clean way to do the power switch. Thanks!
 
#2
The first problem that comes to mind is that you will have mains AC voltage and your small signal circuitry right next to each other, inducing hum. Also do not know if the switch you are wanting to use is rated for mains voltage and the current you are drawing.

A solution may be to have a relay switch the mains, but then you would have to have a low voltage power source that is always on - so probably no good.

So, the simple solution that most use is to keep small signal far away from AC line = separate power and volume.

Cheers,

Chris
 
#3
If you feel antsy about the switch in the pot handling the current, you could always use an appropriately rated relay with a 115VAC coil to switch the real power.With the proper case to resonate, I always thought that the in-pot switches made a neat and characteristic noise when actuated.
 
#4
yep - I've used them on a number of builds. No issues. And the ones I use are all recovered from old consoles. The issue Chris raises is mitigated by bringing hte power wiring in at 90 degrees to th3e signal wiring. In my case that means the power wiring running along the side of hte chassis to the back of the sw/pot unit, while the signal wiring runs across the front of the chassis and to the side of the sw/pot.
 
#6
Maxwell Smart said:
... I see a lot of people with separate power switches in their builds but none with a combo power switch / potentiometer. I was wondering if there's any special potentiometers that would have to be used,...
Click to expand...

Those combo switchs will work OK in an amp the has line level inputs. Those amps don't have much gain and don't really pick up hum from the AC wires being close to signal wires

But I would not wire a high-gain amp that way. High gain is (1) One that has a phono cartridge input. (2) microphone preamps (3) guitar amps. In those cases we want many inches of separation and even then use shielded wire.

For radios the combined switch/pot works well.
 
#9
DF96 - Disadvantage is also that the pot gets more use so the track wears out more quickly. I would use a separate switch.

CORRECT

BinaryMike - Pots combined with switches tend to be lesser-quality components, which often means poor tracking between left and right channel gain.

CORRECT
 
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