Scott 299b outputting sound to speakers with vol fully down

malvo

Active Member
So I have a nicely restored 299b that sounds great and classes up my living room. The 299b doesn't have a headphone jack, but this hasn't been a problem around here as I've never needed or wanted to listen on headphones here at home. Until recently.
Not long ago my schedule changed and suddenly I find myself with some prime listening time in the evening after everyone else has gone to bed, but that time of day around here requires headphones. No built-in headphone option for this amp seemed a trivial fix: tape out from the Scott into my stand-alone headphone amp works and sounds great.
But here's the problem: even with the volume dialed all the way down on the Scott, there's still perfectly audible music on the speakers. The Scott's volume pot otherwise seems to function perfectly, nothing scratchy, no dropouts or off noises. The audible music on the speakers isn't loud, mind you, but it's also not anything approaching silent. Domestic peace in my home demands silent.
Does this problem of audible music even with the volume all the way down sound like something that can get solved reasonably easily? Or should I leave it be and work out another way to get some headphone time at home?
I don't claim any expertise at all with vintage tube gear (other than I like the way it sounds), and so I'm grateful for any and all input.
 
It should be no sound at full attenuation of the volume control. The fact that there is some sound indicates the vc is not working properly. You can try cleaning it but you may have to change it out. While that is not a very hard operation, sourcing a new vc is. There are some superior replacements but the fit will probably be off and require customization. You can measure the original pot to verify if it is defective.
 
Malvo -- Here's a test to try: temporarily (either by soldering temporary leads or by using clip leads) connect the center terminal of each section of the volume control, to the chassis. The chassis connection can be made anywhere, but the other end must go to the volume control center terminal. This will disable the volume control, making it permanently in the full down position. Now play the amplifier. Is the minimum volume sound level still there as it was before the temporary leads were connected? If so -- and many times it still will be -- then the problem is not the control.

If the sound does in fact go fully to zero with the temporary leads installed, then check the connections to the control to make sure they are all good, and try cleaning it as well. Short of those measures resolving the issue, the control will likely then need to be replaced.

More often however, installing the temporary lead will not change the minimum volume level at all, indicating that the problem is not the control. This is not an uncommon problem with some Scott units. The problem actually traces to a combination of:

1. Use of a "hot" signal from a digital source, that can often reach peak levels of 2 volts or more (sources of the day typically only produced an output of .50 volts), and

2. Lack of sufficient shielding for the wiring connecting the center terminal of the volume control to pin #2 of the input tube of the power amp section (6BL8), relative to the hot signals presented .

If this is the case, the remedy is to first ensure that all wiring connected to pin #2 of the 6BL8 tube is of the shielded type, but most importantly, wire wrapping the 220K resistor connected between the volume control center terminal and pin #2 of the 6BL8 tube with an insulated wire that has one end grounded. Using small gauge wire will make the task easy enough. The lead should wrap the resistor close enough as to no longer being able to see the resistor. One end of the wire wrap lead is grounded at a convenient point, the other is terminated closely at the resistor body, connecting to nothing. The resistor itself should be located as close to pin #2 of the tube socket as possible.

The presence of a 220K resistor between the volume control and the input grid of the 6BL8 allows the resistor to effectively become an antenna within the chassis, allowing it to pick up any stray signals being radiated within it. Since the resistor appears after the volume control, you still hear sound even with the volume control set to a minimum setting. By wrapping this resistor in both channels, the antenna effect is removed, allowing a minimum volume control setting to then actually produce zero volume when set to a full down position.

I hope this helps!

Dave
 
Dave and all, thanks for the responses. I will run this test of the volume control later in the weekend and report back.
 
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