[nate911]

I've had my Minimax built for more than a year, and it's been working great for the most part. A few recent problems that I'd like to look into though.
 
1. One of my tube lights does not come on very often; sometimes it comes on only after the amp has been on for a while. Very intermittent-I suppose I should order another LED? I've already tried reflowing the solder on that LED.
 
2. When I have my 12AE6 tubes in, the left channel has a hum, quieted by cupping anything over that tube. This is not present with the 12FK6 tubes. I'm guessing one of my 12AE6 tubes is bad?
 
3. When I turn the amp off, the last split second the output has power, there is a harsh static right before it turns off. I'm not sure what exactly what would be causing this.
 
Thanks!

 

[tomb]

Quote:

I've had my Minimax built for more than a year, and it's been working great for the most part. A few recent problems that I'd like to look into though.
 
1. One of my tube lights does not come on very often; sometimes it comes on only after the amp has been on for a while. Very intermittent-I suppose I should order another LED? I've already tried reflowing the solder on that LED.
 
You probably need another LED.  Even if it is making contact with the PCB pads and solder, it could be that it was over-exposed to heat from the original soldering and has developed an intermittent connection inside the LED body.  I see this a lot and it sounds like what may have happened.  If the amp was built according to the BOM and specs, the current is set at 1/2 of what the LED is rated, so I doubt seriously it is burning out or wearing out.
 
2. When I have my 12AE6 tubes in, the left channel has a hum, quieted by cupping anything over that tube. This is not present with the 12FK6 tubes. I'm guessing one of my 12AE6 tubes is bad?
 
Yep - this is pretty much the symptom of a bad tube ... or bad connections on the tube pins.  You might try polishing the tube pins, or bending them all outward ever so slightly so that they make much tighter contact with the tube socket.  If none of that has an effect, then yes - the tube is bad.
 
BTW, if it's a Beezar tube that's gone bad, I'll replace it.
 
3. When I turn the amp off, the last split second the output has power, there is a harsh static right before it turns off. I'm not sure what exactly what would be causing this.
 
This is usually a tube issue and may very well be related to #2.  Keep in mind that the relay-delay is protecting your headphones on power-up, the worst scenario for potential damage.  However, there is nothing special that ramps down the capacitor charge left in the amp upon power-down (except for the onboard resistors).  I would always caution that you have the volume turned down before starting any power-down or power-up, simply as a rudimentary standard of prudent operation.
 
As for the specific cause, it goes back to whether the tube has sufficient connection to ground and is able to dissipate its voltage through the resistors on the PCB when powered-down.
 
Thanks!

 

 

[nate911]

Thanks for the reply.
 
1. Yea, that's what I thought. I'll need to order another one then.
 
2. The pins look fine and its making good contact. It is a Beezar tube; I'll PM you with the details.
 
3. The static is still present even if the volume is turned all the way down on the amp. At quick glance, everything looks fine on the PCB; I'm not sure what else I could do or look for.

 

[tomb]

Quote:

Thanks for the reply.
 
1. Yea, that's what I thought. I'll need to order another one then.
 
2. The pins look fine and its making good contact. It is a Beezar tube; I'll PM you with the details.
 
3. The static is still present even if the volume is turned all the way down on the amp. At quick glance, everything looks fine on the PCB; I'm not sure what else I could do or look for.

It sounds like a bad tube - I'll send you another as we noted in PM's.

 

 

[nate911]

Anybody else have ideas about the static when turning the amp off?

 

[jdkJake]

As TomB stated, more than likely artifacts caused by the power supply caps discharging. Since you are powering things down at that point, why does it matter?

If TomB thinks it can be minimized by changing out the tube, IMHO, you would be wise to heed his advice and change the tube and go from there. He has more than a little experience with the amp.

Tubes are funny creatures. Full of potential mechanical and physical imperfections and limitations. The fact they can even come close to meeting the established performance consistently across thousand of issues is quite impressive and is a testament to product engineering.

 

[nate911]

Quote:

As TomB stated, more than likely artifacts caused by the power supply caps discharging. Since you are powering things down at that point, why does it matter?
If TomB thinks it can be minimized by changing out the tube, IMHO, you would be wise to heed his advice and change the tube and go from there. He has more than a little experience with the amp.

Tubes are funny creatures. Full of potential mechanical and physical imperfections and limitations. The fact they can even come close to meeting the established performance consistently across thousand of issues is quite impressive and is a testament to product engineering.

The amp had not always done that; that's why I was concerned. It seems to not have any adverse effects, so I suppose it's fine.
He's in the process of taking care of that for me

 
Very interesting devices indeed...
Thanks!