[gsteinb88]

So, ive built two Millets, and both have the same problem. I will go into detail about one of them, as they are different.
Configuration:
-Alps Blue Velvet (i havent figured out how to use the ground pad...)
-HA3-5002-5Z Buffers
-Neutrik PCB mounted 1/4" jack
-Neutrik RCA jacks connected via 26ga SPC wire, ground terminals jumpered and then connected to pad
-Unregulated 24v Wallwart (I have a TREAD on the other one, nothing on this one)
-12AE6A or 12FK6 tubes
So in detail, i get a very loud buzzing when i turn the amp on, but that goes away after a few seconds, so im not concerned about that one. However, whenver there is a cable connected to the neutrik RCA jacks, i get this humming in the background like it is picking up EMI or something to the effect. When i have no cables connected, i get basically a black background with a more than tolerable amount of noise. When i try to listen to music the humming is completely intolerable, and ruins the music. I can hear the music behind it though.
Any help would be appreciated, thank you.
-g
EDIT: I should add that while i get EMI hum with wires on only the millet, when it gets plugged into a source, the hum gets much louder, and becomes a ground loop hum, i think. I have a hammond metal enclosure with plastic endplates. Any idea where the ground loop could be coming from?

 

[MisterX]

Quote:

-Neutrik RCA jacks connected via 26ga SPC wire, ground terminals jumpered and then connected to pad

What pad?

If you are referring to the ground in pad that would explain your problem (because it is not connected to anything

)

 

[tomb]

Couple of possibilities maybe:
1. You have the latest run of boards where the ground pad on the input/output positions was not really tied into the board's ground plane.
2. Your Alps pot probably isn't grounded either if it's tied into one of these pads. In any event, you run a wire from one of the Alps phillips screws to a good ground pad on the board.

I was going to agree with the ground loop problem until you said plastic end plates. If your power plug is not isolated from the case, that can cause it. However, chances are nil that's happening with plastic end plates.

If you think you have the newest boards and are unaware of the mistakes, check DIYforums.org for NeilR's corrections. There's a sticky with a board photo and lines drawn showing how to correct it.

If it's none of those, you might post a photo. Maybe someone will spot something.

 

[gsteinb88]

GAH! I have the newest boards and had no idea about the problems. Thanks a million.
-g
EDIT: Thanks about the alps pot, but i would bet that the main problem is the input ground pad...
EDIT2: Amazing. The fk6's just sound amazing.

EDIT3: Only problem with ungrounded alps is that it buzzes when i touch the shaft. I will ground it in the morning, for now its time for me to enjoy music

 

[n_maher]

Way to go man, just kick back and enjoy the music!

 

[Polaris111688]

While we're on the topic of Millet help, what would explain a Millet's bias reading as 0.5V for one channel and 2V for the other channel? I'm dealing with this on a friend's Millet that I helped him build. I do not have pictures at the moment, but the setup is using a STEPS set to 24V, and my implementation of the PPAv2's diamond buffers.

 

[n_maher]

Quote:

Originally Posted by Polaris111688 While we're on the topic of Millet help, what would explain a Millet's bias reading as 0.5V for one channel and 2V for the other channel? I'm dealing with this on a friend's Millet that I helped him build. I do not have pictures at the moment, but the setup is using a STEPS set to 24V, and my implementation of the PPAv2's diamond buffers.

I can't think of anything, short of the trimpots either being bad or not soldering in properly, but I wouldn't think that this would cause the bias to be low. I'd start trouble shooting it like any other amp, review all the solder joints, look for shorts, cold joints, etc. Then look for parts oriented incorrectly and while you're at it I'd have your friend take the DB's out of the circuit and try just using regular buffers so that you're not owrrying about those being a potential problem. And pictures would certainly help.

Nate

 

[gsteinb88]

Sounds like bad trimpots or bad buffers to me. Try testing the V at the buffer in (tubes in, buffers out). I know this is the bottom left pin on the HA3-5002's, but im not sure for the other type. I had a problem with bad trimpots in my first one, and i ended up frying more than a couple buffers. Have him touch the dDB's and see if they are hot.
-g

 

[NeilR]

If you still can't bias it correctly with the buffers removed, I would look very carefully at your CCS. A low bias voltage is indicative of too much current flowing. There are only two parts involved in the bias point (other than the tubes themselves): the CCS and the trim pot. Also make sure your volume POT is soldered in correctly, or temporarily jumper the grid pins to ground with some alligator clips.

Measure the voltage drop across the 1K resistor (R5) between the CCS and the supply voltage. That should work out to the current draw of your chosen CCS. It should be about 0.56V if you are using a .56ma default CCS.

 

[steinchen]

you have to have buffers installed when biasing the tubes since the bias testpoints are tied to the buffers output

 

[NeilR]

Quote:

Originally Posted by steinchen you have to have buffers installed when biasing the tubes since the bias testpoints are tied to the buffers output

I guess it should be assumed that without buffers, the bias is measured directly from the input of the buffer (or the tube's plate pin). I should have said that.

I know why it is usually measured at the buffer output, but I have never gotten a measurably different value using 3 different meters. In any event I would think almost any meter would give a reasonable if not 100% accurate value when read directly from the plate... accurate enough to determine that the tube is biasing correctly.

 

[Polaris111688]

I'm going to see if I screwed up soldering my DB board. The Millet and STEPS are fine as far as I can see. The ALPS pot is soldered directly onto the amp board, so orientation cannot be wrong. Next time I meet up with my friend (hopefully soon, as he has everything built thus far), I'm going to install a pair of spare BUF634Ps first and see if that fixes the problem, as well as check the CCS and trimpots.

 

[gsteinb88]

Make sure you install the jumper for the buf634's if you have the latest revision (hate to see someone else go through what i did...)
-g

 

[NeilR]

Quote:

Originally Posted by gsteinb88 Make sure you install the jumper for the buf634's if you have the latest revision (hate to see someone else go through what i did...) -g

The jumpers are for the OPA551, not the BUF634.

 

[gsteinb88]

Gah, yea, typo. I was thinking of my buffers, which do not require jumpering.