My millett doesn't amplify

[~AZ~]

I got finished putting my millett together and everything sounds good except that with the volume all the way up it drives my headphoines to barely acceptable levels.

I checked the R4 positions and didn't have 22.1k but jumpered them to make sure and still have the problem. I am running it off a tread and the bias is set to about 15. I have an alps pot and nuetrik jack soldered directly to the board.

any ideas?

 

[Uncle Erik]

Are you sure you have the pot hooked up correctly?

I don't know which board you're using, but on most amps, the input (off the RCA jacks) goes directly to the pot, and then the output from the pot goes to the board.

Yours may be different, but it sure sounds like 100% of the input is bypassing the pot and going into the amp. Also, make sure you didn't hook up the leads backwards on the pot. That happens, too.

Let us know how it turns out-

 

[NeilR]

If you have a later board, did you ground the ground pads per this post?

 

[~AZ~]

I checked the soldering on the pot and can find no problems and I do have the latest revision of the board and have grounded the pads.

 

[mb3k]

Try some simple tests... swap/change the tubes, check values of R4, let the tubes heat up for 10 mins first before listening, check the volume level of the source.

EDIT: I see you've checked R4.

 

[Clutz]

Here is my guess - I don't hav ea lot of time so I'm sorry if this has already been suggested.

Are you sure the input RCA jacks are wired correctly? The outer wire is ground, the inner wires are signal (e.g. connect the inner part to either L or R pad, and the outer part of the wire to ground). If you did this the opposite way, you wouldn't get any signal amplification (I don't think) because you'd be amplifying the ground (return) instead of the signal (incoming).

That's my thought. I'm not sure how wiring the pot up incorrectly would result in no volume gain. I would imagine that would result in either no sound, or a tonne of volume gain - but not just an unamplified signal.

 

[NeilR]

R4 should be 22 ohms, not 22K. You said you jumpered it, but you might want to double check other resistor values.

 

[~AZ~]

I only have one set of tubes, right now I am testing with my Zen player with the volume turnes all the way up. I checked the values of the other resistors and can't find anything wrong...I wired the rca input from this diagram http://www.diyforums.org/images/mh_jan.png

I am stumped.

 

[nikongod]

is your source outputting a suitable output voltage? ie 1/2 "scale" or more.

 

[dhp]

what are your R6 values? Can you give us a top down picture?

also, what are you using for your buffer values?

 

[~AZ~]

I'm not sure what 1/2 "scale" means but I have tried hooking it up to my zen and my 0404 soundcard and still have the same problem. In the R6 spots I have Vishay Dale 221 Ohm 1/4W and am using the 634P buffer. I am trying to see if I can get my friend to come over with his digital camera to post a pic.

 

[thedoctor]

here's what i usually do :
- check the input/output ground from the RCA and to the output jacks. make sure those 2 are connected to GND.
- bypass the vol control altogether by shorting the input to the tube's grid.
- check the CCS and make sure it's regulating 0.5mA of current.
- try and use different bias voltage and see if it helps.
- check the buffer section and make sure those are working. swapping them for another chip is a good way to test.

and a digital multimeter sure helps. tell me you have one.

 

[dhp]

how much voltage is going into those tubes? have you tried biasing them?

 

[~AZ~]

I double checked all my soldering and fixed anything that even looked funny. I swapped the buffers and still have the same problem except that when I check the test points for the bias I can not adjust it to lower than about 19v. The amp itself sounds great other than the volume and I took compared the volume coming out of the amp to the volume of the headphone hooked directly into my mp3 player and it is actually softer out of the amp.

Sorry this is my first project so I am a little thick on how to check the voltages at different points...can anyone explain how to check the voltage going into the tubes or anywhere else I should check?

thanks again for all the help.

 

[NeilR]

If you can't get the bias below 19V then you aren't getting enough current flowing through the tubes. Measure the voltage across R5- it should be about 0.56V if you used the .56ma CCS. Make sure R5 is a 1K resistor.

(put your meter on DC volts and put the probes on both leads of R5)

If you get the same results in both channels (can't bias below 19V, low output) then it is probably not a bad part, it is something you did consistently to both channels.

You can also remove the buffers and check bias. This will elimiante the buffer from the picture. Without the buffers, you need to check bias at the tube plate instead of the test point. The tube plate is pin 7. Red probe to pin 7, black probe to ground.