[rreynol]

Quote:

Originally Posted by JWFokker The trim pots don't seem to make any difference when I adjust them. Maybe I'm not measuring the bias correctly? What should I have the multimeter set to when I measure it, and should the amp be on? I previously measured the bias when it was off and got reasonable numbers, but the trim pots don't change anything. When I tried measuring it when it was on, I didn't even get a measurement.

Multimeter should be on DC volts and I'd imagine that the amp should be on (why would there be current flowing with it off? if it is, you may have another problem). Do you have the tubes plugged in? When I first put mine together, I had a 'duh' moment and couldnt figure out why the trimpots had no effect on the tp voltages. Then I realized that I was dumb and plugged the tubes in. Also, I noticed that you really don't see the voltage changing much until the trim pot gets to a lower half the of the range. Hope that helps. If not, someone else will probably have some suggestions. I, too, am a tube noob.

 

[rreynol]

Quote:

Originally Posted by pabbi1 Please bear with me... Per Drew, C7 is the output capacitor and it will see a DC voltage equal to the "bias" voltage setting. C2 will see the difference between the bias voltage and the power supply voltage. If my incoming is 26v, and I have a "low" gain - say 10v. and C2 is the difference between incoming and "bias" for C7 (in my case, 50v caps), then I have 16v hitting C2. Is this exactly what I DO NOT want to happen. Having a 50v C7, wouldn't I rather have a higher gain (16-20) hitting C7, to then dial it down for a safe range for C2? What am I missing - other than I ought to have 25-35v caps for C2? I can absorb some burned tubes - I have 6 on hand.

I see your point. You are probably right but I'd also suggest looking at the data sheet for your trimpots and find out how many turns it is and use that to figure out what you should probably have them at when you first plug in the tubes (1/2 the number of turns, etc). Or you could just measure the resistance of the trimpots and set them for 2.5 KOhms, etc. That's probably the best suggestion. If you plan on using botique caps for C2, it might be hard finding 25V-35V caps that fit the board. Saying 10V or 16-20V isnt really the gain. Gain technically has no units. What you're speaking of is voltages being dropped across the tube, across C2/C7, etc.


Edit: Oh and if I'm totally wrong, I apologize.

 

[dviswa]

Quote:

Originally Posted by JWFokker The trim pots don't seem to make any difference when I adjust them. Maybe I'm not measuring the bias correctly? What should I have the multimeter set to when I measure it, and should the amp be on? I previously measured the bias when it was off and got reasonable numbers, but the trim pots don't change anything. When I tried measuring it when it was on, I didn't even get a measurement.

There are 2 oblong shaped pads, one labeled as bias. I have been measuring the bias voltage across these 2 pads. Surely you are not measuring when the amp is switched on are you. BTW, the pots keep turning even after the slider has reached the end. Turn the pot back a few turns and you should see some change.

 

[mrdon]

I am soooo close to completion but....upon initially testing the unit with a PCDP conected I am getting music from the left headphone speaker but not the right. What I hear in the right side upon power up is a sound similar to a TV starting up, sort of a hum that builds, and then cuts out.

1. Is it a matter of adjusting the bias?
2. Is my 24VDC, 400mA wallwart powerful enough to power the 12fk6 tubes?
3. Do my tubes need to be matched?
4. Something else?

 

[drewd]

Quote:

Originally Posted by mrdon I am soooo close to completion but....upon initially testing the unit with a PCDP conected I am getting music from the left headphone speaker but not the right. What I hear in the right side upon power up is a sound similar to a TV starting up, sort of a hum that builds, and then cuts out. 1. Is it a matter of adjusting the bias?

Yep!

Quote:

2. Is my 24VDC, 400mA wallwart powerful enough to power the 12fk6 tubes?

Yes, no problem there.

Quote:

3. Do my tubes need to be matched?

In a perfect world, yes, that would be nice, but I don't think that you'll get any takers in the tube vendors who will want to match a couple of $3.00 tubes, so don't worry about that.

Quote:

4. Something else?

I think that you covered all the bases!

-Drew

 

[mrdon]

Thanks Drew for the advice. I'll try it this afternoon. BTW, please ignore my post over at DIYForums.org.

Quote:

Originally Posted by drewd Yep! Yes, no problem there. In a perfect world, yes, that would be nice, but I don't think that you'll get any takers in the tube vendors who will want to match a couple of $3.00 tubes, so don't worry about that. I think that you covered all the bases! -Drew

 

[drewd]

Quote:

Originally Posted by drewd In a perfect world, yes, that would be nice, but I don't think that you'll get any takers in the tube vendors who will want to match a couple of $3.00 tubes, so don't worry about that.

I should qualify that to say that you should, at the very least, use tubes from the same manufacturer. I've got 40 or 50 12FK6 tubes here from Radio Electric Supply and I see that amongst the different brands, those of the same brand are extremely close to each other in performance. They were nice enough to write the results of the tester on the ends of the boxes.

Mixing and matching brands is probably not a good idea, from a sonic point of view.

-Drew

 

[Buzzerbro]

Here is my hybrid amp. It has Nichicon Muse 470uF caps on the outputs and the rest are Elna. The tubes are 12FM6. The pot is an Alps 50K. This is my maxed out version.

Bias is set to 14V. It sounds very good and the bass is nice with my AKG K240S headphones.

[size=xx-small]If the photo is not visible, its because I have exceeded GeoCities allocated Data Transfer. Sorry[/size]

 

[MASantos]

Quote:

Originally Posted by USER NAME: I'm not sure where we are supposed to post our pictures so I copied this from to construction thread. I was just fooling around in the shop trying to think of a good chassis solution for my Millet Hybrid. It's not done yet. It's just a fully dressed chassis with no board inside. I tried to add some carbon fiber to the mix, but it just looked awkward. I'm never satisfied. Maybe I should start looking in to aluminum? Anybody have any constructive criticism?

Hello,

I would replace those arched metal things( are those kitchen door knobs?) on the sides of the tubes with wooden ones.

otherwise the amp looks amazing. I will also build mine in a wood enclosure, bot only in september....

Manuel

 

[mrdon]

Still have no music coming from right channel in headphones. Tried to bias via DREWD's advice but I realized I've never biased an amp before!

Anyway, how do we noobies perform this task? Do we need voltmeter or will a multimeter do the job? Step by step instructions with pics would be very helpful!

Quote:

Originally Posted by rreynol Multimeter should be on DC volts and I'd imagine that the amp should be on (why would there be current flowing with it off? if it is, you may have another problem). Do you have the tubes plugged in? When I first put mine together, I had a 'duh' moment and couldnt figure out why the trimpots had no effect on the tp voltages. Then I realized that I was dumb and plugged the tubes in. Also, I noticed that you really don't see the voltage changing much until the trim pot gets to a lower half the of the range. Hope that helps. If not, someone else will probably have some suggestions. I, too, am a tube noob.

 

[rreynol]

Quote:

Originally Posted by mrdon Still have no music coming from right channel in headphones. Tried to bias via DREWD's advice but I realized I've never biased an amp before! Anyway, how do we noobies perform this task? Do we need voltmeter or will a multimeter do the job? Step by step instructions with pics would be very helpful!

You will need to measure dc volts across the left and right bias test points. Both a multimeter and a plain old voltmeter will work.

 

[mrdon]

Quote:

Originally Posted by rreynol You will need to measure dc volts across the left and right bias test points. Both a multimeter and a plain old voltmeter will work.

What setting do I set my multimeter on? Could you walk us through the process RREYNOL?

 

[BlazerFRS]

You'll need to set it to DC volts.

 

[drewd]

Quote:

Originally Posted by mrdon Anyway, how do we noobies perform this task? Do we need voltmeter or will a multimeter do the job? Step by step instructions with pics would be very helpful!

I don't have my camera handy, but here is a description on what to do:

To set the bias voltage, pick the channel that you want to adjust. Locate the two test points on the PCB. Set your voltmeter to DC volts and the scale to something over 30V. Turn the amplifier on and put one probe on each test point. If you have not adjusted the voltage yet, you'll probably find that it reads quite high. Using a flat bladed screwdriver, start turning the screw on the pot that is associated with the test points you are measuring. If I recall correctly, you'll want to turn clockwise to lower the voltage. It should take between four and six turns before the voltage starts dropping, then it will drop fairly quickly. If the voltage does not drop, turn the other direction.

With a 24V power supply, the initial bias setting should be 12V. Match the two channels to within 0.1V and then listen!

-Drew

 

[shut85]

i've gotten all the parts on the board except for the components like the power and inputs and outputs, i assume that s1 and s2 are for the switch but i only see a v_in pad and was wasnt sure how to connect the power exactly. wouldnt there be a v+ and a v- from the power to connect to the board, so where would the two go? also is there a certain way i have to orient the led on the pads next to d2la, like is there a + side and a - side?