[[AK]Zip]

Since I am sure there are a quite a few people with MH amps who had someone build it for them, but come to a point of when they want to try different tubes. I did this writeup for a member on here who thought it would be a good idea to just go ahead an post it publicly so here it goes.

Your MH should look pretty much just like mine except you might not have the diamond buffer board and probably different caps. The location should be pretty much the same for everything. You need to adjust biasing in the Millett Hybrid every time you chance your tubes.

You need:

flat head screwdriver
multimeter
Millett Hybrid board
Tubes

Here is a picture of the board with the buffers out so that you can see better since I have DB and they take up a lof of room. Do not remove the buffers like I did. I highlighted the area's you will be using in blue. Also, read all the way through this once and make sure you understand everything before starting.

First turn off the amp and remove the current tubes.

Then locate Bias Adj Left and Bias Adj Right.

Take the screwdriver and turn the pot counter clock wise and that will set it to max resistance meaning that not much voltage goes to the tubes. Install the other tubes and turn the amp on.

Give the amp a few second to warm up. While its doing that take your multimeter and set it to V or DCV and make sure your multimeter probes are in the voltage socket (This is very important). If your multimeter only has V then it will automatically do all the settings. If you have a cheaper/older multimeter then set it to DCV (direct current voltage). Set it to 20 which will be 20volts. (see picture below for a visual).

Your multimeter will have a red wire (+) and a black wire (-/ground).

Locate L Bias and R Bias (TP1 and TP2).

Depending on how your amp was built there is either something soldered there or just holes.

If its just holes then insert the red point from the multimeter into the non broken up hole mark (look below for a marked picture). The black point into the other.

Do 1 side at a time.

While you have your points in the holes take your screw driver and start to turn the pot clockwise.

Default voltage is 12v. I have my tubes set to 16v which is what I found to be optimal for me. You can play around with this and see what works best for you. Just make sure its between 12v and 17v. Also, make sure left and right are set as close to one another as possible.

Once you set them as close together as you can just leave the amp on for about 24 hours and test it again. The voltage should drop because as heat increases voltage decreases. Just readjust both side and check back again in about 12 to 24 hour and adjust if needed. After that they will fluctuate depending on room temp, but it will not be big enough to need to keep readjusting.

If I left anything out or if you don't understand something please feel free to PM me again and I will add it or explain it in a different way.

Good luck and enjoy. Just make sure don't short the test points with the multimeter test tips.

-Alex-

 

[Samgotit]

Thanks!

This will be very handy when I finish the diamond buffers.

 

[SHLim]

Thanks!

 

[BradJudy]

This might be a dumb question since my MH is my first piece of tube hardware...

Can biasing be done with headphones connected, or should they be disconnected? If they can be connected, can they be playing, or should biasing be done with the source off?

 

[[AK]Zip]

Quote:

Originally Posted by BradJudy This might be a dumb question since my MH is my first piece of tube hardware... Can biasing be done with headphones connected, or should they be disconnected? If they can be connected, can they be playing, or should biasing be done with the source off?

I would keep the headphones disconnected. The source can be connected though. Playing music doesn't make a difference.

-Alex-

 

[jbloudg20]

Quote:

Originally Posted by [AK]Zip Default voltage is 12v. I have my tubes set to 16v which is what I found to be optimal for me. You can play around with this and see what works best for you. Just make sure its between 12v and 17v. Also, make sure left and right are set as close to one another as possible.

Any particular reason you chose 12-17 volts? Right in the orignal article, Pete has waveforms for 12.5 volts and 19.5 volts, with 19.5 volts being less overall THD, but with more 3rd harmonic distortion.

EDIT: Snip pics from my reply

 

[Comfy]

Thanks

 

[[AK]Zip]

Quote:

Originally Posted by jbloudg20 Any particular reason you chose 12-17 volts? Right in the orignal article, Pete has waveforms for 12.5 volts and 19.5 volts, with 19.5 volts being less overall THD, but with more 3rd harmonic distortion. EDIT: Snip pics from my reply

12v is what was set for the MH as the default. Take a look here: http://www.diyforums.org/build.html

Also, I choose 16v for my own because its what I found to be the nicest sounding and talked to others who confirmed it (16-17v).

-Alex-

 

[NeilR]

I have a suggestion for a minor change to your instructions.... I built my Millet and measured bias as you show in your picture. I shorted the two bias points with a probe slip and blew a 5002 buffer. With two probes within 1/8" of each other, it is not as hard as you think, especially when your attention is on the meter display and the pot screw.

There are small ground vias around the perimeter of the board. A better way is to put the negative probe in one of the perimeter vias, and the positive probe in the bias pad. It is much harder to slip and short something if only one probe is in this area.

 

[[AK]Zip]

Quote:

Originally Posted by NeilR I have a suggestion for a minor change to your instructions.... I built my Millet and measured bias as you show in your picture. I shorted the two bias points with a probe slip and blew a 5002 buffer. With two probes within 1/8" of each other, it is not as hard as you think, especially when your attention is on the meter display and the pot screw. There are small ground vias around the perimeter of the board. A better way is to put the negative probe in one of the perimeter vias, and the positive probe in the bias pad. It is much harder to slip and short something if only one probe is in this area.

Thats a great suggestion. You see the gray and black wires coming out of the hole up top? those are my test wires that I still haven't gotten connectors for. I only took the picture the way I did because this was originally written for a member on here and he suggested I make it public.

-Alex-

 

[NeilR]

You may also want to stress, since you are directing this at newbies, that the probes must be interted in the meter's Voltage socket. If they are inserted in the current metering socket, that will short the buffers and destroy them.

 

[[AK]Zip]

Quote:

Originally Posted by NeilR You may also want to stress, since you are directing this at newbies, that the probes must be interted in the meter's Voltage socket. If they are inserted in the current metering socket, that will short the buffers and destroy them.

That's a good one. I edited my original post.

Thanks.

 

[NeilR]

Quote:

Originally Posted by [AK]Zip Thats a great suggestion. You see the gray and black wires coming out of the hole up top? those are my test wires that I still haven't gotten connectors for. I only took the picture the way I did because this was originally written for a member on here and he suggested I make it public. -Alex-

I saw those wires and wondered if they were for bias testing. I socketed mine too. I think your efforts here are great because a lot of non-DIYers will end up with these amps and everybody who owns a tube amp should know how to bias it.

 

[Samgotit]

This will be the first thing I do when I get my "like new" Fluke 75 I got off ebay for $26.00.