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Millett UniAmp Build Thread
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Author:  Bluesman [ 08 May 2009, 20:15 ]
Post subject:  Millett UniAmp Build Thread

I am kicking this off because as of this week several people have their pcbs in hand. And even if some of us don't get our build started immediately, we can still lay the foundation by coming up with a parts list.

Maybe others are like me in that they gather parts over a period of time because of cost. These amps can run into several hundred dollars, depending on what components we buy. So a gradual spending curve can be good.

I'll post what I have been working on. Values and such that are either documented on Mr. Millett's site, or on the pcb itself. I suggest everyone put the Peter Millett Uniamp Project Page in their favorites and refer to it often.
Attachment:
Millett-UniAmp-Schematic.pdf
Attachment:
Millett-UniAmp-hookup-schematic.png

Attachment:
Millett-UniAmp-chassis-template.pdf

Peter Millett UniAmp BOM (Bill of Materials)

We can also share knowledge of suppliers that may not be commonly known to everyone. And anything else helpful. And if I make a mistake in a listing or comment, please speak up. I'm not one to mind at all if someone points out an error I make.

Resistors
R1, R3 - 47K 1/2W
R2 - 910R 1/2W
R4, R5 - 47K 1W The pcb is marked 5W Mills, but documentation says 1W.
R6 - Open
R7, R9, R12, R13, R16, R19 - 1K 1/2W
R8, R10 - 470K 1/2W
R11, R21 - 100R 1W
R14, R22 - 400R 5W The pcb is marked for 12W Mills and 400R-5W. Audiocostruzioni in Italy uses wirewound 6W 1k and 6W 680R paralleled.
R15, R20 - 33K 1W The pcb is marked 5W Mills, but documentation says 1W.
R17, R18 - 220K 1/2W
R23 - 100K 1W The pcb is marked 5W Mills, but documentation says 1W.

Two 100R 1W off the pcb are recommended if you operate power tubes in triode.

Capacitors
C1, C6, C8, C9, C12, C13 - 100uf/450V Electrolytic
C2, C3 - .22uf/630V Film
C4, C10 - 330uf/50V Electrolytic
C5, C7 - .33uf/630V Film
C11 - 47uf/450V Electrolytic

Nowhere does the documentation say 630V film caps are needed, but I'm judging the voltage by the size alloted on the board and from other similar circuits. Plus that's a very common rating for film caps.

Tube Sockets
Z1, Z2, Z3 - Octal pcb mount tube socket
Z4, Z5 - Octal chassis mount tube socket

Vacuum Tubes
VT1, VT2 - 6SN7
VT3 - 5U4
VT4, VT5 - KT88/EL34/6L6----300B and 2A3 possible with B+ and bias resistor change

Transformers
T1- Phase splitter- Sowter 3575-C recommended
T2- Power transformer 800VCT(400-0-400)@200mA, 6.3VCT@4A, 5V@3A
T3- Push-pull output-6.6K recommended by Mr. Millett for EL34s, 6L6s, and KT88s

L1- Choke 5-10 henries recommended. Resistance is probably critical, but not specified in the documentation. I'm going with the Hammond 193J.

CL110 - thermistor
V230LA10 - varistor

Various switches and connectors


All these are for one channel, so we'll need to double numbers shown for pcb mounted parts. If building a single chassis design you can use one power supply. Anyone know if we can use a single choke for both pcbs?

I have purchased all resistors but R14, R22, and R23. Got some really nice 1% PRP from Sonic Craft. They have every needed value in stock in the 1/2W and 1W.

I also got what I think are great deals on ebay on some Rifa and Nichicon Muse electrolytics in the values needed. Comparable to what general purpose caps cost at Digi-Key or Mouser. And free shipping. Can't beat that with two sticks.

Author:  Bluesman [ 09 May 2009, 07:04 ]
Post subject:  Re: Millett UniAmp Build Thread

I should point out that using a single power supply for both boards would dictate a power transformer with the current draw capabilities to perform that task. So compensate for that when compiling your parts lists.

Another thing, NFB can be applied before T1, so in that case R6 would be used. Not sure now to determine the value.

Someone correct me if this is incorrect.

Author:  Endspec [ 09 May 2009, 10:39 ]
Post subject:  Re: Millett UniAmp Build Thread

I am interested in running them from one power tranny too, Would you just split the wires and feed the two boards and still unse two rectifier tubes and two chokes?

Author:  Gio [ 09 May 2009, 10:45 ]
Post subject:  Re: Millett UniAmp Build Thread

Hi Greg, thanks for getting this all together. I think for the time being we will try keep the BOM on the first post up to date by editing it as required. That way we won''t get a bunch of versions floating around. My boards are on the way so I will be able to chime in more shortly.
Cheers

Author:  Bluesman [ 09 May 2009, 11:41 ]
Post subject:  Re: Millett UniAmp Build Thread

Gio, Great idea! And thanks for the edits you made. Much neater.

All,
I did notice that the pcb has a terminal for a CT on the 6.3V tap. Can someone say if this means for sure the power tranny has to have a 6.3V secondary with a CT, or is there some way around this? The Edcor power tranny mentioned for the possible group buy does NOT have a CT on the 6.3V. So that will have to be addressed if the CT is a MUST.

Cheers, Greg

Author:  Gio [ 10 May 2009, 11:46 ]
Post subject:  Re: Millett UniAmp Build Thread

Bluesman wrote:
I did notice that the pcb has a terminal for a CT on the 6.3V tap. Can someone say if this means for sure the power tranny has to have a 6.3V secondary with a CT, or is there some way around this? The Edcor power tranny mentioned for the possible group buy does NOT have a CT on the 6.3V. So that will have to be addressed if the CT is a MUST.

As far as I can tell, it does not look like the CT is being used to raise/bias the heater voltage. The CT is shown as going to ground. I will ask Dan who has already built a UniAmp.

I will also ask him about the ratings he used for the choke.
Cheers

Author:  Bluesman [ 10 May 2009, 17:58 ]
Post subject:  Re: Millett UniAmp Build Thread

Thanks, Gio

Mark has built the UniAmp, as well. He bought a pair of these boards, so maybe he and Dan can chime in and help us fill in the BOM wherever there's some uncertainty or something has been overlooked.

Later, Greg

Author:  Bluesman [ 13 May 2009, 19:02 ]
Post subject:  Re: Millett UniAmp Build Thread

Just touching base with my own progress update.

Received the aforementioned resistors and film caps from Sonic Craft. Used my lunch break at work the last couple of days and got them on the board. The soldering with this huge board is so easy. It's nice to have an abundance of room when trying to get the solder tip where it needs to be.

Here's a pic of what I have done. Can't wait to get the rest of my components.

Image

NOTE: When looking at the side of the board marked "TOP" at C2 position, you are actually looking at the side that will be facing down in your chassis. The capacitors, etc., will be mounted to this side and be hanging upside down. The sockets and tubes will be mounted to the side labeled "BOTTOM", that is if you want them to protrude through the top plate of your chassis.

Author:  mwhouston [ 14 May 2009, 03:13 ]
Post subject:  Re: Millett UniAmp Build Thread

Very good start. Clean as.

Author:  Bluesman [ 14 May 2009, 20:52 ]
Post subject:  Re: Millett UniAmp Build Thread

Bluesman wrote:
Mark has built the UniAmp, as well. He bought a pair of these boards, so maybe he and Dan can chime in and help us fill in the BOM wherever there's some uncertainty or something has been overlooked.

Quoting myself here to point out that the statement that Mark has built the UniAmp is not true. While he has the amp/amps, they were actually built by Dan.

Sorry about that.

Cheers, Greg

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