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Old 11th December 2010, 11:41 PM   #1
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Default 1625 pp amp layout - comments invited

Hi guys - ok, the build is underway.

You are looking top down from the back of the proposed layout. Chassis is simple folded 1.6mm aluminium. Ends will be in solid rimu (podocarp timber). Width is to end up with a finished 43cm wide, 34cm deep (it goes on a 35cm deep shelf). The red outline is the perimeter of the top.

The chassis is 6.5cm high.

12SL7s on the left - they will have RCA inputs directly behind them in line with the speaker posts you can see. Sockets will mount directly to the chassis top plate - they drop in and are locked by a circlip.

The 1625s are on a 8cm centre grid - close, but not tight. Sockets will mount directly to the chassis top plate from underneath, on small stand-offs.

The iron is (screen top to bottom, rhs), 5.5h choke, 16.5h choke, PTX. OPTs sit behind the 1625s. All direct top mounting again except the PTX obviously drops through. An IEC power input socket with filter will sit behind the PTX

Underneath will be 2 x ccs circuits for the SL7's in ltp configuration; 4 x mosfet sf circuits between the ltp's and the 1625's.

Note all terminations are on the top of the chassis rather than the back - no depth to work with!

Cooling is via a mesh screen at hte back and radial drillings in the top plate around the tubes.

Straight power amp - no volume controls required.

Rip in guys...
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File Type: jpg amp layout.JPG (73.8 KB, 375 views)
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Old 12th December 2010, 03:27 AM   #2
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Hello,
Looks nice.
Perhaps you are braver than me. I would try the layout with clip leads to verify that there will be no induced hum or cross talk among and between the transformers and tubes with your proposed spacing.
1625ís are nice tubes top cap and all.
DT
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Old 12th December 2010, 04:25 AM   #3
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Thanks DT - yeah, bit of a soft spot for top caps - last amp was EL36 (6CM5) with 6J7 front end...

I have the option of putting the smaller of the chokes as a through-chassis fitting. The layout is based around keeping the power iron at one end of the chassis, the input tubes at the other end.

Cheers
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Old 16th January 2011, 11:03 PM   #4
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Default Tranny orientation.

It looks like your power tranny is in the same orientation as the outputs.
I think the windings are effectively concentric.

Rotate the power tranny 90 degrees.

I would never use a drop through tranny as they tend to induce current into the chassis and if you use more than one chassis earth connection, then hum will be induced.
I prefer to join the mains to the chassis at the same point as the input connector and that is the only electrical connection to the chassis.

The other way is to join power earth to chassis and float the audio apart from one connection from the insulated input connector earth to chassis (preferably to the same point as the mains earth is connected next to the input connector).

Need to be a bit careful how the earth cabling is run, if it goes close to power transformer or chokes, it picks up current from induction.

You may be able to get a couple of covers and stand it up.
If you do, turn it round so it's 90 degrees to the output trannys.

Look at the ST70 layout, tou will see the way the windings go, the power is 90 rotated to the outputs.
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Old 16th January 2011, 11:20 PM   #5
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SSSHHHH
let's not let everyone know what a great audio tube the 1625 is. It is still a bargain.
I recently finished my single-ended 1625 with one 6sn7 and it competes with the KT88
single-ended my friend built at the same time except he used a 6sl7. Upon hearing mine with the 6sn7 he converted his over to 6sn7.
So my recomendation is drop the 12sl7 and go to a 12sn7or 6sn7, it allows a much greater current flow

John
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Old 17th January 2011, 12:02 AM   #6
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thanks guys - marcus yeah, the earth scheme is subject to a LOT of thought at the momment - I stuffed up the last one I did and I'm determined to get it (mostly) right first time this time. Will definitely be floating the audio. Gotcha on the trannie orientation - thanks for that.

Fattie - yay for topcaps!!!! I'm using the 12sl7s because I have them and will be buffering them with a mosfet follower - they won't have to handle big current into the 1625 grids that way, and I keep the tube count down. Miles may cringe a little (or a lot) but I like the look and feel of minimal. Performance issues are dealt with under the top-plate and with sand.

Cheers!
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Old 17th January 2011, 12:58 AM   #7
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Quote:
Originally Posted by marcuswilson View Post
It looks like your power tranny is in the same orientation as the outputs.
I think the windings are effectively concentric.
Could you elaborate on that? I don't see it
I would say the windings of the OPT are vertical (i.e. on the picture) and the PT windings are horizontal?
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Old 17th January 2011, 01:12 AM   #8
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yep and that may fix MOST of any cross-induction, but the best solution is, as marcus says, to reorient the ptx 90 degrees. THat way, the magnetic field from windings on the ptx are even less likely to cross the opt cores
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Old 17th January 2011, 01:48 AM   #9
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Quote:
Originally Posted by aardvarkash10 View Post
yep and that may fix MOST of any cross-induction, but the best solution is, as marcus says, to reorient the ptx 90 degrees. THat way, the magnetic field from windings on the ptx are even less likely to cross the opt cores
Well, if I may hijack this thread for this one question, I started on my amp and was about to drill the holes for the opt, now I'm unsure if I did it right, see pic:

Click the image to open in full size.

After reading these posts I was going to turn the opt's 90 degrees.

(After all that's how it was in the donor amp..)
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Old 17th January 2011, 01:56 AM   #10
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nope - I think you are fine. THe ptx magnetic field is mainly propogated perpendicular to the long side of the ptx (so running from the bottom of your image to the top) so your opts are not in the main field. Just be careful with the wiring to your smoothing caps (brehind the ptx) as there will be high current pulses in htat wiring which MAY induce noise in the opts.

It a black art really... :-)
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