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Oddwatt Push Pull Tube Amplifier Projects
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Author:  bronx [ 22 Dec 2017, 11:17 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce, Eric, thanks for the recommendations on the power supply.

I'm looking at chassis options for my build. I found some nice enclosures at diyaudio.com, and I made a quick mockup with Sketchup to see how the components would lay out. (see pics)

The chassis is the 2U Galaxy, and is about 13" wide and 11" deep, 3" tall: https://diyaudiostore.com/collections/g ... 2174804228

I was thinking the "quasi-heatsinks" on the sides of the chassis could be used for the LM317's
Power tubes (6L6s) are about 3.1" center-to-center, and the power tube center to edge of the transformers is about 3.1" as well.

Though it looks nice, I'm thinking it may be too small. I'd appreciate any feedback.

Attachment:
Oddwatt_small_chassis_front_v1.png


Attachment:
Oddwatt_small_chassis_v1.png

Author:  gofar99 [ 22 Dec 2017, 21:39 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, I personally would go for a bigger chassis. This is particularly the case when you try to stuff all the parts that go inside. Crowding causes two major problems IMO. First is heat build up. This is the killer of components. Second is that is hard to diagnose and trouble shoot any problems that arise. A third one is also frequently encountered, hum and noise because things are too close together. You chassis could fit the build, but I would much prefer about 2inches more in both length and width.

Good listening
Bruce

Author:  eshazen [ 23 Dec 2017, 08:23 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

I used a Hammond 17 x 12 x 3 and it worked out pretty well.

Image

Author:  gofar99 [ 23 Dec 2017, 15:42 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, Nice looking build. That is about the size I used on the Stereo 50 a while back.

Good listening
Bruce
Attachment:
Stereo 50 Feb 07 2013B.jpg

Author:  bronx [ 26 Jan 2018, 17:38 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

A couple BOM questions:
1. The 470pf capacitor for the feedback circuit. What type of cap should should I use here? Ceramic, Mica, or Film? Also, what voltage rating should I use?
2. On the amp schematic, the 220uf capacitor, would a Nichicon audio electrolytic be a good choice? I was thinking the UKA series (High Grade Audio, High Temp)
3. On the amp schematic, what proximity should the 10UF capacitor be to output transformer? Does the length of wire between the cap and the output transformer matter for performance?

Author:  gofar99 [ 27 Jan 2018, 11:18 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, I use mica or poly caps for the 470pf. Anything over 25 volts is fine. The 220uf should be 50 volts (25 is OK). Since it is in the signal path a good quality capacitor should be used. I use Silmac II caps, but many others will be fine...just not the 15 cent types. Ideally the 10uf capacitor ought be close to the output transformer as it acts as a low impedance path for any signal swings that may occur. Up to about 6 inches should be fine if the transformer is either the one specified in the project or one that has at least 18 gauge wires. BTW, the ground end should go to good ground as well.

Good listening
Bruce

Author:  victorzwk [ 30 Jan 2018, 06:16 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Quick question. On a stereo build with a shared power supply, would it be interesting to add a fuse to protect each channel separatelly? For example a 0.5A fast blow on the 450V input of each OPT? Thanks!

Author:  Soundbrigade [ 30 Jan 2018, 08:31 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

YEPP!
In my first serious tubeamp build (6550-Se monoblock), I used a fuse between PSU (B+) and OP tranny in each amp as I was strongly recommended to do so, so I guess that EACH amp half should have its transformer and OP tubes protected separately.

Author:  gofar99 [ 30 Jan 2018, 14:26 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi I agree, in a stereo build to fuse each set of outputs. If one tube fails then there is less chance of other things going with it. Another design feature in the amps is the use of 2 parallel resistors in the B+. This is not because it is cheaper....it is because if you have a slow failure one will always blow first the then the other. It is one place where 20% tolerance parts are fine and actually needed. You could of course use a 110 and a 91 in parallel and have the same effect. Also the size of the resistors is important. Normally this type of failure (slow) is not encountered, but it can. A slow failure may not blow the fuses. In a stereo build you will need to halve the resistance values. I would probably have the B+ string divided into two and the resistors and the cap that follows them for each channel. My personal stereo builds actually have two complete power filter strings...one for each channel. It will reduce cross talk...but is actually overkill for anyone who is not as picky as I am.

Good listening
Bruce

Author:  victorzwk [ 31 Jan 2018, 07:57 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Great, will order the fuses and fuse holders!

Meanwhile I will be finishing the cabinet for my Stereo build + Forewatt.

Testing the fitting of the cabinet parts.
Attachment:
Cabinet.jpg


Finishing the parts. Glossy finish for the top plate and Matte for the rest.
Attachment:
Cabinet Parts.jpg


Finished home made output transformers.
Attachment:
OPTs.jpg

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