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Old 6th September 2013, 05:30 PM   #11
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Blaireau View Post
Novar sockets are rather known as magnoval (large noval) in europe.
Actually, the pins of Magnoval tubes are bigger in diameter. You can use a Novar socket for a Magnoval tube, but the pins of the socket will stretch and no longer be able to make good contact with the Novar tubes after that.

Quote:
Originally Posted by holland View Post
Bigger PSU filter caps, snap in?
Yep. The PSU caps are snap-in types. The footprint supports the use of caps up to 25 mm in diameter with 10 mm pin spacing. The caps I use in my build are 220 uF, 400 V, 22 mm diameter.

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Originally Posted by TheGimp View Post
If the power resistors were mounted on the same side as the tubes, they could take advantage of close proximity to the metal plate/housing the PWB was mounted to in order to dissipate heat better. This might help with thermal management.
That's an interesting idea. My approach is to include some ventilation holes under the power resistors. I've seen this done in much of the HP/Agilent test equipment as well. The holes allow the resistors to breathe pretty well. The only resistors that dissipate any significant power are the resistors biasing the output stage. They dissipate about 1.4 W with the 6LR8 tubes. They're rated for 5 W...

I've measured the temperature of the various parts after a couple of hours of operation. I was mostly concerned about the caps, but they had barely gotten lukewarm. The hottest components were the 5 W resistors (no surprises there). If I recall correctly, they were running about 80~90 deg C (they're rated for 250 C!).

The board measures 3.8 x 5.8 inches (just shy of 10 x 15 cm). So 0.2 inches (5 mm) smaller on each side compared to Tubelab's board. It is possible to physically fit the parts on a smaller board, but I wanted a reasonable spacing between tubes and wanted the electrolytic caps to be a safe distance away from the hot resistors.

Note: If you open the dimensional drawing of the board in a new window and zoom in, it becomes easier to read the dimensions. Sorry for the small print...

~Tom
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Last edited by tomchr; 6th September 2013 at 05:38 PM.
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Old 6th September 2013, 06:48 PM   #12
Kyle K is offline Kyle K  United States
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The magnoval sockets I have (Russian I believe) will not grip the pins of a novar tube well, even when brand new, although sellers often say they will work. Just inverting the socket and tube will cause the tube to fall out. Real novar sockets are hard to find.
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Old 6th September 2013, 06:58 PM   #13
tomchr is offline tomchr  United States
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The Novar tubes have 0.038" diameter pins. The Magnoval uses 0.046" diameter pins. So I'm not surprised that the Magnoval sockets won't make good contact with a Novar tube (regardless of what the eBay seller says).

I bought ceramic Novar sockets from a reputable supplier - ESRC in Florida. They seem to make good contact with the tube pins. You can get the tubes there as well.

~Tom
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Old 7th September 2013, 04:55 AM   #14
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I must say that I totally ignored the difference betwen pin's diameter of both supports. I simply thought the name was different, depending of which side of the Atlantic ocean you live on.
Thanks for the info.I'll remember it.
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Old 7th September 2013, 06:07 PM   #15
tomchr is offline tomchr  United States
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No worries. I'm glad we got things straightened out.

~Tom
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Old 8th September 2013, 06:29 PM   #16
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Quote:
Originally Posted by tomchr View Post
If any of you have comments or requests for tweaks, now would be a really good time to speak up. I plan to send the board out for fabrication early next week.

~Tom
Hi Tom,

I noticed that for this new version you added some partial Schade local feedback via R3 (150Kohm), this is slightly too low based on an existing schematics nearly identical to yours from Kegger. He is using 270Kohm after many hours of listening test. Here is a link to the thread and his latest schematic.

The "$250" Amp? - 6LU8 Compactron SE, UL - AudioKarma.org Home Audio Stereo Discussion Forums

p.s. I have completed a few days ago the Kegger 6LR8 amp and it sounds great.

BR,
Eric
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File Type: gif 6LU8 latest fine tuned by kegger.GIF (15.6 KB, 617 views)

Last edited by e_fortier; 8th September 2013 at 06:31 PM.
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Old 8th September 2013, 10:16 PM   #17
tomchr is offline tomchr  United States
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I tried 100k, 150k, 220k, and 330k. I found I get the best performance at 150 kΩ, but I'm open to taking another look.

~Tom
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Old 8th September 2013, 10:28 PM   #18
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Quote:
I noticed that for this new version you added some partial Schade local feedback via R3 (150Kohm),
Quote:
I tried 100k, 150k, 220k, and 330k. I found I get the best performance at 150 kΩ,
Tom copied mine ...post #2

Spud: what, again!?!

And I copied Kegger's

Spud anyone?

There are experts on this forum who have told us dozens of times that this type of feedback can not be used with a triode driver. There is theory to back this statement up, but it does work quite well here. I tried several resistors and I also settled on 150K. The value seems to depend on your speakers and music choices more than anything else.
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Old 8th September 2013, 11:30 PM   #19
tomchr is offline tomchr  United States
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In addition to the Schade feedback, I tried feedback to the cathode of the output stage. That's what I used on my 6LU8 Spud, but I actually find that the amp works better with the Schade feedback.

I really like the sound with the triode coupled 6LR8 tubes. That's a rockin' li'l amp!

And yes, I allowed myself to be inspired by Tubelab, Kegger, and a few other designs on the Internetz. Thanks.

~Tom
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Last edited by tomchr; 8th September 2013 at 11:32 PM.
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Old 9th September 2013, 01:54 AM   #20
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Quote:
Originally Posted by tomchr View Post
In addition to the Schade feedback, I tried feedback to the cathode of the output stage. That's what I used on my 6LU8 Spud, but I actually find that the amp works better with the Schade feedback.

I really like the sound with the triode coupled 6LR8 tubes. That's a rockin' li'l amp!

And yes, I allowed myself to be inspired by Tubelab, Kegger, and a few other designs on the Internetz. Thanks.

~Tom
Thanks for letting me know about the 150K vs 270Kohm, I might give it a try. The 6LR8 does make nice music .

p.s. To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.

I also allow myself to be inspired by other.

Rgds,
Eric

Last edited by e_fortier; 9th September 2013 at 01:57 AM.
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