Novar Spud :: A $200 sweep tube Spud with PCB - diyAudio
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Old 5th September 2013, 06:32 AM   #1
tomchr is offline tomchr  United States
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Lightbulb Novar Spud :: A $200 sweep tube Spud with PCB

I have received several requests for a circuit board for a sweep tube based amplifier. Most of these center around my 6LU8 Spud, but the board for that amplifier would need significant rework to be in shape for professional fabrication. So I decided to take new look at the various options for a sweep tube Spud. The 6LU8 is a nice tube. It’s has a triode and a power pentode in one bottle. In addition to the 12-pin Compactron version, it’s available in a 9-pin Novar version labeled 6LR8. This tube is an excellent candidate for a Spud amp.

Another Novar triode-pentode combo tube is the 6KY8. The 6KY8 and 6LR8 share the same pinout. The 6GF7A is a Novar tube with a small signal triode and a power triode in one bottle. The pinout of this tube is the same as for the 6LR8 and 6KY8 – except for the screen grid pin - which, in the 6GF7A, is simply left floating. Convenient that…

After much research, experimentation, and optimization, I have come up with a design that supports all three tubes. With some creative wiring, one could use a 6LU8 as well. Note, however, that the 6GF7A runs best at a lower B+, so rolling all three tubes through is not an option without changes to the power supply. I ended up using 250 V B+ with the 6GF7A and 300 V with the 6KY8 and 6LR8.
With the 6LR8, I get about 6 W in UL mode and 4 W in triode. The 6GF7A provides about 2 W (sounds really sweet too!).

The total parts budget (including the cost of the PCB) starts at just shy of $200 (not including enclosure and shipping charges).

So without further ado, I bring to you the Novar Spud.

~Tom
Attached Images
File Type: png NovarSpud_AMP_1p0.png (32.4 KB, 1533 views)
File Type: png NovarSpud_PSU_1p0.png (30.4 KB, 1507 views)
File Type: png NovarSpud_PCB_1p0.png (79.6 KB, 1488 views)
File Type: jpg NovarSpud_1p0_ProtoTop.jpg (275.2 KB, 1503 views)
File Type: jpg NovarSpud_1p0_ProtoComponent.jpg (370.6 KB, 1455 views)
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Last edited by tomchr; 5th September 2013 at 06:37 AM.
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Old 5th September 2013, 07:18 AM   #2
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hello ,

The support of the tube 6LU8 is it the same that that of the EL 84?

If c is the case, very intéréssant.

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Old 5th September 2013, 06:19 PM   #3
tomchr is offline tomchr  United States
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The 6LU8/6LR8 is a triode-pentode Compactron. It looks like the pentode section of the EL84 has about the same max plate dissipation (12 W vs 14 W of the 6LR8) but I think that's where the similarity ends. They use different sockets as well. EL84 is a Noval tube, 6LR8 a Novar tube.

Now, you could probably take a medium mu or hugh mu triode, an EL84, and build a similar circuit.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 5th September 2013, 08:31 PM   #4
tomchr is offline tomchr  United States
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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
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 6th September 2013, 04:59 AM   #5
12B4A is offline 12B4A  United States
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3"x5" board? That would make a nice tube amp 'mirror' to the Pass ACA diyAudio kit. It might fit an ACA size chassis w/o the heatsink (Galaxy Maggiorato?)
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Old 6th September 2013, 05:43 AM   #6
tomchr is offline tomchr  United States
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3.8 x 5.8 inches, actually. Or just shy of 10x15 cm.

~Tom
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Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 6th September 2013, 03:32 PM   #7
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Quote:
3"x5" board? That would make a nice tube amp 'mirror' to the Pass ACA diyAudio kit.
It won't fit in 3 X 5. I have a board that is nearly identical to this one. It is 4 X 6. You could physically stuff all the parts on a 3 X 5 board, but it would be cramped and heat buildup would be an issue.

I found a 6LR8 with a crack in the glass running up the entire side of the tube. I did my best to blow it up. Didn't happen. That little amp still plays even with the cracked tube.
Attached Images
File Type: jpg PCboard.jpg (318.2 KB, 508 views)
File Type: jpg Cracked.jpg (169.0 KB, 372 views)
File Type: jpg 500volts_signal.jpg (351.8 KB, 333 views)
File Type: jpg 500volts_no_signal.jpg (293.7 KB, 309 views)
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Old 6th September 2013, 05:31 PM   #8
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Quote:
EL84 is a Noval tube, 6LR8 a Novar tube.
Novar sockets are rather known as magnoval (large noval) in europe.
Tubes like EL504, EL519, EY500, and many others use this kind of support.
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Old 6th September 2013, 05:53 PM   #9
holland is offline holland  United States
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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
Bigger PSU filter caps, snap in? No reason other than I have a few 470uF 400V snap in caps lying around, that I should probably use before they dry up.

It's not really a convincing argument, but worth a shot.
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Old 6th September 2013, 06:00 PM   #10
TheGimp is offline TheGimp  United States
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Tom,

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.
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