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Old 5th August 2012, 02:50 AM   #1
roline is offline roline  United States
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Default This SPUD's 4 U

I started to play with a tube headphone project for a little set of Merit universal output transformers and ended up with a PCB idea for a Quad SPUD in PP using 6LU8 tubes. I've used 6JN6 to 6LB6 tubes in other amps, this is my first SPUD amp.
I only have one SPUD, so for spacing trials I used a set of 6JN6 sweep tubes. The idea is to place the electrolytic caps on the back side of the PCB.
With the 6CG3 damper diode tube, it is just too crowded in the center of the PCB.
I like to use solid state diodes in the power supply but use a damper diode as a delay pass device for the B+. I don't have room for the 12 piner, are there any 7 or 9 pin diode tubes that can handle 200ma with 20 volts across them?
I found 6203 and 6AY3/6C19P as candidates, are there any better choices? I do not like to have the top cap, I would prefer to keep it simple.
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File Type: jpg TRY1.jpg (829.6 KB, 413 views)
File Type: jpg SPUD_6AF3_C.jpg (707.3 KB, 394 views)
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Old 16th February 2013, 03:37 PM   #2
roline is offline roline  United States
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Default QUAD SPUD PP AMP

Finally did a toner transfer PCB and started stuffing.
I did a universal PCB to allow for playing with different combinations.
It is designed for cathode bias or adjustable fixed bias, I chose adjustable fixed bias.
Output configerations to choose from are triode, pentode and UL, I chose pentode with the G2 at 150V.
I have a set of 6.6K 40watt hammonds to try with it and a 600Vct 200ma tran for power. Still missing a few components, so another order is needed...
So for those who have played with the 6LU8, how much power have you squeezed out of the amps in what configuration?
Attached Images
File Type: jpg back1.jpg (575.1 KB, 276 views)
File Type: jpg top1.jpg (473.1 KB, 252 views)
File Type: jpg P2060119.jpg (890.0 KB, 247 views)
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Old 21st February 2013, 11:50 PM   #3
roline is offline roline  United States
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Default QUAD SPUD PP AMP

Final components received today and the little quad spud is up and running. I had a Hammond 372-JX on the shelf from another project, it is a little overkill but works great. I ran out of CL80's and will need to order a few more. That should drop the voltages closer to the designed values. First stage output is running 20 volts higher that plan which raises the cathodyne from being centered. I can play with the bias or change the plate resistor.
I had some small 8K-8 outputs to use to fire it up. I've left the GNFB off of this combo.
I have a set of Bogen MX30A iron that I've cleaned up, just need to dust them off and lengthen the tube side wires. This trans has a built in feedback winding to play with instead of using GNFB.

It is configured now as an adjustable fix bias amp; running around -18 volts on the grid bias. Tubes are running at 385 Volts @ 45ma 17.3 watts. That is above their 14 watt rating, but no signs of stress and they are still an available inexpensive tube.
In cathode bias mode the raised cathode voltage to about 18V would reduce the tube dissipation to about 16 watts.

With the Bogen output iron (6.6kRa) I'm expecting performance to almost be on par with the KEG-EAR amp when running the 6P3S, 6P3S-E and 6L6 tubes (1650H output iron); also the little 6GV5 sweep tube amp with this same power iron.
Attached Images
File Type: jpg spud on top.jpg (661.8 KB, 80 views)
File Type: jpg spud on bottom.jpg (681.6 KB, 68 views)
File Type: jpg IMG00967-20121024-1135.jpg (166.4 KB, 44 views)
File Type: jpg bogen mx30a pic.jpg (501.8 KB, 32 views)
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Old 22nd February 2013, 12:18 AM   #4
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I think they're called spuds because they're a 1 tuber.
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Old 22nd February 2013, 01:03 AM   #5
roline is offline roline  United States
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So an "Honest to God" Spud Amp would have to be a mono channel?
A stereo amp would have to be at a minimum a "pair of spuds" or "dual spuds" for a stereo SE amp, why not a Quad Spud? The 6LU8 is as cheap as spuds if you look in the right places!
Sounds like a hill of spuds to me.....
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Old 22nd February 2013, 01:44 AM   #6
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interesting....SPUD means by my definition, that only one tube stage is used. This means no cascading. The first tube does the whole job.
Thomas shows one with the 6HS5 here: VinylSavor: Tube of the Month: The 6HS5
Here is one with the 6С45П: 6s45pe Eintaktverstaerker
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Old 22nd February 2013, 02:45 AM   #7
Fenris is offline Fenris  United States
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"SPUD" means a single tube (or "tuber") per channel. The only was to practically do it was with a SE dissimilar triode or pentode tube, and the sweep tubes are the most logical ones to use. While technically no longer a single tube (per channel) amp, your amp uses the traditional Spud tubes. I did something vaguely similar with a 13DE7 and I've thought about using a pair of compactron sweep tubes for a PP guitar amp, but haven't left the design stage.
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Old 22nd February 2013, 06:16 AM   #8
tomchr is offline tomchr  United States
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My 6LU8 Spud:
Neurochrome.com : : Audio : 6LU8 Spud

~Tom
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Old 22nd February 2013, 11:06 AM   #9
roline is offline roline  United States
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Based on the consensus, I would have to reclassify my amp to a Quad Spud TUBED PP Amp, or Quad SPUD'd PP Amp or just a Quad 6LU8 PP Amp for those who wish to leave all in one tube=complete amp stage.
There is another sin in the design, I used sand, for a zener G2 160V reference and in the power supply.....
I'll find out this weekend with the Bogen iron what it sounds like, the end may justify the means....
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Old 22nd February 2013, 11:20 AM   #10
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Quattrorissole?
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