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Old 28th December 2009, 03:09 AM   #1
tomchr is offline tomchr  United States
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Default 6LU8 Spud - Request for comments

Folks,

Inspired by the Spud Anyone? thread I decided to make my debut in the tube circuit design world a Spud based on a 6LU8 tube.

Attached circuit has been playing in my living room for weeks now and I really like it. It's quite good with the Edcor XSE-15-8-5K OPT's and even better with CXSE-25-8-5K OPT's. My plan is to use the smaller transformers for the Spud and use the bigger ones for my next project.

The Spud runs off of +300 V B+ and delivers 4.4 W, 2 kHz into 8 ohms at 1.7 % THD. The bandwidth measures 48 Hz ~ 31 kHz at full power. Clipping happens just shy of 5 W.
The cathode current of the pentode section is 60 mA resulting in approx. 16 W dissipated in the tube (plate + screen). I have tried lower voltage, higher current as well as the opposite. 300-ish V, 60-ish mA seems to be the sweet spot.

I also tried running the triode on a CSS. I didn't like that much. I used the 10M45 and it seemed to make the input stage run out of breath rather quickly. I only got about a watt out of the amp before clipping and the CSS didn't provide any appreciable difference in sound quality so I opted for resistive plate load on the triode. Resistors beat the CSS on price and complexity every time...
In addition, I tried plate-to-plate feedback with 100 kOhm from the pentode plate to the triode plate. Using this feedback scheme I got higher distortion (1.5 % THD + 1 W, 1 kHz vs 0.7 % with cathode feedback), so after trying a couple of different resistors I dropped that topology.

For power supply I use an Antek 1T250, which yields a B+ of 290 V after rectification (5AR4) and filtering (Pete Millett-style source follower). I'm running the tube heaters on 6.3 VAC. The DC level of the heaters is set by two 100 kOhm resistors to the voltage of the cathode in the left channel. This should bias the heater at roughly the same potential as the cathode, thereby, minimizing hum.

Any comments or suggestions for improvements?

Thanks,

~Tom
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File Type: pdf 6LU8_Spud.pdf (15.6 KB, 612 views)

Last edited by tomchr; 28th December 2009 at 03:24 AM.
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Old 28th December 2009, 03:21 AM   #2
mach1 is offline mach1  Australia
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There's a huge thread on a 6LU8 ultralinear spud at the Audiokarma forum. It uses local feedback, which is an option I would seriously consider.
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Old 28th December 2009, 03:34 AM   #3
tomchr is offline tomchr  United States
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Mach1:

I'm familiar with the Audiokarma thread. I read through it when I designed mine. The final schematic is on Page 41 of the thread. They ended up with 270 kOhm plate-to-plate for local feedback. I chose to follow George's (tubelab) topology of the Simple SE and make the local feedback via the cathode.

Any particular reason that you suggest I revisit the p-p feedback?

Thanks,

~Tom
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Old 28th December 2009, 04:34 AM   #4
mach1 is offline mach1  Australia
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I have a 7802 amp with cathode feedback that still benefits from a little bit of plate-plate feedback: you don't have to use one feedback mechanism to the exclusion of the other.

390k or 430k would probably be a good starting point for the feedback resistor seeing you are already employing CF. 100k is way too high, even when not employing any other type of feedback. When using a 470k or 330k grid resistor, a good rule of thumb is to size the feedback resistor at around five times the value of the plate resistor of the driver tube.

The main issue will be whether you are left with enough gain to drive the output valve to clipping. If not, you can try taking feedback from the ultralinear tap to give yourself a little bit more headroom.
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Last edited by mach1; 28th December 2009 at 05:00 AM.
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Old 28th December 2009, 04:46 AM   #5
mach1 is offline mach1  Australia
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One more thought: using a 100k feedback resistor would change the dc operating point of the driver tube considerably (increasing current by approx + 2mA or around 50%).

When comparing both types of feedback, this will have introduced another variable, making any valid comparison problematic.
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Last edited by mach1; 28th December 2009 at 04:48 AM.
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Old 28th December 2009, 05:05 AM   #6
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Pictures?

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Old 28th December 2009, 04:49 PM   #7
tomchr is offline tomchr  United States
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Mach1: I understand that I don't have to rely solely on one feedback loop. And I certainly have the ability to play a little. My question is: What should I expect to get out of it? Better stability of the amp? Better sound quality? Lower distortion? Something completely different?

Pictures. Well, sure. I just realized I don't have many, but I've attached a couple of pictures showing various stages of the development. On the picture showing both channels, I'll let you take a guess as to which channel was built ad-hoc and which one I planned out on paper first...

My first experiments was done on a 'peg board' (my term). I pounded brass nails into a piece of plywood at strategic places and soldered the circuit to the nails. Works well... But when it came time to get the second channel going, I opted to move the contraption to a piece of scrap PCB material. I put some standoffs on it and ran the point-to-point wiring underneath. It's truly an unsafe deathtrap. Keep kids and pets away. But it works...

~Tom
Attached Images
File Type: jpg Spud_DeathTrap2000.jpg (132.3 KB, 910 views)
File Type: jpg Spud_GlowPic.jpg (69.2 KB, 867 views)
File Type: jpg DT400-2W-Triode.jpg (273.9 KB, 1595 views)
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Old 29th December 2009, 02:47 AM   #8
mach1 is offline mach1  Australia
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Tom,

On reflection, plate-plate feedback in your current circuit will not work optimally, as the input tube in this type of circuit acts as a current source, and should therefore present as high an output impedance as possible. Leds generally have a low dynamic impedance (in the order of 10 ohms), which adds (mu + 1)Rk or 590R to the input triode's 16k plate resistance, while a 620R cathode resistor adds (mu + 1)Rk or about 37k, representing a 200+% increase over the led implementation. This massive difference in output impedance would be quite significant to the performance of the circuit.

If you were to revert to a cathode resistor I would expect the beneficial effects of plate-plate feedback (lower distortion and lower output impedance) to be more apparent.
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Last edited by mach1; 29th December 2009 at 02:50 AM.
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Old 29th December 2009, 07:39 AM   #9
tomchr is offline tomchr  United States
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Mach1: Sounds like you were involved in developing the one on Audiokarma... On their Spud, one could bypass the 620R cathode resistor and lower the impedance that way. I'm a little surprised they didn't do that. It would have increased the gain as well. But maybe the distortion goes up also. That's actually an option I did not try. The triode section seems to be happy with 2 V Vgk. I deduced that from the data sheet and decided on LED biasing right there.

I like the LED biasing. I actually measured the dynamic impedance of the LED I'm using. At 21 ohms it's not a perfect voltage source but quite close.

Thanks for your input. I'm always interested in learning new stuff.

~Tom
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Old 29th December 2009, 08:35 AM   #10
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5W TV sweep spud - I like it!

Cheers!
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