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6LU8 Spud - Request for comments

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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?




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Joined 2009
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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?


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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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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Joined 2009
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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...



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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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Joined 2009
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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.

Final schematics, test results & pictures

Can't believe it's been almost a year since I finished the Spud. Anyway... I finally got around to get my test results and pictures organized. See attached.

The chassis is made from 18 mm think walnut. It measures 290 x 290 x 95 mm. The Antek 1T250 is enclosed in a steel 'bell' from Antek. CA-100 is the part number if memory serves... Top plate is 1.6 mm aluminum. Bottom plate (not shown) is 1.6 mm aluminum perf plate.




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Here is clearly seen the effect of too low primary inductance of the Edcor XSE-series transformer. The distortion values are very good at higher frequencies, but jump quite high at low end.

Even the fairly priced GXSE-series would improve the performance essentially.
You are correct, sir. The XSE transformers do not offer much in the bass region. This is clear from the "specs" published by Edcor as well. But for less than $20 each they're very, very hard to beat on the price/performance ratio.

Also note that most listening volumes are in the 10 mW range with 85~90 dB/W*m efficient speakers...

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But for less than $20 each they're very, very hard to beat on the price/performance ratio.

You are obviously right. Below $20 may be difficult to find better choice.
But by spending $26.10 you will get a lot better transformer (GXSE10-8-5K) and better low end performance. Since the overall performance of the amplifier seems excellent, why not make the low end similar too ?
...and the CXSE25-8-5K would be even better. I'm sure units by Plitron or Electraprint would be better still. At some point the optimization has to stop. I wanted an amp with a small footprint and didn't want to spend more than I had to. I got what I wanted out of the project so I stopped where I did. I gained the knowledge and confidence in my tube amp design skills that I wanted. I am working on a different design now. If you are to reproduce my design nothing prevents you from choosing different output transformers.

I wanted an amp with a small footprint and didn't want to spend more than I had to.

Then your decisions have been fully right. The main thing is that you are satisfied with the performance of your amplifier.

My comments concerning the transformers you used and your test results
should be understood as a general discussion about the various things having effect to the performance of the amplifier.

Can you tell me the test equipment you have used to measure and plot the distortion vs. frequency results ?
Beautiful amp Tom. Nice to see u too use those cheap Edcors. I use the exact same ones in my shop amp. I use a bunch of them in guitar amps, and they are great for the price.
I have doubts the CXSE25... will be much better than the GXSE10... For more correct flux in the core, the OT should be sized accordingly and an oversized ditto may not always give the best results. (Ref Lundahl.se)
Great looking amp Tom. I am just now getting ready to wire mine up. Here are some pix. I am still trying to find an Oil Pressure gauge and Oil Temp gauge for the two holes in front.


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