The "$250" Amp? - 6LU8 Compactron SE, UL

Tag moar!

sled108 said:
Wow...What can I say, But Check this out...Enjoy...
http://www.youtube.com/watch?v=3ifmd5aC5Sk
Click to expand...

Stick some cool tags on that utube thing!

Well, wait then others might know the power of the $150 cake pan Sub-Compactron --> and we would all be out of work!

I enjoyed it. I wonder if a shorter clip with lower volume would help?

Kinda funny though, you tweak the amp, and then you have to start tweaking the utube video!

But I really like that we got to see and hear it in any event. Really makes it real for us.

Dan
 
Thanks for the link. I have been working on something similar only planning on using LED bias for the triode and fixed bias for the pentode. I wonder if it would be detrimental to run the cathodes of both channel's triode through the same LED to increase the current and improve linearity.

Yes great thread but I need a couple of weeks of free time to go through it all. :)

mike
 
Here are a couple links (to this thread) showing my SUPER build running a sorta fixed
bias for the outputs then LED bias for the front end.

http://www.audiokarma.org/forums/showpost.php?p=1989263&postcount=272

http://www.audiokarma.org/forums/showpost.php?p=2041839&postcount=295

Not sure how the shared LED bias would work for both channels, my gut tells me they
would bleed the other channel through and feed it into the entire amp but if you want
to try it I'd be curious how well it works, for what it is worth mine works just fine now
biasing one triode with the LED and I rolled in a bunch, this amp runs really really nice. :music:

-------------------

Here's a few more pic's, including the underside when most of the wiring was done and
the basic schematic, can't say this was the final schem as I just don't remember now. :)
 

Attachments

  • 6L68_ACTU_SWTCH_NBP_nv.GIF
    6L68_ACTU_SWTCH_NBP_nv.GIF
    19.7 KB · Views: 249
  • 6LR8_drvr_top_1000.jpg
    6LR8_drvr_top_1000.jpg
    76.5 KB · Views: 231
  • 6LR8_drvr_under_1000.jpg
    6LR8_drvr_under_1000.jpg
    58.4 KB · Views: 154
  • 6LR8_back_991.jpg
    6LR8_back_991.jpg
    97.8 KB · Views: 237
  • 6LR8_under_991.jpg
    6LR8_under_991.jpg
    125.6 KB · Views: 247
Hey Mashaffer, it is a VERY good sounding amp. From low to high, very detailed, smooth as all get out, and dynamic. Kegger did a great job on that one.
 
Kegger said:
Here are a couple links (to this thread) showing my SUPER build running a sorta fixed
bias for the outputs then LED bias for the front end.

http://www.audiokarma.org/forums/showpost.php?p=1989263&postcount=272

http://www.audiokarma.org/forums/showpost.php?p=2041839&postcount=295

Not sure how the shared LED bias would work for both channels, my gut tells me they
would bleed the other channel through and feed it into the entire amp but if you want
to try it I'd be curious how well it works, for what it is worth mine works just fine now
biasing one triode with the LED and I rolled in a bunch, this amp runs really really nice. :music:

-------------------

Here's a few more pic's, including the underside when most of the wiring was done and
the basic schematic, can't say this was the final schem as I just don't remember now. :)
Click to expand...

Kegger,

I have been running LED bias for many yrs now after spotting diode biasing being used in a commercial phono stage.

I thought resistor bias with bypass would be better so I changed it over and listened! Well! As fast as I took the diodes out was as fast as I put them back in! And I have never used resistor biasing since other than in a feedback loop of a PP amp.

I was looking at your schematic and I see your 6LR8 triode section looks similar to a 12AT7 I don't know what your running for current through it but if its just a couple of ma. You could try pulling some more current through it to get well into its knee that is its locked forward voltage. Otherwise there can be some fluctuation of it forward voltage drop.

Another thing that can be done is to run a bypass cap as many LEDS ACR with increase with frequency.

I use a C4S current source with an LED biasing my 811-10 driver stage. And love the combination.

Since my 811-10 runs near 0 volts bias in Class A2 I have connected the filament to ground with diodes as well. When I did this the amps dynamics went through the roof! And I got a phone call from my neighbor that my 10 watt SET was moving the pictures on her walls I was playing some Jazz with acoustic bass. Needless to say I lived in a Duplex with our living rooms separated by two garages.

One can bias SETs with even a LM317 Regulator as cathode bias as well. and run a bypass cap across the regulator to eliminate its ACR which eliminates the sonic signature of the regulator as well while the regulator maintains the DC bias.

I have done this with my CDP to its discrete regulator for my analog stage out and the effect of bypassing with a 1uf cap was stunning. Later I increased the supply capacitance so that I could use just a diode to drop the ripple over in its bulk supply. I then bypassed the regulator series pass transistors with a Muse 47uf bipolar for an even fuller warmer sounding bass response.

So yeah, I'm a big fan of LED biasing.

Heres what I did in my amps.

I have since added 47uf Muse bipolar caps across the 811's diodes as well.

img0001jy2.png


Great work on your amp!

Cheers

SET12
 
Yah I don't like to have to use bypass caps on cathode resistors, when I can avoid
it I do, the LED biasing here works rather well, I put those plug in deals so I could do
some "LED rolling", I found some that work rather well with no needed extra current to. :thmbsp:
 
Kegger said:
Yah I don't like to have to use bypass caps on cathode resistors, when I can avoid
it I do, the LED biasing here works rather well, I put those plug in deals so I could do
some "LED rolling", I found some that work rather well with no needed extra current to. :thmbsp:
Click to expand...

Why don't you like bypass caps? Though I rarely use resistor bias anymore.

I myself prefer zero feedback and with out the cap you have local feedback! Granted it isn't much usually but I can hear 1 db changes in feedback myself in global PP amps. I spent time with some VAC's that were adjustable.

Did you find some LED's with like a Imax of 5 or 10 ma. most that I have found were 25 ma. Imax, there has been huge discussion of LED biasing on diyaudio.com and else where there.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=137447

SET12
 
Sorry didn't realize there was a recent post.

I find it sounds better when u don't bypass cathode resistors, I haven't experimented
with bypassing LED's, I tried bunch of LED's found nice differences in the sound, I also
cranked up the current in them an found no improvement (on this setup) I actually do
like a little local feedback but try an not use global unless I am building a pentode amp
and can get no other type of feedback to work well. (I generally do UL or Triode)

Yah I did find some low Imax LED's, can not remember now where or what there current
was but was pretty low, I've been all over DIYAUDIO reading all the LED stuff, in the end
it came down to just doing it as there was a lof of conflicting reports.
 
Last edited:
Also just an update on the standard 6LU8 build

I believe Squidward has all his parts on order and should be coming in shortly so he can do
the varification with the BOM he has finished. ;)
 
Thanks for the update. :)

I'm slowly gathering parts for my build. Yesterday I bought a couple 6BY5GA rectifier tubes and some sockets. I just wish I could model this tube in PSUD2.

Jeff
 
I have taken a look at the official schematic (after I did my own back of the envelope pass at the VAS stage). What I came up with is red LED bias (1.7V) at 5mA and (IIRC) 32K plate resistor. I notice that the official design uses 2V @ 3.2mA and 50K plate resistor. The 2V bias obviously give a little bit more headroom (unless the output stage overloads first). But I wonder if anyone has tried a bias point similar to the one I came up with and what your results were.

If the 2V/3.2mA is a better (more linear for example) bias point I see another possible option. That is to put a 100 ohm or so resistor from the cathode to the LED to drop another 0.3V. This should also provide a little bit of local NFB (degeneration) which might even improve linearity somewhat.

Any thoughts.

OH, BTW, I am using the 6LR8 (9 pin version) in my project.

mike
 
I don't want to comment much on "alternative" designs as I'd like a few more to build
the amp first then experiment, BUT yes I too have used LED's, but I found low voltage
ones so I could stick with larger plate loads which does make the driver perform better.

By all means post your results and alternatives but I've built a few of these including
a paralell version running the 6LR8's with driver LED bias, amp performs rather well to.

What I'm getting at, ED/Squid is working with his parts order to build another amp an
then post the BOM when it's working, I don't want to put much more of my "different"
configs out there till he at least has that up so others can build a amp then change it.

But I do encourage others to experiment and post there findings. :thmbsp:
 
Last edited:
Thanks Kegger, I don't want to confuse things of course. If it would be better I will post this rather different project in a different thread. But since I am here can you suggest an LED that approximates the official 2V/3.2mA bias within it's linear range? I am not really familiar with all of the types I just know that the reds are considered a good one in general.

mike
 
You must log in or register to reply here.
Back
Top Bottom