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

Here is the PCB info... part list and schematic, all pretty cool stuff.
Its going to be a few weeks before I get a report on the bench test.
I ordered up some tubes from AES and a new PCB from Mr. Chong. Im thinking of using the bigger Edcors next time.
 

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justinsweber said:
Here is the PCB info... part list and schematic, all pretty cool stuff.
Its going to be a few weeks before I get a report on the bench test.
I ordered up some tubes from AES and a new PCB from Mr. Chong. Im thinking of using the bigger Edcors next time.
Click to expand...

(thanks for posting) I saved it..

Hey double check your cathode resistor on the output tube,
there may be a typo on that schematic. (but might not)

(if running full screen voltage)
It should be between 680 and 750ohm, depending on plate V.

If your running the 180ohm listed there your running the tubes
into meltdown territory. But since it's running zeners to drop V
it will be different and draw much less then our original.
(so can you list your plate an cathode voltage)

Other then that and not having that feedback scheme wired, I
see nothing that looks out of place, one tweak I'd probably run
is drop the cathode resistor on the driver down to 680 verse 1k.

One last thing, I would suggest an larger H choke then the 1.5H
suggested in the PDF, something like at least 6H but more 8H, +.
(If trying to use a single LC stage, multiple you could use a 1.5H)
The RadioDaze choke used in the original build would be fine there.
 
Kegger said:
Hey double check your cathode resistor on the output tube,
there may be a typo on that schematic. (but might not)

(if running full screen voltage)
It should be between 680 and 750ohm, depending on plate V.

If your running the 180ohm listed there your running the tubes
into meltdown territory. But since it's running zeners to drop V
it will be different and draw much less then our original.
(so can you list your plate an cathode voltage)
Click to expand...

The pic of the PCB shows the 180R, so no typo, I think.
Cathode & plate voltage will tell the story.
 
Tom McNally...Really?....I checked out his Amps years ago, before I built the 6LU8....I thought that Workmanship looked Familiar...He's big into 'Darlings'....
 
sled108 said:
Tom McNally...Really?....I checked out his Amps years ago, before I built the 6LU8....I thought that Workmanship looked Familiar...He's big into 'Darlings'....
Click to expand...
He builds lots of designs that would be familiar to you, I think. Scroll down that webpage- something for everybody!
All very clean builds, from the look of things.
 
I edited my response on the new circuit and the cathode resistor there.

I took a more "awake" look at the schematic, he's running zeners to drop
the screen voltage, that will drastically lower the plate current, so it may
be just fine and correct for that lower screen voltage.

Still, if the plate and cathode voltage can be measured, we can see what
exactly everything is being ran at..
 
Since I've been Running this through the Klipsch, I've also been doing some Tube Rolling...Anyone else have Opinions on This?..I've tried 3 different Sets so far...Smaller Ones and Medium Ones that I'm not sure of the Make...But, the Best so far are the Large Bottle RCA's, Very Crisp, Very Nice...I'll Post Pics of them if any of You out There are Interested...Or, maybe even if You're Not..
 
My own measurements... Think I did it right

Pin Voltages
1 6.6V
2 158VDC
3 3mv
4 300VDC
5 3mv
6 280mv
7 conected
8 134DC
9 12VDC
10 6mv
11 2.5VDC
12 6.6V
 
It's not super ridiculous, but yur on the plus side of 18w dissipation.

Now that tube is rated for 14w in the TV environment, which stress
the tube more then we tend to do in audio an you can usually push
the specs for our use, I would keep an eye on them, watch them in
low/no light an see if you have any red glowing plates.

If there stressed to much up the cathode resistor to about 240-270
ohm to drop off some current there. (and remeasure)
 
Sorry to be late on this discussion...

Mr. Chong's PCB design looks nice. However, the zener chain locks the screen to a constant DC potential, so the ultralinear tap doesn't buy you anything (the amp appears to be running in pentode mode, despite the screen tap's connection).

Am I missing something?

It appears that R15/16's connection to the tap could be moved to the plate supply without really changing circuit operation. If pentode mode was Mr. Chong's intention, connecting the screen regulator directly to the plate supply would slightly reduce the DC current in the OPT primary, giving you a bit more magnetic margin in its core.

PCB population option...
I guess you could leave out the screen regulator (D1/2/3/4 and C7/8), change cathode resistor R13/14 to 680 ohms and R15/16 to 1.2K to make it an ultralinear design.

-Dan
 
Lowering that screen voltage (be it regulated or not) but still connected to the
ultralinear tap does not make the amp run pentode.

In other-words voltage on the screen does not determine pentode or ultralinear.

There are a few (very few) output transformers that have a "separate winding"
for the voltage tap to the screen but are still AC coupled to the plate for an UL
connection, UL is in AC voltage terms not DC terms.
 
Hey, just came across this thread yesterday, and I am seriously thinking about making one of these. I love the sound of tubes in a system, but I really can't afford to go out and drop $1k on a McIntosh or something, as much as I would love to. I just have a few questions:

1. Is this only a power amp, or is it integrated as well? From what I've seen so far I think its a power amp, but I remember seeing a post somewhere where a guy posted a PDF about a chassis plan with tone controls, phono imput, etc (I wish I could tell you where, but I don't remember, it was either in the 30's or late 20's, page that is, and yes I do run default posts-per-page; i remember there was an issue with that as well :P). Is this actually in the plan, or was this just a tweak? If it is just a tweak, have you guys thought of planning a pre-amp to pair with this?

2. I've only read about the first 35ish pages so this may have come up afterwards, but has Wiggy got back on board? I am a next-to-complete noob when it comes to electrical anything, I can solder alright but thats as far as I know at the moment. I'm sure I could struggle my way through building this, but if wiggy ever got around to making a how-to for this it would be immensely helpful.

3. I have a pair of Altec Lansing Design 5's and some old speakers I got from a Sylvania SC-18 console, but I have no idea if either of these are high efficiency. IIRC someone said that anything with 93db would be just fine to use with this amp. How would one tell what sort of db's their speakers can get if there is no info on them on the web and they do not have any sort of acoustics meters?
 
1.) Just a power amp, but it could be converted to a full integrated amp with some effort. A source switch, an attenuator, and a phonoclone would probably be good enough.

2.) Not sure.

3.) Not familiiar with your speakers. I think a minimum of 90 would be needed, more sensitivity would be better.
 
Just completed my 6LU8 Compactron SE...

I posted my version of the 6LU8 compactron SE here on the DIY thread, http://www.audiokarma.org/forums/showthread.php?p=5336184#post5336184.

The sound is quite nice, although I switched from .33mkf to .1mkf caps in the interstage, as it had too much bass for my tastes. I also run it a tad hotter: 320V B+.

Anyway, just wanted to say thank you to Kegger, Wigwam Jones, Squidward and everyone else who contributed to this wonderful thread!!!

Cheers, Paul.
 
That C1 Cap makes a Big Difference in the Sound of this Amp...I thought that the 1% Electrocube's would be Fine...But, after hearing Ero's in my Phono and Pre, I put a pair of them in This..What a Difference! They Lifted the Veil, Way Chrisper. Now, I'm going to try some Wima MKP 10's which are Well Spoken of...Any other Suggestions out there for a Sweet .33uf 250v Cap?
Oh yeah, and what started me on this Cap Swapping thing was Changing the .33uf 1% Daytons to Ero's in my 6DJ8 Phono Stage...It had the Same Effect, so you can cross off Daytons from my List..
 
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Agree wholeheartedly Sled..

I've been suggesting for some time (and here to) that a good quality poly or oil cap in
the first supply spot can make positive difference on the sound to, as can using oil or
poly in all the spots and or at least bypassing any electrolytic with a good quality cap.
 
All righty Then....The Edcors are on Their way, only a Month or so till they get Here...So, for now I'm doing an Interesting Side by Side...On One Side; The 6LU8 Amp fed from the 6V6 Pre and a Fisher KM-60 Tuner into Klipsch KG3's...On the other, or should I say, right next to them; The ST-70 fed from the Marantz 7T and a Pioneer TX-9100 Tuner into DMS3000 Bozaks...Well, it would seem that Efficiency Counts in these matters...The Klipsch at 94dB beat what ever the Bozaks might be, I've never been able to find what that is...Maybe 90dB tops...So, with Closely Matched Tonality and Volume, at 'Elevated Conversation' levels; 12:00 on the 7T and 3/4 of the 6LU8's Output makes me Figure...If the 6LU8 only does 6-7 Watt/ Ch. then it must be doing only around 5-6 W/ CH. at this 'Have to Talk Loud' level...Not Bad for a SE Spud Amp... And, it would seem that the First Watts and Efficient Speakers, Really Count...
Topped Out, the 6LU8 matches the ST-70 to 12:30...Where, the KT77's are just Coming on Cam...
 
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