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

After connecting good speakers and putting it though the gauntlet from Bach to Beatles to Marley to Cooder to Miles to Jimi, I am stunned at how good these chubby little tubes sound.

I want to test voltages but I can't stop listening to it.

I'm not running through a pre or attenuator of any kind. I'm using a source with adjustable output. (A PC with an M-Audio 1010LT card playing 24/96 flacs) At full output, the distortion is a bit too much but dialed back a bit it just sounds excellent. The volume is more than enough for me with 92dB speakers. Plenty of bass too.

I recently gave my turntable away to my sister when hers died. I am definitely looking for a new one to hear with this.

BTW, I checked the packaging on those fuses and it says 1A. All of the fuses say .1A.
 
Not surprised you'd get a little distortion with 92db speakers. That is a lot for that amp to drive. That is weird about the fuses. When Squid had the prototype at my house last year, I remember the highs being very sweet.
 
GOYA said:
Thanks for all your work on this Kegger and all the others too.:D
Click to expand...

You are welcome, as to the distortion at full output from your sound card, that
is probably to be expected, I believe to drive the amp to full output it's in the 1
to 1.5v volt range, your sound card may and probably goes higher then 2v, so it
should push the amp past it's rated clipping point.(not a problem, just dial back).:music:

92db speakers with decent impedance curves should get plenty loud and perform.:thmbsp:
 
I finally checked the B+ last night. It looks like I'm way low at 250v. It still sounds very good. I tested both with input signal and without and it made no difference.

I'm going to recheck with a different VOM tonight.

(I'm using the cheapo Harbor Freight DMM)
 
Are you using the triortal (sp) PT? I am building the box for my amp this Thursday so I am thinking it is time to start ordering parts.
 
Double check your voltage, if your still low paralell in say 1-1.5uf caps at a time on the
first cap till you get what you want. (nice poly's or oils would be good here)

What cathode resistors are yu running and what is the voltage on the cathode of each
of the output tubes you are using?
 
Yes, I'm using the AnTek toroidal. I never tested any voltages on it before last night.
As far as the other components of the PS, I got a 7uF motor run from Surplus Sales of Nebraska, the Radio Daze chokes, Dayton polyprops for the bypass caps and the 150 uF caps are some well known name brand that I can't remember at the moment.

I got some small caps to adjust the 7 uF for proper B+ but I don't know if I have enough to make up 50V. Once I find the correct figure for that first cap, would it have any benefit to get a motor run of that value to replace the 7 uF and the smaller paralleled caps?
 
You'd be surprised how little extra it takes most of the time on that first cap to adjust
the voltage. Actually paralelling caps is a good thing and I do it quite often, you lower
the ESR an a "blend" of different caps can be beneficial in other ways for filter an tone.:D

So paralell them on up and keep um that way..:yes:

Now it would be nice to see what current your drawing in the outputs, that "could" explain
why your voltage is low, if your outputs are drawing more current that is.

So if you could measure the voltage on each output tubes cathode and exact ohm resistor
you used let's see where were at before adjusting the voltage on the supply..
 
Kegger said:
Now it would be nice to see what current your drawing in the outputs, that "could" explain
why your voltage is low, if your outputs are drawing more current that is.

So if you could measure the voltage on each output tubes cathode and exact ohm resistor
you used let's see where were at before adjusting the voltage on the supply..
Click to expand...

Left channel

666 Ohm - 26.6V
_______________
Right channel

679 Ohm - 24.3V


This part seems weird. Maybe I'm measuring incorrectly but both channels measure .02 micro amps across the cathode resistor at idle.
 
Not sure how your measuring current, but no big deal. (meter goes in series for that)

Calculating it though one tube is basically 40ma an the other 36ma, so go ahead and
get your voltage up then with the extra caps on the supply. :)
 
Kegger said:
Not sure how your measuring current, but no big deal. (meter goes in series for that)

Calculating it though one tube is basically 40ma an the other 36ma, so go ahead and
get your voltage up then with the extra caps on the supply. :)
Click to expand...

Ah, in series. That makes sense.

I have extras on the cathode resistors. I think I'll change out the 666 ohm for one that measures 677.

What would be the maximum capacitance when I increase the 7uF input cap?

Also, the voltage can be lower for paralleled caps, right?

The ones I've got are .33 uF 250V. I think I'll need to get some 1 uF.

EDIT: correction of previous post - I got the motor run cap at TEDSS. The mounting clamp for it came from SS of Neb.
 
Last edited:
No the voltage is lower when you series them, I'd suggest 400v rated caps.

Max cap would probably be around 30uf, my guess would be you'd need 15uf
combined total max to reach the 300v value. (if everything is as it should be)
 
Parts are ordered and I should have them to work on it this weekend.

I know one of the great things about working with tubes is that few things have to be exact but is it unusual that this amp sounds very good being so underpowered?
 
Is there a final BOM yet?

I've been following this thread for a long time and am wondering if there will EVER be a complete list of parts to order for this project? I'm sure there are other tube noobs who are anxiously hoping for the same thing. Have I missed it somewhere, or has it been decided that this project isn't suitable for us? :scratch2:
 
GOYA said:
Parts are ordered and I should have them to work on it this weekend.

I know one of the great things about working with tubes is that few things have to be exact but is it unusual that this amp sounds very good being so underpowered?
Click to expand...

You've still got decent voltage and current so the amp works fine,
but when you get to 300v you'll have a bit more power here though..:thmbsp:
 
jimbofish said:
I've been following this thread for a long time and am wondering if there will EVER be a complete list of parts to order for this project? I'm sure there are other tube noobs who are anxiously hoping for the same thing. Have I missed it somewhere, or has it been decided that this project isn't suitable for us? :scratch2:
Click to expand...

Our BOM maker has been having health issues so he's still trying to finish it then post it.
(I to wish the BOM was done for everyone to see it)

Several "new" builders have had success here with reading the thread an parts suggested
plus with help from others, the BOM is still something we want to do though in this thread.
 
Also, you can look at the schems as figure out what you need.....that and stuff that has already been mentioned.
 
jimbofish said:
I've been following this thread for a long time and am wondering if there will EVER be a complete list of parts to order for this project? I'm sure there are other tube noobs who are anxiously hoping for the same thing. Have I missed it somewhere, or has it been decided that this project isn't suitable for us? :scratch2:
Click to expand...

Sled108 posted some of the parts he ordered back in post #631.

I've decided to try the 6LU8 in Push-Pull configuration, so I'm going to require different power and output transformers.

Jeff
 
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