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

I've heard of beer goggles but I didn't know that beer muffs existed. :)

The bolero's were not for me, but they sounded almost acceptable on a couple of jazz tracks. What they did do well is show that with a moderately efficient speaker, you don't need a lot of power to get good output. If I had to pinpoint an issue, I think the altec woofer was being asked to play entirely too high and it became shouty/sibilant in the upper midrange. The lack of real low bass was expected wtih the small cabinets, so that didn't worry me too much.

They were significantly better than the Zu Druids, though. :)
 
To bad you didn't get a chance to hear them Dave when most were gone.
(Not saying they were the best speakers I've heard)

But I sat and talked to Ross an asked him specically about the tweeter, I
believe what he said was they use a microphone diaphram for the tweeter
and it is very sensitive pretty much falls apart after a few watts, Rob had
sat down and turned the volume so it was adaquite at the listening position
in the front chairs, he seemed to like what he was hearing so I sat down an
what I heard was not the same sound we had earlier at the levels we were
pushing the speaker, it had none of the nasal properties and not a trace of
sibilance, it was all gone. Quite a nice sound to listen to.
 
Guys, this weekend is out for me - I'm wrapping up things at my previous digs and getting the last of my stuff moved into a new place. However, all should be done after this coming weekend, and then I'm all yours. In the meantime, don't let my absence slow anyone down - have fun and I'll just be envious from the sidelines!
 
Kegger,

A belated thanks to your welcome. Got my sleeves of GE 6LU8s and some nice compactron sockets from ebay, so at least that basic requirement is taken care of.

I'm blind so wonder if you could do me a favor and write up a list of the pinouts -- the picture on the first page doesn't do me a lot of good :-).

Thanks,

Jim
 
Here yu go!

Not a Problem, JIM

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

Triode Section,

2 is the Plate

10 is the Grid

11 is the Cathode

Pentode Section,

4 is the Plate

6 and 7 are both the Grid, (use either or both)

9 is the Cathode and G3 connection

8 is the Screen/G2,
_________________________
(Heaters are pin 1 and pin 12)

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

I commend you for being able to build things like this with no sight, I hope someone is reading you the schematic. :thmbsp:
 
Kegger,

Thanks much, and I will surely get somebody to help me with the schematic. Actually, oce the university is back in session I plan to go and hire an EE student with a copy of Visio to draw this and a feww other schematics up for me so I can work on getting them to print nicely on my high-res tactile printer. If that works well enough, a lot of other possibilities will open up. Once I can "see" schematics and characteristic curves, scope plots, etc., I'll be back in business. (I was an EE before I lost my sight, but without something to "look" at, the words can be a confusing jumble to wade through.)

Eventually, I'd probably like to turn this project into a pair of PSE monoblocks, but we'll see how that goes.

Thanks again,

Jim
 
A quick project update from my end:

I got to hear the prototype for the first time last weekend. It performed well under unfavorable conditions. I'd still like to hear it in my main system for a true evaluation, but it's a solid performer for sure.

The circuit has been nailed down, and we are ready to proceed with the next stage of the project. I expect that we will have the photo shoot/build document done by the end of August. I will be ordering off-the-shelf chassis parts for ease of replication, tho I expect a lot of builders will do their own thing as far as that's concerned. There will be a comprehensive Bill Of Materials (BOM).
 
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Just to echo what Keeger said, I felt the little 6LU8 made the Altec Bolero's voice sing very nicely at a relaxed listening volume and I remembered hearing the same explaination from Ross about the tweeters as Kegger mentioned in our chat. This would account for their performance when driven like a "rock" speaker and than listened to at moderate levels and enjoyed. To add a bit more, Ross also speculated that this is probably why so few of these exist today, people just over drove them and fried the tweeters.

Hey Keg! Look forward to hearing them as I have had a broke down MA-1 Mono Amp for some time know and the vinyl just keeps stacking up. It seems like everytime I go to call Ralph at Atma-Sphere I get side-tracked.

Happy (first watt) Listening! :beatnik:
 
Looks Good!

Just sticking my head in again. Looking awesome! I can't wait for that BOM so I can start getting things together. :music:
 
Ive ben out of the loop for a while but things are settling down here and my builup of tis amp should start bac up again in a coupple of weeks.
 
I read this from front to back....man is my head swimming. I have a bud who is an electrical engeneering student...I think I'll get his help and build two LOL
 
Hey guys, I now have my Jensen Imperials tri-amped so maybe in the near future the amp can make its way back to my house and we can swap it into each position and see where it sounds best. I promis to supply the food and drink. Cheers.
 
Been reading this thread for quite a time -- your original idea and bringing it to fruition is much appreciated! I plan to build this amp and will probably do two before I'm done. I've done quite a lot of radio and audio repair but never built anything from scratch except a piece of test equipment built from a kit.
It's really kind of exciting to do a "built from scratch" amp! So, I'll keep tuned and begin assembling some parts. Thanks again!
 
I stumbled across this thread while searching for Compactron tube amp designs through Google -- a very interesting project! I've built a 6T9-based Compactron amp myself, from schematics published in an old GE Hobby Manual (which itself was licensed from Popular Electronics). It's probably similar to the original Nuts 'n Volts schematic you guys found, if not identical.

I've built a few different Compactron-based amps for friends and personal listening. The aforementioned 6T9 amp is used at college for dorm room sound, and provides about 2 Watts per channel, which is quite adequate for something as small as a dorm room with a good set of bookshelf speakers. The wonderful thing about Compactrons is that no one wants them, so they cost nothing (the sockets cost more than the tubes usually).

I'm very interested to find out how your experiments with the 6LU8 turn out...I've had good results with other TV sweep tubes (6S4, etc) so I should expect your results to be good as well -- especially since you guys are using higher-quality parts than I am! I think my 6T9 amp ended up costing me less than $50, since I got all of the parts as cheaply as possible -- the output transformers being the real money-saver: Antique Electronics Supply had (has?) 5k:8 8-Watt rated output transformers for less than $20 each.
 
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Alright the mule/new amp is alive!

Pair of tubes per channel in paralell, so it's Paralell, Single Ended, Ultra Linear... :)

The triode of the first tube feeds into the triode of the second tube, direct coupled
then that feeds the outputs from a cathode follower, it has a switch to use just the
front triode to feed the outputs taking the second one out of the picture, I am using
LED bias on the driver with removable sockets to "roll" in different LED's, it's a combo
cathode bias and fixed bias on the outputs with a smaller cathode resistor then used
in the standard build then an adjustable negative bias supply from there to "balance"
out all the outputs together.

The power supply is quite beefy, running choke input from an ST-70 power tranny to
a Hammond organ amp choke (those are on top the outputs are down below) to that
second organ choke coming from a pair of 5V4G's running in paralell fed from the large
filament tranny underneath. (Standard 5V4's are in there for testing)

It's an old ST-70 chassis mounted on an old Turntable chassis, made for experimenting
with all the easy connect sections down below as previously posted.

Driver board is a heavily modified VTA board just for ease of bias and wiring the circuit.
 

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Looks damn nice Kegger.. very fine work...

BTW.. good to see I'm not the only one that can't stand a tidy work space :D
 
I read this entire thread over the weekend, then registered so I could post. A couple of things:

1) I think I can puzzle out where the lines are supposed to connect in the schematic, thanks for posting it, Kegger.
2) I don't remember coming across a schematic for the PSU. Is it in the thread somewhere, and I just forgot to where it was?
3) **** EVERYBODY TAKE NOTE! **** AES has 6LU8s on sale for $3.05!! I hope they still have some Friday when I get paid ;^)
 
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