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

sled108 said:
Hey Kegger...Any thoughts on how to Kick it up a Notch with the 6LU8's...I've a couple Sets to Spare, and would like to Hear what They can do..Right now the Amp sounds Great, but it seems to be Holding Back a bit...Can I make it Cry and Sing?
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You could crank up the voltage, say 350 should be "safe" to experiment with.

You could parallel another pair of them an double the output, the 5k tranny is
higher then you need but will work, I used the 5k 16ohm tranny on 8ohm load.

You could replace the cathode resistor on the driver with a 2v red LED as well.

Trying a different value feedback resistor may be worth experimenting for you.
(anything from say 50k to 470K, to removing the feedback altogether)
 
mashaffer said:
I don't think anyone has done PP or screen drive yet. ;)
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Designing one from scratch is beyond me at this point, so I jumped on the Pete Millett DCPP band wagon. I already had some 6JM6's for a PP project, so I figured this would be a great way to try them out.:yes:

Jeff
 
Starting my 6LU8 build. Here is a pic of the wood mockup of my chassis. I already have the square aluminum tube cut and will start welding it up Tuesday night. The one thing I am doing diff from the mockup is flipping the front tubes up side down so the flair at the end is facing down. The left and right signal paths will be in each of the front arms and the power supply and rec tube will be in the back arm. The hole thing will be on three spikes. One under each arm. I am thinking about having it powder coated metal fake candy green and using gold spikes.

IMG_3339.jpg
 
From the back, it will be the the PX, in front of that the rec tube and underneith those the chokes. Going forward on each arm there will be the OT then the 6LU8 at the end of the arm.

P.S., the toroidal PX will be standing up facing forward.
 
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...he's baaaack

Hey folks!:banana:

Lots of water under the bridge here, and I'm seriously considering doing something with (most of) the parts that have been gathering dust in my basement in the meanwhile.

I think I'm going to keep most (if not all) of the surplus iron I bought to use for other projects. I'm really liking the looks of using the Antec xfrmr that's been suggested here (note the old link elsewhere on this thread is broken). I've become intrigued by some rather hefty damper diodes that are available cheap (e.g. $1/ea plus shipping). I found out the info from an otherwise broken link that I found on wayback. In particular, the 6CG3 is a real beast.

Some preliminary fiddling on PSUD II with similar diodes, the motor run caps that I have, and theoretical Radio Daze chokes that I don't yet have looks promising.

The problem being that the heaters are current hogs, which would probably be incompatible with doubling up the tubes for more output and still using the Antek toroid. I'm checking into getting another xfrmr just for the damper diode heaters. Too bad we don't have a proper electronics surplus store in this town any more. Or, I could be sane/sensible and just use a couple of 6W4's instead

...naaaah

Seriously, it would be cool to have something sorted out that would work on almost anything, with the correct iron and some other minor adjustments. For instance, the 18W Superlite (a Marshall Guitar Amp Derivative) on GuitarAmplifierPCBs.com has some PSU variations that look remarkably similar to Kegger's PSU for this project. They have a tapped RCRCRCRC filter off the PSU, which provides various voltages to the circuit (the first R is a bleeder resistor).

In any case, I hate like hell paying $20-40 for a 5AR4 or EZ81 when I can get two 6W4GTs or 6CG3s for $2.00. Even factoring in another xfrmr, the sockets and extra whatnot, I think I'm still coming out ahead (at least compared to NOS), and some people think that damper diodes are the sweetest sounding, period.

There's things I have to get out of the way first, and I'll keep you posted on my progress, if any.
 
Figured it out!

Well, one minor thing, anyway...

Turns out that there is a 25CG3, which according to the GE data sheet, is otherwise identical to the 6CG3, and is still available for the same price. Since (I presume) the heater just wants watts, the amperage is correspondingly lower. If I'm getting another transformer for the diode filaments, anyway, I can get it any voltage I want. Antek has a suitable one, or I could even use one from RadioShack for a few bucks less.

Two of these monsters, the sockets and the RadioShack transformer would all add up to about what the cheapest 5AR4 costs, except I end up with sweet-sounding (by some accounts) glass that can handle 5KV@1050ma on each plate, and can output 350ma!

I've done some more fiddling on PSUD II, with 6W4GT and 6D22S, and the results are similar, so I suppose the 6/25CG3 would be as well.

[edit: Turns out that Duncan Amps has the parameters posted for the 6/25CG3. I just have to take a few minutes and add them to the PSUD II database]

And it's sure nice to not have to be bugged by repeated warnings that the diode's current or voltage limits were exceeded. The damper diodes are a lot harder to overwhelm.

BTW, there's also a 25W4GT, and suppose that's kind of moot, as it's 50% less of a current hog than the 6CG3. The 6W4GT data sheet has a voltage drop spec on it, and the 6CG3 sheet I found didn't, even though both were GE. Anyway, the former lists 21 volts at 250ma. The design center is spec is 125ma. Hmm. I guess I'm not comepletely clear on exactly what "design center" means after all. Any actual engineers around here to straighten me out on this?

Anyway, the latter just says "low tube drop", so I presume it's similar.
 
You should be just fine using the "standard" octal based 6.3v damper diodes.
(I use them quite often, our first build used um on the Antek, they are cheap)

There are a ton of same pin-out 6.3v 1.2a heater damper diodes that work fine
on that Antek tranny.. http://www.antekinc.com/pdf/AN-3T275.pdf

6AX4, 6DA4, 6DM4, 6DT4, 6U4, 6W4, and many many others, even those higher
current ones will work if you are using just a pair of 6LU8, 6LR8, yah if you want
to double up those output tubes you'll need extra heater, can add one later to it.

Keep in mind using 2 of these you'll get double the current rating, WAY more then
capable on any of these guy's for use here, no need to be concerned on this build.
 
Made great strides on my chassis last night for this amp. Should have it done in a week or so. I need to start ordering parts. Has anything changed? Are we still using the schems as posted? I need to use a normal PT vs the Toroidal. Which model should I use? Cheers. Pix to follow tonight.
 
Well, here is what I have so far. Will start drilling this weekend. Then weld it up next week. The sides will be filled in and have a motor run oip cap sticking out either side. The chokes will be mounted inside the back of the overhang. The OT will be be mounted on the arms between the 6LU8 and overhang. The ends of the arms will be plated off as well. The rec tube will hang down from the bottom of the overhang and the PT will be mounted directly above that. Those 6LU8 tubes look pretty small out there, maybe 300Bs or 813??? would look better. Then powder coat in large metal flake green (gold flake of course) :D
 

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djnagle said:
Has anything changed? Are we still using the schems as posted? I need to use a normal PT vs the Toroidal. Which model should I use? Cheers. Pix to follow tonight.
Click to expand...

Just noticed this.

A Hammond 270EX should do nicely for a PX. It looks like the 270DX is just shy on current.

I built the schematic as listed and it sounds very good. You might want to upgrade the output iron but that's the only grey area I can think of if your wall voltage is in the 120V range.
 
Hammond 270EX - thanks.

This whole thing has gotten out of hand. It started as a simple project but dang.
 
Well, I have to say good bye to the 6LU8 amp. The dimentions of this chassis is just too big for the small, and great sounding, compactron tube. This is now turned into a JE Lab 6SL7/2A3 amp. Cheers.
 
New-bee here, taking the plunge,

Bought the iron from Antek and Edcor today;

Been reading this thread for a week trying to understand what is in it, how it goes together, and why this amp works.

Edcor told me about 5 weeks, so I have more time to learn.
 
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