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

I'm getting really excited about this project! I actually went down to the cave and started cleaning off the bench!:D

Wiggy, I think you're doing a great job of keeping things on track. Version 1.0 will be a great starting point, simple, inexpensive, and folks so inclined can go from there.

Just a thought to file away, not for now, but for "2.0" down the road...I have an acquaintance who was sorta into tube gear and hi-fi many years ago. He still has his old ST-70 stashed somewhere in a closet. I happened to mention to him about this little project, and he got all excited. "You know what I do for a living, don't you?". Me: "No.".

"I design and manufacture low production run (20-50 units) custom printed circuit boards."
 
Kegger said:
This is no knock on anyone wanting/questioning the use of feedback, just the statement as to where this project is going to start from, (And probably end, with very nice sonic results). ;)
Click to expand...

Let me test what I'm learning - we're talking local feedback here, instead of global feedback, since we're not taking signal from the speaker terminals, but from in front of the output transformers to feed back into the signal path to reduce distortion.

Or am I nuts? OK, well, I know I'm nuts. But am I right?
 
Also, if there is real trouble sourcing sockets/tubes, I believe that the 6LR8 is the same tube with a 9-pin Novar socket. Not sure if that would help source sockets, though. But for those that might have a small tube stash laying around and have some of these, you could easily swap sockets and use these in place of the 6LU8.
 
OK, I"ve been doing a bit of parts sourcing and development on this project, and I think that eventually there will be two logical offshoots from this thread. The first and most important goal will be that we will develop a document that will describe the project in detail, including a BOM, schematic, and step-by-step instructions for the completion of this amplifier. It will contain options for both tube and solid state rectified power supplies. There will be mention of chassis work in that there will be a drill guide of some sort, probably based on a something generic like a Hammond Box.

The second thing to come out of this thread could possibly be a "one time only" group buy of parts and components for those that are interested. We could leverage group pricing on some components, and while it might not be dramatic, for some any saving we got could help. The real advantage to this option would be the possibility of getting a chassis for low cost, if some AKers (like Newman and others) wanted to keep costs super low and do a small run for just this project. We've been discussing options, and it would appear to keep costs super low we might have to do a small amount of fabrication. We'll know more later, and trying to organize a group buy before we have a build document would be like putting the cart firmly in front of the horse, anyway.
 
I'm late to the game, and a wee bit short on funds, but I (think) I'm in.

Need to read pages 2-9 of this thread to be sure. Ironically, I was just thinking "hey, how could I build a DIY tube amp". :D

If we need a bulk of some basic components, I might be able to swing something through work and/or our distributors to get us a price break on some of the more common misc parts. No promises, but I can try.
 
Yes I agree w/you Kegger the short feedbac loop would be very good plate to grid of output tube causing the noise on the Bpluss through the Edcor tranny to cancill out-giving very good sonic results-global feedbac to input stage wouldnt be needed when using 2 chokes one for each channel -thankyou for telling me about the edcor diy stuff I never used one of these before-and the bypass cap would be a mod .
 
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Kegger said:
As far as feedback use with "ringing and overshoot", these are things that arise from inside the
output transformer and then are tried to be compensated by with Loop Feedback, this project
will not be using Loop feedback (I don't like to have to use it and feel a SE amp doesn't need it).
Loop feedback can also Cause ringing/overshoot then more tweaking is done to fix it.

By using the plate to plate/grid feedback we are attempting to send an output transformer the
best signal we can send it with minimal "screwing with it in a harmful way", to me and many of
the single ended projects out there minute amounts of short, uncomplicated feedback is where
the amp will "Sound best" yes that is subjective and not a testing definition.

The amp will not be one of the best bench testing amplifiers out there, but we will make it sound
good, this basic circuit has been used very successfully, also the output transformers have been
well documented as being a great performer along with being inexpensive, some adjusting of the
100K feedback resistors will be played with for best results, but in simulations it is not all that of
a critical adjustment with 100K being a fine "starting" point. (But we are using different tubes)
(Yes a small bypass cap may be tweaked in there as well to experiment with)

Anyone wanting to implement some other form of feedback being loop or other are fine to try it
on there own and post there results here for others who may want to follow your path. Also if
anyone would like to see how others have used/tested the Edcor outputs should travel on over
to DIYAUDIO.com were people like Tubelab among others have put them through the ringer.

This is no knock on anyone wanting/questioning the use of feedback, just the statement as to
where this project is going to start from, (And probably end, with very nice sonic results). . ;)
Click to expand...

You could very easily use the 40% screen tap on the output transformer as feedback ala Douglas' e-linear as the diver stage supply. It works quite nicely :thmbsp:
 
bricktop said:
You could very easily use the 40% screen tap on the output transformer as feedback ala Douglas' e-linear as the diver stage supply. It works quite nicely :thmbsp:
Click to expand...

Agreed and it's one of the mods I mentioned on the first page, could be very cool
to mess around with and change the amp after words to see/hear the difference to.
 
A quick update about the prototype:

To keep costs down, I'm recycling a chassis that I had parted out from long ago. It was a long skinny organ amp. I plan on building the PS on one side, and the signal path on the other. I will hand this off to Kegger after it's built, for tweaking. I will likely be building it next weekend, so if you want to come by, Wiggy, we should work something out.

I also stopped by a metal supply house that Pak mentioned to me. I think I might have a nice top plate for a simple diy style photoshoot build.
 
Squidward said:
A quick update about the prototype:

To keep costs down, I'm recycling a chassis that I had parted out from long ago. It was a long skinny organ amp. I plan on building the PS on one side, and the signal path on the other. I will hand this off to Kegger after it's built, for tweaking. I will likely be building it next weekend, so if you want to come by, Wiggy, we should work something out.

I also stopped by a metal supply house that Pak mentioned to me. I think I might have a nice top plate for a simple diy style photoshoot build.
Click to expand...

You got it. Shoot me a note on when you want to see my ugly mug and if I should bring bribes, er, I mean liquid libations.

Remember, folks, this is the prototype. Nothing to get excited about, we (meaning Squidward and Kegger, and me as excited onlooker) want to see if smoke comes out, and failing that, signal. No need to rush out and buy anything - we don't have a firm design yet, so no parts list.

But this is one step on the path.

Personally, I can't wait!
 
Well, I had a good talk with Pak yesterday about this project. I learned a lot, and am still digesting quite a bit of it, but he did say something that had a very practical application for this project. He basically said that he believed the cathode resistor on the pentode output section would have be differently sized for pentode vs. triode/ul operation. This kind of bummed me out, I had been hoping for the ultimate in flexibility with this amp, but it is what it is, I guess. This doesn't change anything in Kegger's published schematic, as he has the tube running in UL anyway. This makes me predict that the final build will have a switch for triode/UL mode only. The silver lining is that a much cheaper switch will work.

I've been sawing away at the carcass of an organ amp, and I think I've finally gotten somewhere with it. I'd take a pic, but the wife has the camera. I think that it's really ugly, but will make a great test bed. In fact, after the "basic build" project is over, it'll have plenty of room for upgrades (which we could also document). There might eventually be a "Pakked out" version that has dual damper diodes in the power supply, and the output section rigged for pentode operation with a tube regulated G2. We'll see how far the crazy train takes us.
 
Here's a few pics of the chassis of the prototype amp. I gave it a quick spray of primer, and it actually looks much better. It had a cutout for a power transformer that I enlarged, and then covered with the piece of perforated aluminum plate that I plan on mounting the output tubes to. As you can see, I'll probably not need to drill too many holes, as I have an assortment provided for me to choose from!

Enjoy!

http://tubehacker.com/events/wigwam_amp/
 
Squidward said:
Here's a few pics of the chassis of the prototype amp. I gave it a quick spray of primer, and it actually looks much better. It had a cutout for a power transformer that I enlarged, and then covered with the piece of perforated aluminum plate that I plan on mounting the output tubes to. As you can see, I'll probably not need to drill too many holes, as I have an assortment provided for me to choose from!

Enjoy!

http://tubehacker.com/events/wigwam_amp/
Click to expand...

That's awesome! Great photos, too!
 
Joining late, but.....

One option for chassis sourcing might be Macaltec, one of the most accomplished metal workers around here. If he can fill in 2 inch chunks missing from Altec horns and make them look essentially OEM, he should be able to whip up some metal plates with holes cut in them....:thmbsp:

I'll give him a call and see what the options are....

Meanwhile, looking forward to hear how this progresses
 
Cool

Very sweet looking test bed. Pick a hole, any hole.

Please take some pics of yourselves when you guys are doing this. It's always cool to see the "suspects" I mean people involved. Kind of personalizes it if you know what I mean. It's great and all when people post pics of projects but I'd like faces to associate with them if possible!

Have fun you guys. :thmbsp:
 
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