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

ED, and everyone else.

Power requirements, the outputs will run at roughly 48ma each, the drivers at
3ma each, so the total for the amp is 100ma give or take a small amount here.

I would suggest at least one of the chokes Newman pointed out as a fine choice
and I also like the idea of if it's tube rectified the EZ81, so the toroid would work.

I ran some power supply sims, and a small cap (10uf or less) followed by the choke
and cap (of say 100uf) then either roughly a 220ohm resistor (or another choke) and
150uf cap with a 275-0-275 trannie will yield a well filtered approximately 300v supply.
(This will work for both solid state or tubed rectification)

I'll show a schematic of the power supply to help what this looks like to the rest here.

If we buy a "but" load of those chokes, using 2 in the supply becomes cost effective.
And I would run them in an, C,LC,LC supply config so the whole power supply gets a
decent amount of current running through it, splitting it up won't filter it as nice and
the choke wont see much current to really do its job well. (the last LC can be an RC) :)

(A basic Solid State Supply Attached, 10-02-08) (upped the current a bit from original)
(you could skip the second choke and up the cap following the choke then to say 330uf)
 

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Going back to the Amp schematic for this an doing more setup plus thoughts,

I think the same feedback used in the RH amp should be well implimented here, also start
with 100K resistor seems to work nice by providing a decent amount of "friendly" feedback,
an it also has the added benefit of providing us a bit more voltage to the front end, which
in turn puts our driver in a better operating point as well.

When I get a chance I'll update the schematic to show these revisions.
(Schematic updated on page 1)

(EDIT/Update) WE didn't actually need/want as much feedback so the 100K became 270K...
 
Last edited:
These are the outputs that many will likely use, XSE15-8-5K.
http://www.edcorusa.com/products/transformers/xse/xse15-8-5k.html

(same price as the one above, basically $20.00 each)
It is possable that in UL mode the XSE15-16-5K may be a slightly better load
for the output tubes and running that trannie on 8ohm speakers makes, yields
a 2.5k output trannie instead of 5K output trannie, either will work but one is
going to be a better choice then the other, even though it may be slight.
(2.5K could give us a touch more power then the 5K load will)

I'd have no problem suggesting either one of these for this build, and if someone
has the 16ohm version I'd suggest using it, so ED if that's what you have then do
go ahead and build the prototype amp with that one and we'll test it from there.


EDIT: The original posted above 5K output is the one to get.
I do have the 5K outputs and will be using them when the amp comes my way.
 
Kegger said:
These are the outputs that many will likely use, XSE15-8-5K.
http://www.edcorusa.com/products/transformers/xse/xse15-8-5k.html

(same price as the one above, basically $20.00 each)
It is possable that in UL mode the XSE15-16-5K may be a slightly better load
for the output tubes and running that trannie on 8ohm speakers makes, yields
a 2.5k output trannie instead of 5K output trannie, either will work but one is
going to be a better choice then the other, even though it may be slight.
(2.5K could give us a touch more power then the 5K load will)

I'd have no problem suggesting either one of these for this build, and if someone
has the 16ohm version I'd suggest using it, so ED if that's what you have then do
go ahead and build the prototype amp with that one and we'll test it from there.
Click to expand...

Edcor has no NRE fee part for XSE25-8-2.5K, a 25 watt 2500ohms-to-8ohms SE transformer for $30.79/ea. I would imagine they have a similar 15 watt 2.5K-8ohm one already designed (so no fees), though not listed on the webpage. Give them a call :thmbsp:
 
A group buy portion of this could be the, OUTPUT TRANS, POWER TRANS, maybe
a common AMP CHASSIS for everyone to build from. So a chassis might be next?
 
bricktop said:
Edcor has no NRE fee part for XSE25-8-2.5K, a 25 watt 2500ohms-to-8ohms SE transformer for $30.79/ea. I would imagine they have a similar 15 watt 2.5K-8ohm one already designed (so no fees), though not listed on the webpage. Give them a call :thmbsp:
Click to expand...

Gotcha, didn't realize they have other trannies then whats listed, thanks!
 
Kegger said:
Going back to the Amp schematic for this an doing more setup plus thoughts,

I think the same feedback used in the RH amp should be well implimented here, also start
with 100K resistor seems to work nice by providing a decent amount of "friendly" feedback,
an it also has the added benefit of providing us a bit more voltage to the front end, which
in turn puts our driver in a better operating point as well.

When I get a chance I'll update the schematic to show these revisions.
(Schematic updated on page 1)
Click to expand...

Wouldn't the RH style feedback be considered plate to plate feedback?
I had a single-ended 6BQ5 Motorola amplifier from a largish record player that used plate to plate feedback.

It used a 12AX7 driver, I can't recall the value of R. It also had a ceramic cap bypassed across the resistor.

It seems to have some benefits. The older EE audio friend of mine seemed to like the idea of "RH" or plate to plate feedback.
He mentioned a old timer when he was young touting about plate to plate feedback, and it's uses.

I wish I could understand why, or the uses.....but he understood it.....I forget more easily....
 
hey-Hey!!!,
The term plate-to-plate is another way of saying plate-to-grid as the 'forward plate' is connected to the grid of the following tube. The tube is an inverting amp, grid input and plate output...so the plate signal is opposite polarity v. the input, so feeding it back reduces gain, improves bandwidth and linearity.

It is short path, local FB that is pretty much disconnected from the wacky phase shifts we encounter when passing the signal through a transformer. It is stable out into the high frequency, which is good.
cheers,
Douglas
 
I will be following along on this build but may be building a variation of the larger RH807. Great explanations and direction so far.
 
This sounds very interesting to me-I since joining ak love (pacprotecters)ideas,now to this project-like choke filtering a ez81,22uf c1 cap-2nd filtering 100uf-220uf-100 volts bias in open loop reduce a bit to arround 96-97 volts-edgcor will work well-I will look into this tube when I have time -but I believe that it will work well-6au8?
 
5k tranny will give less output -less distortion-2.5k a bit more robust-must see edgcors specs to know which is best match for 8 ohm load.
 
Bruce221 said:
This sounds very interesting to me-I since joining ak love (pacprotecters)ideas,now to this project-like choke filtering a ez81,22uf c1 cap-2nd filtering 100uf-220uf-100 volts bias in open loop reduce a bit to arround 96-97 volts-edgcor will work well-I will look into this tube when I have time -but I believe that it will work well-6au8?
Click to expand...

Bruce,
I looked up the 6AU8 data here: http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_1/6AU8-A.PDF

The pentode has a 3.3 Watt dissipation rating, and I'd qualify that as a signal amplifier device and not a power tube/section. The base is 9-pin mini, like a 6CG7, which would not exclude it out of hand, but the guts inside the glass won't stand the service we've got planned for the 6LU8.
cheers,
Douglas
 
mhardy6647 said:
Wigster... I am far too lazy to read this thread; did you look at this?
http://www.dogstar.dantimax.dk/tubestuf/miniblok.htm

EDIT: this one, too.
www.sparetimegizmos.com/Hardware/Stereo_6t9.htm
http://www.tubes4hifi.com/6T9amp.htm (same design)
Click to expand...

Thanks so much for those links mhardy... the first one and the way it describes the curcuits was a MASSIVE help for me to understand a little bit better about what is happening inside these amps, and why things are done they way they are.... VERY informative!
 
Sockets may be difficult to find for the 12 pin tube... Radiodaze and AES are both out of them... I have bought them on eBay and see a few for sale now. The 6LU8 has a less common 9 pin (Novar) equivalent - the 6LR8. But I don't think (real) Novar sockets are easy to find either... many are Magnoval - a later design with larger pins, which will be loose on a Novar tube.
 
Not sure what vendor I got mine from or how he had them listed, but I did
get some from the Bay a while back without to many problems, I'll look into
how/where I got them from and post a source.

Yes as Tom said there not a big seller so they are tougher to come by.
But they are out there and not a very expensive socket either as well.

We just need to do a bit of research and track um down.
 
It occurs to me to wonder: while figuring out a power supply, would it be possible (or desirable) to have power off/stand-by/on? One of my concerns is tube life. Leaving an amp on all the time shortens life. Turning on and off is also not good for the heater filaments. Is it possible to have a stand-by mode where the power to the grid is cut and the filament powered at a reduced level? This would reduce warm up time AND prolong tube life by limiting the current surge in the filaments when switching the power to full on.:scratch2:

Please be gentle--I'm a total noobie.

Cheers, Jim
 
A standby on the plate voltage would be a good idea, particularly if one opts to use a soiled state rectifier and one doesn't use a 'soft-start' current inrush limiter of some sort.
 
There is an ebay seller named PacificTV that currently has sockets in stock, as well. 29 of the chassis mount versions. I'll contact him and see if he'll work on his price if I buy 24-30 of them. I've purchased from him before and had an excellent experience.
 
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