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

Thanks for the Kudo's guys! I am liking the amp quite a bit and think it looks pretty cool.
It is built on an ST-70 chassis, an old one I had left over, the SE outputs are underneath.
(chokes are on top where the standard 70's outputs would be) 2 tubes a side in paralell.

Not yet has a power supply been broken down, ED and Wiggy will be doing a build then
publishing the power supply schematic, if someone wanted to build one and had a power
supply they wanted to use or know how to build one, it's roughly 300v at say 110ma.,

Kegger said:

Not yet has a power supply been broken down, ED and Wiggy will be doing a build then
publishing the power supply schematic, if someone wanted to build one and had a power
supply they wanted to use or know how to build one, it's roughly 300v at say 110ma.,

Click to expand...

I presume you mean 110ma for a stereo amp?

Not like there's any real possibility of me getting this done before Ed and Wiggy do, and a guy can dream, can't he?

I picked up another sleeve of 6LU8s at AES while they were on sale, plus 4 6LR8s and 4 compactron-9 sockets.

BTW, the compactron 12 sockets I got on ebay were immaculate and extremely well built.

Looking forward to when you guys can get a BOM together, and also some of the variants -- PSE, PP, etc.

JKeep up the great work,

Jim

Sorry, folks - I just grabbed the last of the really cheap 6LU6s at AES (I know this because the site automatically adjusted my order quantity way down). Their regular price is still pretty good, or you might try N9TEW ( http://www.hamtubes.com/tubes/list3.htm ). BTW, anyone please let me know if I'm out of line RE: the sales policy. I have no financial interest whatever in either company, just trying to share where I've found deals. N9TEW has a lot of tubes, many at very good prices.

***Edit - it's now not clear whether I got the last 6LU8s or not - stay tuned for further developments ***

Speaking of which, I've come across a site that has pretty much everything but tubes at makes-ebay-look-exhoribitant prices. Well, they have tubes, just not a whole lot of what I'm looking for. They do have what appear to be good prices on things like used 12AX7s.

Anway, I know that's sometimes not too hard to beat ebay, and this guy seems to be really good, especially on chokes and power transformers. While most of it is used, he says he tests it and guarantees it, so who cares? I found the site when I did some deep internet searching in frustration I after got beat on a big filament transformer on ebay. Anyway, they don't have online stock (just lists that aren't updated all that frequently). You find stuff on the list and ask for a quote. They also have compactron sockets, though they only list prices on PC mount (dirt cheap, BTW). I asked for a quote for conventional ones. In any case, I'll see how it goes and let people know (again, if that's OK per policy on this board).

I fooled with PSUD2 far, far later last night (this morning, actually), than I should have, and I think I somewhat got the hang of it. Who knows, I might be able to build and test a PSU in time, or (now I'm thinking) as an alternative/variation that could be used. Of course, I won't post any details until I actually build one.

Well you can post your power supply questions and we can try an help.

If you have a psudII sim you can take a screen pic an show it to us as well.

OK, I have a very basic question: Does the enclosed design seem to make sense, or am I way off base here?

All right, so that was more like TWO questions

Seriously, while I was climbing the proverbial learning curve with PSUD2, many of my efforts were truly horrible (closer to power oscillators then PSUs). I think I have a handle on it now, but what do I know?

BTW, I went ahead and ordered the (mostly) chokes and transformers from that place I mentioned. I checked with BBB in his area first, and they hadn't even heard of him, which I take as a good sign ;^)

I'll let everyone know how this works out.

He charges $3/each for Cinch compactron sockets, which struck me as a good deal. He is going to pack everything into one, big, flat rate box, which will no doubt have postal employees halfway across the country cursing the day I was born, my ancestry, etc.

Anyway, as part of this big order, I snagged a power transformer that should do it for the filament and B+ (for a simple stereo amp, which is what I'm starting with) It's rated at 230V@190ma and 6.3V@4A. If, by some miracle, someone wants to build a PSU based on my design, I think something comparable should be available easily enough.

Partly in the interest of making a duplicatable design (partly because they were so cheap), I managed to snag two Triad C-40X chokes (which are available new, and don't cost that much) They're rated at 320mH at 600mA.

Interestingly enough, they guy I'm buying all this stuff from asked me what I was going to do with all the low-value chokes I ordered. I told him about how I had seen a PSU design on another discussion board that used a low-value speaker crossover choke, and that using low-value chokes seems to work (on paper, at least) with PSUD2, if you get the other values right. That is, I modeled a PSU with the Triad choke specs before I ordered them, and it seemed to work.

So far, I've modeled only the B+ supply. I had to wing it on the unloaded voltage, and that's easy enough to tweak with the resistors. Likewise if somebody else wants to take a crack at this with a different transformer.

For the filament supply (which I haven't worked on yet), while I've considered some fancy stuff long term, in order to K.I.S.S. for the first iteration, I'l probably just do one or more stages do an RC filtering. The cost/availability of a 3A choke is another consideration, let alone even finding anything that is up to the task of the Triad F-28-U monster I ordered to use when I scale up.

Some explanation is in order. Suffice to say I'm sort of screen capture challenged right now, so I used the built-in printing capabilities of the program to create pdfs. The enclosed graphs need about 125% zoom, while you might want 200% on the schematic.

I don't have the chops to figure out what impedance the circuit will present to the B+, so I used a current load instead. Just for fun I had it step to 220mA for .5 seconds, then down to 110mA

Yes I am using semiconductor rectifiers. Some time ago, I downloaded (sorry, don't remember from where) a nifty article on how to design snubbers. The example used happened to be the 1N4007, and the author came up with what appears to be a very effective CRC snubber for that particular diode.

Of course, in the interest of simplicity, the snubbers will be optional, and I'm not going to even put them on until I'm sure the amp works.

The other thing is the weird values of the caps. I've seen more than one place that it is better to make small banks of low-value caps than use one big one. So, the caps are multiples of 22uF, which frequently falls close to standard values anyway.

Page 1 shows the Radiodaze 8H 150 ohm chokes, and page 14 shows the Antek
toroidal power transformer the prototype used.

And here's a pic of the basic layout to the power supply the prototype is using.
(you could skip the second choke and up the cap following the choke to 330uf)

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

Also for posting pic's of the screenshot from PSUDII, just do a (CTRL, PRT SCR)
then bring up paint an paste it, then use the crop tool to size it an save as a gif.

For starters yes your power supply is a little off base, and I wouldn't be using
those chokes you have there without a LOT LOT LOT of experimenting, those
are really for a sophisticated design doing a "Flywheel" concept, not something
I would suggest trying to build here.

If you post what your thinking before you buy we can help.

I'm posting a scenareo that should work with your power transformer, using 2
chokes in the power supply is not "needed" but if there inexpensive enough I
would suggest two of them, (there values are just tossed in there for now) if
you have a list of available chokes to you just post them here and we'll see if
we can make them work. Using 1 choke an a second RC filter will work as well.

Usuable choke values are like 4h to say 10h and 30ohm to say 200ohm,

Also you do not need nor want to do anything to the filament supply, you do
a basic ac wire up to all the tubes, so no power supply needs to be built there.

(here's the pic of a basic layout using Your power tranny)

Thank you for your indulgence of my misdirected, impulsive n00b enthusiasms, and your kind offer to use your skills and efforts to bail me out of what must look like a bit of a quandry.

And it's very cool that you plugged "my" power transformer into the design and modeled it.

Yes, I have ordered two of the Triad chokes, and others besides. They were cheap, but not in porportion to those Radio Daze chokes, which strike me as a screaming deal. So of course it would be no big deal to slap another LC section on there.

And, while I had my reasons for doing the most work on modeling a PSU with the Triad choke, I have a Philco 6.5H/150mA/200 ohms coming as well. I tried those values too, and managed to come up with something credible looking, after a few iterations of my all too frequent power oscillator type circuits.

It's funny - I honestly did go through this thread from beginning to end before I ever posted anything. Perhaps not reading every word, but enough to get the idea.

Still, I managed to miss the link to the Radio Daze choke, and some of the information/gifs on power supplies in the first few pages. I suppose I just didn't retain it because I've never approached this kind of thing from this viewpoint. While I can read schematics and have some minor practical experience with analog circuitry, I've never actually built any analog circuit this complex (not successfully, anyway - there was that PAIA phaser/flanger kit I never got working), much less tried to work up a PSU design on my own.

And I can't believe I didn't remember using about Paint for screen caps. I actually figured that out to show my son how to capture something from a video game to use as Windows wallpaper (I use MS word and sometimes Fireworks at work for this, and I have neither nor anything comparable on my Windows box).

At any rate, I've got to tell you that I am REALLY going to have to suck it up to build this with an AC filament supply. I guess it will be good experience to break my silicon habits in some ways, and that still won't make it easy. I guess it's all part of the discipline I'll have to show in resisting the urge to tart up my MkI version of the basic design before I even know that I can build something like this that works.

Anyway, thanks for the help you've provided, and further help you've offered, and your generally positive tone. I'll definitely take your word on the "...LOT LOT LOT of experimenting..." bit, though that strikes me as great fun. I have a copy of the ARRL handbook, and they have an article by a guy that pretty much torture-tested a 723-based regulated PSU by rigging it where he could inject AC signals to see how the feedback worked, test the loop filter, etc. While of course these are open loop circuits, I can think of three ways one might test/model the circuit dynamically to see how it responds. Naturally, if I ever have the ability to do such things, it won't be particularly soon.

So, I'll cool it on the DC filament supply, and try another LC section with the Triad choke (just for fun), as well as seeing what I come up with using the single Philco choke.

Oh, yeah - I forgot to ask: is there still some notion of using a motor run capacitor for the first C? There's still an honest-to-God, old school, independent electronics distributor here in Denver (in fact, I used to work there), and I wouldn't have any problem snagging one there. Is 10uF a good number, or is that just what happened to be available. If I recall correctly, there isn't that much of a correlation between capacitance and price in motor run caps.

I essentially transcribed your prototype on PSUD2. I've also added second a second choke to what I'm now calling my "goofy" design, though I finally figured out that you were referring to two Radio Daze chokes in the "rough draft" silicon diode schematic you put together.

Duh! I guess it's not hard to tell I haven't gotten much sleep this week.

I've also done a design with the big choke I've ordered. I've done two versions of both designs, one with 110mA load and the other with 5mA load. In both cases, there doesn't seem to be any effect except the voltage going up.

Been out, let me digest this.

So what chokes do you have available to you and or what were you thinking of buying?

Kegger said:

Been out, let me digest this.

Click to expand...

Not to worry, I don't have any expectations in this regard, especially since I'm constitutionally incapable of composing a short email or a message board post. As always, your feedback and input is most welcome.

Kegger said:

So what chokes do you have available to you and or what were you thinking of buying?

Click to expand...

While I do have a 6.5H choke coming, for my first build, (MKI), I'm struggling mightily to RTFM (as it were) and not tart up the design with extra geegaws or stray too much from the basic design, with the Si rectifier supply.

So, chances are that for my MKI, I'll go ahead and buy choke(s) from Radio Daze. What the heck, they're cheap.

Anyway, I've modeled your version of the PSU using "my" power tranny and two Radio Daze chokes, though the unloaded B+ voltage is kind of a crapshoot. Unless it's higher than I guessed, the PSU basically won't work with two chokes, unless I want to starve the amp.

In case you hadn't guessed, I'm the kind of person that's compulsive about knowing how things work. I've delved into guite a lot of tranformer and choke-related material this weekend (even - believe it or not - cracking some actual books). One very cool resource I found on the web is Turner Audio. Among other things, he has a page that goes through PSU design ( http://www.turneraudio.com.au/powersupplies.html ), step by step, with quite a number of formulas. One thing he explains is how to calculate the low pass frequency of an LC filter (which it is, after all), to make sure it's low enough not to cause problems with the amp.

I built a quick spreadsheet with most of his formulas in it (with some derivations). One formula I used right away gives you the DC impedance (resistance?) of the B+ circuit, which I immediately plugged into PSUd2. Also, according to the formulas, even with just 68uf for C1, ripple voltage is 3Vrms, even before the LC elements.

Speaking of buying parts, there was some talk of buying the 5K16 Edcor so it would give you a 2.5K primary with 8 an ohm speaker. Am I correct in assuming that 5K turned out to be the number after all?

On the schematic:
What is voltage rating on caps?
What does "Pol" mean? No luck so far finding anything "poly..." with 220uF capacity in Mouser catalog.
Power resistor is no problem, and even not that expensive: http://www.mouser.com/Search/ProductDetail.aspx?qs=i9jpE441ig96OzMtnSCr3Q== though I'm sorely tempted to get the cool-looking aluminum heatsinked ones (http://www.arcol.co.uk/uploads/products/hsseries-10.pdf) - they're still not that spendy, and they are 1% after all.
What wattage, composition, tolerance on resistors with no wattage rating on the schematic?

Thanks again.

Here's my admittedly simple view of hitting a target voltage with a given transformer. I'll use the prototype amplifier as an example. The Antek we're using as a power transformer (PT) has a rated output of 275-0-275. If you do a full choke input, you usually get around .9 times the voltage out, or (275 * .9 = 247 ) around 250 volts. If you do a full capacitor input filter for a tube rectifier (say around 40uF), you get around 1.4 times the voltage (257 * 1.4 = 385). If we're trying to hit 300V, it is within the range (250-385), but we need to "tune" the filter to give that exact voltage. When I made the prototype, I made a board of caps that could be swapped in and out to make different values for the first capacitor of the filter. Keg did the actual experimentation to find the right capacitance, but it had been my idea to start with some small amount, like 2uF, and increase the value until the voltage rose to our target voltage of 300VDC. Thus, we can find it by simple experimentation. For our line voltage, the first cap ended up being around 7uF, I think, but Keg can confirm. This is a really nice solution, because by using a smaller cap in this position, it makes it possible to use a higher quality cap, for both size and economic reasons. You also give your first choke a lot of protection. Rectifiers like it, too, because the smaller cap size stresses them out less. Granted, a pure choke input supply has the best regulation characteristics, but in that scheme you need a higher voltage power transformer, and a much beefier first choke. A lot of us feel that the prototype's power supply is a great compromise, high performance on a budget. That being said, tho, I expect that the power supply portion of the amplifier might be the most modded, as I'm sure some people will want to recycle iron from old console amps, etc. Luckily, 300VDC is a very common target for a lot of supplies.

lloydrmc said:

Speaking of buying parts, there was some talk of buying the 5K16 Edcor so it would give you a 2.5K primary with 8 an ohm speaker. Am I correct in assuming that 5K turned out to be the number after all?

On the schematic:
What is voltage rating on caps?
What does "Pol" mean? No luck so far finding anything "poly..." with 220uF capacity in Mouser catalog.
Power resistor is no problem, and even not that expensive: http://www.mouser.com/Search/ProductDetail.aspx?qs=i9jpE441ig96OzMtnSCr3Q== though I'm sorely tempted to get the cool-looking aluminum heatsinked ones (http://www.arcol.co.uk/uploads/products/hsseries-10.pdf) - they're still not that spendy, and they are 1% after all.
What wattage, composition, tolerance on resistors with no wattage rating on the schematic?

Thanks again.

Click to expand...

Yes the 5K 8ohm tranny is the one to use if your primarily running the amp on 8ohm speakers.
(4ohm version on 4ohm speakers and 16ohm version on 16ohm speakers, all the 5K variants)
The amp responds well using 4,8,16ohm speakers with the 8ohm tranny, so it is flexible though.

On the schematic “POL” stands for polarized, electrolytic and showing the polarity if one is used.
A poly or oil cap is just fine as well an may even be an upgrade, so yes try them if you have them.

Voltages are on the schematic, so cap voltages should follow with say a 50% increase or more of
voltage rating on the cap in that position, so if you have 300v then a 450v is more then adequate,
150v at the coupling area would mean say a 225v rating or higher cap, at the output cathode 37v
then a 50v cap would work but I’d probably go at least 75v there, again any kind of cap you want
to use will work, except the coupling cap C1 needs to be a non polarized better quality cap of say
at least a poly, a nice Russian oil cap would serve very well here, same as C5 and C4 if used to.

Resistors are all 1/2watt except were specified, again any kind will work, but I’d use primarily the
carbon film type, maybe carbon comp for the 100k plate load resistors, metal film for the 2.7k RC
filter and wire wound for the output cathode resistors. Tolerance is not critical either, just so they
are fairly equal between both channels should be just fine.

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

(Pic’s attached)
Now if I had the parts you have this is the power supply I’d build, I’m showing 2, if your tranny is
a center tapped one then I’d use the full wave, if not then the bridge is what it is. Your tranny as
you said is fairly low so no tube rectifier and those Radiodaze chokes would not be preferred here
as well, so it’s solid state rectified and using the chokes you have will get the right voltage an be
a nice clean dc supply as well, I modeled it with the chokes in both spots and it works better with
the larger one first, I also kept all your 22uf caps in parallel, if you wanted to use motor runs here
no problem, should work very very well indead.
(you can tweak the first cap size when you build the supply to adjust the output voltage, ala Squid

Squidward said:

Here's my admittedly simple view of hitting a target voltage with a given transformer.

Click to expand...

"imple view" or not, you, do know what you're talking about, after all. My problem was that PSUD2 "wants" unloaded voltage, which is higher than the rated voltage, but I didn't know how much. Turned out to be entirely moot, anyway (see below).

[snip]

Squidward said:

When I made the prototype, I made a board of caps that could be swapped in and out to make different values for the first capacitor of the filter. Keg did the actual experimentation to find the right capacitance, but it had been my idea to start with some small amount, like 2uF, and increase the value until the voltage rose to our target voltage of 300VDC.

Click to expand...

Cool.

Squidward said:

Thus, we can find it by simple experimentation. For our line voltage, the first cap ended up being around 7uF, I think, but Keg can confirm. This is a really nice solution, because by using a smaller cap in this position, it makes it possible to use a higher quality cap, for both size and economic reasons.

Click to expand...

Makes sense. The guy at Turner Audio talks about how you had better have a cap with a high ripple voltage rating (e.g. motor run) if it's right after the diodes and the value is low. PSUD2 certainly shows this, as the I(T1) graph looks pretty scary in this scenario.

Anyway, in the process of checking some cap specs from data sheets linked at Mouser, I came across something I was pretty impressed with.

I used to work at Fistell's electronics here in Denver, which still sells parts to the ever-decreasing pool of electronic repair shops. Anyway, we used to sell a LOT of Nichicon 105C caps. The techs liked to use them because they were unlikely to fail again.

Not that it's a low value, but check this out: - 68uf@450v, temperature rating -20 to +105C, ripple rating 1575 mArms at 105C and 100kHz (the lower values have lower ripple ratings, too). Impressive as all that is, Mouser charges $2.68 each ($2.44 if you buy 10). I don't know anything about their minimum order, shipping, handling, etc., and I've gotten the impression that they're a good place to buy small quantities of parts.

Squidward said:

That being said, tho, I expect that the power supply portion of the amplifier might be the most modded,

Click to expand...

That is highly likely. For my part, as ignorant as I am, I can still pretty much understand the basics of how the PSU works.

Squidward said:

...as I'm sure some people will want to recycle iron from old console amps, etc. Luckily, 300VDC is a very common target for a lot of supplies.

Click to expand...

That is me all over. The challenge is, it turns out that my guy did not in fact have the most suitable transformer in stock. He just stuck a cash refund in with the rest of my order. It's a drag because I'd much rather have the xfrmr than the whole $8.75 I paid for it.

While he did make a mistake, it was a simple and honest one, and I'm still pretty happy with the transaction. It took a couple of days back and forth narrowing down what was in stock, pricing, etc. I ran the PayPal after 9:15 PM on Thursday, and the flat rate box with (mostly) about 30 lbs of vintage iron was sitting on my doorstep today. I do a lot of mail order shopping, and that's about as fast as I've ever had something be shipped from a couple of states away (Ohio to Colorado).

So, since I'm satisfied with the result, I'll let people know that my mystery source is http://www.oldradioparts.com/ He's a one man show, and will be out until the 24th.

At any rate, I may way well just kluge up something. I do have another power xfrmr I ordered that might be made to work, 'til I can get something better or decide to suck it up and buy the big toroid. I certainly have enough different chokes to do some real world experimenting. I just need some caps and diodes to play with. I like your multi-cap board idea. They used to have large quantities of some fairly oddball caps. Maybe I could rig something with some series/parallel jumpers.

In any case, thanks for your advice/insight.

Kegger said:

The amp responds well using 4,8,16ohm speakers with the 8ohm tranny, so it is flexible though.

Click to expand...

Cool. I guess that's what comes with using such a sturdy tube, with an excellent circuit design to boot.

Kegger said:

On the schematic “POL” stands for polarized, electrolytic and showing the polarity if one is used.
A poly or oil cap is just fine as well an may even be an upgrade, so yes try them if you have them.

Click to expand...

DUH! [slapping forehead] I can't believe that I didn't pick up on that. In my defense, my eyes aren't fabulous, I've misplaced all my reading glasses, and my new ones are still on order.

Still, there that little plus is on the schematic. Not as plain as day, perhaps - as plain as twilight?

I have very few components on hand, surely no high-voltage caps to speak of. I'm just searching a couple of surplus places and Mouser. I'm definitely open to other suggested sources.

Given the high instantaneous currents PSUD2 is showing on some of the components in some designs, I wanted to find out about how much of an overload they should be expected to handle for a short period of time. I did some searching over the weekend, and in the process, I came across some publications about resistors, mostly by folks at IRC. Turns out there's a good reason not to use film resistors in certain high voltage applications. They're trimmed by a laser cutting a groove in them (frequently in a spiral), and this presents a failure point. A while back I read about an amp a guy made with 813s, and he had a catastrophic failure in a film resistor. Of course, he's running 800V on the plates!

Again, according to the IRC guy, the resistive parts of other types of resistors can fail (sometimes spectacularly) before the heat even has a chance to travel into the rest of the resistor where it can be dissipated. He said this is why carbon comps hold up so well - they're ALL resistive material, with no grooves, etc. to compromise that. All the power is dissipated throughout the whole device.

Interestingly enough, all things equal, noninductive wirewound (like IRC NAS series) can take even more than a regular wirewound, because they're wound in both directions to make them non-inductive.

So I'm not arguing with your recommendations here - I just thought you would find the above interesting.

I like the idea of a sort of low-key matching. Surely half watt reistors are cheap enough to where I can get some extras of each and so I can come up with pairs that are similar values as you suggest.

Kegger said:

Voltages are on the schematic

Click to expand...

DUH again [slapping forehead again]!!

I'm honestly not this dumb. Well, not all the time, anyway ;^)

[snip]

Kegger said:

(Pic’s attached)
Now if I had the parts you have this is the power supply I’d build, I’m showing 2,

[snip]

(you can tweak the first cap size when you build the supply to adjust the output voltage, ala Squid

Click to expand...

Now I feel horrible, because you've done all this work on my behalf, and guess which was the ONE transformer my guy didn't have after all.

...Murphy's law for ordering parts, I guess.

I may end up going a different route with the caps, too, unless I can find 22uF@450 much cheaper than I have so far. All Electronics has some 47uF@450v for real cheap, and the leads are very short, so I don't know if I want to mess with them. And those high-temp Nichicons seem like they can take an amazing amount of abuse (at not much more money than the above, in absolute terms) - see my reply to Squid.

BTW, Alltronics (I'm constantly getting the two mixed up) has some 3900uF@400v, computer grade, which look awfully tempting, though they would need something like a 30 second soft start. Definitely not MKI version material.

Speaking of surplus places, I just remembered Electronic Goldmine, and they do have what appears to be good deals on Aerovox polypropylene metallized film caps. I just have to see if they have anything usable for this project.

In any case, I'm really glad you are possessed of so much patience and a good sense of humor about all of this. I'm slowly figuring all of this out, and I honestly think I have the chops to do this, given the all the wonderful support I'm getting on this discussion board.

AES had it right, and they still have 6LU8s

Folks, all I can say is it's a really good thing that I don't do this for a living, because - paraphrasing Zig Ziglar - my kid would be even skinnier than he is.

I just now checked, and AES still has 6LU8s at half price ($3.05/each).

They had actually gotten back to me pretty quickly, and the messages ended up as false positives in my spam filter.

At least I didn't go on about what a bunch of bums they are at AES. I just knew there had to be something else going on, and there was. The order was goofed up, and it was all my fault. Truly.

So, if there's any bums around here, then it's me.

Data Sheets

All,
I found some data sheets at: http://www.mif.pg.gda.pl/homepages/frank/vs.html

The GE one looks particularly good.

BTW, I got my tubes from AES. Mostly GE with some Sylvania and a couple of Zenith.

Yes, I know how little that likely has to do with what factory(ies) they actually came out of.