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

Fran604g said:

I'm liking this, It's kinda my speed.
Thanks, folks!

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You like train wrecks?
Just my Sunday morning thoughts,
I am hoping that somebody actually gets it and can actually source out an amp design as requested, where it starts off simple and then after wallets recover people take the design and have group upgrades. For example, a tube rectifier is not necessary as well as more parts and money, 10 bucks here and 10 bucks there add up and all of a sudden it is 400. and rising with custom built transformers.
Group circuit upgrade could be done down the road, like a tube rectifier cicuit and the power supply learning experience continues without forgetting people's wallets, etc.
Our chip amp effort was an unsuccessful mess from trying to group build on 3 different levels at the same time. Most of us including me have a partially built amp in a cardboard box in the garage somewhere because most of us noobs got tired of the headaches from trying to wade through it all. Of course those that didn't need a group build in the first place finished theirs.
Just have to keep it simple, cheap, and everybody on the same page, even those that can barely solder.

Carl

Nice

Congrats Wiggy in getting control of your project. Not any easy thing to do around here from what I've seen so far!

I'll be tagging along for the ride, and I've already learned a bunch from reading the previous posts in this thread. :thmbsp:

As long as it stays relatively simple and inexpensive, I'll be building it as well. Should be fun!

Well you can call it a group build or just a tutorial of how at least 5 people will be
building an amp, as this was Wiggy's "want" then mine and Ed's "YAH I will do that"

So there are definatly a few of us who are going to build this and many parts will be
sourced for others to follow along if they care to, if they can't get something or need
help just ask and I'm sure someone will lend a hand.

CarlV said:

You like train wrecks?
Just my Sunday morning thoughts,
I am hoping that somebody actually gets it and can actually source out an amp design as requested, where it starts off simple and then after wallets recover people take the design and have group upgrades. For example, a tube rectifier is not necessary as well as more parts and money, 10 bucks here and 10 bucks there add up and all of a sudden it is 400. and rising with custom built transformers.
Group circuit upgrade could be done down the road, like a tube rectifier cicuit and the power supply learning experience continues without forgetting people's wallets, etc.
Our chip amp effort was an unsuccessful mess from trying to group build on 3 different levels at the same time. Most of us including me have a partially built amp in a cardboard box in the garage somewhere because most of us noobs got tired of the headaches from trying to wade through it all. Of course those that didn't need a group build in the first place finished theirs.
Just have to keep it simple, cheap, and everybody on the same page, even those that can barely solder.

Carl

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Could be worse, I guess. I'm just seeing where the train is headed...

I came across this and found inspiration:

http://www.electronixandmore.com/project/12.html

hey-Hey!!!,
On the rectifiers, there are indeed a few ways to skin the cat. Starting with a CT/full wave arrangement of 275-0-275 we can run with two UF4007's. These are about the most inexpensive of the fast/soft recovery diodes. IMO, the next level of speed would be Fairchild Stealth diodes in 1k2 PIV( peak inverse voltage ) TO-220 folowed by the SiC Schottky types from Cree.

If we go with a full bridge with the two 275 V coils in parallel, we can use 600 PIV-rated diodes, but we'd need four of them for the full bridge( see the link about rectifier designs I posted earlier ). 600V rated Fairchild Stealth in 600V PIV have recovery times in the few tens of nano-seconds...quite fast, eh?

Any of the suggested Antek power TX's can comfortably run the heater for a 5V rectifier tube like a 5AR4.
cheers,
Douglas

caddisgeek said:

sounds interesting, what about those outside the US and run on 240v, can we play too?

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A transformer has a primary that you plug in to the wall, and a secondary that will provide the high and low voltage that the tubes need to run. You must chose a transformer that has a primary that is compatible with you local AC voltage. Some transformers have primary options that will allow you to run on different primary voltages, some don't. The Antek torriods have a "dual primary" rated at 115VAC. This means that if you have around 115VAC (here), you wire the two primary windings in parallel and it will work. If you have around 230VAC (where you are), you simply wire the primaries in series. I guess having multiple primary options is a plus for the Antek. I know the Hammond "300 series" transformers have multiple primaries as well, but unfortunately they would be too expensive for the scope of this project. Thanks for the reminder that we should keep in mind that we have a global audience!

6lu8

From a quick search online I didn't find a lot of examples of people using the 6LU8. Found a couple PP amps but no SET amps. There is mention of a 6LU8 SET schematic in Glass Audio but I was never able to track it down.

Nice to be a part of something that hasn't been done many times before already.

The plot thickens.... :thmbsp:

Pass the salt.

chasaboo said:

From a quick search online I didn't find a lot of examples of people using the 6LU8. Found a couple PP amps but no SET amps. There is mention of a 6LU8 SET schematic in Glass Audio but I was never able to track it down.

Nice to be a part of something that hasn't been done many times before already.

The plot thickens.... :thmbsp:

Click to expand...

I have a lot of Glass Audio, if you can narrow my search I'll be able to find it more quickly... That pentode section will make a triode very much like a 2A3 with about 50% more gain and 2A3-ish plate resistance.
cheers,
Douglas

I'll weigh in more later. Translating for Douglas takes time, and it's Mother's Day.

Everyone, quit playin' with tubes and the Internet and go tell the women in your life you love 'em!

The kids and I made beakfast for the wif this morning so I am safe for the day, Don't want to spoil her to much ya know:nono:
But I am playing some lp's she loves, The Carpenters and she is snoozing in the recliner now.I I get to crank it some as it is in the other room.

I am reading an article in the May 2008 Audio Xpress magazine by Pete Millet, entitled "The Mighty Midget." He build an SE amp using two 6T10 tubes. The story began when he picked up a couple of midget output transformers in Japan, but he suggests that a pair of Edcor XSE-10-8-8K transformers would work just as well. He used a Hammon 261G6 for a power transformer, but he said he was pushing a bit, and it gets hot. He suggests something with a bit more grunt. For keeping it cheap, he says a Triad N68X plus a 6.3V 2A filament transformer would work fine.

http://www.pmillett.com/midget.htm

He says he is getting about 4.2W output in SE pentode mode. He says he tried other tubes, such as the 6EM7, but found the distortion unacceptable - and he put a $5 price tag on the tubes he searched for (the 6LU8 goes for around $6, I think).

He is using local feedback to control distortion, I think (I'm getting fuzzy on this) by a bias voltage to G3, using an article he read by O.H. Shrade of RCA dating to 1934 entitled "Beam Power Tubes".

It appears he had to design a small IC section to deal with startup thump and shutdown squeal, he's not sure what was causing that.

Thoughts?

Great thread, considering I am pretty low on are learning curve.

For a minute there I thought your choice of a 6LU8 pentode tube was
the same one used in an amp I recently came across.

But this one turned out in fact to be a 6GW8 in push pull, the
era being 1964.

totem said:

Great thread, considering I am pretty low on are learning curve.

For a minute there I thought your choice of a 6LU8 pentode tube was
the same one used in an amp I recently came across.

But this one turned out in fact to be a 6GW8 in push pull, the
era being 1964.

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As I understand it, the 6LU8 is a 'compactron', being several tubes shoved into one glass envelope, while the 6GW8 is a single tube, which can be used as a triode or a pentode. The 6LU8 is essentially two tubes in one socket. Also, it would seem a bit of a difference in price.

However, like you, I'm just starting out here.

Long waay to go.

PakProtector said:

First, there are three power levels of the 275-0, 0-275 Antek power torroid. From smallest to biggest the difference is $10, with the big one capable of about an amp of current from the HV coil.

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Pak says that if we are looking at the Antek torroids, there are three options that have the desired high voltage characteristics that we desire. The Antek model numbers start with "1T", "2T", and "3T". The price difference between the "1T" model and the "3T" model is $10. I believe that Douglas is saying that for $10, you really should go for the increased capacity. In my experience, larger transformers run cooler in a given application. This may be a desirable characteristic, and well worth $10.

I checked Antek's site, and I really only see the 275-0-275 high voltage secondary transformer in a "3T" package. It is the "AN-3T275" model. This might be the most attractive option from Antek.

Second is tied to the current capacity options, and that is using a pair of tube diodes on a 275-0-275 HV coil or putting a half SS diode/half tube diode full bridge on a paralleled set of HV coils.

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Douglas says that we have the option of using a tube rectifier or solid state diodes with the Antek. It is also possible to do a "hybrid" rectifier, where you use both. There are wiring options for both.

If you put the coils in series, we'll only need a single double diode, perhaps a 5AR4 to do the job. A pair of single diode tubes from the damper diode family( such as 6DE4, 6AU4, 6AX4, 6CJ3, 6DN4 ) are quite inexpensive, and in this service are operated very conservatively.

Running damper diodes will take up the supplied 4A 6.3 v winding on the suggested Antek power TX, so a 3A 6.3V tX would have to be added for the two power tubes each needing 1.5A. That could be a Xicon, supplied by Mouser for $7 each. Two damper diodes, and the 3A filament TX are approximately equal to the price of a single 5AR4...

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Douglas says that if we use the Antek PT, and we want to use a tube rectifier, we have a few options. There are standard 5 volt rectifier options, and Douglas mentions the 5AR4. I know that we can probably also use the 5U4 and 5Y3. These are all octal tubes, all of them contain 2 diodes. The Antek PT does not have a native 5 volt winding, however. I think that Douglas is implying that a 5 volt tube could be run off the 6 volt winding, I don't know how tho. If one wanted a 5 volt rectifier, I think it'd be easy to get a separate 5 volt filament transformer.

Douglas suggests that you could use the entire 6 volt winding from the transformer to heat a pair of damper diodes. A damper diode is a very tough rectifier used primarily in TV circuits, they are rather inexpensive too. They eat up a lot of heater current, but they are amongst the best rectifiers ever made. The disadvantage to this setup would be the need for two unusual sockets, and it would eat up all of the 6 volt heater from the PT (hence the need for an additional $7 heater transformer). You'd end up with an amazing rectifier, but I think it would be overkill for this application.

Frankly, the more I think about it, the more I like the 6CA4 or EZ81 rectifier. It's a standard 9 pin tube, so sockets should be cheap and easy to find. I know it will work in this application, I've built a very similar amp and used this rectifier. It uses only 1 amp of the 6 volt winding, leaving 3 more to heat the rest of the tubes. This would be perfect, as a pair of 6LU8 tubes would need 3 amps. Even Douglas likes this tube (tho probably not as much as damper diodes).

This is Wiggy's project, and I'm more than content to let him call the shots. That being said, I might recommend the Antek power transformer with the 6CA4 / EZ81 tube rectifier. The Antek allows for 115VAC and 230VAC operation natively, and being a torroid might not give off as much of a magnetic field as a standard transformer (not that I'd be worried in any case). If a tube rectifier is desired, the 6CA4 is more than capable, and can be operated off the Antek with no supplemental iron for any of the tubes.

If a solid state rectifier option is desired for cost/simplicity, I think that's a great idea, too. I agree that we shouldn't go nuts and make a million options and confuse the project, but I think that allowing a choice between a solid state and tube rectifier wouldn't be that bad. When we get closer to building a Bill Of Materials (BOM), I'll price both options out. When we make a tutorial for how to build the amp, I will include both rectifiers as options.

Any thoughts?

Bless you, sir.

Squidward, still not entirely sure why the toroidal over a standard transformer? Price? Coolativity? Temperature? Just curious.

A "standard" transformer as most people know them are also called "EI" transformers. This is because the laminations used to make them are in the shape of the letters E and I.

The advantage of a torroid over a standard EI transformer is that they are more efficient. They generate less heat, they are smaller for a given power rating, and they give off less magnetic leakage.

Ironically, the advantage of an EI is that by being less efficient, it actually rejects more line noise than a torroid would.

Careful construction practices will hopefully help mitigate any magnetic field problems from any transformer. This means that while a torriod might produce less interference, EI iron should not be ruled out.

The reason that the Antek appeals to me is not because it is a torroid. It just happens to be a torroid. We are looking at a power supply that will need to make about 300 Volts DC. This means that we need something in the 250 to 300 ballpark before the rectifier, if we intend to use a Capacitor input filter (which for this project, we do). The Antek comes in with 275, right on the money. Also, to realistically use a tube rectifier, we need the high voltage secondary to have what is known as a "center tap". The Antek has two discreet windings, each is 0-275. By combining them, we will get 275-0-275, and create a center tap. A center tap is simply a reference directly between two of the winding's potentials. This is a long winded way of saying that it is tube rectifier friendly, and will give us the desired voltages before the rectifier.

Tubes also require "heat", or a certain amount of low voltage power to work. The main tubes to worry about in this project are the 6LU8 tubes. Reading their data sheet, one can discover that they each require 1.5 amps of 6.3 Volt heat. The Antek has a 4 amp 6.3 Volt winding. If we use this winding for that purpose, we still have 1 amp of 6.3 volt capacity left over (which I think we might use for the EZ81 rectifier).

The Antek also has the dual primary, so that we'd have a more global transformer. Our friend down under, for instance, would probably be able to use the same transformer at 220VAC from the wall, and be just fine. This wasn't an initial design requirement, but it's a nice touch.

The cost of the Antek is around $40 before shipping (maybe less if we do a group buy). When pricing out the other iron, I'm not sure that we found as much flexibility at the same price point. I think that the only cheaper option was the PT from musicalpowersupplies.com (MSP). It had it's own advantages, such as a slightly variable primary for 120VAC lines, but no 240VAC options. It had 5 amps of 6.3 volt heater, but that was split across two windings. I bet that the MSP transformer would be run a lot closer to it's maximum ratings, and would produce a lot more heat. We could make it work, but so far the Antek looks like a very attractive option for less than $10 more.

So, long story short, geometry aside, the Antek looks like a winner for price vs. performance. Just my $0.02.

That makes a lot of sense, Squidward, thank you. It's an ugly little spud, though, isn't it? I guess we could put a hat on it or something.

I think the fact that it would do well for 120 as well as 240 volt mains guys makes it a no-brainer. That way we can all build the same amp, right? I mean, aside from any add-ons you propeller-head guys can come up with afterwards.

If we're settled on that, then we have the tubes and the power amp. What's next, the output amps? Or do we have to make a decision on the tube rectifier-versus-diode thing? I'm still a little fuzzy on what it's supposed to do.