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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

21LG6/A P-P for 50+ watts?

ericj

Member
2008-12-08 10:24 pm
I've had sweep tubes on the brain for a couple days, growing out of having compactrons on the brain for a while.

I should preface this by admitting that while i have built many circuits successfully, it's been mostly headphone amps and other low-watt audio gear.

Just one of those, completed anyway, has been a high voltage amp. Alex Cavalli's "Bijou" all-tube OTL futterman headphone amp, 6n1p+6n6p in this configuration. 350v b+ iirc.

I'm not a complete n00b and I'm smart enough to not kill myself with high voltages but i am not used to calculating values on the fly.

I'm near done building a Super Squirrel Monkey guitar amp for my 8yo niece, who recently saved up her pennies and bought her first strat. No kidding. 12ax7 + 6af11, B+ a bit under 200v, probably about 3W (plenty for a practice amp).

So anyway. That's me.

21LG6/A beam power pentode. Rated 28w max plate dissipation in horizontal deflection duty. Not expensive or rare. Scroll down in this pdf for full specs, such as they are for sweep tubes:

http://www.mif.pg.gda.pl/homepages/frank/sheets/191/2/21KA6.pdf

Unusual heater voltage but Antek sells a 100va 20v toroid for about $21 that should power four of them about fine.

I've read a big chunk of the various threads and websites about sweep tubes. Sounds like in audio duty, particularly push-pull class AB, they can often be driven much harder so long as screen current is kept in check (due to the grids being awful close together on sweep pentodes).

I've seen hyperbole about being able to drive 30w sweep tubes in PP to well over 100w/c output but I'm not holding my breath.

I'm well aware that most of the money will be sunk into transformers. But it would still be nice to make something with tubes that don't come with an audiophile price tag.

Also, that GU-50 tubes are cheap and plentiful as well if i want a tried and true tube for 100W p-p at a really high b+. But people have done that before.

I like the look and ideas behind Pete Millet's mono universal PP driver board. Plus, glow regulators, right? I actually have a pair of 0b2 in the junk box just waiting for a reason to glow.

I don't expect this kind of project to be quick, simple, or cheap. I figure i could acquire the boards and tubes, decide on topology, and gradually determine the values for driver board components to achieve my goals.

How dumb is this idea?
 
Not dumb at all, perfectly good idea. In fact I should have a 100 watt prototype up and running with 13GB5's today if it keeps snowing.

The tubes you have will make stupid power, well over 100 watts if fed enough B+ and a proper load impedance. The trick to sweep tubes is to regulate the screen voltage. Most sweep tubes need about 150 volts on the screen grid.

Pete's driver board will work good for this application. It does not include a screen regulator so you will need to make one. Peak screen current can exceed 50 mA on signal peaks, so you need an active regulator, a simple glow tube and resistor is usually not good enough, especially at lowish (for a sweep tube) plate voltages. I usually use a zener / mosfet design. You can copy the design from Pete's Engineers Amp.

In fact there is a lot to be learned about sweep tube amps by reading the entire thread. I created a 125 WPC amp design with minimal modifications to the board using 6HJ5's which were cheap $4 each. At least 10 people built them. The board is compatible with a dozen or more different sweep tubes, but the 21LG6 is not easily usable in that board.

Over 50 watts will be easy on 450 volts with a 3300 ohm OPT.

http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html
 

ericj

Member
2008-12-08 10:24 pm
What i worry about, a bit, is my ability to figure this stuff out. But only because i haven't done that yet.

6hj5 are still pretty cheap. Not quite as cheap as 21lg6/a, but i haven't actually bought those yet. I've just made a brief study of relative prices.

Using edcor iron, it's arguable that the difference between 60w output and 100w output is incremental. $70 vs $100 per channel (about).

Anyway, building small circuits on perfboard is well within my ability, with regard to the screen regulator.

I will try and read more of that thread.
 
The 21LG6/A and the 6HJ5 are both available at VacuumTubes.net for $4 each.

The two tubes have fairly similar characteristic curves, so a design that works for 6HJ5 should work for 21LG6/A too. Unfortunately, the pinouts are not quite so similar as to be readily interchangeable.

http://frank.pocnet.net/sheets/123/6/6LG6.pdf
http://frank.pocnet.net/sheets/084/6/6HJ5.pdf
http://frank.pocnet.net/sheets/084/6/6HD5.pdf (same tube as 6HJ5, different screen V shown)

plate curve pics, 50mA/div Vert., 50V/div Horiz.:
a) 21LG6/A
b) 6HJ5

and the new Crazy Drive (g2 and g1) curves for them (these are somewhat adjustable via a pot for Rg2g1; and one can tune these two tubes to be virtually identical with a little pot tweaking):
c) 21LG6/A
d) 6HJ5

No matter what you pay for a conventional "audio tube", they don't come close to what TV Sweep tubes can do now.
 

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ericj

Member
2008-12-08 10:24 pm
surplussales.com is selling 21lg6/a for $3.50 and states that they have a large quantity available. Rated a bit beefier than 6hj5.

There is temptation to just do exactly what someone has already done and have a good amp at the end, but i wouldn't learn anything. In fact, if anything, what I've proven over the last 15 or so years that I've been building headphone amps and the like is that i'm really good at assembling stuff.

And there's a limited satisfaction there. I mean, lots of people call that work.

It sounds like i could extrapolate a lot from engineer's amp builds for what I am looking at, though.
 
Pete Millett has a driver board using a 12GN7 driver pair that could be used too.

ESRC has the similar 12HL7 on their $1 list. (although these are probably the RCA made ones, which look more like 7 Watt tubes, versus the original Sylvania 10 Watters, perfectly fine for driver tubes though.)

Summer Dollar Days - Vacuum Tube Sale - $1.00 Vacuum Tubes

If you are only looking for 50 Watts output (those 21LG6s will easily do twice that), then there are 38HE7 tubes on the $1 list (and with no plate cap, like the 6HJ5). The 38HE7 usually has series heaters for the two elements in the tube (pentode and damper diode), with the pentode section heater only, available between pins 10 and 12 using 21 V. The pentode is big enough that it should be able to dissipate 15 or 16 Watts without the damper diode operating. (the 12HE7 is on the $1 list also, but generally has the heaters for both sections in parallel, so cannot separate them.)

6CB5, 6EX6, 6CD6GA, 26DQ5 some big Sweeps, on an octal base, for cheap too.

And if you want 200 Watts+, there are 35LR6, 26LX6, 36MC6, 42KN6, 36/26LW6 .....
 
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ericj

Member
2008-12-08 10:24 pm
More power is always tempting. I do have some speakers that would appreciate it - Ohm Walsh 3, Magnepan SMGa, ADS L810, etc.

600v isn't substantially more dangerous than 450v, is it?

Just more money, different ratings on some parts, different values on others.

Maybe i should go big.
 
"Maybe i should go big."

The incremental cost does generally go down with the bigger amps (at least up to 100 Watt out), except for power supply capacitors over 450V. George (Tubelab) is your man for Earth shaker amps. Ebay generally has cheap industrial power transformers for 240V, 440V or 600V. Edcor is good for up to 100 Watt OTs, Hammond has a 280 Watter OT. One could also use two 100 Watt OTs in a Norman Crowhurst Twin design.

Anything over 350V gets me concerned about safety though, getting nasty above there. One could stay at "low" (350V B+ say) voltage by using the biggest TV Sweeps (or parallel Sweeps) at lower power out, lower Zprimary and higher current. Starts eating a lot of heater Watts then however. 21LG6/A is quite efficient on its heater requirements. 20/6LF6 even more so, but expensive, if obtainable at all.
 
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ericj

Member
2008-12-08 10:24 pm
It sort of appears as though a target of 100w is only perhaps $120 or so more expensive than a target of 60w.

I'd be collecting parts in stages, so it would be spread out among several paychecks.

I'm under the impression that I'd be at or near 400v at 60w. I know that as the voltage goes up, it does get more dangerous, but you're in the kilovolt range before you have to start worrying about whether materials that you've considered non-conductive in lesser jobs are in fact plenty conductive at your current voltages.

Like when a ferrite bead becomes a perfectly acceptable path to ground.

So yeah, I know that there are things to worry about, like the max voltage rating of resistors.

For personal safety, with everything over about 40v, I apply the alanis morissette rule.

One hand in my pocket.
 
Paralled tubes can keep the B+ down if wanted. A bit more hassle with matching the tubes or 2X the bias adjustments. And 2X the heater current.

Edcor does make 100 Watt OTs with 1.7K Ohm, 2.2K Ohm or 2.5K Ohm primary, beside the 3.3K Ohm primary one would likely use for a single tube TV Sweep pair at 100 Watts.
 
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ericj

Member
2008-12-08 10:24 pm
Huh, you're right. 600v industrial transformers Are cheap and plentiful on ebay. And I'm sure i can find pretty boxes to cover them.

If the board, board level components, and output iron are the only parts i have to pay full retail for, i think i will satisfy my cheap bastard aesthetic.


Well, I'll probably end up ordering a sheet of metal cut to spec from onlinemetals. but that doesn't count.
 
A 600 volt transformer will make over 700 volts.....now you are starting to get into the scary voltage ranges....where OPT's get expensive. Keep in mind that the OPT sees twice the B+ voltage when operated into a restive load without clipping. I have seen 4 X B+ running a guitar amp into clipping with a speaker load near resonance.

A single 24 watt sweep tube (6HJ5's) can live for years in a 125 WPC amp driven hard (clipping on peaks) because the average power is still in the 10 watt range. A 28 watt tube just adds margin. To get this kind of power requires 600 to 650 volts. Your driver circuit doesn't want or need 600 volts, so you would need a second power supply, a regulator to drop 300 volts, or two 300 volt power supplies wired in series.

After blowing my first version of Pete's red board up twice, I came to realize that two 300 volt supplies IS the way to go. All of the circuitry runs on 300 volts. The driver board, screen regulator and everything else gets 300 volts from one power supply, the 600 volt connection goes only one place, to the OPT's. Two Anteks will get you there.


All of the 125 WPC versions of Pete's big red board were build this way, and all have been reliable. I made three, one is for experiments, it's pretty beat up. The second was built into an amp that I played loudly a lot, but given to my boss when I left Florida. It was the only tube amp that he liked with his Magnepans. The third is still in a box around here somewhere....hasn't been seen since I have moved twice.

Number two and three used 100 watt Edcor 3.3K ohm OPT's as did most other builders. They used one Ebay surplus guitar amp power transformer, and one Antek toroid for power.
 

Albrerta

Member
2007-06-26 3:24 am
A 600 volt transformer will make over 700 volts.....now you are starting to get into the scary voltage ranges....where OPT's get expensive. Keep in mind that the OPT sees twice the B+ voltage when operated into a restive load without clipping. I have seen 4 X B+ running a guitar amp into clipping with a speaker load near resonance.

A single 24 watt sweep tube (6HJ5's) can live for years in a 125 WPC amp driven hard (clipping on peaks) because the average power is still in the 10 watt range. A 28 watt tube just adds margin. To get this kind of power requires 600 to 650 volts. Your driver circuit doesn't want or need 600 volts, so you would need a second power supply, a regulator to drop 300 volts, or two 300 volt power supplies wired in series.

After blowing my first version of Pete's red board up twice, I came to realize that two 300 volt supplies IS the way to go. All of the circuitry runs on 300 volts. The driver board, screen regulator and everything else gets 300 volts from one power supply, the 600 volt connection goes only one place, to the OPT's. Two Anteks will get you there.


All of the 125 WPC versions of Pete's big red board were build this way, and all have been reliable. I made three, one is for experiments, it's pretty beat up. The second was built into an amp that I played loudly a lot, but given to my boss when I left Florida. It was the only tube amp that he liked with his Magnepans. The third is still in a box around here somewhere....hasn't been seen since I have moved twice.

Number two and three used 100 watt Edcor 3.3K ohm OPT's as did most other builders. They used one Ebay surplus guitar amp power transformer, and one Antek toroid for power.
I love my Magnepan and looking for a tube amp for a while, which tubes were on that second one? Maybe it's time to start collecting tubes, and of course get the board. Thanks
 
I have been dreaming up a similar project using Pete's compactron triple triode PP driver board Push-pull driver board and some sweep tubes to build a 100w amplifier. The big read board once I had the power supply all put together was almost too heavy so my thought was to build to separate mono blocks using those driver boards. Still just a thought but something I think I may pursue when I have some time.
 
All of the 125 WPC amps used 6HJ5's for output tubes and 6GU5's for drivers. The 6HJ5 is not a direct drop in, two resistors need to be put in the wrong holes to make this work. Plugging them in to an unmodified board will cause smoke. Details somewhere in the long thread. The 6GU5's are a direct drop in even though they are not pentodes. It's been several years since these amps were built so it would be a good idea to read and understand before proceeding. There were several builders that made the 125 WPC flavor.

I am working on a new, but similar design with a target power output in the 250 to 500 WPC range, but it won't be ready for quite some time, and parts won't be cheap. The OPT's are about $400 each!.........I got mine considerably cheaper, one set surplus, the other set used. At that power level things can get ugly fast, so there likely will be some type of microprocessor bias control, supervisory, shutdown, and watchdog circuit.

The initial 100 WPC test board is on the bench now, but I spent too much time shoveling snow today, maybe it sees power tomorrow. It's all built with cheap expendable parts. $1 tubes, $20 OPT's......
 

ericj

Member
2008-12-08 10:24 pm
I've seen / read about the compactron triple triode driver board, and it looks like a great, simple, design.

Which is sorta not what I am looking for. If I am spending $100+/c on output iron, I want CCSes, etc.

Given the high voltages, I think I'll skip perfboard and build the screen supply on a board that I'll etch and drill myself.

Tentatively I'm sold on the 6HJ5. It depends on what i figure out for prospective available-in-stock iron. If i settle on something that is designed for tube amps it'll have 6.3v secondaries, making it pointless / stupid to spend another $20 on a transformer for 21v heaters.

I'll dig through the red board thread to gather details.
 

ericj

Member
2008-12-08 10:24 pm
Seems like another possibility with the 21lg6/a is that i could use a switching supply for the heaters.

A Meanwell 24v 3A supply would cover it easily and costs about $14. Adjusts down to 21.6 per the datasheet. might want a resistor to drop that to 21 even across filaments.

Just dreaming. I do wonder if anyone else has tried using switchers for odd heater voltages though.
 
I do wonder if anyone else has tried using switchers for odd heater voltages though.

Yes, I have used 24 volt Mean Well switchers from Jameco to run 26LW6's and I got a pair of 36 volt Mean Wells for 36LW6's and 35LR6's...I have a lot of those tubes.

I will be working on a guitar amp in the 20 to 50 watt range that will be completely powered by batteries or one of those 12 volt 6 amp replacement power bricks from Amazon.
 

ericj

Member
2008-12-08 10:24 pm
Cool.

I realized today that it would be easy/cheap to build a basic guitar amp into some old but not great Utah 3-way speakers i picked up at a thrift store a few years ago. They didn't make the cut for music listening, even after re-cap, but it would be trivial to throw together a tube input stage, tone stack, and a 12v class-d power amp.

Of course, i don't really play, so doing that for fun AND profit means extracting money from some random guitarist.

.:Sent by pneumatic tubes