Battery Powered Tube Amp(Senior Design)

[Boilermaker88]

Hello fellow amp gurus,

I'm a Senior here at Purdue University in the Electrical Engineering department. My Senior design proposal has been focused on inventing a battery powered tube amp. I was wanting to see if I could get some feedback from this community on my project. I was thinking of using a lithium ion battery, a switching boost power supply to get the crazy high B+ voltage, an E88CC preamp tube because of the 90V voltage, an ECC83S power tube for its low heater power, and was thinking about a 1 Watt output.
So far I have a general idea about how to approach this project, but I have very little exact info on things such as calculations and part numbers. I am trying my hardest to learn how to play with tubes in an amplifier. I have been using op amps and MOSFET's my whole college career.
I figured this community would be eager to help a kid out with your experiences and sources. My bookmark folder has 25 different pages where I have been doing research. Any info would help guys.

Thanks in advance.

[Enzo]

60 years ago there were no transistors, and portable radios and things had to use tubes. In fact when we refer to the high boltage for the tubes as "B" voltage it is a term left over from the battery days. The B battery was the high voltage one. The A battery for the heaters, and the C battery for the bias.

50 years ago I used to work on some of those radios. A portable radio was the size of a workman's lunchbox and half of that space was battery.

I see two directions you could go. You could go for a modern design using current production parts, as if you could go into production on the product if you wanted. The other direction would be to build a one-off using old parts and not caring if any more duplicates could be built - just a proof of concept project.

The main drawback I see is heater current - a typical small tube wants about 2 watts per heater. A power tube more like 5-10 watts. That is a lot from a battery in anything like portable size. Of course if convenient size is no issue then go old school with mutliple batteries. In the old days they made tubes specifically for battery operation. Tubes with low voltage heaters and lower current heaters. They also developed lines of super-miniature tubes for portable equipment and car radios. They developed tubes that didn;t need high voltage as well.

Then we used 45-60-90 volt batteries, but now we can make efficient compact switching supplies instead. A switcher for B+ is perfect. Other than the power tube for driving a speaker, the B+ voltage could be even higher than 90v if you wanted. Not a lot of current through preamp tubes.

I don't know where I put it but somewhere I have a survey of sub-miniature tubes I did for a project. RCA really didn;t get into that much, but the GE tube book is full of them. www.tubesandmore.com has many of them at reasonable prices.

SOme are shaped like a 12AX7 but half the diameter, and have pins for small sockets. But there are smaller ones still that look like an overgrown NE2 lamp - bare glass bulb with a row of maybe 5 wires coming out the end. They were soldered in place. These were small, like T2 or T3 bulb, maybe 9-10mm diameter.

Look up 6AK4, a little triode. Look up 2E35, 2E36. little power pentodes - we are talking milliwatts here though. 6BF7 twin triode.

6286 is a tiny triode - 1.25v heater @.14A
6788 pentode
8071 triode
7963 twin triode
6021 twin triode

There are others. I find these small tube types fascinating. Imagine a whole functioning tube amp that would fit into a tube screamer pedal.


1 watt is still plenty loud. You could make a 1/4 watt amp that still makes plenty sound. If you designed it for headphones only you could reduce power output even more. REmember those little solid state amps built into cigarette packs?

[unparalleled]

Easiest way would be to mod a small 12V-120v or 240v inverter. Just Pick up on the HV DC before the chopper circuits,( even remove the chopper components from the inverter ) and use that to provide your HT supplies. You will be able to get a lot more than 1Watt!

I can send you a simple buck/ boost power supply circuit. Just drop me an Email : nospamcaraudioshp@xtra.co.nz (remove nospam)

[Boilermaker88]

60 years ago there were no transistors, and portable radios and things had to use tubes. In fact when we refer to the high boltage for the tubes as "B" voltage it is a term left over from the battery days. The B battery was the high voltage one. The A battery for the heaters, and the C battery for the bias.

50 years ago I used to work on some of those radios. A portable radio was the size of a workman's lunchbox and half of that space was battery.

I see two directions you could go. You could go for a modern design using current production parts, as if you could go into production on the product if you wanted. The other direction would be to build a one-off using old parts and not caring if any more duplicates could be built - just a proof of concept project.

The main drawback I see is heater current - a typical small tube wants about 2 watts per heater. A power tube more like 5-10 watts. That is a lot from a battery in anything like portable size. Of course if convenient size is no issue then go old school with mutliple batteries. In the old days they made tubes specifically for battery operation. Tubes with low voltage heaters and lower current heaters. They also developed lines of super-miniature tubes for portable equipment and car radios. They developed tubes that didn;t need high voltage as well.

Then we used 45-60-90 volt batteries, but now we can make efficient compact switching supplies instead. A switcher for B+ is perfect. Other than the power tube for driving a speaker, the B+ voltage could be even higher than 90v if you wanted. Not a lot of current through preamp tubes.

I don't know where I put it but somewhere I have a survey of sub-miniature tubes I did for a project. RCA really didn;t get into that much, but the GE tube book is full of them. www.tubesandmore.com has many of them at reasonable prices.

SOme are shaped like a 12AX7 but half the diameter, and have pins for small sockets. But there are smaller ones still that look like an overgrown NE2 lamp - bare glass bulb with a row of maybe 5 wires coming out the end. They were soldered in place. These were small, like T2 or T3 bulb, maybe 9-10mm diameter.

Look up 6AK4, a little triode. Look up 2E35, 2E36. little power pentodes - we are talking milliwatts here though. 6BF7 twin triode.

6286 is a tiny triode - 1.25v heater @.14A
6788 pentode
8071 triode
7963 twin triode
6021 twin triode

There are others. I find these small tube types fascinating. Imagine a whole functioning tube amp that would fit into a tube screamer pedal.


1 watt is still plenty loud. You could make a 1/4 watt amp that still makes plenty sound. If you designed it for headphones only you could reduce power output even more. REmember those little solid state amps built into cigarette packs?

THANK YOU SO MUCH!

This is exactly what I have been hunting for. I have been turned to the mini-tubes. Those would be attractive if I could get a 1/4 Watt output approved. A 12AX7 tube, referenced here http://lib.store.yahoo.net/lib/thetu...JJ-ECC83-S.pdf , needs 2.4W to work. I was thinking of using 1 of the triodes as preamp and the other as the power so I could "cheat" on heaters since they share one. I do not know exactly what type of Power output that would give me though. I have not looked too much at power tubes. My instructor said something about scaling the project back to just making a preamp. If that was the case, then it would simplify my project greatly.

Easiest way would be to mod a small 12V-120v or 240v inverter. Just Pick up on the HV DC before the chopper circuits,( even remove the chopper components from the inverter ) and use that to provide your HT supplies. You will be able to get a lot more than 1Watt!

I was looking at inverters, Kodak flash bulb circuits, and switching supplies. I figure I could get the power, it is just using the power wisely and doing it with a battery. I am still looking for solid, design type information to start putting some details on paper. I have been looking at tons of National Semiconductor switching regulators and such too.

[martin manning]

Not battery powered, but relevant to your quest, I think...

one 12ax7 complete amp problem

[Enzo]

Easiest way would be to mod a small 12V-120v or 240v inverter. Just Pick up on the HV DC before the chopper circuits,( even remove the chopper components from the inverter ) and use that to provide your HT supplies. You will be able to get a lot more than 1Watt!

Power has to come from somewhere. It would be easy enough to build a 10w or larger amp. The problem is getting a battery to power it. We must assume we won't be hauling a car or motorcycle battery around for it. We'd also want more than a few minutes from a small battery.

I've recently seen some high voltage switchers, but can't recall where. I'll see if I can find them. And I would not be at all surprised if Nat Semi has application notes on just what you need simply done.

And I think scaling back to a preamp only might be a wise option, though less of a challenge.

[Boilermaker88]

Power has to come from somewhere. It would be easy enough to build a 10w or larger amp. The problem is getting a battery to power it. We must assume we won't be hauling a car or motorcycle battery around for it. We'd also want more than a few minutes from a small battery.

I've recently seen some high voltage switchers, but can't recall where. I'll see if I can find them. And I would not be at all surprised if Nat Semi has application notes on just what you need simply done.

And I think scaling back to a preamp only might be a wise option, though less of a challenge.

My battery option that I was looking at was a 50 Amp Hour Laptop battery. I was looking at a 30 minute amp life too.

[Robert M. Martinelli]

Hm, since you're willing to obtain about 1W, how about a 12AU7 used in a push pull configuration? The max plate dissipation is around 3-4 Watt and the heater currents are not so high a, so it would fit the bill better than a "normal" output tube.

P.S. to Unparalleled - we' re talking 'bout battery powered amps, so the problem is not the voltage, as you could easily build a step up even from 9 to 1000 VDC, the problem is the current, as tubes use much more power than BJTs or mosfets to yield the same output power. This one-watter, when finished, will probably result in "eating" some 10 to 15 Watt ( 2 Watt each tube for the heaters alone, assuming some 12A_7 will be used )

[Steve Conner]

Hi all,

I recently read about the Russian 1J24B pencil tubes. They seem to run happily off 48V or 90V HT and 1.2V heater at only 13mA, and are available cheap on Ebay. I bet you could make something nice with a bunch of 1J24Bs as a preamp, and a push-pull 12AU7 power amp.

On 4hv.org, an electronics forum that I help to run, we have lots of boost converter circuits, and there's also a simple self-oscillating inverter called the Royer oscillator. You can actually use this to generate HT for a tube amp from a 12V battery, using a filament transformer run backwards. I once tried this with a Champ-style amp, and like Bob says, the power consumption was up in the tens of watts. It's unregulated of course, but tubes don't care. For extra geekpoints, you can use a 400Hz filament transformer from an aircraft surplus store, or salvage the ferrite transformer from a computer power supply and go ultrasonic.

(The Royer oscillator also gets sloppily referred to as the Mazzilli oscillator or ZVS on our forum.)

[Boilermaker88]

Hm, since you're willing to obtain about 1W, how about a 12AU7 used in a push pull configuration? The max plate dissipation is around 3-4 Watt and the heater currents are not so high a, so it would fit the bill better than a "normal" output tube.

P.S. to Unparalleled - we' re talking 'bout battery powered amps, so the problem is not the voltage, as you could easily build a step up even from 9 to 1000 VDC, the problem is the current, as tubes use much more power than BJTs or mosfets to yield the same output power. This one-watter, when finished, will probably result in "eating" some 10 to 15 Watt ( 2 Watt each tube for the heaters alone, assuming some 12A_7 will be used )

I checked out this data sheet on the 12AU7 here http://www.tubebuilder.com/images/tu...2au7aspecs.pdf . I found 330V @ 0.022mW which is 7.26W for the cathode and 12.6 @ 150mA for the heater which is 1.89W. Add the preamp power calculations of about 4W and I'm looking at roughly 7.26W + 1.89W + 4W = 13.15W as a rough estimate.

I could get away with a 30Watt/hr 12V battery very easy. I looked into RC batteries and they seem to be too small for my purpose. I envisioned a laptop battery being in the base of the amp, separated by some sort of air cooled vent between the battery and amp. Most amps I see have the chassis shaped in a rectangle shape like a battery.

[Enzo]

I guess the form factor matters. At first I envisioned your project as a small hand held chassis design. But it could just as easily be a small combo portable. The laptop battery makes some sense for the handheld, but in a small combo, you could have a couple gel cells or lead-acid batteries tucked in there.

There are numerous battery combo amps that could easily be made from tubes instead of SS circuits. Crate made some, PV made some, EH made their Freedom amp.

Freedom Amp

[martin manning]

There is also the Zvex nano amp, which runs from a 12VDC wall wart, drawing 500ma, so it could be battery powered. Very cool. Details are here:

http://zvexamps.com/amp_view.html

And this, with a schematic included, using two 6021 sub-mini tubes like the Zvex. In fact it looks very similar to the Zvex.

http://jjs.at/electronic/class_a_subminiature.html

Lots of links here, including data sheets for several tube types:

http://amps.zugster.net/projects/subminiature

MPM

[Boilermaker88]

I wanted to thank everyone again for their insight and help. I have decided to use a 5W design off of the classic AX84 amp and power it at 1W. I had a meeting with a Professor and he helped get me a solution. He advised to design 10 times more power than you will be using due to the 3x crest faster of audio sign waves. So, I will be using my 1W design budget with a 5W design. I will be using the 12ax7 preamp tube and EL84 power tube to get my 1W out. He OK'd my power budget for a 30 minute runtime and advised turning down the B+ voltage as many here have. So my biggest amplifier experimenting will be trying to get the 1W I need with as little power as these tubes will take.

I really liked the idea of using the minitubes, but I could not find a solution to get my 1W output that was as easy as using existing designs. I wish I had the time to design my amp from scratch like this and make this project even more outstanding. I am going to have to keep my focus on the power supply and battery.

I will try and keep this thread updated with my trials and tribulations. It is defiantly bookmarked.

Wish me luck folks.

[Enzo]

Does your prof know that guitar amps are not hifi? Crest factor? Guitar amps are not made to REproduce sound, they are primary elements of sound production. As that they are not intended to faithfully reproduce the input signal. That contrasts with a hifi amp that DOES expect to faithfully reproduce the inpit signal at its output.

The whole point of a guitar amp is that it lends its own character to the passing signal. How the amps overdrive and smash the signal is part of their charm. We love the sound of the soft clipping in a tube amp. We love the resulting compression. SO if there is some concern over headroom, I hope we are not trying to make this a distortion free amp. You can always plug your guitar into one of the PA system channels to hear what clean and accurate sounds like. Or plug into your home stereo directly. Thin and lifeless.

[schnipgloss]

Just to let you know a battery powered tube amplifier is very possible and it can still sound amazing. I go to a little crap school called UT, (Toledo OH, not the cool Texas one) and I'm in the EE department. Some of my friends got in a group and created a battery powered vacuum tube amp. The thing runs off a 12V moped battery. We jammed on it in the lab today for around 3 hours and it was still working. Voltage had only dropped to around 11.5 volts after 3 hours. They had to wind their own transformers to get the 12V to 300V step up and they had to create a feedback network to keep the power supply from going crazy. I sounded amazing. There is no noise from the transformers or anything. You get noise only when you stand to close with the guitar.

But in summary, it is possible to make a load amp that can blast 2 12" woofers and sound amazing. Takes a lot of time and effort, plus they had been building tube amps for two years now. Oh, and it can run on AC wall power and recharge the battery in the same go. Quite amazing for a one semester project.

[Enzo]

You're a Rocket?