Battery amp?

[Leevi]

Has anybody built a battery tube amp? The result must be a low hum amp?
The low DC voltage lift should probably be done by using pulse techniques and transformer
which requires electronics in power supply. I think there should be lot of batteries
rather rechargeable in order to cover the big power consumption.
/Leevi

[tubesornothing]

I seem to recall pignose did something like this...

[phsyconoodler]

Using old car radio tubes is a good way.

[imaradiostar]

It's funny how threads can come up in a timely fashion- been thinking about this a bit recently. I was thinking an 8-12 watt 6aq5 based battery amp with a single 10 or 12 and an output to power a few basic pedals would be perfect.

I figure a 12ax7 for the preamp and a 12ax7 for the PI should be fine- maybe this is a chance to use up some of my 12 volt pentodes- who knows. Total filament current with just 2x 12ax7's and 2x 6aq5's in parallel is only .78 amps at 12-14 volts. Using 6bq5's would bump it up to only 1 amp- a reasonable draw for some very affordable valve regulated 12 volt batteries- or perhaps a pair of 6 volt ones. I looked at a pair of powersonic ps-670's.

I was originally planning to use the transformer from a computer power supply in reverse- connect the secondary to a big mosfet (or two) and a simple cmos or 555 timer based oscillator set to free run at 25-40khz. This would live in the bottom of the combo chassis, away from the audio works and shielded with steel. Depending on how you connect it you should be able to get around 320 volts DC at a more than reasonable current.

Then I realized there is probably a much simpler solution- I already have at least 5 DC-AC inverters I've received as gifts over the years- more than I can ever use. One of the small 100 watt ones could be slightly modified and connected as a voltage doubler to provide about 320 volts DC under load. These things are already more or less regulated so it'll only take the current it needs to provide constant AC output while maintaining decent efficiency.

The aforementioned valve regulated batteries are nice because you can "float charge" them. When you're not using your battery amp hook it up to a constant voltage of 2.2 to 2.3 volts per cell and the batteries will stay charged and ready to go for your next outdoor jam session.

jamie

[PRR]

> Has anybody built a battery tube amp?

All pre-1956 car radios.

BBC's "Radio House" (main headquarters and many studios) was originally ALL battery power. The basement was mostly batteries.

> pulse techniques

That's still a "hum", just at another frequency. A 400Hz vibrator is easier to filter than 50/60Hz, but also more audible. 25KHz "may" be easier to filter, though you can get some very odd hetrodynes.

> must be a low hum amp?

You can get "zero hum" with simpler cheaper techniques.

[Leevi]

Jamie

Then I realized there is probably a much simpler solution- I already have at least 5 DC-AC inverters I've received as gifts over the years- more than I can ever use. One of the small 100 watt ones could be slightly modified and connected as a voltage doubler to provide about 320 volts DC under load. These things are already more or less regulated so it'll only take the current it needs to provide constant AC output while maintaining decent efficiency.

Did it cause hum?

PRR

You can get "zero hum" with simpler cheaper techniques.

What techniques do you mean?

/Leevi

[imaradiostar]

Dunno, haven't built it yet.

I would generally trust PRR's judgement then try something anyway just to learn for myself.

was 400 hz the norm back in the day? I know it's a standard for military/aircraft use but that's 115 vac stuff.

PRR, What kind of transformer would you use to get the B+ supply? Batteries?

jamie

[PRR]

> What techniques do you mean?

Just the usual things we must do when eating AC wall-power, just done very well.

B+ filtering. Heater-lead twist/layout or properly filtered DC heat. Ample separation between power transformer/wiring and sensitive stages.

The good old professional audio gear (Ampex, Gates, German radio) did not hum.

I built a Champ a little too small. I used very good B+ filtering, simple but clean AC heater layout. Volume down the hiss was, of course, very low, and hum was undetectable. As Volume came to max the his of course rose, and I could see hum on the 'scope, but could not hear it in any guitar speaker (and even with a deep-bass speaker the hum would not be heard above the intrinsic hiss.) That was with the chassis open under fluorescent lighting; chassis closed should be better, and we try to stay out of fluorescent lighting. A larger chassis would help by allowing the PT to be further from sensitive signals, the heater routing could have been a bit better, and DC heat is an option.

[stingray_65]

http://www.tubesandmore.com/

go to  "tech supplies" and then "batteries"

they have batteries up to 67.5 V for old Zenith transoceanics

check out sopht amps

http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

he has schematics for "space" tube amps.

[Leevi]

So there is an amp using 12V B+, interesting!

Another motivation for battery amp are the street musicians.
At least here in Europe in some countries it's not allowed to use any wall power for street playing.
/Leevi

[HotBluePlates]

go to  "tech supplies" and then "batteries"

they have batteries up to 67.5 V for old Zenith transoceanics

Hmmmmm.... 67.5v * 4 = 270v. Coincidence that you see that occasionally as plate voltage in data sheet typical conditions?

Or how 'bout having 270v available from your B battery, but moving the "ground point" in a resistor string from + to - on the battery, so that you have 250v from the ground tap to the plate, and -20v from the ground tap down to the - terminal? Now you have 250v and 20v fixed bias.

The numbers arrived at on the sheets weren't an accident, and neither was the fact that you didn't necessarily need a huge B+ unless the size of the signal output needed dictated it.

I really need to go build myself a 230-250v amp...

[imaradiostar]

I know the original purpose was for a hum-free amp but I don't think batteries are needed to achieve that, as mentioned above.

On the other hand- I did some experimenting with a tiny "100 watt" inverter today. I use quotes because it doesn't really do 100 watts. It's more like 50 watts and that's only with the car running or battery on a charger and floating around 13 or more volts. This little inverter uses a pair of mosfets to drive a tiny transformer at something like 40 or 50 kHz to create about 135 volts DC then uses an h bridge to swap the polarity with about a 25% duty cycle- pretty mediocre. After playing with the thing a little bit I realized the entire "outlet" side of the inverter was electrically isolated from the 12 volt side so I cut the board in half, leaving the transformer secondary free to play with.

Now I have a full wave bridge connected to the whole transformer secondary. It's making 277 volts at about 40ma (7k ohm cement resistor and a 20uf cap) and isn't breaking a sweat. The battery is at about 12.3 volts (not a full charge) and the inverter is drawing a little over an amp. It would appear that these are very usable voltages and currents for small push-pull or SE amps. I tend to favor PP but to each their own.

I might see if I can borrow some valve regulated batteries from work for a proof of concept. I think this could be a really fun project for me. I'm in school and have little time to play. Now I can have a little amp and play guitar between classes or after work without sacrificing the tube tone I like so much!

The concept is likely to extend to other inverters as well. I'd wager that cheaper ones are the better ones to use as they're less likely to use current feedback and muck with the simplicity of it all. You can get some pretty high powered inverters for chump change at wal-mart or on ebay- probably good enough for a larger amp or even a tube car stereo!

jamie

[imaradiostar]

A little update for any that care.

I hooked this thing up to my version of the "hoso" amp- it's got 6aq5's and a few circuit changes but it's largely the amp that everyone here should be familiar with by now. With the plate voltage (inverter output) at about 275 volts the amp idles at around 71ma, measured with a 1 ohm 1% resistor between the inverter and the amp. The battery side of the inverter is drawing 1.75 amps, scaling upward to about 2.5 amps at full clip. This puts energy wasted in the inverter at around .5 watts- pretty slick.

I was NOT running the filaments from the battery for this test- only the high voltage supply using the inverter. I left it running for quite a while- there was little battery voltage drop but of course this was a large automotive type battery, not a smallish VRLA or gel-cell. Total resting current draw (including filaments) for a functional "battery amp" should be about 2.5 amps. Full load will approach 3.5. Some sort of "standby" arrangement that allows the inverter to fire up unhindered and limits inrush current would help in a final version.

How does it sound? Pretty nice, really. The only odd (but not really bad) thing is that the supply is ROCK SOLID. The B+ supply only sagged about a volt between 70ma and 95ma. Seriously. I'm kinda tempted to knock down the values of the 1k screen resistors and the shared 1k screen filter dropping resistor to see how much power it'll make, given the chance. It would be easy to add a "sag" resistor to make things sound more like what we're used to.

The inverter is a "stupid" device. It has a center tapped primary connected to a pair of mosfets that grab each leg of the primary for 5us at a time- that tells me it's a 100khz inverter! The end result is that output voltage scales with input voltage- duty cycle is always 50%. This means it maintains a constant output voltage relative to the battery or supply running the inverter. I put a charger on the thing while playing and voltages and currents went up a little bit but I'm not really worried about it on a cathode biased amp. It could be a much bigger problem for fixed bias though. I had to crank the gain on the scope up REALLY high to see the pulses at the first filter cap- kindof surprising really. I guess with such high switching frequencies things even out nicely- the AC looks mostly like DC to the caps. Of course I did manage to blow a bridge rectifier before swiping some ultrafast diodes from an old computer supply!

Anyway, hope this is helpful to someone. I think I'm going to build a little combo amp with a small pedalboard that covers the tubes when not in use and make a vinyl cover so I can leave it in the trunk of my car for when I need it. I figure a little amp like this and a few basic pedals and I've got most gigs covered!

jamie

[PRR]

Thanks for the notes on car "120VAC" inverters.

> about a 25% duty cycle- pretty mediocre.

"Has" to be that way. There's two kinds of loads: lamps and computers. Lamps need 120V RMS. Computers need ~~160V peak. A 120V Sine works fine (of course). But a good sine is VERY hard to make at high power. (The 50% and 72% ideal efficiencies of Class A and Class B are really about the fact we test with sines.)

It works out that a pulse with 160V peak and reduced duty-cycle gives 120V RMS.

So both lamps and computers are happy.

At higher prices you can get step-approximation "sine". For more money you get a filter. You can get as close to a sine as you want to pay for.

> it maintains a constant output voltage relative to the battery

Voltage-correction is another added cost, and not warranted on $49 inverters. "120V" stuff usually works 105V-130V, 24%. Car voltage does not vary much past 11.6V-14.4V. Over 14V burns modern batteries, under 11V means you may not have enough charge to re-start the car.

For THIS chore, DC-DC, you don't even want a sine. We do it in home because that's all the utility company offers (because vast networks don't handle anything snazzier than a sine). We would prefer a nice square-wave. As you note, with flat-top waves the caps don't have to do much storage, unlike a sine where we peak-catch and then live on the caps for 80% of the time.

A ~~25% duty pulse is also pretty good.

6W inverter loss is pretty good. It does however mean this would be a poor choice for a very small amp, like a 1-Watt.

Yup, you may need fast diodes.

275V is pretty good for the larger size of amps you would reasonably run on battery. 6V6(6AQ5) has one of the lower heater demands for its output, a good choice.

> 2.5 amps. Full load will approach 3.5

A good-size car battery is 50AH. You can play about 20 hours, but then the battery will be FLAT. Car batteries can't take deep-discharge. 10 hours may be safe. 4AH exit-light batteries are readily available, will take many deep-discharges, give you an hour or so. Lawn-tractor and motorcycle batteries are in the middle.

[imaradiostar]

Thanks for the reply PRR. The inverter shuts down when voltage gets too low (so you can still start your car) so that should be enough protection- when the volume goes away it's time to shut it down and recharge. I'm planning to use a valve regulated lead acid battery- I use them at work for various projects and I like that they're happy "float charging"- I usually use a constant current supply (cheap ones for LED use work well) to charge them with zener diodes across the batteries to prevent them from overvolting once they're charged. I'll probably land at about 12 to 20ah worth of batteries. They're all rated at a "20 hour rate" but I figure 3 to 5 amps should be fine for reasonable play time. Failing that, deep cycle batteries are meant for this kind of use and other than weight are an excellent choice.

6aq5's are not an accidental choice. 

The prototype will look something like a bassmaster yba-2b with added cathode bypass caps and a "gain control" between the first and second stages. This makes for 2 12ax7's and 2 6aq5's. Nice low current draw- should be less than the amp I used to test. I'll probably bias it a little colder too- I should be able to get idle current for the whole amp down around 60-65 ma.

Fast diodes are a must-have. I blew up a cheap-o bridge rectifier after about 35 seconds of playing. After that I used the pair of diodes from the discarded half of the inverter and another two from a switching power supply and built a new bridge- no problems since then.

The switching supply can pull a load it can't start. Any suggestions on how to minimize the inrush current?

jamie

[kagliostro]

Hope this isn't off topic

about car receiver tube I find this

http://www.mif.pg.gda.pl/homepages/frank/sheets/079/1/12AB5.pdf

and the attached schematic of an audio amplifier using that tubes




Kagliostro

p.s.: the guy who give me the schematic noticed that there is something that seems wrong in the PS schematic

[imaradiostar]

The 12ab5 looks like a 6aq5 with 12 volt filaments, or at least very close to one. Not a bad choice! With two of those and two 12ax7's you'd only draw .7 amps of current. It's a little bit less than the .75 you'd get with series 6aq5's.

The schematic you showed is much more of a hifi amp design than a guitar amp. It uses feedback into the cathodes of the output tubes- I've never tried it on guitar because it requires an output transformer with extra windings.

jamie

[kagliostro]

Hi Imaradiostar

the guy who given me the schematic has put some mod in that amp (in the pre section) as to use it with a guitar but he is not very happy of the result because the amp doesn't sound enough as he want an he is thinking to put another preamp stage on

here the message attached to the schematic

I removed the inputs and added a switched jack (grounded when no plug  in it) right at the grid/2.2M R3 joint.
Lowered R4 to 2k4 (which looks very center bias on the scope) and have  a switchable paralled 1k5 to bias it very hot.
Added a 470 pf bright cap between C3 & C4 for more treble at low  volume settings.
The amp will only create clean sounds with average pickups & I'm  hoping distortion pedals will work well with the amp.

Note, even though it is a Push Pull amp, it is not as loud an a Champ  or Kalamazoo. I think it would need another triode for the preamp or a  splitter that provides gain to fully drive the 12ab5's. Loud for bedroom volume though.

True measured voltage on the 12ab5 plates is 276v & screen grids are  
273v.

Also, the power supply section of the schematic is incorrect-don't use  it.
For example - 280v at C10B and 285v at plate of 12ab5 can't be  correct. I Believe C10b goes to the preamp & screens and C10A goes to  the plates.

I'm thinking that some of the unwanted result can be due to the output arrangement

what do you think about ?  

Kagliostro

[PRR]

> ...something that seems wrong in the PS schematic  12AB5 pp amp 02

Yes, the plate and G2 feeds have been reversed.
 
> will only create clean sounds

At a glance, you need more than one triode stage to get from guitar to 6V6-like power tubes. See 12AX7 Champs and DeLuxes.

It seems to need 11.5Vrms at 12AB5 grids, V1A has gain near 50, it needs a full 0.23V to reach full output, and more to get "not clean" output. While a guitarist "can" make 0.2V peaks, amps are usually scaled to require less than 0.05V (50mV) to reach clipping.

> some of the unwanted result can be due to the output arrangement

It is "tame", "hi-fidelity". Guitarists usually don't use a lot of NFB. It will be a little more gain and a little less tame with the cathode windings taken out. But it still won't be a "normal guitar amp". It needs more gain.

> 12ab5 looks like a 6aq5 with 12 volt filaments

It's apparently a 6V6 made for a small bottle, and 12V, but the heater consumption is 10% less.

> The switching supply can pull a load it can't start. Any suggestions on how to minimize the inrush current?

What's the inrush current for a good cap and a square-wave? About infinity. Insert some resistance. How much simple loss can you stand, 20V? Then 20V/80mA= 270 ohms 5W resistor from rectifier to first cap.

That still gives about 1 Amp start-surge. Several dozen Amps back at the 12V section. If it still won't start, you may have to switch in 1K for starting, and use a relay to reduce to 100 ohms once the tubes pull current (a handy delay factor).

Still, it "should" start computers and such, which are cap-input. Maybe that H-bridge you took out did some current limiting? Also maybe your wires are "too good". A car cigar outlet has several feet of the skinniest wire, you may be doing "the right thing" with a short length of fat wire, which may be "too good".

[kagliostro]

Today I was looking to my schematic archive

to find an FBT schematic

so I've find that FBT in 1968 has in production a 15w combo with a PP of 6aq5

here the schematic, if of interest

Kagliostro