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#1
Hey guys, I wanted to post this here for any curious folks who might want to give this a shot.

Back around March of this year, Cathbard asked me about designing a busking amp for him. He wanted it to be all tube, but be able to run off a 12v auto battery. I thought it was a pretty interesting idea so I volunteered to put together a design. Below is a link to the schematic.

Disclaimer:
This is an initial prototype design. I have not built this circuit for testing, nor do I intend to. It is offered to you as-is, without any guarantees, for your own testing and tweaking purposes.

Now that this is understood, a little about the amp:

The amp utilizes 3 12U7 noval sub-mini dual triodes (2 for the preamp and 1 for the phase inverter) and 2 12K5 7-pin sub mini power tetrodes (class AB push-pull power section). As mentioned, the amp runs on a very low power supply at 12 volts DC. This can come from an automotive battery or a wall wart.

Since the power supply starts with DC voltage, only one transformer is needed: the output transformer. I've chosen the Hammond 125A because of it's broad primary Z capabilities which could come in handy should you want to tweak the power amp. It is a very small 3 watt transformer. My design configures the OT for a 2.1k primary Z but there are many options in case one wants to experiment.

I don't know exactly how much power the amp will put out, but am imagining it to be along the lines of 1 watt. This could possibly be a killer bedroom amp.

Anyhow, the schematic is linked below. I just finished it and I'm tired, so let me know if you notice any mistakes like duplicate part numbers, etc. Also, feel free to ask questions. This amp should be dirt cheap to build, so I hope some folks take the plunge and try it!

12v Tube Amp Schematic
Last edited by CECamps at Aug 1, 2011,
#2
chalk me up as interested in hearing how this turns out for anyone who builds one
Call me Dom
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#4
Quote by AcousticMirror
I already shocked myself.


I'm expecting you to have parts procured by the end of the week and a functional prototype by the beginning of the following week. Please report back hastily.
#5
is this only one watt?

what's impedance of the output transformer?

i'm making mine to drive headphones.

craig can you get 12.6v off a 12v battery on a lm338?
Prs se Holcomb is the answer
Last edited by AcousticMirror at Aug 1, 2011,
#6
An amp where B+ actually stands for "battery".

Gilchrist custom
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#7
Are these tubes optimized for 12v, or 300v?
"If you're looking for me,
you better check under the sea,
because that's where you'll find me..."
#8
They are designed to run between 12V and 30V. They are what used to be in car radios back in the day. That's what gave me the idea, I remember the old valve radio my father had in his truck.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#10
*snore*

I seem to remember a 12V all tube build somewhere on this forum somewhere before... :p
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#11
Quote by Ippon
With Min hopped up (sugar high) as a result of the munchies, he might have a prototype by Wednesday.



i dunno if the drop out on a 12v input is enough for the 12v regulator to maintain regulation...
Prs se Holcomb is the answer
#12
So not to sound like a 16 year old with a nissan sentra but could something like this actually be used in car audio? If so I might have to make one to show one of my friends how powerful a watt actually is. He swears up and down that his "good" car audio amplifier brands don't overrate the wattage, cause it's "only" 1000 watts.

Sorry to bring such a negative subject into this but I'm just so sick of all the self proclaimed "techs" around here telling me I'm wrong about speaker loads, watts, and even what a low pass filter is, just because they've opened up an amp without shocking themselves or installed an amp in a vehicle.
Guitars:
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'77 Harmony
Roadworn Starcaster
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Amps:
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Epiphone Valve Junior combo
#13
ya you could.

i think cars used to have valve stereos all the time.

but not really 1 watt ones though.

probably ran a couple more output valves for like 3-5 watts.
Prs se Holcomb is the answer
#14
Yeah, I figured they'd be a little higher wattage, but even at like 4 watts that'd be around 96-100 db within one meter of the speaker so shouldn't that be like deafening levels especially when you're in an enclosed space?

edit:I just had a thought, if I wanted to find out the output power of an amp couldn't I take the voltage going to the speakers then divide that by the speaker load to get the current then multiply the current times the voltage? Kind of like finding the dissipation of a tube?

So: Current=Voltage/Speaker resistance then Dissipation=Voltage*Current. Right?
Guitars:
LTD F-50
Yamaha EG-112
'77 Harmony
Roadworn Starcaster
Gretsch G5120

Amps:
Vypyr 15
Epiphone Valve Junior combo
Last edited by grungebaby at Aug 1, 2011,
#15
The one in my father's truck was fairly loud but nothing like a car stereo of today. Granted, I was pretty young at the time so my memory isn't too reliable but I remember being impressed by the volume of the transistorised one that he had in his next truck. The valve one sounded better but you know what those first transistor radios sounded like.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#16
Well I almost go to sleep everytime I have to endure the loud competitions just because I'm used to the way a tube amp kinda fills a room, my friend that I've know since I was like 9, I'm 15 now, has kinda caught on that I know a little bit about quality sound so he'll actually take my advice from time to time, I told him he should try and find a tube power amp for his system. He said they don't make them anymore, if I were to make one for him would there be any enviromental considerations i.e. heat problems? Also is my thought thing in my edit correct?
Guitars:
LTD F-50
Yamaha EG-112
'77 Harmony
Roadworn Starcaster
Gretsch G5120

Amps:
Vypyr 15
Epiphone Valve Junior combo
#17
Sadly the watt rating on amps has very little to do with the actual voltage and current outputs which is why solid state items like hifis, car radios, computer systems and home cinema systems can have such huge watt ratings and not come close to the amplification of a tube amp.

http://en.wikipedia.org/wiki/Audio_power
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#18
Yeah, but by using the whole P=VI formula I could get a reading of the voltage of my friends "system" on his ideal settings and then throw that into the formula and if I'm right have a power rating for whats actually going the the speakers. But to not get so far off topic I want to here how loud one of these will be so I can decide if I want to build one or beef one up, who's making one anyway?
Guitars:
LTD F-50
Yamaha EG-112
'77 Harmony
Roadworn Starcaster
Gretsch G5120

Amps:
Vypyr 15
Epiphone Valve Junior combo
#20
I've temporarily taken the schematic down as Cath asked me a question that made me realize I hadn't finished the power supply design. I'll put it back up when complete.
#21
grungebaby, I've fixed quite a few car power amps in my time and am yet to see one that was capable of getting even close to the power they claimed. Given the transistors they used at the voltage rail that the buck booster was able to supply it was physically impossible. We are talking between 5 and 10 times lower than advertised. The car stereo market is a cesspool of scammers scamming the gullible.
There's no real way of telling how loud this amp will be without testing it. I have said all along with my discussions with Craig that I intended to run it through a 10" speaker, probably one of the cheap Signature series Weber Alnicos. The aim is for it to be a busker amp so it only needs to be as loud as your voice or a harp really - or some dude tapping out beats on a plastic drum. I think through a 10" speaker it should be fine for that. The Webers aren't that efficient really I don't think but they are low wattage and are crappy enough to work well in an open backed cab.
I should be able to make it so it's easy enough to carry around on a bus. A bit bigger than most busking type amps but I think it will be worth it.

Edit. The heaviest part of it will be the battery. The idea was to use a motorbike battery. I need to come up with a harness for the battery that will fit anything from a posty bike battery to a Harley battery, depending on your priorities or wallet size. You'd just charge it with a normal car battery charger.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Aug 1, 2011,
#22
Quote by Cathbard


Edit. The heaviest part of it will be the battery. The idea was to use a motorbike battery. I need to come up with a harness for the battery that will fit anything from a posty bike battery to a Harley battery, depending on your priorities or wallet size. You'd just charge it with a normal car battery charger.


What about a li-ion rechargeable (Laptop battery?) or something along those lines instead of a lead-acid?

Conventional automotive batteries are designed for heavy initial surge, then recharge, not slow steady depletion. (Deep cycle marine batteries the exception of course)

Laptop type batteries OTOH are designed for exactly the type of draw you will be experiencing...and you can get them (or build them) in higher voltages which may be beneficial. And chargers are still readily available.

Just a thought.
#23
Laptop battery is a very interesting idea. It's a shame no standard has been settled on for size and shape. The idea has merit though. I've probably got a couple laying around here. Wonder what their voltages are.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#24
Quote by Cathbard
Laptop battery is a very interesting idea. It's a shame no standard has been settled on for size and shape. The idea has merit though. I've probably got a couple laying around here. Wonder what their voltages are.


Here's an idea...18v cordless tool rechargeables? Readily available, designed for constant draw, quick chargers available, vehicle chargers available, quick to change out etc etc...

Somthing like this ?

Thoughts?
#25
power tool batteries also very readily rehoused into a more amp suitable enclosure
Epiphone Les Paul Plus Top
Jet City JCA5212RC (SLO Modded)
Ibanez WD7 Wah
Mad Professor Sweet Honey Overdrive
TC Electronic Flashback Triple Delay
TC Electronic Trinity Reverb
#26
power tool batteries would certainly seem the best option for portability/availability/charging etc.
just whether they put out enough power/last long enough.

anyway, if this was easy enough to follow/cheap enough, id be happy to try it
#27
oh, a side note to uk builders, the 12u7 and 12k5 tubes are not very easy to source on this side of the pond...
Epiphone Les Paul Plus Top
Jet City JCA5212RC (SLO Modded)
Ibanez WD7 Wah
Mad Professor Sweet Honey Overdrive
TC Electronic Flashback Triple Delay
TC Electronic Trinity Reverb
#28
The only real downside I see to power tool batteries is that with continuous use they get a very sharp drop off, I know it ticks me off when my drill stops mid screw, I can only imagine if something like that happened with my amp mid jam/song.
Guitars:
LTD F-50
Yamaha EG-112
'77 Harmony
Roadworn Starcaster
Gretsch G5120

Amps:
Vypyr 15
Epiphone Valve Junior combo
#29
Quote by Cathbard
grungebaby, I've fixed quite a few car power amps in my time and am yet to see one that was capable of getting even close to the power they claimed. Given the transistors they used at the voltage rail that the buck booster was able to supply it was physically impossible. We are talking between 5 and 10 times lower than advertised. The car stereo market is a cesspool of scammers scamming the gullible.
There's no real way of telling how loud this amp will be without testing it. I have said all along with my discussions with Craig that I intended to run it through a 10" speaker, probably one of the cheap Signature series Weber Alnicos. The aim is for it to be a busker amp so it only needs to be as loud as your voice or a harp really - or some dude tapping out beats on a plastic drum. I think through a 10" speaker it should be fine for that. The Webers aren't that efficient really I don't think but they are low wattage and are crappy enough to work well in an open backed cab.
I should be able to make it so it's easy enough to carry around on a bus. A bit bigger than most busking type amps but I think it will be worth it.

Edit. The heaviest part of it will be the battery. The idea was to use a motorbike battery. I need to come up with a harness for the battery that will fit anything from a posty bike battery to a Harley battery, depending on your priorities or wallet size. You'd just charge it with a normal car battery charger.



cath and craig couldn't you guys use a quad of 9v volts through a voltage doubler?

i dunno if that would drain the batteries to fast though?

you'd get 18v to the heater regulator to 12v and then whatever you needed for the b+
Prs se Holcomb is the answer
#30
Quote by grungebaby
The only real downside I see to power tool batteries is that with continuous use they get a very sharp drop off, I know it ticks me off when my drill stops mid screw, I can only imagine if something like that happened with my amp mid jam/song.


Yep, set up that way to save the batteries. But with a portable of this sort, there's no reason one couldn't have 2 battery sockets on it, and a spare or two charged/charging in the vehicle or ready in a bag.

Once you determine runtime expectancy (at home, with a few friends and beer...), just change them at a predetermined time.

Also, I suspect most power tools draw significantly more power than the amp in question, so the batteries should last a good long time. I'm hoping someone will give us a theoretical power draw calculation, because I'm too lazy to do it (and I don't have the data yet...)

I have a little SS Fender mini-deluxe that's rated at one watt, runs off of a single 9v battery and the battery lasts for a VERY long time. Presumably the amp in question will use more power than that, but given the sources we've considered I'd say runtime won't be a real concern.

Quote by AcousticMirror
cath and craig couldn't you guys use a quad of 9v volts through a voltage doubler?

i dunno if that would drain the batteries to fast though?

you'd get 18v to the heater regulator to 12v and then whatever you needed for the b+


I suspect it's a power limitation problem.

An Energizer 522 9v is rated at about 220maH with a half amp constant discharge to 4.8v (Not sure we could go that low...)

By way of comparison, a Ryobi 130429002 is rated at 3 Amp Hours, more than 13 of the 9v's...(and it's an 18v battery pack)
Last edited by Arby911 at Aug 1, 2011,
#31
Speakers are inductive, the power of the output is difficult, with a resistive load (where the current and voltage are in phase), the power is P=EI, 1=12/I I=1/12A or .08A for the output approximately. Add this to the 150mA per tube (3 tubes I believe?) + .450A per power tube, and you get 1350mA for the heaters, plus the power stage and the preamp to 1400mA to be round. With a 6500mAh lithium battery, you get around 4-4.5 hours, closer to 4 due to the end of life curve. So after 4 hours of play, charge it up.
#33
Quote by Cathbard
They are designed to run between 12V and 30V. They are what used to be in car radios back in the day. That's what gave me the idea, I remember the old valve radio my father had in his truck.

Back in the day, car radios used vibrator tubes in an inverter circuit to step the voltage way up... there may have been some low-voltage tube radios out there, as innovations in simplicity, but I've never heard of them. just saying...
#34
The 12U7 is a pretty old valve and so is the 12K5. They were specifically designed for automotive use. It stands to reason that a lot of car radios would have been using them.

That battery arby posted looks very interesting. The power tool one was only 2.5 amp hours. You are looking at about 1.5 amps just for the heaters alone. You'd want to get a couple of hours out of it at least.

Min, Craig's looking at the voltage double type of idea. ie some type of step up regulator. I'll just wait to see what he comes up with.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#35
Quote by Cathbard
The 12U7 is a pretty old valve and so is the 12K5. They were specifically designed for automotive use. It stands to reason that a lot of car radios would have been using them.

That battery arby posted looks very interesting. The power tool one was only 2.5 amp hours. You are looking at about 1.5 amps just for the heaters alone. You'd want to get a couple of hours out of it at least.

Min, Craig's looking at the voltage double type of idea. ie some type of step up regulator. I'll just wait to see what he comes up with.


yay i'm so smart.
Prs se Holcomb is the answer
#36
Yeah, when I started that schematic, the power supply was where I began. The regulators on there were basically a placeholder and I was going to re-address them after the amp design was finished. But I forgot.

I've been working all day but I'm going to try and get a revised schematic up later tonight. I'm thinking a trusty LM2577 step-up regulator is probably what I'll go with for the B+ supply.
#38
Quote by forsaknazrael
gleee.

could i use an old valve junior OT? or should I just get the Hammond?


No, the VJ is single ended. This is a push-pull amp. You can use any PP OT with a (roughly) 2k primary Z to build it as designed.
#39
Power tool batteries aren't really going to be that great of an idea.

Their operating voltage is extremely varied. Take a "12V" lithium ion battery. It will spend most of its life time at 10.8V. But it can run between 12.6 max all the way down to 0 if it has no electronics to shut it off. It should be shut off at 7.5V though.

They are also fairly low power, so your not likely to have it last for long.

Laptop batteries are usually 12V. Most also are only 3-6Ah. They will last longer than power tool batteries, but they are also designed to be used in conjunction with a boost converter.

If you are going to go with a lithium battery you need to use an 18-20V battery and use some type of converter to step it down. The minimum safe operating voltage with those lithium ion batteries is about 12.6V, so you should always be safe. Now getting the voltage to step down is another story. A buck converter is the best idea here for battery life, but it can have its drawbacks. Can be noisy.

So a 12V car/motorcycle battery is probably best. It will stay in the 10-14V range more than likely.

*cough* The Ryobi battery is going to run like a champ. The 18V stuff would work great with a buck converter to get down to 12V. You would essentially get almost 4.5Ah out of said battery. If your really interested in a lithium ion battery pack and circuit let me know and I can design something for you.....
Last edited by XgamerGt04 at Aug 1, 2011,
#40
Quote by XgamerGt04

*cough* The Ryobi battery is going to run like a champ. The 18V stuff would work great with a buck converter to get down to 12V. You would essentially get almost 4.5Ah out of said battery. If your really interested in a lithium ion battery pack and circuit let me know and I can design something for you.....


Why don't you draw something up and I'll add it to the OP as an option for folks who might be interested?
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