Building a kid's tube amp

Tube or solid-state, this section goes to eleven!

Building a kid's tube amp

Postby Dixie Guitars » 11 Nov 2015, 03:08

Greetings,

This is my first post, so I'll start with a little introduction.

I am an avid DIY-er but I do not have much experience with electronics. I recently pretty much finished building two guitars for my 5-year old (both small scale ES-335 type). I built everything form scratch, including home wound humbuckers. The guitars are finished in a sense that you can plug them in and play, but there are two additional things that I wish to do.

====================================

Project #1 - small solid state amp

One thing is a built-in solid state amp that would enable my kid to play without plugging the guitar into an amp. Ideally I would like it to run on 9V. Ideally I would install two mini speakers around the center block and the electrified sound would come out of the sound holes. There would be an ON/OFF switch under the pick guard and a thumb wheel for gain, also under the pick guard.

I've spent about 3 weeks searching the internet for schematic diagrams and breadboarded some circuits that didn't really give me acceptable results. After getting frustrated I went to the guitar store to check out some ready made mini amps and the only one that was not too loud was the Marshal MS-2, but the sound was horrible.

What I want is an amp that would produce clean sound as well as overdrive and would not get too loud when my kid (like all kids) turns all the volume pots to max.

====================================

Project #2 - battery powered tube amp

The other thing that I want to build is a kid safe tube amp. It needs to be battery powered (I was thinking 12V DC) and be a bit louder than the built in amp, but also not loud enough to bother neighbors when volume is maxed out.

Again, I searched the internet and also sent out some emails to people I found on the internet and at this point I am confused. I am finding diagrams that show me that tube amps can run on 12V DC, but I also got some email replies informing me that this was not really possible. I also found some diagrams that run on a 12V battery, but there is a voltage pump that raises the voltage to 60 or 80 V DC.

If the sound was OK, I think I could even make it a hybrid amp, if all tube is not possible.

====================================


So, I guess I should start by first asking, are these two amps that I am looking for possible to build or is my criteria too limited?



Also, so it doesn't come as a surprise later, I wish to disclose the fact that I might be making money at some future point, selling these amps and guitars. I initially started building just for my kid, but so far anyone that has seen the guitars has been telling me to start a business. Even my kid's music teacher got excited and said he has a bunch of people who would want to buy such guitars for their kids. I really enjoyed building the guitars and decided to take everyone's advice and eventually start building guitars and amps for kids.


Thank you in advance for your help.
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Re: Building a kid's tube amp

Postby lolbou » 11 Nov 2015, 14:51

Dixie Guitars wrote:Project #1 - small solid state amp
The speaker is the limiting thing, I think. I have an Orange Micro Crush here (as bench amp), and I've added a speaker out at the back to use it with an external cab. The built-in speaker sounds like shit, but it sounds fair in the Marshall 1912 cabinet... Any amp would sound "shkrink shkrink" in such speakers I think... I've salvaged a pair of 10cm 8ohms speaker in an old Philips stereo that sounded good and quite powerful. I'm gonna have a try at a cabinet and mini amp with it. Maybe you could try and seek good salvaged speakers?

Dixie Guitars wrote:Project #2 - battery powered tube amp
The 12AX7 heaters current is 150mA when wired in series and fed with 12.6V :wink: . You need a tough battery! I you have one, starved tube overdrive channel could work, but I don't think a starved output stage exists, so hybrid seems fair if you don't wanna mess with high voltages...
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Re: Building a kid's tube amp

Postby Dixie Guitars » 11 Nov 2015, 15:29

lolbou wrote:The speaker is the limiting thing, I think. I have an Orange Micro Crush here (as bench amp), and I've added a speaker out at the back to use it with an external cab. The built-in speaker sounds like shit, but it sounds fair in the Marshall 1912 cabinet... Any amp would sound "shkrink shkrink" in such speakers I think... I've salvaged a pair of 10cm 8ohms speaker in an old Philips stereo that sounded good and quite powerful. I'm gonna have a try at a cabinet and mini amp with it. Maybe you could try and seek good salvaged speakers?

One thing I don't understand... I've salvaged some mini speakers from an old iMac. When they were in the computer the speakers sounded great. When connected to a breadboarded amp they sound terrible, and have absolutely no volume. Logic would suggest that speakers should be fine and that the amp is no good. But when I connected a sightly larger speaker I got more sound out of it, even if that speaker was rated at a lower wattage.

OK, I understand that a larger cone on a speaker will push more air. But I just thought that the new submini computer speaker are some kind of new technology and that you can get more sound out of them, which seems to be the case when they are in a computer.

Perhaps I should be looking for an oval speaker?

lolbou wrote:The 12AX7 heaters current is 150mA when wired in series and fed with 12.6V :wink: . You need a tough battery! I you have one, starved tube overdrive channel could work, but I don't think a starved output stage exists, so hybrid seems fair if you don't wanna mess with high voltages...

After a lot of searching I did manage to find a circuit for an amp called LIL WATTER. Here's a link to the PDF.

http://craftsman.gtfm.org/wp-content/uploads/sites/2/2014/04/hybrid-amp.pdf

Does anyone have any opinions or practical advice for me, regarding this circuit?

Thanks...
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Re: Building a kid's tube amp

Postby deltafred » 11 Nov 2015, 16:39

You need your speakers in a suitable enclosure otherwise you will lose what little low end they had when in the Mac and could possible damage the speakers if you push them too hard in your attempt to get enough volume. Look up Thiele Small parameters if you are interested in this but it is doubtful if you will be able to find the parameters for OEM speakers.

What lolbou says about tubes, they generally consume a lot of power just idling so you will need a lot bigger battery than for a comparable power solid state or IC amp. Far better would be to build a SS or IC power amp with a tube simulator in the preamp. I am sure the guitar guys here will be able to advise on the best overdrive/distortion circuit to incorporate (I am primarily bass player who happens to play a bit of guitar so am not best placed to advise on this).

If you are going to build amps to sell then you really need to be sure that they comply with your country's electrical safety regulations. Having tube voltages (several hundred volts) could possibly make this difficult.
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Re: Building a kid's tube amp

Postby Dixie Guitars » 11 Nov 2015, 18:47

Thanks, deltafred.

This is all very useful information for me. I'll definitely read about those Thiele Small parameters.

Do you think it will be a big challenge to find suitable speakers that will fit into an ES-335 style guitar and get decent bedroom volume sound?

Regarding the tube amp, I don't think I mind a bigger battery. But from what you're saying I think it would be a good choice to do a hybrid amp with a single tube, like the LIL WATTER (unless you guys can recommend a better one).

Thanks again.
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Re: Building a kid's tube amp

Postby deltafred » 12 Nov 2015, 14:23

I don't think you are going to get very much volume from guitar mounted speakers before it feeds back especially on a 335 style guitar. A far better bet (IMO) would be to build a micro amp for bedroom use. The problem that I see with trying it is you will have 2 holes in your newly built guitar if you decide it is not going to work.

I would definitely go with a solid state output for your stand alone (larger) amp. Personally I would go solid throughout, starved plate preamps don't sound that much like properly biased tube preamps (IMO, YMMV of course). I get the impression that starved plate tubes are often included so they can say the amp has a tube in it.
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Re: Building a kid's tube amp

Postby Dixie Guitars » 13 Nov 2015, 03:17

deltafred wrote:I don't think you are going to get very much volume from guitar mounted speakers before it feeds back especially on a 335 style guitar. A far better bet (IMO) would be to build a micro amp for bedroom use. The problem that I see with trying it is you will have 2 holes in your newly built guitar if you decide it is not going to work.


Thanks for sharing your thought, deltafred.

I understand what you mean, but I still want to try before giving up.

I would not be cutting any holes in the guitar. This is a specially built guitar that has a removable back. It's a kid's guitar, so it's a "rugged" short-scale ES-335 that can be described as a cross between a high end cigar box guitar and a Gibson ES-335. I'll post pictures...

My idea is to mount a speaker on each side of the center block and have the sound come out of the f-holes. I read about the Thiele-Small Parameters and I've concluded that it's too much for me to study at this time. So, I guess my best bet would be to experiment with oval speakers. Those typically don't have any enclosures inside TV's, as far as I know, so I guess it should work, somewhat.

I don't want that sound to get too loud, anyway. Just as long as it sounds like an electric guitar, should be fine.

I didn't get any feedback when using the Marshall MS-2, even when holding the amp over the pickups. The pickups aren't even potted, yet, but I will pot them in wax, so it should be safe to go even a bit louder than the Marshall. At least it's what I think.

The thing about the Marshall is that I really didn't like how it sounded, not even with my regular guitars plugged in.

I'm hoping someone here could recommend a schematic for a slightly better amp, with just a tad more volume than the Marshall.


deltafred wrote:I would definitely go with a solid state output for your stand alone (larger) amp. Personally I would go solid throughout, starved plate preamps don't sound that much like properly biased tube preamps (IMO, YMMV of course). I get the impression that starved plate tubes are often included so they can say the amp has a tube in it.


I plead guilty to that last comment. I kind of just want it to be a tube amp and I just want to have fun building one. If the whole thing doesn't work out I'll of course be forced to do a solid state. But, again, I want to try.

There are two schematics that I found.

This one is called Guitar Amplifier for Headphones, but I see it also has a speaker.
http://www.ecp.cc/mini_guitar_amp.html.
I just don't understand the capacitor and some resistor values on the schematic, as most have just numbers without the units.

Image

Can anyone guess what the missing units are?


The other amp is the LIL WATTER hybrid that I mentioned earlier.
http://craftsman.gtfm.org/wp-content/uploads/sites/2/2014/04/hybrid-amp.pdf
Is that anything on this schematic that would be considered a red flag?
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Re: Building a kid's tube amp

Postby deltafred » 13 Nov 2015, 11:33

Your speakers need to be on a baffle otherwise you will have no low end. Try this - get a piece of card and cut a hole in it the size of the cone. First hold a speaker in free air, then place it behind the hole and listen to the difference. My very simplified explanation of what is happening - it's all to do with wavelength - the baffle prevents the low frequency sound nipping round the edge of the speaker and canceling itself out, the higher the frequencies don't have enough time because the wavelength is shorter.

If you could mount the speakers behind the f holes so there was no direct path round the edge of the speakers that would work fine, or a box that had the speaker on the back and the f hole as its speaker aperture.

The units on the schematic are ohms for the resistors and micro farads (uF) for the capacitors.
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Re: Building a kid's tube amp

Postby Dixie Guitars » 13 Nov 2015, 15:21

Great information, deltafred. Thanks a million.

Well, last night I found a TV on the street. I happened to have my tools on me and I spend about 2 minutes to get both speakers out. They are 8Ohm, 6W, oval speakers with pretty large diameter magnets. I believe someone threw out the TV just so I could salvage the speakers. Now I have no choice, I really need to work on this.

I believe I will be better off using just one of the speakers. I believe most of the schematics will be for 8Ohm, so can't use both. And it's kind of tight around the lower f-hole because of the pots an wires, but there should be plenty of room behind the top f-hole.


Thank you also for defining the values of the components.

What about the transformer? Do you know what value is that?


Thanks, again.
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Re: Building a kid's tube amp

Postby PhillyWill » 13 Nov 2015, 15:41

Regarding the output transformer - you need to follow the first link in the article: http://www.sophtamps.ca/mambo/index.php ... &Itemid=37
There are a number of somewhat more informative schematics at that site as well.

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Re: Building a kid's tube amp

Postby Dixie Guitars » 20 Nov 2015, 01:10

In light of the events in Paris, I simply didn't feel like spending time on any online forums, for a while.

I did make some progress, though.

Since I live in a big city I can eventually find practically everything on the streets. In the past couple of days I went around my neighborhood, on my bike, and found a whole bunch of discarded TVs. I opened them all up and now I have about two dozen oval speakers that I can experiment with, to see which sound best. Most of them can fit pretty nicely inside the guitar.

I already tested some with the Marshall MS-2 and I have to say, although I don't love the sound, some of the speakers actually made that amp sound a bit more "serious" (for lack of better words).

My main concern with that amp is that the clean sound is not much louder than the sound produced acoustically (I think part of the issue is the fact that the guitar has home-wound pickups that aren't as loud as standard). I wish I could get a clean sound as loud as the overdrive sound and that the OD sound is then not too loud. A thumb wheel, instead of a clean/OD switch would also be preferable.

So, I'm asking for opinions.


Do you guys think that It's worth pursuing, making a built-in amp that will sound noticeably better than the MS-2 (and also addressing the loudness issue, above), or should I just make my life easier and put the MS-2 in the guitar?

Thanks...
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Re: Building a kid's tube amp

Postby deltafred » 20 Nov 2015, 11:17

I have my doubts about a built in amp and speaker/s. If you get enough volume to be useful you will probably be close to getting feedback because of the vibrations transmitted from the speaker to the bridge and pickup.

I wound a pickup for the first guitar I built and found that the output was very low so I added a small transformer. It was a 240v input, 12v output, I fed the pickup into the 12v side and took the 240v side to the jack. It boosted the signal voltage pretty well and the output level was just about what a commercially wound pickup gave.
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Re: Building a kid's tube amp

Postby okgb » 20 Nov 2015, 15:28

It sounds like you may have the gumption to just get going, so mock it up on the bench and find out for yourself!
I don't think you're going to find a battery powered tube circuit with allot of clean headroom and volume,
Even if you keep asking , you know, physics !

Some takimine guitars used a circuit they called " cool tube " but that was a preamp

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Re: Building a kid's tube amp

Postby Dixie Guitars » 21 Nov 2015, 16:21

deltafred wrote:I have my doubts about a built in amp and speaker/s. If you get enough volume to be useful you will probably be close to getting feedback because of the vibrations transmitted from the speaker to the bridge and pickup.

I have the same doubts, absolutely. At some point there will be feedback but I'm still curious to see how much volume I'll be able to get before feedback.

If this was a guitar for a client I would advise the client against it and not take on the job as I would not want to deal with complaints. But since this guitar will just stay at home I've decided to experiment and have fun working with my kid on a project. If nothing else comes out of it, he will what feedback is all about, LOL.

deltafred wrote:I wound a pickup for the first guitar I built and found that the output was very low so I added a small transformer. It was a 240v input, 12v output, I fed the pickup into the 12v side and took the 240v side to the jack. It boosted the signal voltage pretty well and the output level was just about what a commercially wound pickup gave.

This is a pretty neat idea and I just made a mental note to possibly use it in the future.

At this time the bridge pickup (alnico 2) is not as loud at the neck (alnico 5). So, I am going to rewind a new one with slightly longer pole pieces, which will give it more winds.

Both pickups were noticeably louder before I put the covers on. The covers are lids from Altoids boxes, obviously a choice to make the guitar look like a home made kid's guitar. I believe the covers on commercial pickups are from a non ferrous metal and I guess it makes a big difference, as the Altoids are interfering with the magnetic filed. But I'm keeping the Altoids just because I want to maintain the looks.

Thanks for you comments.
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Re: Building a kid's tube amp

Postby Dixie Guitars » 21 Nov 2015, 16:40

okgb wrote:I don't think you're going to find a battery powered tube circuit with allot of clean headroom and volume,
Even if you keep asking , you know, physics !

Darn physics always get in the way of what I'd lie to do...

Question...

I found some schematics of battery powered tube amps that have a voltage boost circuit, that bring the voltage to about 80VDC.

How much do I need to worry about this high voltage in an amp used by a kid?

I know about that "death capacitor" and understand how a player can get shocked if a voltage "leak" ends up on the grounded parts of the guitar hardware, and then the kid somehow completes the circuit to get shocked. Obviously if the kid puts his hand on a grounded radiator, while holding the strings, does it. But that's an amp using AC, and that's not properly grounded.

So, are there any potential scenarios I should really be worried about (apart from a kid opening up the amp)? Perhaps touching the tip and the shield of the guitar cable, before plugging it in?
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Re: Building a kid's tube amp

Postby deltafred » 21 Nov 2015, 17:35

Dixie Guitars wrote:The covers are lids from Altoids boxes, obviously a choice to make the guitar look like a home made kid's guitar. I believe the covers on commercial pickups are from a non ferrous metal and I guess it makes a big difference, as the Altoids are interfering with the magnetic filed.

Using ferrous metal for the covers is going to shield the strings from the magnetic field so attenuate the output signal. Isn't there some plastic or nonferrous box lid you could use for the covers?
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Re: Building a kid's tube amp

Postby Dixie Guitars » 21 Nov 2015, 18:03

deltafred wrote:Using ferrous metal for the covers is going to shield the strings from the magnetic field so attenuate the output signal. Isn't there some plastic or nonferrous box lid you could use for the covers?

I guess it would take some looking around to find plastic candy boxes/covers that would be the appropriate width for the string spread. I was already thinking of doing that, but these things take time to find. The other issues is that my kid has his mind set on Altoids and in general, when his mind is set there's no way of changing it.

With this build the most important thing to accomplish is to make my kid happy and have a guitar that works "just fine" for his level. But I'll be doing more builds in the future...
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Re: Building a kid's tube amp

Postby Thomas_H » 22 Nov 2015, 12:35

Hi guys,

This is a very Interesting thread, I start thinking about this and had some ideas...

I also had a MS2 and the difference between clean and overdrive was really a problem, but if you look at the schematic the clean circuit goes directly into the power IC, while the overdrive has a fet before it. The ruby, on the other hand, uses the fet all the time and you can control the clean to distortion with you guitar volume, when the gain is not maxed. I would say that you could use a simple voltage charge pump, as the Max1044 or the icl1776s to increase headroom, they are very common in some 18v pedals.

For the tube amp, it is a bit difficult to obtain a battery that would last for a long time, but have you already seen Greg's stuff?

Link


The amp is very simple, and he uses a SMPS to obtain 200v available at http://www.tayloredge.com/storefront/SmartNixie/PSU/index.html.
Of course 6v6 would not be the best output tube, I read somewhere the 6ak6 gives you 1w with only 150mA at 6.3V for the heaters, you just need to find another tube with same current and see available heater voltage, with another 6.3V you could probably use a 12v battery as Greg did. For a 12ax7 you would need to run the heater in the 12v configuration so you would need a 18v battery...

Another approach would be take a look at some tube amplifier radios, they use some odd tubes that work at voltages as low as 45V like here:
http://www.hoofbags.me.uk/geekiestuff.html using DL94 and DAF96/91 tubes.

As for the altoids covers, have you thought about making your own cover? The Altoids tin could be used as a mold as they are embossed, so you could make a plastic cover and paint it with chrome spray, it would look like that gibsons with the gibsons name on the pickup covers. Maybe you won't need to melt the polymeric material, only heat it up until it's soft and moldable, and use two altoids covers, one as mold and the other to punch the heated polymer in the mold like a press.

Good luck with your builds, I hope to see some pictures!

Thomas

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Re: Building a kid's tube amp

Postby Dixie Guitars » 22 Nov 2015, 23:33

Thomas_H wrote:This is a very Interesting thread, I start thinking about this and had some ideas...

Thanks for joining in.

Thomas_H wrote:I also had a MS2 and the difference between clean and overdrive was really a problem...

Thanks for explaining the theory behind it.

Since I'd like the built in amp to be ready for a Thanksgiving party, I guess I'll just use the MS-2, now. Then I'll work on building something better and upgrade the guitar.

Thomas_H wrote:For the tube amp, it is a bit difficult to obtain a battery that would last for a long time...

I was hoping to use a 12V LiFePO4 battery from http://www.batteryspace.com/128vlifepo4batterypacks.aspx. I think a couple of hours of battery life would be plenty for a kid. Roughly speaking, how many Amp hours should it be to run a hybrid amp with a single tube, for 2 hours?

Thomas_H wrote:have you already seen Greg's stuff?

Yes, I've seen his videos. His amps seem to be good and I want to build some for myself, but I'm still worried about the voltage being pumped up, for a kid's amp.

Thomas_H wrote:Another approach would be take a look at some tube amplifier radios, they use some odd tubes that work at voltages as low as 45V like here:
http://www.hoofbags.me.uk/geekiestuff.html using DL94 and DAF96/91 tubes.

Definitely, thank you for that ink. I like some of the stuff there and really like those thumb tack boards for soldering the connections, perfect for this project.

Thomas_H wrote:As for the altoids covers, have you thought about making your own cover? The Altoids tin could be used as a mold as they are embossed, so you could make a plastic cover...

That's a very good idea. Thanks.

But unfortunately, for this small guitar, I am using Altoids Smalls, which is not embossed. But I would like to use your idea for a future build, which could use a full size Altoids box (perhaps a bass).

Thomas_H wrote:Good luck with your builds, I hope to see some pictures!

Thanks, and yes, I will definitely post some photos.
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Re: Building a kid's tube amp

Postby Thomas_H » 23 Nov 2015, 22:10

Hi,

I think a couple of hours of battery life would be plenty for a kid. Roughly speaking, how many Amp hours should it be to run a hybrid amp with a single tube, for 2 hours?


Well, Greg stated somewhere he could play during 3 hours or so...
For one tube, let's say a 12au7 the heaters would be consuming 150mA in the 12V configuration, if you use the same battery that Greg, a 12V 7000mAh used for home security systems, it would give you 46 hours if you only consider the heaters. So, you could probably use the smaller one with 600mAh and still have 4 hours of fun. But there are some extra stuff draining your battery, like the tube plates (let's say 5mA) and the IC you are using.

I done something similar in a Altoids tin using the ruby amp as output for my headphones, but you could do something better:
Image
Sorry for the bad picture, it was a long time ago.

I understand that you are concerned about the high voltages, the space charge tubes are the option, I guess I will build a real battery tube amp for myself :lol:

As for the altoids tin, if you have no problem on buying some extra boxes, there are different kinds of small boxes, in some of them the word small is embossed while in others the word altoids, I have an Altoids Collection, and I managed to put a cmoy with a small battery inside of one of those smalltoids.Unfortunately that small 12v garage door opener batteries only have 50mA. I ended up putting it in a normal size box, at leat the 9V battery would fit.

Thomas

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