Building a kid's tube amp

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

Re: Building a kid's tube amp

Postby Dixie Guitars » 30 Nov 2015, 15:19

Just to get back to the previous discussion about piezo pickups...

Again, Phil, thanks for explaining all that.

phatt wrote:Some of the earliest barcus berry piezo PU's where simply a G type clamp that mounted on the side of the body of a guitar and (If I recall) you could alter the pressure and position of the PU for different effect on tone and output...

As to the rubber muting tone **Yes** that is the whole idea as these piezos have a huge hi frequency response and direct mounting can be problematic and cause all sorts of problems,, one being massive feedback...

OK, so I think I could come up with a small internal clamp and mount the internal piezo between two pieces of rubber. Perhaps rubber from a bicycle inner tube would be fine. Or perhaps an old rubber gasket, or perhaps even cork. I just want it to hold up over time. Some rubber materials become brittle over time. Any idea which exact material would be my best option?

phatt wrote:For guitar the most common setup now is to have the piezo element directly under the bridge bone which maximizes the pressure and hence they have substantial output signal. these are often shrink wrapped giving just a little cushioning which helps even out the ride,,, :mrgreen:

My bridge piezo is sandwiched at the base of the archtop bridge, with epoxy. Now I understand that I would have been better off using some kind of material that becomes like rubber when it dries up.

Can anyone think of a material that I can handle like glue and then dries up into rubber?


phatt wrote:They do of course have a completely different response to mag PU's and a lot of circuit tricks are still needed to wipe off excessive hi frequency response...

Good PU info here;
http://www.ozvalveamps.org/pickups.htm

The article you linked to says...

If we assume as above that the [piezo] pickup has a capacitance of 500pF, then connecting a capacitor of say 10 times that value, 5000pF will cut the signal to about one-tenth (one eleventh actually) or a bit more than -20dB compared to the unloaded output. One volt peak from the pickup becomes about 100mV peak at the amplifier which is similar to the output of a magnetic pickup anyway, so this shouldn't be a problem.

But two capacitors in series like this acts somewhat like a transformer which results in a better match to the amplifier input resistance, hence less low frequency roll-off.

Am I understanding this correctly..? I could try adding an internal cap of about 10 times the capacitance of the piezo, and it should be connected in series.

Is that correct?

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

Postby Dixie Guitars » 30 Nov 2015, 17:20

Dixie Guitars wrote:My bridge piezo is sandwiched at the base of the archtop bridge, with epoxy. Now I understand that I would have been better off using some kind of material that becomes like rubber when it dries up.

Can anyone think of a material that I can handle like glue and then dries up into rubber?

Actually, I'll answer my own question here.

I think the solution to this might be silicone. Or I also found a roofing material caller Liquid Rubber.

Do you guys think that would work?
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Re: Building a kid's tube amp

Postby guitarzan425 » 30 Dec 2015, 03:28

Thomas_H wrote:
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.
Thomas


That Altoids amp is so cool!
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Re: Building a kid's tube amp

Postby Dixie Guitars » 17 Jan 2016, 02:49

Hello again,

First of all, Happy New Year to all.

I haven't been posting for about a month because I started Googling for additional information and eventually decided to educate myself first, before posting and participating in this forum. So, I started as a newbie with minimal knowledge of electronics and now I can say that I have a better understanding of how things work, so hopefully I won't be one of those that need to be spoon fed every step of the way. Nevertheless, I'm still new to this so I still need help.

I finally breadboarded a solid state amp that I am quite happy with. It is basically a modified version of this:

Image
http://i2.wp.com/www.circuitbasics.com/wp-content/uploads/2015/04/LM386-Audio-amplifier-with-Bass-Boost-2.png

With a JFET input buffer from the Ruby Amp.

When studying the Ruby Amp schematic I noticed that the JFET pins were not labeled. After some more Googling I found this:

Image
I connected the FET the way it is here and it worked. However, after yet some more studying about how JFETs should be wired I learned that the Drain should be connected to the positive. However, in that hand drawn schematic the Source is connected to the positive. I also learned that for most FETs the D and the S are interchangeable (the exception being those FETs that have the Gate closer to the Source). So, I decided to rewire my FET, so that the Drain is connected to the positive. But it didn't work. When I rewired it back to the way it was, then it worked again.

So, this is the confusing part for me, now. I don't understand why it doesn't work when I connect the Drain to the positive, as I believe is the correct way.

Any ideas?

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

Postby mister707 » 05 Mar 2016, 18:45

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Re: Building a kid's tube amp

Postby deltafred » 05 Mar 2016, 19:12


I gotta ask, what has Popular Mechanics got to do with the thread topic?

I know there is a picture of someone playing a guitar on your screen grab but there is no way I can read the text. (If you were not a previous poster I would consider reporting your post as spam.)
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Re: Building a kid's tube amp

Postby mister707 » 05 Mar 2016, 19:54

¿Spam?
I hope you can read the circuit diagram.

Image

Image

Image

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Re: Building a kid's tube amp

Postby deltafred » 05 Mar 2016, 21:16

That's a lot better. :thumbsup

The guitar she is playing looks like a solid body Ovation. A guitarist I played with years ago had one exactly like it, I always liked the sound of it but he hated it and got a 62 reissue Strat (that did nothing but snap strings where they passed over the bridge).
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Re: Building a kid's tube amp

Postby blackcorvo » 27 Jun 2018, 21:40

I saw the title for this post, and I wanted to chime in!

If you want an interesting option for a tube amp, you can look up old devices that had built-in tube amps. Transcievers, police radios, old tube radios, intercoms, etc.
I recently got myself a "Talk-a-Phone LM-5 Master" tube intercom, which I converted into a small guitar amp:

https://www.youtube.com/watch?v=O_HXrMHrEqU

Do keep in mind that sometimes, such old devices DO NOT have a built-in isolation transformer, which poses a risk of electric shock. So, before plugging it into the wall, get yourself an isolation transformer! And I also recommend using a fused power strip, for extra safety while testing things.

A safer (and more readly available) option is building one from scratch. One such option I recommend is this project, developed by Handmades.com.br (a brazilian DIY forum):

https://www.altanatubes.com.br/download ... 20V1.1.pdf - (schematics on pages 5, 6 and 7).

It's simple, uses tubes and a power transformer that are easy to find, and relatively cheap compared to other, more common projects. There's many videos of this circuit being tested, such as:

https://youtu.be/bMwMGjNODCw?t=1m39s

Just to give you an idea of how it might sound.
Good luck with your builds!

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