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#1
So, whilst waiting for opportunity to continue my current guitar build I've been looking at amps, battery powered amps and stuff, been trying to get serious with my music, including really getting to know my gear. Anyway I've decided to have a go at building a battery powered all tube amp based off Sopht Amp's 12k5 12V low voltage tube amp.

I've added in a gain control and a 3 band tone stack and a master volume. Now I am looking for some advice to add an effects loop. Would I be able to just break the circuit between the tone stack and the volume control (red star on the schematic) or would it be necessary to buffer (I think that's the correct term) it with another triode? If I can just break the circuit is there anything I need to do with the return signal to limit it in case it damages my output tube? If I do need to run the return into another tube, do I need to use both of the triodes in the second dual triode tube (using the 12U7 like Sopht's design) or can I leave half the tube unused?

Obviously the non-tube option is preferable to increase battery life when running on batteries but I'll take options and advice on both methods.
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#2
oh... best include the schematic...



All the components do have values but I don't have the parts list digitised yet.
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Last edited by GABarrie at Jun 9, 2011,
#3
Going hybrid for buffers and effects loops is a great idea.

Also, I wouldn't jumper the wiper to the ground lug of the gain and volume pots. As of right now, you're shorting the signal out to ground.

You may even want to use an emitter/source follower to the output rather than use some iron to power the speaker. If using a MosFET buffer, you'll want to put the ground lug of the mid pot to a reference voltage.

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm
http://www.geofex.com/Article_Folders/mosboost/mosboost.htm

Also, for some circuit ideas, here's an amp I just threw up on the breadboard.
Attachments:
Hybrid Headphone Amp.pdf
#5
Agh, thanks for pointing the shorting issue out. I think I see where I got confused, stole the tone stack from the ax84 schematic and the shielding grounds after each wiper. I'm still getting the hang of this, last time I did electronics was 7 or 8 years ago and was one term of my technology course at school - and all I dealt with was a 555 timer and we didn't even get taught what the pins were for, just given a layout, some stripboard and told to build it, at least I learnt to solder.

As for going hybrid that may be my best option although I wanted to try keeping it all tube. If I went hybrid would a jfet transistor be enough to buffer or would I need an op-amp? Again, new at this and trying to learn so if I say something noobish be gentle with me :p
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#6
Oh, another thing, looking for something cool and unusual to use as an enclosure, like the beer can amps, any suggestions?
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#7
I would use either a JFET or MOSFET source follower, you can direct couple the gate to the plate, you will not be able to hear a difference except that the tubes won't be loaded down by the tonestack or load.

I would say just find something laying around.
#8
got some 2N5457s lying around. they do the trick?

And some updated schematics two different versions actually, one with the solid state buffer, one with a second 12U7. Was considering trying the tube one out on a breadboard to examine battery life, so I can keep the design all tube if its not gonna die after 30 minutes. I can also experiment then with tone and volume levels if I switch the whole of the loop section off including the second tube with the contact break switch in the return plug, or just bridge the effects loop break and keep the tube as a second preamp stage.

the All Tube schem:


the Hybrid schem:


It's likely that there are flaws in both of these still, as I said, my electronics experience is low, and I'm still learning as I go.
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#9
You still will want a buffer in the hybrid scheme, the output impedance of a tonestack tends to be high and variable. On the return of the hybrid scheme, place another resistor from the base to B+, a 5 to 1 ratio of the top to bottom resistor will work, or even 10 to 1.

If you wanna keep the 3 tubes scheme (assuming you're using dual triode tubes for the preamp) you can even going hybrid, rather than using a bjt to boost the return, use a cathode follower before the send and just a triode stage to return. The input impedance of a tube is much higher than a bjt and will a do nicer job for the return.

The all tube scheme you've got will work pretty decent, you might wanna avoid a 12AX7 or other high mu tube before the tonestack, a 12AU7 will work perfectly there. You could even use a 12DW7 to get a high gain input stage, and a low gain, low output z triode before the tonestack.
#10
I'm using 12U7s (they're pretty much 12AU7 but the 12v operation equivalent as far as I can tell) for both the preamp and the buffer tubes.

http://www.nj7p.org/Tube4.php?tube=12U7

gonna work up all the component values as best as I can in the morning and then work out a parts list, already got the transformer and the tubes ordered though so apart from the battery its all fairly cheap components from here on
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#11
http://www.freewebs.com/valvewizard/index.html

His preamp book is great, and you'll learn a lot about designing graphically with triodes.

You may want to use to 6v lantern batteries in series, a little more expensive, but have a good bit of energy in them, additionally, Lithium batteries have a pretty good end of life curve (more usable time from them) and they tend to last longer.

Here's another document written by the same guy, he knows a good bit of stuff.
#12
have my eye on a 6500mAh 12v lithium battery but its gonna cost me as much as the 3 tubes >.<
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#14
The problem with lithium is that their operating voltage will vary a lot. Your looking at 12.6 voltage full down to about 7.5 volts at cutoff. And you will have to buy a charger and also make sure the battery bar cut off circuitry already. You can discharge a lithium ion battery once, but it may never work again.

They are great when you don't have to have a specific voltage, say like power tools, but if you need an exact voltage then you need to have a regulator and enough cells so your lowest voltage still gives you what you need. I don't know how the heaters will take 7.5 volts well.
#15
http://www.maplin.co.uk/aa-size-battery-holders-31427 like the 8AA version of that? :p btw thanks for the link to those resources, added his book to my wish list
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Last edited by GABarrie at Jun 11, 2011,
#16
Yeah, that 8 AA will work great, and I didn't know that, thanks Matt. He has a good site, too, definitely bookmark worthy for tube amp stuff. Don't forget about the resource thread for places for parts, and the Amp Building Resource thread for some information and more links.
#17
Tweaked the schem for a standby switch, not sure how necessary it is for low voltage tubes but they werent very easy to get hold of so I'd like to preserve as much tube life as possible.

Ordered about half the components now including some funky colored LEDs (I don't do basic green or red), still need some poly film caps (damn they are way more expensive than all other types of caps), jack sockets but they won't be hard to get, the battery pack when I decide what I'm using, thinking it may just the 8AAs for now and take it from there. Also need some tube sockets, some heat sinks and an enclosure. Still looking for something unusual to use as an enclosure but so far no luck.

Also got a breadboard so I can start testing component values and designing the layout.

Pics coming as soon as I have parts in stock

Also debating putting some basic effects in like an OD with true bypass to give me a poor mans overdrive channel. If I do then I may set up a bank of 9V batteries for power, 1 for effects and 2 in series with a voltage regulator for the amp.
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#19
all parts ordered, should have it all by the end of next week allowing for some transatlantic shipping and then the pictures can start
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#20
You don't need a standby switch even for HV designs, only certain types of equipment (not guitar amps) need standby switches. You might as well just add a Mute switch by shunting a grid to ground. The standby switch these days is just convention.

You can just add (clean) gain stages with MosFETs/JFETs/BJTs to distort the tubes more.

Good luck!
#21
But the standby switch gives me the option to have another funky cool colored LED lol, designed an overdrive circuit that uses LEDs in the diode loop so I get those to glow too. I just like shiney things.

Decided to just build the enclosure out of plexiglass and got some 7mm ply to build a cab with an 8" speaker.

Incidentally do I need to do anything special with wiring the cab or just jack socket to speaker?
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#23
Quote by kurtlives91
If you want glowing and shi t just replace the cathode resistors with appropriate LEDs.

Might as well build a whole led circuit then, might be overkill :p
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#24
You could use 16 Gauge speaker wire, a bit much though considering the low power of the amp.

The cathode LED VS a cathode resistor is a viable option. No extra parts in comparison.
#25
Going to go with the current schematic, keeping the standby switch in though if nothing else it will cut all power to everything except the heaters and conserve battery life, rather than just muting it
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#26
In the grand scheme of things your standby switch will do nothing to lengthen battery life. If you've got two 12U7 tubes at 12V then your looking at 300mA draw. Your tubes with 12 volts on the plates will be drawing something in the microamps to a few milliamps.

Throw in the draw of the 12K5 at 450mA and you're looking at 750 mA total draw out of the battteries. Your going to have very little battery life even with lithium AAs. You may get an hour or two out of those batteries, and much less than an hour out of alkaline batteries. The higher the discharge rate on most single cell batteries like AA AAA, the less capacity you get out of them overall.

You would be better off finding some type of Lead Acid battery and charger or something else along those lines. These tubes were designed for car radio, so it makes a lot of sense to use those with them. It will probably cost a little bit more up front, but not having to constantly buy new batteries would be nice. Also, rechargeable lithium batteries will not last as long as non rechargeable ones.

If you were still looking at rechargable lithium ion batteries, you could always look for a 15 or 18 volt battery and have a 12 volt regulator. This way you could get one like that 6500mAh battery you talked about and always have the right voltage.
#27
for the moment going to be running 4 Duracell Procell 9V batteries (2 lots of 2 in series, then wired together in parallel) that will give me 1100mAh at 18V into a 12V regulator which will do till I get a more permanent battery solution, building in a DC in power supply too so will only effect me when I am away from a wall socket. As often when I travel it's a space issue rather than a power point issue.
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#28
Are they alkaline batteries?

If so your still running into the issues I discusses above. Alkaline batteries can be given a capacity rating, but that rating will never be much above say 10mA or so discharge current. But as you increase the discharge current their useable capacity plummets.

Obviously you'll be able to use it for a little while... Its just one of the little tricks they don't tell you about with alkaline batteries. But it is a pretty cheap way to run it for awhile, just giving you a heads up since it seems like most people don't know much about batteries... and all I do now is work with batteries haha.
#29
cheers for the battery tip it will have to do for now

few noob electronics questions:

1) what program to you guys draw up all your lovely schematics in?

2) if a schematic has a bias (i.e. a 4.5V bias in a 9V circuit) does that mean a +4.5V DC connection instead of a +9V? And if it that's the case, is a voltage divider the best way to achieve it?
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#30
There are a couple freeware ones. I've taken a liking to ExpressPCB, there is also Eagle and DIYLC (DIY Layout Creator)

A voltage divider is a good way to accomplish a 1/2 supply voltage. Two 10k's in series with a 100uF cap will make a pretty stable Vb. Another option if using OPAMPs is to put a non-inverting buffer following the voltage divider, this gives the divider some more oomph. But if not including OPAMPs, there is no real need to use a buffer.
#31
so i was right about a bias is a different voltage in the circuit then? :p
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#33
cool, cheers mate, beginning to get the hang of ExpressPCB too
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#34
Apparently Thomann aren't getting my VHT in stock, possibly ever.

So I've decided to build my own following this project. I'm currently designing it (need to sort out tube biasing and tweaking the tonestack), but does anyone have a link to a good schematic I can use/adapt for putting a spring reverb tank into the design?
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#36
at this point I don't know how to identify a good one :p already got my reverb tank bookmarked
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#37
ok, got the pcb soldered, all the wires stripped to connect up all the periphery components (pots, jacks, tube sockets etc.) gonna solder it all together tomorrow evening and test it out, turns out the project box I bought isn't big enough to fit everything on top and fit stuff to protect the tubes so having to order a bigger enclosure, gonna mount it all in cardboard for the moment, hopefully on Friday I can have it working and get some sound clips for people
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#39
It's ok, got everything I needed

Hoping to get time tonight to finish it, will let you know how it goes
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#40
ok, so following a horrendous day in relation to my daughter being a nightmare and about 2/3rds of a bottle of wine to take the edge off I gave up after I soldered the power supply the wrong way round, gonna try get more done tonight
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