I was just perusing the internet and came across this site http://www.junkbox.com/electronics/lowvoltagetubes.shtml about certain types of tubes that were meant to be run at low voltages (both on the heaters AND the plates). 20v or less. Most of the info at that site is about building low voltage tube radios radios, but there is no reason on can't build instrument amplifaction stages with these tubes. The sopht amp uses common space charge tubes (12u7 in the preamp and I forget what the power tube is). AFAI understand space charge tubes will sound  different than "starved cathode" designs that use regular tubes in a low voltage mode. Anyone know of any other designs, specifically preamps or overdrives, that use space charge tubes. I searched google, but got a lot of useless repsponses. The idea is to use a space charged tube circuit as the preamp to a chip amp. I've got one chipamp that runs at +-24, and another at +18 that I'm fooling with the preamp sections on. Of course I can use a regulator to get whatever voltage I need for the space charged tube preamp design.

Cheers,

DDC

Big congratulations to Armstrom! That Hurricane you built is way awesome! And I'd also like to say that all the entered amps were very awesome in their own right... Good job to all the other contestants! There is DEFINITELY some major Mojo goin' on here!

Hi all... It's been a while since I've posted here, but I've been lurking around, reading posts and what not. I've been working on a new ~18watt combo amp using the Qkit #105 board and the Hitachi HA13118 IC. It's together enough to enter into this contest (which is a VERY cool idea, but the way), but not completely finished yet. I'm having some issues with hum (prob a ground loop), and I'm going to replace the current preamp (the SSTS preamp I posted about in the preamp forum a year or so ago) with a modified Run Off Groove "Peppermill" that I have breadboarded, but haven't put on perf yet. I'm also fooling around still with the tone control. I started with the Simply Wonderful circuit, but now I'm probably going to build some variation of the BMP tone control. The metal chassis and face plate is from a scrapped radioshack brand PA amp I found at the goodwill. It had a good xformer in it, and also had the rectifier and heatsink. I built the amp body myself from som scrap lumber I had in the shed. I covered the speakers with some left over window screen, and I covered the top with "tweed" from some old curtains. I made the red leather handle myself as well from a "sample" I got from the IKEA sofa department (I won't tell you how I got it). I've made all the connections easily changeable by using screw terminal strips, so this amp can be modded VERY quickly. Another possible mod will be to make it be battery-powered (with a 12v sealed lead acid battery). Anyhow, here's some pics:

Overall front shot (Note that the top is not actually completely on it):


Shot showing the top:


Overall front shot showing my pedal board too (gratuitous!):


Shot from the back showing the speakers (2 awesome vintage eight inch speakers from an old organ):


Shot with the top off:


Shot zooming in on the "guts" (hitachi board at top center, SSTS preamp board at bottom right):


Here's the Peppermill on my breadboard. Mods are added swichable diode clipping (asymetric) and added switchable gain boost to the BS170 mosfet section. This makes it a VERY variable preamp, going from squeaky clean to really dirty:


Well, I hope you like it. I don't have any sound clips yet, but, like I said, the sound isn't quite "there" yet for me, and I'm still working on it!

Cheers,

DDC

I haven't yet used a TDA chip, but I hear good things about them. Actually, you could probably use most any chip meant for car audio applications with good/acceptable results.

I would think the main concerns for such an amp would be 1) Preamp construction and 2) battery type and charging method.

Simplest build is like the little gem/noisy cricket that many of us have built (including me). In this case preamp section is a single FET gain stage/buffer stage, and power amp is half watt LM386. Battery power needed is only 9v, so we use rechargeable/throwaway 9volts. The power output is low, so the current draw is also low, and there is no complicated type of charging circuitry needed.

More power requires more amperage. In this case you are left looking at these 12v rechargeables that are some kind of lead acid  or gel lead acid varieties. In this case you want to look at the AmpHour rating of the battery. A 12v 8AH battery will produce 12volts (more or less) at 1 Amp for 8 hours. This means yo can run a 12 watt amplifier for a total of 8 hours from that battery. If your circuit has a higher wattage (say 24 watts) that means it draws more current (2 amps instead of 1), you'll get a run time of 4 hours off the same battery. I'm pretty sure that as long as the AH rating of the battery is as big as the current draw of your amp, your amplifier will yield maximum power. As the AH rating gets bigger than your current draw, you just get longer life at that max power, not more power.

    Charging the battery is another matter. For lead acid batteries, you need some kind of "smart" charger that varies the voltage supply according to how fully charged the battery is. If you don't do this, the battery will quikly become useless. The most simple is to just buy a cheap outboard wall wart type charger, and extract the battery and charge it externally when it is low. You could also build in some kind of power jack with switching to enable the wall wart to plug into the back of the amp. You should make it so that when the plug is in, the thing won't turn on. Most difficult is to build the charging circuitry into the chassis itself, and just plug mains power directly into it. You will need and mains to 18v transformer, DC rectification bridge, and a battery charging circuit (these can be purchased as modules). This setup has the advantage of the fact that you can make the amp switchable from mains power when plugged in to battery power when unplugged.

  The preamp can be any kind of circuitry you like. You can run it at 12volts, but I would definately use some sort of regulator IC to control the amount of power it will have to disperse, especially if used with high AH batteries. Another possibility is to use some sort of current isolating transformer. Otherwise you will have to use bigger more expensive resistors and have caps rated for higher power. Make sure you use a regulator IC that is rated for higher power too, and it will probably need a heatsink or to be mounted to the chassis.

Cheers,

DDC

I'm working on a 12volt portable using the HA13118. It's available as a kit from q-kits or other online retailers. Max voltage is 18 volts @ 1 A to put out 18w through a 4 ohm speaker. If you run it of a 12V sealed lead-acid battery you should be able to pul 10-12w depending on the amp-hour rating of the particular bat you choose. You can get smallish sized rechargeable sealed LA bats from many outdoor suppliers for $10-12 or in the ~$20ish range if you also opt for a charger wall-wart (not a bad idea). I've salvaged mine from one of those "million candle power" flashlights that hada busted bulb...
  I got the amp module built, and have tested it directly with a 12v source. It has good output, and doesn't really need a preamp to get volume, but I think it could use a pre for some tone shaping/distortion character so I'm gonna add one...
  I've been a bit pressed for free "amp buildin' time" lately, so I haven;t really worked on this in a couple of months... I'm not sure when I'll get a chance to finish it, but I'll definately post it here when I finally get around to it.

Yes, Armstrom is correct there. You will definately blow the regulator chip if your supply rails are too high for its specs. By the way, I failed to mention that if your power amp uses a bipolar supply, you should attach the regulator between the positive supply rail and the center tap ground, not between the positive and negative rails.
Cheers,
DDC

When I was just learning how to build my first amps, I too was baffled at how to power preamp sextions which, if you are building anything over 12-15 watts, require lower voltages than the poweramp section will. The best solution I've found is to use a voltage regulator chip to supply the preamp. These are readily available (even from radioshack), and are very easy to use. You attach one pin to the input votage (higher voltage), one pin to ground, and then the specified output voltage up to the specified number of amps (500 milliamps is all you need for most preamps) comes out the third pin. I've successfully used these chips to power the preamp in all the amps I've built (three).

Hi all, These are the schematics I used to build my solid state Harp Amp. I should say a couple things about it:
1) I used mainly scavenged parts for the power supply (except the 10000uf 35V filtering caps and the 12v regulator for the preamp power), so that's the reason you see two transformers. You should note that the trannies are wired in series such that there are +, -, and ct taps on the secondaries. It's vital to get the proper phasing here, so you must test the output with a multimeter before permanently wiring here, or you will get phase cancellation (effectively producing 0v out either tap). Figure 1
2) The prof tweed was tweaked with some info I gained from the DIY stompbox forum. The following changes were made and are shown as notes on my schematic: I changed all the DC coupling caps to 0.1uf. I changed the caps in the speaker sim section to 5n8. I changed the cap in the feedback loop to 0.01uf. The first two mods allow for more bass, a mod that is normally done to guitar amps to help "voice" them for harp. The last mod actually increases the gain a bit, which I found necessary to make this circuit work properly as a preamp. Also you should note that I am powering the thing at 12v, which means the fets should be biased at around 6v (it was originally powered by 9v). Of course, I biased the fets "by ear", which I always find more satisfactory. Figure 2
3) The lm1875 poweramp board is a kit obtained from qkits.com. I won;t paste the schematic here, but this is the link to where you can purchase it from qkits: http://store.qkits.com/moreinfo.cfm/QK50 The kit is high quality: I found the board to be really well designed, and the components to be of good quality. If you think it is necessary, you could always use higher quality components (1% metal film resistors instead of the stock 5% carbon ones, tantalium caps instead of the stock electrolitics), but who knows how much difference that would make. I really appreciated the on boarde fuse protection. You will need to provide your own heatsink, and make sure you isolate it from the rest of the chassis (the chip is not insulated). Also, there is an on board zobel network, which is prevents oscillation when driving difficult speakers or when pushing signal over long lengths of cable. No schem

Finally, please note that this amp was designed and intended for amplified Harmonica tone. That means it's really quite dirty, although I plan to add a clean channel in the near future. It may work for guitar or bass, but I don;t play either, so I can neither confirm nor deny it's efficacy in either case. Also, I decline to show schematics of the jacks and switching, as I did not make any, and these are often a matter of personal choice anyway. I wired a true bypass switch to the preamp as a place saver for a clean channel switch, and I made a effects send/return loop before the master volume control, but these are not requirements for a working circuit.

The amp can be seen and heard here: http://www.youtube.com/watch?v=RiV78_rUv8c

And here are some of the "guts". First and overview, then a zoom of the prof tweed circuit, then the lm1875 board from qkits...

okay, here are a couple of pictures of the outside. Note the harmonica in the first pic 

Well, I have just finished my first amp build with some success! It's a Professor Tweed modded for 12v, slightly more gain, and tweaked for harmonica (better frequency response) that drives an LM1875 power amp module. I built the prof tweed and power supply on vero, and used the lm1875 mosule from qkits.com. I salvaged the chassis from a defunct old tube radio, got the transformers, jacks, leds, wire, and pots from a pair of old 8-track hifi tape decks, and got the heat sink, mains jack, mains cable, and mains fuse, from an old computer power supply. I sused new switches.

I spent maybe $10 to purchase all the salaged materials from Savers or the Goodwill, and I spent another $10 on the new parts, and about $13 on the lm1875 kit. All told that's about $30-$35 bucks for the whole shebang--not bad if I do say so myself!

Here's a YouTube clip of the amp in action, and I'll follow up with some close up stills, and I'll throw some schematics up in a new thread over in the schematics forum.

<object width="425" height="350"> <param name="movie" value="http://www.youtube.com/v/RiV78_rUv8c"> </param> <embed src="http://www.youtube.com/v/RiV78_rUv8c" type="application/x-shockwave-flash" width="425" height="350"> </embed> </object>

YouTube link: http://www.youtube.com/watch?v=RiV78_rUv8c

Hey man, Both your amps look and sound great! I just finished my little lm1875 job, using a modified professor tweed as the preamp. It works and sounds good, but I'm still "tweaking" it a bit. I've got the prof set for TOO much gain, so I need to mod it a bit (luckly I socketed the caps in the FB loop), and I blew the currently limiting resistor on my power LED, so I need to rewire that too. Once I get those things fixed, I'll post some pics and sounds too...

Congrats on the successfull Build!

oh cool, I'm a *nix man myself!  :tu: I'll def check out the clips when I get home (can't do that sort of stuff at work!)
About the speaker wiring, I was already aware of the effects to impedance from Parallel and Series wiring of speakers, I was asking more about the effects to power rating. I know that if you have a need for, say, a 2kohm resistor that will dissipate 1/2 watt of power (say the current limiting resitor for an LED between your power rails), but all you got is 1/4 watt resistors, you can wire two 4kohm 1/4 watt resistors in parallel for the equivalent. My question is: does it work the same way with speakers. For example if you have an amp that will push 30 watts into 4ohms, and you have two 15watt 8ohm speakers, will their sum power rating in parallel now be 30watts or will it still be 15watts. On the surface, it seems to me that they ought to now be able to handle 30watts, since if they are in parallel, they should each be taking exactly half the current presneted to them (assuming that the hookup wire, etc. leading to each is identical in length and resistance). If you wire speakers in series, will the power rating change? I don;t think it should since they will both see the same amount of current as if they were wired independantly. I am basiclly seeking confirmation of these thoughts from people who are more sure in their knowledge than me!

Cheers,

Isaac

sorry, meant "Armstrom", not "Armstrong"!

I'm curious to know the answer to this question too, as I think I'm in about the same spot as Armstong there (by the way, Armstong, are you the same Armstrong who has been posting on the valvecaster and the Prof Tweed over on the DIY Stompboxes forum? If so, I read over there that you are building a Harp amp. So am I! It seems that we are trying to build almost the exact same set up!)

Another thing I'm curious about is wattage and parallel wiring of speakers. Do they act like resistors where wireing them in parallel increases the overal wattage capability whilst decreasing the overall resistance? ie. if you wire two 8ohm 15watt speakers in parallel, does the amp "see" them now as on 4ohm 30watt speaker?