The Hundred-Buck Amp Challenge

I think you will need to set some required low frequency response (maybe 40 Hz for guitar?),

The lowest note on a guitar is around 80 Hz. Many guitar amp specific OPT's are specced from 80 Hz to 4000 Hz.

And cheap 480 V industrial/control transformers off Ebay are usually a few hundred Watts at least.

True, you can find industrial control transformers cheap on Ebay. The supply and price is not constant and they are real expensive if purchased new.

And the Watt limit should be taken as a minimum, since its easy to get a couple of $1 sweep tubes to do 50 Watts in P-P.

I thought about this for a minute. Even though I probably could make a screen driven sweep tube amp using some 98 cent 6BQ6GT's that rocked 50 watts or more for under $100, it would be a one trick pony. How often would it get used.

This thread started as an offshoot from another thread that was about low powered practice amps. I know that it is against my grain, but maybe this should be about making a great amp in the 2 to 5 watt range that might do 10 watts when driven to extreme saturation. The amp should be able of running all day long without melting down, or heating up the room. The bulk of the money could be spent on making the thing sound good.

There is actually a decent demand for these. Just look around at the botique guitar amps selling for $1000+ that make 5 watts or less. If they are used on stage or in the studio, they are miked. The 1.21 Giggawatt amps are being used by bar and club bands that need to look good and don't play through a house PA.

The whole amp could be done with just one odd tube type, like 6LU8, 6LR8, 8AL9, 6AL11.

There is not enough gain in a 6LU8 or 6LR8, been there tried that. They are OK for HiFi spud amps when driven by a 1 volt CD player. A guitar puts out 50 to 400 mV.
 
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I think you will need to set some required low frequency response (maybe 40 Hz for guitar?), or the amp/OT will just be a tweeter amp. Real easy to do a 400 Hz and up amp. (use air core even) Similarly the HF end needs some minimum, or the amp will be 1000 Hz limited. And the Watt limit should be taken as a minimum, since its easy to get a couple of $1 sweep tubes to do 50 Watts in P-P. And cheap 480 V industrial/control transformers off Ebay are usually a few hundred Watts at least.

The OP was 2-5W Challenge
The challenge is to build a low-powered practice amp for electric guitar. Power output should be in the range of 2-5 watts.

Being an avid guitar player, and reading multiple forums regularly, it is very common for even 5W to be considered " To Loud " for home practice amps.
This is especially true multi-family dwellings.

Lets not forget the el-cheapo 6AV6 or 6AT6 (Ala Tweed Harvard) for an addition gain stage if needed, those are available world wide cheap.

Edcor makes a tasty/hefty OP transformer very cheap that would easily accommodate low powered 6AQ5/6005. Though I am thinking PP as well.

examples
XPWR024-120 Power trans $20.00 (175-0-175) at 60mA center tapped and 6.3V (3.15-0-3.15) at 2A center tapped.
XSE10-8-5K SE output $17.22
GXPP10-8-8K PP output $25.10
 
"There is not enough gain in a 6LU8 or 6LR8, been there tried that. They are OK for HiFi spud amps when driven by a 1 volt CD player. A guitar puts out 50 to 400 mV. "


Point taken on the 50 to 400 mV input. But how about the twin pentode tubes with a CCS loaded input pentode. 6AL11, 6G11, 6AD10, 6BY11, 6T10 Or there's the triple valve tubes like 6AF11, 6BD11, 6AS11, 11CF11
 
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A twin pentode compactron would definitely be a contender, although compactrons are not common outside the US.

Edcor makes a tasty/hefty OP transformer very cheap that would easily accommodate low powered 6AQ5/6005. Though I am thinking PP as well.

Sounds like 2 of us are on the same track. I happen to have a big box of 6AQ5's and a bigger box of 6AU6's. Both are available world wide. I have some XPP10-8-8K OPT's too, if I don't find something else that works.
 
Interesting,

I've been in Key West for the past week and just got back this morning at 4am (1000 mi barn-burner drive).

A couple more tubes to consider are the 6P1P(/-EV) which is a 6V6 derivative, along with the 6P2P(/-EV) which is similar to the 12AX7.

Creative use of transformers looks interesting.
 
I expect 6J6 in phase splitter and a pair of 6BA6 in output will give nice Fender-style overdrive. Instead of 6BA6 I may use 6AQ5, but they would sound more Marshall-like than Fender-like. Also, 6P30B are good candidates for output, and they don't need tube sockets. In such case I may go with 6N16B in phase splitter.

For preamp I will use 6N17B, they are very like 12AX7, but tiny and don't require tube sockets.
 
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Joined 2010
Last year our little group ran a similar competition for a 6V6 amp (stereo) housed in a container that came from Ikea.

ray

LMAO,
6V6 Entries

Brilliant

Just a thought, what about a mains Tx as Op Tx..toroid maybe/
Post #2 in this thread
Link:

http://www.diyaudio.com/forums/tubes-valves/79104-power-transformers-output.html

I suppose you could get a small cheap Op tx for the same money...might sound better.

Then again OTL .....only very low power use the speaker as cathode load.

Regards
M. Gregg
 
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$100! Way too much money! Use say four 70V to 8 Ohm distribution transformers connected in series on the primary and series parallel for the output. That is under $11. Use an old fashioned dual triode 6N7 in a socket for $10. I would pull the transformer out of an old CRT TV set. That is free since they are not HD! If that isn't allowed use 3 surplus or Chinese 12 VCT 1 A or so transformers. The first one is a bit higher current and steps the voltage down for the filaments and also drives the other two to get from 12 volts back to 240 V ct. That is under $20!

Now to really save money use a 20K/600 ohm transformer as a phase splitter! $2 buys a cheapie!

Now a $.99 12ax7, and a few more parts under a dollar leave only the choke and filter capacitor. Skip the choke and use power resistors, a 10 uf 400 volt capacitor is under a $1 so the CRCRC may be your best filter value. At .125A 47 ohms is a nice value. The chassis should be an old cookie tin. Even if you include the price of the cookies around here that is $2.99!

So You might be a hair over $50! Of course this would be a 10 Watt amplifier!
 
$100! Way too much money!.......So You might be a hair over $50!

Yes, I could build a $50 amp too, with all new parts. In fact I am ordering the parts to do just that. I am not sure that too many GOOD guitar players would keep it for long though. Of course you are welcome to build a $50 amp and prove me wrong.

Now a $.99 12ax7

Show me where I can buy a 12AX7 for $1 and I will use them. I can get an output tube capable of 75 watts per pair for $1, but 12AX7's seem to be about $5 each.

Next question for pondering......

All things being equal (they are not really, but...) would it score points to use octal output tubes over miniature tubes? Size matters you know.....
 
Some random thoughts (yes, I know, working without the proper tools again...:D)

I'm setting aside the compactrons and loctals for now, so that this project can easily be duplicated outside the US. I'm also shelving the unusual filament voltages, to utilize more mainstream transformers. So at the risk of reinventing the wheel, I'll be using an octal pentode for the output, which is readily and inexpensively available in both 6.3 and 12.6 volt versions.

I've settled on a 6W6, which is available NOS for less than $5.00. It also sounds excellent when strapped as a triode, so I'll incorporate a switch to select between the two modes in the output stage. The SE output stage will also allow lots of tube rolling - 6K6s, 6Y6s, 6l6Gs, and so on.

I'll be using a variable gain control for the input stage, and a master volume for the entire circuit. Between the mode selection, tube variations, and stage adjustments, people will hopefully be able to get any sound they want. You can play the last stanza of Hendrix's 'Third Stone From the Sun' at deafening volume into your headphones, or when the wife gets home, softly play Duane Allman's 'Little Martha' to her through a speaker.

I'm leaning toward new-production power and output transformers from Edcor; I like the company and their products. Don't know if they're available through Farnell for our co-conspirators across the oceans.

My biggest unknown at this point is the driver stage, and whether it will be triode or pentode (or another switchable triode-strapped pentode?). It will be octal; I have a personal idiosyncracy of trying to keep tube basing uniform throughout a circuit. Octals are also easier to wire for less-experienced builders. I'd love to use some of my stash of 2C52s for the driver stage, but they've become rare and expensive. Too bad -- the mu of 90 matches many *AX7s, and would make the circuit implementation a snap.

Now to start adding up costs so far, and see how much I have left to flesh out the first stab at this design...
 
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Joined 2002
I'm leaning toward new-production power and output transformers from Edcor; I like the company and their products. Don't know if they're available through Farnell for our co-conspirators across the oceans..

Anything coming from the US arm of Farnell to Europe automatically involves a £25, (+vat), handling and shipping fee. Direct from Edcor may well be more reasonable. :)

:edit:

An effects loop would be nice, but by no means essential.
 
A question for the musicians and instrument techs: how popular are pedals / effects boxes, and should I incorporate the ability to use them with this little practice amp?


Great Question,

Personally,
I always lean towards a fairly clean dynamic amp that distorts well at maximum volume. When I want bedroom levels I just plug in the old Pod multi-effects and go with it.

Most certainly I will never get the perfect Trainwreck or Dumble tones, but hey those are not bedroom amps either:D
(Not married with kids amps either)

Iron Options

Difficult part is naturally the iron cost and availability world wide. Music Power Supplies has gotten very popular on many guitar amp DIY sites due to having many low cost options as well.
I can honestly say that these transformers work and sound just fine.
Sales and Information - Musical Power Supplies Inc.- Quality Tube Amplifier Parts You CAN Afford

Unfortunately his PT's are a tad more than the minimalist units needed for this challenge. He does however have a few comparable OT's PP & SE Edcor units.

I checked my supplies and have everything I need except the chassis but I have been kicking around a twist on that as well.
 
My proposed parts expenditures so far (all new parts from established vendors):

Edcor XPWR024-120 Power Transformer $20.01 (Edcor)
Edcor XSE-10-8-*K Output Transformer $17.22 (Edcor)
[4 x] UF4007 Fast-recovery diodes $ .44 (Newark)
3-Wire wire-in power cord $ 7.99 (AbTec)
Panel-mount 5x20 fuse holder $ 1.50 (Pacific TV)
Ceramic 250v GMA fuse $ .25 (Cascade Surplus)
[3 x] Chassis-mount octal sockets $ 6.00 (Cascade Surplus)
6W6 Pentode, NOS $ 4.10 (Pacific TV)
[2 x] 6SF5 Triodes, NOS $ 8.40 (Pacific TV)
[4 x] 1/4” Panel-mount jacks $ 3.72 (PartsExpress)
[2 x] Panel-mount SPST toggle switch $ 3.00 (PartsExpress)

Total so far – $72.63

Still need to fire up PSUDII and TubeCad / SEAmpCad to figure out caps and resistors, and still need to add pots, chassis, and misc. terminal strips, grommets, knobs, etc.

It's going to be pretty tight, but I think I'll be able to come in at $100 for a complete amp using all-new, over-the-counter parts. Of course, if I scrounge through my existing stock, the only thing I MIGHT have to buy is the transformer set. I could probably build the design I'll be working up for nothing.

Therein lies a good part of the challenge – can your final design be built by someone who has to purchase every component? And of course, it has to sound magnificent...;)
 
Another annoying question. Anyone have a problem with using a mosfet follower in the signal chain like the PowerDrive circuits that I use in my HiFi amps? All active gain stages will be pure tube but a mosfet follower to drive the output tube grid can be used to eliminate blocking distortion. They are also useful to drive the tone stack. Just trying to maximize the bang for the buck equation.
 
All active gain stages will be pure tube

By all means! If you can build a better mousetrap...

Just trying to maximize the bang for the buck equation.

And that's exactly what this exercise is all about. I have no delusions that my entry will
sound like anything but 'geese farts in a 55-gallon drum' (borrowed from Leo Kottke), but I'll have fun lashing it together. I'm really looking forward to the innovations that experienced and knowledgeable people like yourself will unveil.
 
I have no delusions that my entry will sound like anything but 'geese farts in a 55-gallon drum'

I never heard that one before. I have heard and built some stuff that I thought sounded terrible, but other people liked it. About 15 or 20 years ago I used to make custom guitar amps. All were sold by word of mouth and some of my weird stuff is what spiked sales, but once a lot of the local musicians have the same sound, no one wants it any more. I didn't have any new tricks and sales eventually died off. I think the last guitar amp that I actually completed was about 15 years ago. It's time to make another one, or two, or three, but first I got to find all the parts of my Stratocaster and put it back together.

I made several versions of the Turbo Champ. In fact I don't think that any two were the same and the schematic I posted was drawn from memory after the fact, but should be pretty close. I think the big "sound trick" on that amp was using the Hammond 125CSE OPT which is just a bit too small, and using a switch on the output to force a mismatched output. This allows the amp to distort at a lower volume, which may be useful here.

I also made a bunch of P-P amps that were in the 20 to 50 watt range. The "sound trick" on that one was to use two different output tubes, but set the idle current the same. Common combinations were 6L6GC and EL34, or 6L6GC and 6V6GT, or EL34 and KT88. This gives the power of a push pull amp, but the fat harmonics of a SE amp.

Just thought I would toss these out there for people to play with. I have posted both on this forum in the past.