The Hundred-Buck Amp Challenge

Disabled Account
Joined 2004
Most certainly a great designer.

Stop licking azz - he's not a member of the jury. :D
BTW, moderators can change polls results at will.... food for thought. :D:D:D

What I'm thinking is, if only local Fdbk will do it, then a switched capacitor output impedance converter could replace the OT and HV xfmrs in one stroke with just a few caps and Mosfets. (Similar to the Berning switched OT scheme, but this uses a HF capacitive voltage multiplier instead). This would be both ultra cheap and light weight. But global feedback is out of the question with it. Of course, it won't have any transformer distortion effects either. But maybe the search begins now for the most distorting capacitors.

Pretty interesting, why don't you build it?
 
"Pretty interesting, why don't you build it? "

It will almost certainly sound too clean. No hysteresis, no saturation effects, DC lowpass up to switching freq./N stages. Maybe if I soak the caps in some nasty polar dielectric fluid like ammonia or something. The issue would likely boil down to whether one can produce all the required "effects" in just the tube circuitry I think.
 
"It might be interesting if you combined positive and negative feedback- that will certainly provide a way of tuning the overload characteristics... "

Interesting point. Maybe could enhance the overload effects with a small ferrite inductor somewhere too. A fair amount of research may be required here, but the economy could pay off stupendously. A feather-weight tube guitar amp. Not power limited either (well, only by the tubes used).

(The impedance converter power capability going up directly with the switching freq., combined with the need for x Nstages higher freq., makes for near kilowatt level capability in the smallest design)
 
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Having no familiarity with guitar amps, can they be built without global NFdbk? Say using only local Fdbk from the OT primary?

Many vintage guitar amps had no feedback at all. Those that used feedback usually returned something from the OPT secondary to the driver. Marshall added the "presence" control into the feedback path to alter the tone.

I am thinking the vintage Motorola approach using the OP secondary as the driver.

I remember that.....I am sitting inside a Motorola plant as I type this.....
I have a few reverb tanks too, but I think new ones will break the bank.

It might be interesting if you combined positive and negative feedback- that will certainly provide a way of tuning the overload characteristics...

Or creating them. It's on my list of things to try. I experimented with positive feedback in a guitar amp years ago but it was tapped from the OPT secondary. That made things highly dependent on the speakers.

Your comment about every dog getting one bite in the original thread gave me two more names for guitar amp controls. Add the "bark" and "bite" knob. Bark transitions from negative feedback to positive and goes from "whine" to "howl". The bite knob accentuates the mid range and goes from "rodent" to "pit bull".

I just sent off my first parts order. $169+ shipping to CE distribution, but that includes two speakers. More parts will be ordered, but items from the southwest take over a week to get here.

Several people have specified an Edcor power transformer or OPT. We have 8 weeks to build, test, debug, and tune this amp. It takes 4 to 8 weeks to get a transformer from Edcor, so they aren't a player here.

In a big guitar amp a good part of the "tone" is dependent on how the speaker deals with overload and break up. This isn't going to be a factor with 1 or 2 watts.
 
I remember that.....I am sitting inside a Motorola plant as I type this.....
I have a few reverb tanks too, but I think new ones will break the bank.


CE has the Mod series tanks, but the $13.00 price tag plus shipping eats a big hole in the budget.

Or is shipping a consideration?
Edit, CE offers 3 day shipping for price of ground shipping on orders over $100.00
Typically orders from place on Monday arrive Thursday AZ to IL.
 
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Edit, CE offers 3 day shipping for price of ground shipping on orders over $100.00

I ordered today (Tuesday) morning Florida time, which means before they opened. We will see if I get my parts on Friday.

I didn't order a reverb tank, maybe next time. My first amp is going to be a traditional Fender type copy without reverb using tubes from the dollar menu at ESRC. It will have SE and P-P output stages all built on turret board. It is just to make something to test OPT's. Turret board is too expensive for use in the final amp. The final design will be built on perf board or PCB depending on how much time I have.

Speaking of OPT's. There are two at CE / AES that fit the price range. They have a P-P OPT with multiple taps rated at 8 watts for $16. The # is P-T291. I think it's around $18 at AES. They have a SE OPT, also 8 watts for $13, a few $ more at AES. The # is P-T31. One of each are on order.
 
I ordered today (Tuesday) morning Florida time, which means before they opened. We will see if I get my parts on Friday.

I didn't order a reverb tank, maybe next time. My first amp is going to be a traditional Fender type copy without reverb using tubes from the dollar menu at ESRC. It will have SE and P-P output stages all built on turret board. It is just to make something to test OPT's. Turret board is too expensive for use in the final amp. The final design will be built on perf board or PCB depending on how much time I have.

Speaking of OPT's. There are two at CE / AES that fit the price range. They have a P-P OPT with multiple taps rated at 8 watts for $16. The # is P-T291. I think it's around $18 at AES. They have a SE OPT, also 8 watts for $13, a few $ more at AES. The # is P-T31. One of each are on order.

With the slow edcor shipping aspect, I may end up going with an allied unit
perhaps the 6K49VG @ $23.31 Borderline to what I was thinking originally but still in the ballpark.
 
I have a 6K49VG and a 6K88VG on order. I have 6K56VG and 6K7VG already but they are probably outside the price range of this project. I might be inclined to turn up the power on one or more designs......just to see what happen of course.

These transformers are all Hammond made units in disguise. I have been using the 6K56VG and 6K7VG in my HiFi amps and they stand up to extend operation well in excess of the current specs. I used the 6K7VG in 50 watt guitar amps back when they were $39! I have one in my SSE amp running two KT88's at 100 mA each plus the driver tubes. That is about 220 mA from a 150 mA transformer. It gets too hot to touch after an hour or so but doesn't fry. It has been in use for over 5 years and saw daily use for 3 years.

Don't forget Antek. I think they are going to be the price / performance leader even if the darn things are ugly. World wide operation is also a plus. Just ordered 3 tiny toroids too. $25 each for a 50VA unit that includes 2 6.3V windings.
 
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It sounds like you guys are MUCH farther along with your circuit details than I am. I'm still trying to minimize the cost of the major components. One of my main concerns was the chassis -- a standard chassis from Hammond or Bud could easily absorb half of the total budget.

I was looking at everything from bread loaf pans to surplus 30-caliber ammunition cans, when I stumbled across a DIY amp builder who used a section of galvanized steel framing for his chassis. These metal studs and headers are are available in most standard lumber dimensions, and look perfect for a small amp chassis.

The nominal 2x4 section (actual 1 5/8" by 3 5/8") might not be quite wide enough for the footprint of some transformers. I'll probably go with the nominal 2x6 section material for my chassis. The great thing about these materials is the price -- $5.28 for a ten-foot length! The non-perforated headers, which are actually better for chassis use, are only $3.40.

If your chassis measured 1 5/8" tall by 3 5/8" deep by a foot long, you'd be able to cut nine chassis sections (allowing for waste) from one header section. That's a galvanized steel chassis for less than 40 cents. Add a couple of end caps and a bottom cover cut from stock of the same gauge
(or a discount-store baking sheet), and the price per chassis probably comes to less than a dollar.

Type the term 'metal framing' or 'steel studs' into your search engine, and you'll find many different profiles and sizes available. This may be the absolute cheapest material to use for fabricating an amplifier chassis.
 
The great thing about these materials is the price -- $5.28 for a ten-foot length! The non-perforated headers, which are actually better for chassis use, are only $3.40.

If your chassis measured 1 5/8" tall by 3 5/8" deep by a foot long, you'd be able to cut nine chassis sections (allowing for waste) from one header section. That's a galvanized steel chassis for less than 40 cents. Add a couple of end caps and a bottom cover cut from stock of the same gauge
(or a discount-store baking sheet), and the price per chassis probably comes to less than a dollar.

Congratulations you have out cheaped me, a truly amazing achievement!
 
It sounds like you guys are MUCH farther along with your circuit details than I am.

Not really....yet. I am getting some likely candidates for power and output transformers ordered from the places that take a long time to deliver. The only circuit details I have are all mixed up inside my head right now, but I do plan to sit down with a hot iron and a box full of tubes this weekend. 8 weeks will go by really fast when you have a full time job that wants 50+ hours a week.

I tend to leave the chassis and cabinet until last. I will make the amplifier circuit using bench power supplies, then make the power supply after I find out just what the circuit likes to eat, then based on what the furball on the bench looks like, bake a box for it.

I found some zinc plated steel "C" shaped sections about 6 inches wide by 2 inches deep by about 8 feet long in the scrap pile behind the warehouse buildng. They look like the stuff used up in the cieling out on the factory floor for all the ethernet cables. The steel was thick and I smoked a drill bit trying to drill it so I tossed them back. I can't afford to break a Greenlee punch on that stuff.

I will probably use some sheet aluminum or even PC board for mounting everything. This depends on the budget left over after the important stuff is figured out. The outer box will be made of wood.

It was stated early on that all HV electronics must be protected from touch, and it was stated that cabinet and speaker does not count in the final cost. This leaves ample ambiguity as to what is and isn't "cabinet". A little creativity could allow for a zero dollar chassis, if there isn't a chassis. Some of my test amps have been made with sheetrock screws holding the tube sockets to a piece of wood with the parts sky wired between them. Maybe this needs to be defined in the rules.

That is another area where the individual build will differ from builder to builder. I have been selling Tubelab amp PC boards for 6 years. I get pictures from builders. I am amazed at the wide varieties of chassis / cabinets that people use. I have seen nice looking amps made on baking pans, and I have seen amps where the builder spent over $1K just on the enclosure!
 
Disabled Account
Joined 2004
Chassis counts? Not a good idea for several reasons. 1) Safety (to save costs an unsafe chassis shouldn't be an option) 2) The fact that many will get them for free or make their own. How can you expect people to count everything? Should I also count the screws? Most kits in the market don't include the chassis anyway. Tubekit didn't have a chassis. 3) We will see very odd looking amps, possibly ugly and cheap looking. :D

Edit - Oh, amps should also have a name. Don't use pistol shrimp. It's taken. :D

Deadly pistol shrimp that stuns prey with sound as loud as Concorde. Over 200 dB.
 
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I am in a bit of a disadvantage in the cost department in relation to the US, most times getting the parts across the border rules out buying the part. Try find a 6V transformer with enough current at a reasonable cost, even sockets get pricey unless you are OK with Ebay as a source (Not one of a kind deals but parts that are consistently available, maybe have to include shipping?). Could I use junkbox parts that I have laying around and use the cost of a comparable part available to you guys? Maybe use a stock list of parts and their price as a way to nail down the value of a part we may have sitting around?
 
Some time ago, I set myself a AU$100 challenge - to build a 6W amp with speaker for less than $100 Australian. As some of you would be aware, transformers are a challenge here in Aust so I used a voltage multiplier and speaker line transformer. The schematic is attached, and the meandering thread descibing the process can be found here: And now for something completely different!!
 

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The Chassis,.. Hmmm,..
An air filter body from the auto junkyard. Or a cylinder head valve cover. Maybe a round aluminum bake pan fitted to a small aluminum wheel rim. A small beer keg or big paint can. Paint roller tray. Toaster oven. Copper flashing and duct tape. Eh..., a toy wagon or toy baby carriage or canister vacuum cleaner, easy to move around. A wire bird cage, good ventilation. A pet carrier, has a handle on it and the holes pre-drilled. Portable propane barbeque. Some baking grills made into a box. Aluminum mail box. Stove pipe. Metal Air ducting. Lawn mower frame with wheels. Ah, got it, a brass outdoor lamp fixture with windows. Off to the junkyard....
 
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