The Hundred-Buck Amp Challenge - diyAudio
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Old 13th June 2011, 05:32 AM   #1
bst is offline bst  United States
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Default The Hundred-Buck Amp Challenge

This challenge was spun off from another thread here: http://www.diyaudio.com/forums/tubes...uitar-amp.html

The challenge is to build a low-powered practice amp for electric guitar. Power output should be in the range of 2-5 watts. The amplifier project need not include a cabinet or speaker.
Total cost of the completed amplifier is not to exceed US $100 ( or equivalent value in non-US currency).

The power supply section can be solid state, but the signal stages must be all tube. Semiconductors may be used for biasing, current control, etc.

The amplifier must be reasonably safe. No line-powered (non-isolated) designs will be considered, and proper enclosure of high-voltage circuitry is required.

Parts may be sourced from any outlet available to the general public; the object is to develop an amp that anyone can build in the months to come.
Parts from vendors who have severely limited supplies or short-term discount pricing should not be used.

Suggestions for additions or limitations to the challenge rules will be entertained until June 20th, 2011. At that time, the rules will be 'carved in stone', and added to this thread.

A poll will posted, and challenge participants will be asked to vote on the design they consider best overall. On August 15th, 2011 the winner of the design poll will have the cost of all materials for his/her amplifier reimbursed, up to the amount of $100. In addition, $250 will be donated to diyaudio.com in appreciation for providing this forum.

I think this is going to be a whole lot of fun...

Last edited by bst; 13th June 2011 at 05:36 AM.
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Old 13th June 2011, 05:41 AM   #2
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I am in.
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Old 13th June 2011, 06:12 AM   #3
tomchr is offline tomchr  United States
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How about using a Compactron? Some of them have three tubes in them. A few triodes and a pentode for power. Just a suggestion...

I'm a hockey player not a geetar player, so I'll watch from the sidelines as you guys fight for the prize.

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Last edited by tomchr; 13th June 2011 at 06:15 AM.
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Old 13th June 2011, 07:06 AM   #4
AJT is offline AJT  Philippines
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how about this one? a 5F1 wannbe

Click the image to open in full size.

i got the 2 x 12LW6 tubes for a $1 each at rogalski's...
the 12AX7 cost P350 or about $7.90

the irons, a power traffo, a single ended opt, 4k:8ohm, and a choke for about P2500 or about $56....

easily less than a hundred green ones....
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Old 13th June 2011, 07:51 AM   #5
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This will be interesting to watch.
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Old 13th June 2011, 12:59 PM   #6
bst is offline bst  United States
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Quote:
How about using a Compactron?
Sure! Lots of interesting multi-section combinations in these envelopes:
Compactron Tubes: A Junkbox Guide

Or how about space-charge tubes, as used in auto radios of the '50s and '60s:Space Charge and Other Low-Voltage Tubes

Or tubes with higher-than-normal heater voltage, whose filaments might also serve to provide a bias reference... Or familiar pentodes hiding in bottles with different pin-outs and heater voltages, e.g. 16GK6...

There are hundreds of possibilities; the combination just needs to be easily duplicated by anyone who decides to build your design.
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Old 13th June 2011, 01:03 PM   #7
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I'm not going to build, but I have a few ideas. Greinacher voltage double a Triad N-68X (under $11) using UF4007 diodes to get the B+. Follow the Fender Champ 5C1 schematic, with a 12SJ7 and a 14C5 as the signal tubes. "Rat Shack" has cheapy 12.6 VAC filament trafos and something from Edcor takes care of the O/P trafo.
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Old 13th June 2011, 01:13 PM   #8
Trout is offline Trout  United States
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Now this sounds like FUN!!

I have built my fair share of guitar amps from leftovers.
Pondering jumping in myself!

Now to decide, simplest, or most bang for the buck.

Last edited by Trout; 13th June 2011 at 01:22 PM.
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Old 13th June 2011, 01:27 PM   #9
bst is offline bst  United States
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Here's another tip for cutting costs without compromising performance. A linear pot can easily be converted to an audio-taper pot by simply bridging its terminals with a 1% resistor. I use this trick all the time to save five bucks or more on volume controls.

Find a 50-cent military-surplus linear pot, add a 50-cent high-quality resistor, and for a dollar you have a pot which will equal or better many "boutique" audio-taper log pots. Here are a couple of articles which explain how:

About halfway through this article: http://www.geofex.com/Article_Folder...s/potscret.htm
...or the first section of this one: ESP - A Better Volume Control

Last edited by bst; 13th June 2011 at 01:28 PM. Reason: correct link URL
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Old 13th June 2011, 01:34 PM   #10
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Quote:
how about this one? a 5F1 wannbe
Why not?

Too bad I can't find one of my "Turbo Champs" (sold the last one 10 years ago). They were quite popular and really screamed. Schematic enclosed. Probably over $100 at todays prices, but swap the KT88 for something cheap like a Chinese 6L6GC

Quote:
Hey, bst, you wanna make something and play guitar, or count the pennies spent in each resistor?
Anyone can go down to Sam Ash (Music store chain) and plop down their credit card and walk out with a Marshall stack. It would a challenge to make a good sounding guitar amp for less than $100. Yes it is possible to spend more than $100 on the speaker, and that is a big part of the sound, but we aren't counting speaker $$$$.

Quote:
Power output should be in the range of 2-5 watts.
Now I'm dissapointed. I'm not one to turn things down.
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File Type: jpg TurboChamp1.jpg (119.8 KB, 5393 views)
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