The Hundred-Buck Amp Challenge

I did a little digging around on my old PC and found some clips from the 6AQ5SE amp I built in 2005.

I have always been a fan of those babies.
This one had used a single 12AX7, a 6AQ5 & 6CA4/EZ81 rectifier. It had a cheap OT from Triode Electronics.
TF103-48ul
FENDER CHAMP STYLE 5W OUTPUT TRANSFORMER TF103-48 Single Ended Ultra-Linear 4 / 8 Ohm MADE IN USA

Close Mic recorded with Shure SM57, a hint of reverb added post with cakewalk software.
12" speaker & Fender Strat
Basically a modded AA764 Champ circuit.

A Little Noodling

Cranked for Feedback

I always found that particular OT a bit weak especially on the bottom end but it was cheap & worked.
 
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taj

diyAudio Member
Joined 2005
Not trying to pee on the party tray or anything, but here's my 2-bits:

What a weird contest. :scratch1: The elusive goal with practice amps is always in creating one that sounds decent while still being cheap and quieter than a "normal" guitar amp. But, nobody has mentioned sound as a criteria here.

If participants are the judges, as described in post #1, then I fully expect the winner to be an exceptionally creative and impressive circuit, that has a 50% chance of sounding like a cat retching when you play a song on a guitar that's plugged into it.

If the judges were, instead, DIY guitar amp builders (i.e. real guitar players) who judge the amps based on their sound, then you've got a real guitar amp contest, IMHO. But then you can't expect them to build and audition them all and cast their ballot in just a month or 2.

And if it's not about guitar sound, then what's it about? Circuit design? Then why a guitar practice amp? What a weird contest.

Also, why 2-5 watts? There are plenty of good practice amp designs out there that push far less than 1 watt (12UA7 or 6CG7 as output). If you happen to want Eddie Van Halen's sound, even 2 watts will practically blow your windows out. I'd suggest "5 watts or less" rather than 2-5 watts.

..todd
 
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There is already a fellow who beat you to it (and tried to push his own kit here and some other boards) - goes for USD 85 with speaker included or $65 without
MTA-1XE DELUXE DIY TUBE GUITAR AMP KIT w/8" SPEAKER | eBay

BTW, to be fair to Europeans one has to keep in mind that iron shipping from US would cost $43 so using all Edcor will raise the price significantly.
For UK-based people this may work as cheap SE OPT - VT1417
For the rest of EU - TUCH66-5kOhm sec. 4-8-16 Ohm S.E., has many more OPTs, PTs and chokes (unfortunately won't ship to non-EU countries)
 
Disabled Account
Joined 2004
Size matters you know.....

Yes, it does. A low power guitar amp should be....small. People using Frankenstein techniques and oversized chassis would get zero points from me.


I've settled on a 6W6, which is available NOS for less than $5.00. It also sounds excellent when strapped as a triode

Yes, it does.

If the judges were, instead, DIY guitar amp builders (i.e. real guitar players) who judge the amps based on their sound, then you've got a real guitar amp contest,

Amen to that. Good tone is good tone though. Most people here don't have a clue how to build a good guitar amp but they have a love for music - they will be familiar with a great electric guitar tone. :)


Rules:

Two pics (one showing the innards), an audio or video file, schematic and part list should be posted with each amp.
 
A couple things that really stick out in my mind on building guitar amps in general has to do with a aspects not included in the challenge.

The speaker.

For example,
take a common 5W SE Champ, or a 14W Tweed Deluxe or even a 100W Marshall, the end tone is greatly affected by speaker choice.

Often guys find that all of the design mods and tweaks possible will never compete with a speaker intended for a specific design & music genre.
Size, type of magnet, and so on.

How will the end design be judged? on a scope? dummy loaded? mathematical theory?

Recording;

If recorded clips are used, there can also be a 2 fold disadvantage.
Some guys can make darn near any amp sound fantastic, others like myself not so much.

Recorded clips can be improved through clever use of the various recording software's. For instance, the clips I posted above have a bit of reverb added post recorded.
It is fairly easy to doctor EQ's as well.

Just a bit of food for thought.
 
There have been requests for practice, low power, quiet, or whatever guitar amps posted here and on the guitar amp builders forums. I have been getting e-mail for guitar amps ever since Tubelab was started. Yes there are schematics for low powered amps using 12AU7's, 6SN7's and others all over the web, and I expect a few entries will be based on those designs. In reality an "amp designer" just needs to copy his favorite preamp circuit and then copy Fender's reverb driver but use a tiny real OPT and you have an amp.

There were 3 threads going last week all asking about low powered guitar amps when someone posted a commercial message pointing to his Ebay ad for a $69 "kit". He even went as far as saying that a guitar amp could not be built for that price, no way....... I replied "wanna bet?" and here we are.

I fully expect the winner to be an exceptionally creative and impressive circuit, that has a 50% chance of sounding like a cat retching when you play a song on a guitar that's plugged into it.

That's always a possibility since the "sound" of a guitar amp is a more subjective quality than say tube VS solid state, CD VS vinyl, and so on. Two excellent guitar playing amp builders would have totally different sound criteria if one was a jazz player, and the other a metal head. Most people here would not vote for the "cat in the clothes dryer" amp.

If enough people listen to the amps and vote we should come up with something. If most of the people who play this game post videos or audio clips of their amps along with full schematics, parts lists, and construction details, then a potential builder can pick the sound they want and go build it, whether or not that particular amp won the contest.

That is the real reason we are here. The choices for tube HiFi designs are already rather limited, but the choices for new and unique guitar amp designs are very slim. The guy that is organizing this thread was one of the people asking for a good design. Lets give him some choices.

I have not completed a new guitar amp design in about 15 years. I have 7 complete working HiFi amps in my lab right now, a few more that work but have no cabinet. I hate to admit it, but I do not have a working guitar amp at all now. That was the little voice inside my head that made me say wanna bet!

My guitar playing skills have degraded a bunch, since I haven't played in a few years either, so its about time to make something new. Given some play time I expect to have more than one design to enter, and nothing will be published unless it could wear the Tubelab name. At least one will be a "creative and impressive circuit" but I think most builders want simplicity.
 
Disabled Account
Joined 2004
Yes Trout, the winner in the tone department will be the one having the best sounding speaker and microphone preamp. :D

This is just for fun but we will sure learn something along the way.

EDIT: Listen to the first audio clip, good recording and a great clean sound but when he cranks up the amp distortion doesn't sound that good. It's not that easy, even if your recording gear is good.

http://www.manortronics-force-gauges.com/Valve_Amp.html
 
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Question here.
Having no familiarity with guitar amps, can they be built without global NFdbk? Say using only local Fdbk from the OT primary?

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.
 
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That won't stop me from trying.:D

Just to cheat a bit, I'll try to hire a really good guitarist to play for the recording. With me playing, I'm sunk no matter how good the amp is.

Funny, for some reason several years back I remembered you making a comment along those lines (being Hifi) But I suspect you might be/are a pretty good guitarist.

Most certainly a great designer.



Thinking out of the box, Fender Reverb Driver? Nahh I am thinking the vintage Motorola approach using the OP secondary as the driver. I still have a copy of the setup used in one of their 3 channel consoles. I have an old tank out of the Motorola, but its not available and re-purposing would be outside the rules.

The tough part is the tank cost, my $$$ figures are currently to close to call on that yet..