The Hundred-Buck Amp Challenge

[tubelab.com]

Are there any largish speakers that would sound good with this project?....I can't help but think that a 5 watt tube could drive the right 12" or even 15"(?) ......Look for low power rating and high efficency.

The difference between a guitar amp system and a HiFi system is the intentional distortion imparted by the guitar amp system.

A HiFi attempts to REproduce recorded music faithfully without alteration (OK, the SE guys like a pleasing level of second harmonics).

A musical instrument amplifier may intentionally PRODUCE harmonic distortion as part of PRODUCING music. The level and type of distortion will depend on the type of music and instrument being played.

A keyboard player may prefer low levels of distortion and a solid state amp may be the preferred choice.

A rhythm guitar player tends to play chords (multiple notes simultaneously). This requires an amp that can play clean, or distort in a subtle manner without high levels of IMD.

A lead guitar player tends to play one note at a time, and may desire moderate to extreme levels of harmonic distortion. IMD is not a factor with a single note. Generally the amp will be played into its compression range. This is where increasing the drive level will produce more distortion without increasing the volume significantly. A skillful guitar player can change the distortion by changing how hard he picks the strings, or by rotating the volume knob on the guitar. What "sounds good" to a lead player in a metal band would kill a rhythm or keyboard player. It would cause a lead player in a blues band to turn it down.

There are 3 major distortion sources in a lead guitar system. The preamp, the power amp, and the speaker. Achieving the threshold of compression isn't hard when you are driving a Marshall stack to 11 so that all 3 sources are active. Finding a speaker that will distort on 2 watts and live at higher power levels isn't easy either. The mechanisms of speaker distortion is beyond my feeble brain, but speakers that distort easilly tend to have strong magnets, thin paper cones, and small voice coils wound with thin wire. The cone exhibits "breakup" where sections of the cone are not moving in concert with other sections. This can be seen by sprinkling talcum powder on the cone and cranking it up (don't do this on your expensive speaker). The EV speakers in my 2X10 box have seriously thick paper and 3 inch voice coils. 2 watts can't possibly cause speaker distortion. Some 12 inch speakers distort in a pleasing manner on 5 watts, they also tend to be quite loud on 5 watts.

A lead amp like a Marshall will have excessive preamp gain with a volume control on the input. It will have a master gain control between the preamp and the power amp, and some users will use a resistive attenuator between the amp and speaker. This allows the preamp and power amp to be operated at the threshold of distortion / compression without being extremely loud.

I'm thinking that most guitar specific speakers will not exhibit significant distortion at the power levels we are discussing here. This does not mean they will all sound the same. Each speaker will still have a unique sound signature created by the type of paper and materials used. The frequency responses will all be different, and will change with input level. The box that the speaker is mounted in greatly affects its sound, as does the type of wood and material used to cover the wood. Intentional damping material is almost never used and cabinet resonances can become your friend. Tiny boxes don't resonate well in the guitar frequencies. Neither to huge ones, they go boom and sound like the thumpa-thumpa car down the street.

[sprinter]

That was a good explanation. Interesting point about intermod being a greater factor in chord playing. I didn't realize that the speaker played a large role in system distortion. I really thought it was all in the preamp and power stages. Sounds like this particular challenge is going to be aimed more at the heavier distortion lead playing styles because there just won't be enough power for cleaner playing to sound good. Is that fair to say?

It sounds like it would be pretty hard to design such a low power, low cost amp that would sound good for multiple styles. However, there are some commercial amps that do both clean and drive channels well. They must use fairly linear speakers in order to maintain a good clean sound at large volumes, no? I imagine that they must rely on the amp stages for distortion when desired, but maybe that's a design compromise that is made, and must have sufficient power for good clean sound, which really should sound fat and full, IMO.

Hey, how about a speaker cab that has a clean speaker and distortion speaker that are switchable with the clean/drive channel selection? Okay, it was just a thought...

[TheGimp]

one interesting thing about guitar amp speakers is that the motor (coil and magnet) is designed to over-travel compared to a traditional speaker. The over-travel produces a non-linear (distortion) response in the travel of the voice-coil and cone.

This is one reason regular speakers don't last as well when used for guitar speakers. Their motors are not designed to handle the abuse.

[ChrisA]

Originally Posted by tomchr

How about using a Compactron? Some of them have three tubes in them. A few triodes and a pentode for power. Just a suggestion...

There is an endless supply on old TV tubes that sell for $1 or $2 each. The price of the tubes will not be the biggest part of the budget. Transformers always cost 2X the price of the tubes but in a $100 amp I suspect the winning design will have a transformers costing maybe 10 to 20 times the price of the tubes.

One comment about the rules. I think the $100 must be the __delivered__ price total that incudes shipping. Otherwise I could get the cost down by buying from 20 different suppliers and paying over $100 in shipping.

[Printer2]

Originally Posted by TheGimp

I've driven an original Celestion Greenback with 3W and it sounds really nice. But the Greenback is only rated at 25W (20 for the new ones) and they are expensive. Does anyone make a lower power 12" speaker? The only one I've found is the Celestion G12 Blue and it is way pricy for me (> $250).

Since this is a bedroom amp, I would think something like the MOD 6-15 would be nice.

While WGS has some nice 12" speakers I will be using a 10". In the spirit of this contest I think this speaker will do nicely, it is affordable and readily available to all.

10" Veteran - 20 Watts | Warehouse Guitar Speakers

Speaking of power supply sag effecting the sound of an amp and if we do not use the exact same PT then the amp will not sound similar, lets get real here. If someone uses the TRIAD MAGNETICS - N68X
Output Voltage:115V
Current Rating:0.435A

TRIAD MAGNETICS|N68X|Isolation Transformer | Newark.com

or uses the HAMMOND - 187D120 - POWER TRANSFORMER
Output Voltage:115V
Current Rating:0.250A

HAMMOND|187D120|POWER TRANSFORMER | Newark.com

I do not think they are going to hear much difference in a 5W amp. There are many different transformers used in making a Marshall 18 Watt amp and yet they all have the distinct sound. Heck, if these two transformers were to make such a difference to the sound then we better spec only one tube manufacturer for our build as others can change the sound.

I want to use a 70V 10W line transformer for my build. Will another manufacturer's change the sound some? Sure. But no worse than using RCA's rather than a set of Chinese tubes. Will the same circuit sound different with different guitar players. I would think so. Should that stop us from having a little fun and nit picking things to death? I really do not care if I win the contest or not. I just want to see a lot of different designs that someone may decide to build after this has run its course.

And on 12AX7's not being affordable, just depends on how you want to spend your $100 budget. I was going to use another tube type but though the 12AX7 might be a better choice since it is pretty much availible to anyone. And $10 is not outrageous.

12AX7 / 7025 / ECC83 Tube Types

I have no problem with someone who wants to put together a kit for $70 on the net and get people building something. But I thought this was about giving an alternative to that route. I thought this was about coming up with the best design we could that someone else could also within the budget constraint.

Now a quick question because I do not recall if it was mentioned, is shipping or taxes to be included in the budget?

[ChrisA]

Originally Posted by sprinter

My 4x12 comment was just to make a point, of course, but I do have a question: Are there any largish speakers that would sound good with this project? I know that most builds will be purposefully small in size, but I can't help but think that a 5 watt tube could drive the right 12" or even 15"(?) speaker nicely. What do you more experienced guitar amp guys think? Is there such a speaker?

I have a 12" Emanance "Legend". It is about 100dB and it sounds good with my 10W amp that I never turn up loud. The amp is a single ended EL84. I built it as a combo with a Jenson Mod 8 inch inside. The 8" mod sounds good but the Legend sound much nicer especially when clean.

I also have one of those $148 Fender champ 600 amps that is 6V6 powered and the Legend sounds better than the built-in Fender speaker

I paid about $100 for the Legend. So it does not really work with the spirit of this contest. for this contest Mod is good but also look at having Weber build a small custom speaker.

[tubelab.com]

I think the $100 must be the __delivered__ price total that incudes shipping.

Impossible. There is too much variability even for people inside the US. The people in other countries are screwed! I ordered $190 worth of parts from CE / AES the shipping was over $20 for UPS ground, and it will take 8 days to get here. Someone in California could have the same parts delivered for $6 in 2 days. Not counting the cost, 8 days out of 8 weeks is a big deal to me. Someone in another country will have to wait on customs too.

Otherwise I could get the cost down by buying from 20 different suppliers and paying over $100 in shipping.

If you WERE making this thing for resale, or making parts kits, you would do exactly that. Just for this project I am looking at 8 suppliers so far. I want everything here before I start building. It would cost me up to a week if I had to order just 1 part in mid project, unless I spent stupid money on 2 day shipping. I would rather spend that money on parts I didn't use.

Why a 4 ohm speaker? Designed for a Solid State amp?

I fixed a friends Peavey with a similar speaker. The amp contained 2 chips, a quad opamp input stage and a chip amp output stage. The owner was hooking the little Peavey up to a 4X12 cabinet with clip leads and blew up the chip. I found a bigger chip with the same pinout.

An array of Cheap full range speakers?

I looked at those. The resonant frequency is 150 Hz which is well up into the guitars range. It might sound good, but it might not. It will be a very strange impedance VS frequency plot that a tube amp with limited voltage capability might not like. I decided to pass on them. Still looking at speakers though.

This is one reason regular speakers don't last as well when used for guitar speakers. Their motors are not designed to handle the abuse.

I learned this one at a very young age. Take the audio stage out of an old TV or radio, connect a guitar, and play it through the original speaker. It will last about a week, max. In high school we got a bunch (like dozens) of those priceless 15 inch field coil speakers from Hammond leslie cabinets. I blew every one of them up. Why? the voice coil was bottoming out because my 15 watt SS amp had good low frequency response and I was slapping the bass guitar strings too hard. Made a cool popping sound though.

Transformers always cost 2X the price of the tubes but in a $100 amp I suspect the winning design will have a transformers costing maybe 10 to 20 times the price of the tubes.

Don't forget pots, switches, and jacks and tube sockets. My spreadsheet runneth over. I am all over the dollar tube lists though. I am still considering a 5 tube amp! (among others) It will be 8 to 10 days before I have enough parts on hand to really test anything for sound.

[sprinter]

Now a quick question because I do not recall if it was mentioned, is shipping or taxes to be included in the budget?

Taxes? Geez, I hope not. Oregonians would have a nearly 10% advantage over Washingtonians right there. Ouch. Some of my best friends are from Washington. Including me.

Shipping? That would depend on the sources and the sources of parts shouldn't be part of the equation as long as the parts are available enough to be "legal". IMO.

[sprinter]

I fixed a friends Peavey with a similar speaker. The amp contained 2 chips, a quad opamp input stage and a chip amp output stage. The owner was hooking the little Peavey up to a 4X12 cabinet with clip leads and blew up the chip. I found a bigger chip with the same pinout.

Hey, how did it sound after the mod? Did the amp do justice to the cab? Do you remember the Peavey model and the replacement chip?

I blew every one of them up

You must have been very popular with the teachers!

[Printer2]

OUTPUT TRANSFORMERS: FENDER CHAMP STYLE 5W OUTPUT TRANSFORMER TF103-48 Single Ended Ultra-Linear 4 / 8 Ohm MADE IN USA $19.95

Shipping to North Dakota $11.95
Shipping to Canada (across the border) $32.95
Resistors can be bought in town here in packs of four, $1.99 (1/2 - 1W), not sure how much shipping I should tack on.