My homemade amp...

[chrisn]

Hello... I'm new to this board and another member suggested I write a little article on my home made tube amp...so...

About a year and a half ago I finally made the decision to jump head into building an amplifier. I'd talked about it with a friend for quite a while and I made up my mind to just dig in a do it...

When I started the project I had almost no electronics experience so I started with a couple effects boxes and did plenty of reading and studying...I can't say I'm all that knowledgeable but I have a solid foundation on the basics

I've based this amp off of the AX84 P1 Extreme. If you're not familiar with the AX84 project go check them out at www.ax84.com



My amp is class A and runs around 12 watts or so. I run off of a 6L6 power tube and 12ax7 preamp tubes. I built the amp to the original design first then began to tweak and modify. I've ended up with a modified tone stack (I removed the mid control and just put in a hi/low mid switch to accommodate the added gain knob) and I added a "cascode boost". I'm not completely sure what exactly a cascode boost is but I do know that its not simply an extra cascaded gain stage like you might modify old 2 channel Fenders, this supplies much more gain!

I've also added a large ceramic resistor on the tube heater lines to drop the voltage. The power transformer is designed for 115V so everything runs a little bit hot. There is a trick to transform your line voltage from 120V down to 115V you could find if you search around for "vintage voltage adapter" or "viltage" I just haven't bothered with it yet and since the slightly higher voltage doesn't seem to effect the amp performance I'm not worried, it is, however; fairly important that you run your tube heaters in the specified range or you could burn out your tubes long before they ought to....

My understanding is that for this amplifier design there is not much difference between a tube rectifier and going solid state so I just used diodes as per the original plan...though a tube recto would be pretty cool aesthetically...it might make its way in there one day...

the cascode boost is on a switch so I retain full original functionality then with the flip of a switch I have gain out the butt...this thing screams in ways a 12W class A tube amp hasn't any right to...

As far as sound goes it can go quite sparkling clean with that wonderful full-bodied tube warmth then roll on a little gain and you've got excellent blues tones. With a little more gain you get great classic rock tones. Flip the gain switch into dual-tube cascade mode and you can get some metal goin...

I run the power through a Furman PL-Plus conditioner and I have very little hum or noise. I was very fortunate for a first timer, though I did keep wire runs and signal paths in mind when laying everything out and I did spend some time chop-sticking wires around to find the quietest spots.

I play through an Avatar 2x12" cab with Celestions...one Vintage 30 and one of the G12H30's

Volume wise its plenty for me. You could play it to jam and gig (especially if you mic the amp to gig). It'll keep up as long as your drummer isn't insanely loud and you're not competing against another guitarist with more powerful amp...



if anybody has any questions I'd be happy to answer...

[ranchtooth]

Thats a nice little amp you have there! And some good fungineering as well, its a nice little package. I'm hoping to do something like this myself in the near future... Could you give a rough estimate on the cost of parts for a setup like you have there?

[chrisn]

thanks for the comments!

for everything shown there i probably spent about $300...then $250 on the cabinet and i completely lost track of what i spent on getting set up with a soldering iron and wires and misc components used to build the effects boxes and stuff...which I ended up dipping into those extra parts when I did mods....

Doberman Music is a company owned by the guy that hosts AX84...he is selling the P1 Extreme kit for $255 right now....I bought it on sale for like $190 I think, keep in mind that was about a year and a half ago...

It comes with pretty much everything...all the components, an eyelet board, single jewel light, transformers, chassis, pots and knobs, power cable....if you buy that kit you don't have to buy much more yourself except for solder and wires and mounting standoffs for the eyelet board....

you can pay extra to have the chassis pre-drilled...i ended up getting a table top drill press from harbor freight for $30 on sale and then you need chassis punches for the tube sockets which you can get at harbor freight for $15...or you can buy greenly's which are ridiculously expensive....

i did some extra mods that add some money and i ditched the Sovtek 6L6 and EH 12AX7 for JJs...the extra gain stage ran about $50 after buying all the caps and resistors and paying shipping, I think I had to go through 2 different suppliers for that stuff....I also added an extra light...the red means "on" and the green means "go!"...the kit came with an amber light...but I wanted a red power light and a green standby light....

If you just build the straight amp and don't take into account tool costs you can get out for under $300...beyond that you could upgrade and modify to your heart's content....

I will say that I was offered $400 cash on the spot for as-is condition by a friend who was pretty impressed with it....and I've been told I could fetch easily $700-$800 if I built it into a nice cab with good speaker....

building an amp is more expensive than buying a similarly spec'd one from a mass producer...but then again there is a while other satisfaction that this comes with that is hard to put a price tag on...

[hammeroff]

Welcome to the boards man! Great looking project there.

Volume wise its plenty for me. You could play it to jam and gig (especially if you mic the amp to gig). It'll keep up as long as your drummer isn't insanely loud and you're not competing against another guitarist with more powerful amp...

Must resist...urge

That's why these go to...

Must....

ARGGHHH!!!!

ELEVEN

Sorry.

[Big K]

great amp project

[johan]

I was going to start a new thread 'Getting into Amps' but saw this one.

This is the story. When I bought a reburbished old fender amp, I got hooked just documenting myself before the sale. I learned that old tube amps maybe leathal to work with, but also that building your own really shouldn't be that hard.

But with only the basic knowledge that a resistor 'resists' electrical current I started out on my electronic adventure by working on my old strat after reading John A's website. I plotted out my path as follows: 1) passive electronics (guitar wiring) 2) active electronics low voltage project (effects at 9 - 24V) and 3) a low wattage guitar practice amp.

Currently well into phase 2 and was even considering a tube based pedal for a while but I'm looking forward still. I was complemplating the possibility of starting off really cheap with the conversion of tube radio to guitar amp. $200 is no money at all for the kit, but I'm on the other side of the ocean where the fun starts $150 later.


So Chrisn, your posting is most inspiring. Beautiful work.
Any amp builder on the EU continent that know of affordable suppliers of starter kits?

thanks
j

[ChrisK]

BTW,

Your amp is a single-ended class A design.

Both a single-ended (output stage structure) amp and a push-pull (output stage structure) amp can be class A (bias point).

Both a single-ended amp and a push-pull amp can be class A/B.

Class A/B on a single-ended class A design occurs when you over drive it (or design it wrong).


Here's some fun stuff;
www.allenamps.com/

[sumgai]

Chris,

...........
Class A/B on a single-ended class A design occurs when you over drive it (or design it wrong).

Gee, I wasn't aware that by driving the input signal to non-linearity levels, the bias point would change enough to cause the output stage to be re-classified. And here I've been living and working all these years under the impression that setting the bias point is what determines the class of operation. Kinda makes me feel lucky to have lived so long while being so badly informed!

OTOH, I do consider it a wasted day if I don't learn at least one new thing.


sumgai

[toddw]

Man you know your stuff Chris. Not too many people would have realized that. Engineer or professor?

oh, and the only reason I knew:

www.aikenamps.com/ClassA.htm

Chrisn,

I started in the same place, but built essentially the P1ex with a spring reverb and power scaling for my first amp. I'd start smaller given the experience, but it is a cool amp. I build my kids small practice amps after that which were smaller, simpler, and more fun to build. Next amp will probably be push pull with a toroidal PT I have sitting around and a couple of KT88's. Got a couple of guitars to build first.

Todd

[UnklMickey]

sumgai said:

Chris,

...........
Class A/B on a single-ended class A design occurs when you over drive it (or design it wrong).

Gee, I wasn't aware that by driving the input signal to non-linearity levels, the bias point would change enough to cause the output stage to be re-classified. And here I've been living and working all these years under the impression that setting the bias point is what determines the class of operation. Kinda makes me feel lucky to have lived so long while being so badly informed!

OTOH, I do consider it a wasted day if I don't learn at least one new thing.


sumgai

it's all about the definition.

Class A = device conducts for 100% of the cycle.
Class AB = device conducts for morethan 50% but less than 100% of the cycle.

drive a Class A single ended amp into saturation, and it's still Class A. (but the output will be distorted.)

drive a Class A single ended amp into cutoff, and it's now Class AB. (and the output will be distorted.)

[toddw]

drive a Class A single ended amp into cutoff, and it's now Class AB. (and the output will be distorted.)

Th a t i a ver rus desc on f ow *drive down . . .* will sound

[sumgai]

unk,

Before we go any further, I gots to know.......

How do you drive a Class A amplifier into cutoff?!

By definition, it is on 100% of the time, and can't reach cutoff due to large swings in the input voltage. If it does, then the bias was not set to 100% conduction in the first place. That is what the bias is for, to set the tube(s) so that it/they can never go into cutoff (in Class A).


(punt)


sumgai

[ChrisK]

Class is the intent of the design.

Bias is the instantaneous sum of all things affecting bias such as the DC bias point and the applied grid signal.

Depending on how something tubular is fixed biased (fixed grid or cathode) may well determine the effective bias shift achievable, but if the effected bias swings wide enough, saturation and/or cutoff occurs.

.....in pre amps as well as power amps.


Effective bias is the actual result of all things related.
Effective class is the actual result of all things bias.

[sumgai]

Chris,

No argruments with what you said, you're correct of course.

However, you suggest that an input signal can go large enough to force an otherwise properly biased tube into cutoff. I submit that such a large voltage swing would be the height of folly, and extremely out of design spec. Not only would this require that the preceding preamp stage produce a prodigous amount of voltage, but it'd need to so while not distorting the signal. Classification assumes a clean incoming signal, else all bets are off. (Of course, the signal itself may contain distortion-like products from earlier processing, but we're speaking of the preamp itself needing to not alter the signal in any way, shape or form.)

Besides, unless the whole signal, P-P, is more positive than the cathode, the cutoff won't be 100%, thus rendering a Class C condition at worst, and more likely, a Class AB₂ condition.


sumgai

[ChrisK]

Just because something can be done doesn't mean that it (ever) should be done.