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Old 2nd August 2009, 11:36 AM   #1
chrish is offline chrish  Australia
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Default Never done this before: Guitar Amp!

Have about half a dozen tube amp projects under my belt now and wanted to try something a little different. My son, 13 years old, has been playing acoustic guitar for a couple of years. He has just started showing some interest in electric guitar. His mother (we are divorced) was kind enough to buy him a cheap ebay special guitar and amp package. I thought "here is an opportunity"! So we have started a father and son project to build a guitar amp. After some investigation, I decided that if I was going to build an amp, it may as well be one that would last him a few years if he decided he wanted to jam with his buddies etc. Well, went for a [/URL]AX84 guitar amp 20W PP power amp (2 x 6V6) combined with a AX84 Lead Pre Amp. Not sure what to expect, I am not an electric guitar player, but the info on this site is great. Fantastic chassis layout diagrams and layout diagrams to construct turret boards etc. This makes it much easier to do the construction with my son, as we can look at the picture of the chassis layout, compare that to the schematic, then drill/solder etc. Makes it a little easier for him to follow what is going on as we learn together.

I wanted to build with parts that I had on hand, so all of the iron is similar, but not identical to that specified. The only extra iron I purchased was the output transformer that I got for a good price on a business trip.

Well, I could not help myself but to modify things a little. As this is a 20 Watt amp, and I have read this is quite loud, I have put a switch on the back to be able to switch between pentode (designed configuration) and triode to knock the output down a little for practice. I have also built PP amps that worked OK at low level with one output tube removed (SE). This might be an option as well to reduce output for practice.

The schematic calls for a 7.6K 25W PP transformer. The one I got a deal on is a 5.5K 50W with 4 Ohm and 8 Ohm taps. I have put a switch on the back to be able to switch between the two to see what works best. THe speaker it will be driving is a Celestion Rocket 50 12" 8 Ohm.

Cabinet will be MDF with the chassis hung upside down at the top.

We are most of the way through the amp construction. The power amp side is wired up and tested. The pre amp is wired from the input to the output of the second gain stage and is also working. Just have to wire up the tone stack. Will have to wait a few weeks, as I see my kids every second weekend and we are completing everything together!

Also, for previous projects I have been using regular hookup wire and solid core 'wire wrap' wire. Somewhere on this board someone mentioned some teflon coated wire, so I ordered some from ebay. What a difference! No more melted insulation! Makes teaching my 13 year old how to solder much easier!

Anyway, some photos below. Will keep posted with the progress.

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Old 2nd August 2009, 09:16 PM   #2
jmilch is offline jmilch  United States
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This is absolutely inspiring. Thanks for the great pictures, and please keep us posted. I am really thinking about taking the plunge and starting another project similiar to this.
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Old 2nd August 2009, 09:29 PM   #3
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Warning....

Guitar amps are addicting.

They will soon outnumber the HiFi projects.

We won't even get into the negative-WAF of them...

Cheers!
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Old 3rd August 2009, 12:23 AM   #4
jjman is offline jjman  United States
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Pulling one tube in a PP output is taboo, at least in the guitar amp discussion board world. It creates a DC imbalance in the OP tranny and other stuff.

I had a guitar amp with a triode switch and it provided almost no reduction in volume. It only reduced the bass somewhat. Others say they like it and the type of output tubes may be important there. Many factory amps have this feature.

What’s that tranny in the middle? A choke? Reverb? It’s pretty big.

Hopefully there is some sort of “master volume” on it so he can have tube distortion w/o having to blast it. I think OP trannies should have a higher Z secondary for headphones-only operation. Call it the “parental” output jack.
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Old 3rd August 2009, 11:09 AM   #5
chrish is offline chrish  Australia
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I have built a PP EL84 amp that you could pull one tube from to get SE. Did not even think to try it until I saw someone on this forum mention they had tried it... Not a big deal, if it works fine, if not...

I have pumped a signal in to the power amp with a signal generator and scoped the output. It looks like quite a difference in output between triode and pentode, but will wait and see what that equates to in the real world.

The middle silver transformer is a choke between B+1 (centre tap of output transformer primary) and B+2 (screen supply).

There are two gain stages with pots, Bass Mid Treble tone stack, Presence and Master volume. If you can see on the photo, these control labels are upside down, as the amp will sit upside down in the cabinet when mounted.

The "core project" chassis layout allowed for an extra 9 pin tube. I have installed this socket and wired up the heater for it in case of future mods. I am interested in how I would organise a headphone output for this. I know his mother would be grateful!

Will keep posted on the progress. It will be a little slow, however, as I an doing this with my son (ie, letting him do most of the work under guidance), teaching as I go, and i only see him every second weekend.

Chris
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Old 3rd August 2009, 12:25 PM   #6
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Awesome! Looks very neat

I'm working on a guitar amp myself at the mo, I'll post something when it's done.

Nice to have a father-son project that works so good
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Old 3rd August 2009, 01:33 PM   #7
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This is a pretty interesting topic, I'm trying to build my own tube guitar amp too. I already build SS guitar amps, sound awesome! But I hate MDF, I use solid 20-22 mm spruce for cabinet and laminated front.

With 20W your son will have enough (so do you!), always place a master volume, and that's it!

You could use a couple or 6L6 with 5.5k in OT primary (I think Fenders have 5.6K...), cathode biased, like old twins, I think that will give you about 25W.
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Old 3rd August 2009, 06:49 PM   #8
ChrisA is offline ChrisA  United States
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Default Re: Never done this before: Guitar Amp!

Quote:
........ Well, went for a [/URL]AX84 guitar amp 20W PP power amp (2 x 6V6) combined with a AX84 Lead Pre Amp. Not sure what to expect, I am not an electric guitar player, but the info on this site is great. .....

I wanted to build with parts that I had on hand, so all of the iron is similar, but not identical to that specified. The only extra iron I purchased was the output transformer that I got for a good price on a business trip.
....

Cabinet will be MDF with the chassis hung upside down at the top.

MDF is really the wrong material for a guitar amp. No one uses it and for a reason (1) It is not very strong, (2) It's way heavy for something that get hauled around a lot and (3) it is sonically "dead"

MDF is good for stereo cabinets that need to be dead and don't get moved around much. For a Guitar amp go to Home Depot and buy some #1 quality pine and make box joints. Or use 1/2 inch birch plywood. But NOT MDF. Hardwood works too if you want the spend the $$ on Maple or Oak. But Fender built with pine. Pine 1x12 are cheap and very easy to work with and light weight when complete.

This is a very complex amp for a first project. The thing about guitar amps is not simply getting them to work. It is making them sound like you want. What kind of music does your son like? The skill you will need to develop is to listen to the amp and then know what to change so make the sound "right". This is MUCH harder than just building a HiFi amp where of course the goal is to add "nothing" to the sound. Here you are shaping or creating the sound. You are actually building a musical instrument and taste and judgement come into play here. Expect some weeks of "tweaking" where you swap out caps and resistor values and maybe even tube types and bias levels.

Also, 20W is going to be very, very loud. People across the street will be able to hear the guitar inside their house. But, you say "I'll just turn down the volume." The sound you are crafting happens when the various stages and the speaker "overload" and add huge amounts of harmonic distortion. This does not happen at low volume. You just have to put in the ear plugs and play.

Or,... build a much smaller amp. May I suggest the next project? Look on the same AX84 site at the "firefly". It's a one watt amp. Or look at the High Octane, put build it with the option to use a very small output tube.


QUICK FIX for volume problem: This is easy. Buy a few 4 or 8 Ohm power resistors rated for 50W (At least double the amp's power rating) or so and mount them to a sheet of aluminim or a heatsink and screw that to the bottom of the inside of the case. Hook up this (via a switch) to the speaker so that 7/8 or 3/4 of the power goes through the resistors while keeping the total seen by the amp at 8 ohms. This will let you turn up the volume and distortion without blowing the glass out of the windows.

OK it's fun to blow the windows out. For a while. But I find what really matters the most to the sound is Practice, Practice, Practice, Practice, ... and then more Practice. and much of this is done in the evening or at night. He can't be waking people up and will need a low volume way to practice. hence the Firefly. He needs the bigger amp but most hours or playing time are done at low practice volume levels. So the amp that gets used the most is the one that sounds good at "bedroom" volume levels.

Another idea is to build a headphone adaptor for the 20W PP amp. Most headphone connect just before the Phase Inverter. Make sure it is capacitor coupled (to keep the 300 volt supply inside the box.) and use a voltage divider to send about one volt to the headphones. I like those plastic "Cliff" jacks that are switched. Stereo jacks provide more switches in the jack. Plugging in the headphones should ground the input to the PI, pulling the phones plug should re-connect the PI to the tone stack and un-ground it. With the phone plugged in the mater volumes controls the headphone volume. Rig the voltage devider so that loudness in the phones matches the loudness in the speaker. Must be done by experiment.

You can get by with just one HiFi amp. But not with just one guitar amp.

You are going to have to learn to ply guitar a little yourself.
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Old 3rd August 2009, 11:16 PM   #9
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Quote:
Originally posted by ChrisA


This is a very complex amp for a first project. The thing about guitar amps is not simply getting them to work. It is making them sound like you want. What kind of music does your son like? The skill you will need to develop is to listen to the amp and then know what to change so make the sound "right". This is MUCH harder than just building a HiFi amp where of course the goal is to add "nothing" to the sound. Here you are shaping or creating the sound. You are actually building a musical instrument and taste and judgement come into play here.

Also, 20W is going to be very, very loud. People across the street will be able to hear the guitar inside their house.

I'm not sure how complex could it be... but sure his son gonna LOVE it! 6V6 break up pretty easily, and combined with a crucnhy preamp, won't be necessary to push it to its limits! Believe it, being 13 with a tube amp is really cool, the Celestion will sound great (I would go for an Eminence combo 1 x 12" or 1 x 15", and and external cabinet if you already have that Celestion). The point is that will give his personal sound, and using some stompboxes he'll complete the sound he wants! 6V6 PP 20W Fenders sound like glory, and really loud! Will do the job for a pub show.

And that's all about... play loud!
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Old 5th August 2009, 10:05 PM   #10
rongon is offline rongon  United States
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Really, really nice project there. Way cool.

I'll agree that MDF is not the right stuff for making a guitar amp cab. Plywood would be better. For a guitar amp, you want to build in a nice coloration, one that you like. Don't think "hi-fi". A guitar amp is a tone generating machine, not a faithful reproducer of audio sources.

The speaker and cab probably make a more dramatic difference to the "tone" of a guitar amp than anything else. Those great Fender combo amps (Deluxe Reverb, Twin Reverb, etc.) sound great because of their cabs and speakers. Take a Deluxe amp and run it through a different cabinet (or the same impedance, 8 Ohms in this case) and it will sound TOTALLY different.

In the preamp, I've found that the tone stack is all important. Choice of tubes and output topologies seem secondary to me. Even pentode vs. triode gain stages don't seem to make a huge difference.

The Fender tone stack is very similar to the Marshall. They differ only in detail.

I find the Fender stack has a "quacky" kind of sound, with a peak around 3500 Hz (I think it is...). The Fender seems to be the one with boomier lows and glassier highs.

The Marshall stack is "honkier" with more "bark", with the mid-peak set at a lower frequency and the midbass-peak at a higher frequency.

I prefer the Baxandall(sp?)-style Ampeg tone stack. Meatier, more mids. Smoother overall, but I'm a jazz player, so I would prefer that...

My favorite tone control trick is the tweed-era Fender "Presence" control in the feedback loop of the output stage. As a matter of fact, I was thinking that a simple Tone control (rolloff the highs) coupled with a Presence control would be all that is really needed.

Naturally, I prefer guitar amps with as little negative feedback around the output stage as possible. The less NFB the juicier and looser the sound, but noise will become an issue at some point.

Aspen Pittman's Guitar Amp Book is a great way to get a lot of schematics and ideas.

Here's a cool tool: http://www.duncanamps.com/tsc/
That's Duncan's Tone Stack Calculator.

Good luck with this. I know you'll have a lot of fun with it.
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