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Old 2nd December 2007, 01:26 AM   #1
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Default General guidence building guitar amp

I want to build a guitar amp for my grandson but have several issues to overcome ignoring cost for the moment). I would welcome any advice offered.

Issue 1: I am not a guitar player and so have no access to an instrument for testing tone etc. Other than finding a cheap guitar to use for this purpose is there any other way to do it? I live in a small town so finding a junk guitar is a one in a million shot.

For example could I have my grandson record some licks directly from the guitar's output and play that back trough an attenuator into the amp under test? would this signal be anything like what the real thing would be?

Would signals from my electronic keyboard be of any use at all? Would there be for example any way to interpret what the amp does to other voices (piano, strings etc) that would correlate to how it would sound with guitar.

Issue 2: Or maybe consider it 1a. Since I am not a player I need to learn from scratch what is needed and desirable in an amp. I have done a lot of google searching and am learning quite a bit but if you have any must see sites on electric guitar for dummies I am all eyes.

Issue 3: Given that his interests run toward punk my inclination is toward a Marshall style circuit of maybe 15W tops. He will not be playing any large venues so a monster amp is not needed. In fact for a lot of scenarios I think that a 1/2 watt amp would be perfect so that he can get into output stage distortion at reasonable volume levels.

Two possibilities that I am considering... Building two seperate output stages which he can switch between using the same input, tone and inverter stages. The other possibility is to diddle the biasing or B+ of one or both output tubes to generate distortion at lower level. For example starving the plate voltage or biasing one of the output tubes out of balance to the other in order to induce (at least 2nd order) distortion.

What are your thoughts on this?

mike
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Old 2nd December 2007, 10:19 PM   #2
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1: I build guitar, bass and harp amps and I use well recorded clean soundclips to test and prototype. I play bass, and live with a guitarist and have access to several others, but the clips are repeatable. I also have a test tone that's recorded at the head to be able to normalise levels.

Not sure if KB would give useful result, but I've never tried it.

2: Desirable properties? Well, it depends what you're expecting from the amp. Thrash metal, blues and country can all be very different. Get some indication from your grandson what he likes and who his fave players are and google their gear. Then clone it. This will get you closest.

Once it's finished and presented to him, especially if he lives close, you can always go there with a soldering iron and a handful of parts to try other things.

I find it's also worthwhile to have two very different channels in a guitar pre, both with decent ranges of adjustment and tending to meet in the middle. This way, with adjustment you can have a wide tonal spread, and a few extra pots and a couple of tubes probably adds $30 to the build.

3: See earlier comment re fave players and gear. Punk isn't an area I have any familiarity with, but I don't think you can go wrong with a Marshall 18W EL84 based amp.

The Marshalls are PP AB1 cathode biassed. With some thinking and relays to switch, I can see several possible combinations (I haven't tested these and can't see me doing so in the near future)
A: stock biassing
B: SY's Red Light District LED cathode biassing
C: fixed bias - less compressed sounding, sharper overload
D: fixed bias single ended - both tubes receive DC bias, but only one gets signal
E: variable offset biassing - gut is saying use this in cathode mode and add small DC offset to grids.

You can add a <2W o/p stage in addition to the normal output. After the pre add a 12AU7 or 12BH7 in PP, one grid grounded and use a cheap 5W line driver transformer for PA. Usuallt about 10K p-p and $5. Add a 200R resistor across the secondaries of both output transformes to prevent damage to either if a speaker isn't connected and it gets signal.
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Old 3rd December 2007, 12:02 AM   #3
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Thanks for the advice Brett, that is very helpful (though I will have to digest some of that a little bit). It is about a 17 hour drive to his place so tweeking trips will have to be well planned.

When they were visiting here last I had an experimental speaker set up and we played his guitar into it by taking the headphone output of his SS practice amp into my Peavey mixer amp and into the speaker. Judging from his playing for me he tends toward heavy but round distortion somewhat like the typical 70's rock band in tone which is why I was also thinking Marshall designs. I suppose of the styles that you mentioned Metal would probably actually be the closest as he is very fond of Christian Metal. He even is familiar with the different types of metal (I didn't know that there were types).

Another thought that I had for overdriving the output tubes at lower volume was to use an L-pad type of affair to cut the output to the speaker while maintaining the load on the amplifier. So we would be driving the output tubes to say 20 watts and dissipating about 19 of them in mongo resistors. It would take some heavy duty resistors but would have the advantage of retaining the sonic character of the existing output stage.

Now D: sounds interesting. I presume that you would have to bias toward Class A though to avoid really trashy distortion no?

I am not sure what you mean by E:.

What you are describing with the 12AU7 output stage sounds to me like something akin to a LTP driving a 70V line transformer. I thought that line transformers were SE. How would the transformer be connected?

mike
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Old 3rd December 2007, 12:29 AM   #4
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Quote:
Originally posted by mashaffer
Another thought that I had for overdriving the output tubes at lower volume was to use an L-pad type of affair to cut the output to the speaker while maintaining the load on the amplifier. So we would be driving the output tubes to say 20 watts and dissipating about 19 of them in mongo resistors. It would take some heavy duty resistors but would have the advantage of retaining the sonic character of the existing output stage.
The LPad type deals have been used in commercial iems for the same purpose, called Powersoak and other brands I forget ATM. Use a 50/100W item as they can get hot, especially if turned down for bedroom levels.
Quote:
Originally posted by mashaffer
Now D: sounds interesting. I presume that you would have to bias toward Class A though to avoid really trashy distortion no?
No it'd be direct biassed up in class A as per normal SE.
Quote:
Originally posted by mashaffer
I am not sure what you mean by E:.
With the amp in normal AB cathode bias, add a small DC voltage to one of the grids of the output stage to deliberately misbias it.
Quote:
Originally posted by mashaffer
What you are describing with the 12AU7 output stage sounds to me like something akin to a LTP driving a 70V line transformer. I thought that line transformers were SE. How would the transformer be connected?
Mike I've used the line drive transformers in the article linked below. Perhaps you can find something similar locally. Cheap is good here. they're connected as a typical output stage, centre tapped. Futher thoughts are the AU7 may have to high an Rp for these, but then maybe they'll distort more into them. Experimentation would be needed.

As I was typing I though of other modes:
F: Class A PP triode
G: Class A PP pentode
either could be fixed or cathode biassed. G may not make a big difference, but F is probably worthwhile for ~5W output.

I dunno if you're interested in speakers, but for heavy and clear, noth thrash wall of noise stuff, the EV EVM12L is an amazing driver. Two, stacked vertically to avoid horizontal phasing and get the drivers nearer his ears in a semi closed back cab (vertical slot maybe 40% panel area) with a bit of damping on the walls to reduce colouration and it'll go very loud on small power. The last harp amp I built was very similar to this except 6SL7/6SN7 pre tubes and a custom tone control that the player liked.
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Old 3rd December 2007, 12:56 AM   #5
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Thanks for the clarifications.

My first pass is to use a salvaged 16ohm 12 inch organ speaker that I have on hand. It appears to be AlNiCo and is the one we played with when he was here. At that time I had it in an N-frame box (like a Ripole but not as squished down). I had built this for testing a subwoofer idea. Obviously the frequency response above about 250Hz was quite ragged but it sounded quite good anyway. This speaker should be about right for an 18 watt amp given what most organs used for amplifiers.

mike
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Old 3rd December 2007, 01:47 AM   #6
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Great Mike,

Here's the link I forgot at the end of the last post.
http://home.alphalink.com.au/~cambie...ls_6CM5amp.htm
http://www.altronics.com.au/index.as...=item&id=M1115

Cheers
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Old 3rd December 2007, 02:02 AM   #7
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Mike, I am a player and I have built a number of amps and continue to build and experiment.

Here are a few things I suggest:

1. Tone is extremely subjective and a single cap can change the amp quite a bit (as I am sure you know). When I build amps for people, I have them test it and tweak accordingly.

2. Find out what his favorite player is, and what gear they use. Copy from that.

3. A lot of people use a (somewhat) clean amp, and then front end it with pedals. Does you grandson do this as well?

4. Using a cheap guitar to test the amp is fine, but it will no where mirro the tone of his guitar - or his playing.

5. I have used WAV recordings of straight guitar playing and pumped them through the amp. This is find for initial testing, but I do not use if for tone shaping.

6. A lot of tone is in how the player plays. Other than that, the order of importance for an amp is: speaker (very important), output transformer, circuit topology, tube selection.

7. A marshall 18 Watt is a great amp that breaks up easy and gives great tone. In combination with power scaling, the player can get driver stage OD at bedroom levels. See www.18Watt.com.

8. The ax84.com site has a number of different amps. My first amp was the firefly - a 5W amp. Not much tone because of poor OT selection.

There are a number of great sites for amp builders:

http://www.el34world.com
http://ampgarage.com
http://www.18watt.com
http://www.ppwatt.com
http://www.ax84.com

d1
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Old 3rd December 2007, 04:44 AM   #8
rickl is offline rickl  United States
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I just did this.

My son and I built a black face Champ clone. 6SL7 -> 6v6 SE.

We used a 5y3 for a rectifier.

About 3.4 watts measured.

What I did and not sure if it will work for your grandson was to get the enclosure complete and have him solder it up.

I found a Fender SS amp on Craigslist. Pulled out the PCB, punched three tube socket holes, drilled some holes for the transformers and terminal strips.

He had to dig through my parts bin for resistors (measured them on the DVM) and caps.

He wired it and soldered. It was *very* hard not to do it but he loves his amp.

Plus it is small enough he can carry to his friends.

I'd see what brands he likes (Fender, vox, ...) and try to emulate one of them.

Good luck!

rick
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Old 3rd December 2007, 01:16 PM   #9
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Rick, I am curious, there are a couple of things about the champ that I wonder:

1. champ's rectifier makes the amp sound "saggy" and thus power chords tend to be mushy and ill-defined. Great for blues but not so good for rock/metal - how does this work for your son?

2. how does the small amp hold up to the volume of a drummer?

d1
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Old 3rd December 2007, 01:47 PM   #10
AuroraB is offline AuroraB  Norway
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Location: Norway, -north of the moral circle..
I'm about to venture into the same direction as Mike here, but had some slightly more power in mind...
From the "signals" I've been trying to read ( without directly promising that I'll go ahead and do it ) I've been gathering info about the Vox AC-30...
As for the power level, I think one can be sure that the higher power levels will be much wanted in between, like "home alone, at last- what can this thing do??!!.............". If going to the extent of building a fair quality amp, thjere isn't much difference in going from 5-10W to 30-50, concidering the actual total work involved in such a project.

Switchable power levels is a feature I've been thinking of. Maybe as simple as triode -pentode switching, or something more fancy? I really don't like the power dump resistors...
( PS: even if I hold an EE, I'm not into guitars either- i got stuck with saxes and licorich sticks )
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