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Old 4th May 2006, 03:33 AM   #1
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Default Tube-buffered Gainclone for a guitar amp?

Some friends of mine heard that I was bodging together an amp, and having bragged that they're supposed to sound as good as a tube amp, they of course asked the following:
"Could you make one for a guitar?"

So, here's the question:
Is it possible to modify a Gainclone or, better still, make a tube-buffered gainclone that could amplify a guitar or bass guitar? If I remember correctly, the voltage out is very low (under 1 volt), but the high input impeadance on gainclones and tubes might allow it to work anyway. Of course, standard speakers would have to be used; I'm learning about speaker design as well, and I'd likely use a FR speaker to simplify design and make it sound better. (Hypothetically, some Fostex FE127's or the like in a simple plywood enclosure. With two Fostex FE127s, this thing would be LOUD.) They'd likely be using some external "stompboxes", but I'm not opposed to building in a reverb.
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Old 4th May 2006, 03:40 AM   #2
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1) If you want to go to the effort to put a tube stage in a guitar amp, don't make it a buffer. Make it capable of producing a range of clean-to-dirty sounds. Look at the input stages of commercial tube amps or commercial distortion stompboxes.

2) I'm baffled by your FE127 idea. Guitar speakers will be cheaper, play louder, and sound better IMHO.
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Old 4th May 2006, 03:47 AM   #3
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Quote:
Originally posted by leadbelly
1) If you want to go to the effort to put a tube stage in a guitar amp, don't make it a buffer. Make it capable of producing a range of clean-to-dirty sounds. Look at the input stages of commercial tube amps or commercial distortion stompboxes.

2) I'm baffled by your FE127 idea. Guitar speakers will be cheaper, play louder, and sound better IMHO.
You're probbably right about the speaker drivers. The FE127's were simply named due to high efficiency.

Out of curiousity, how would one go about making such an input stage? I have'nt the foggiest clue.

EDIT:
I found what looks like a decent guitar preamp stage. I don't know much about tubes, but if I remember, they have a pretty high output impeadance. Perhaps it could be used with a LM3875 used as a simple buffer?
http://www.ax84.com/index.php?pg=pro.../ax84_m311.pdf
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Old 4th May 2006, 08:06 AM   #4
sangram is offline sangram  India
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Good entry-level guitar amps use mostly solid-state chipamps, my MG30RCD uses a 5-leg chipamp like 1875 or probably TDA 2030.

The problem is that guitar players need tone, not fidelity. Tube amps provide gracious clipping (tube front-ends provide some of this) that results from an overdriven output stage into vintage speakers with stiff surrounds and uneven, peaky frequency response.

Since the goals of guiatrists and hi-fi enthusiasts are completely different, maybe your energies are better spent elsewhere.

There are a lot of good ideas on the web with repositories of tube power and preamps, even some of the botique and high-end heads and combos (Like the VOX AC30 and the Marshall JCD heads) circuits are available, in case you want to pursue that route.

Like leadbelly said there is more merit in putting in a 'proper' guitar preamp with overdrive, but a tube output into a vintage speaker cannot be replicated very easily.

The circuit you linked to is a full guitar amp, about 5 watts output AFAIK, perfectly fine for gigging. The preamp section cuts from the point after the second half of the AX7, or if you also want the tone control, then you'll need a buffer as that section needs to look into a highish impedance.

The P1 is a pretty good amp, but it won't be fully dirty, as the article says. More of the warm 60s blues tone...
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Old 4th May 2006, 05:06 PM   #5
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Quote:
Originally posted by sangram
Good entry-level guitar amps use mostly solid-state chipamps, my MG30RCD uses a 5-leg chipamp like 1875 or probably TDA 2030.

The problem is that guitar players need tone, not fidelity. Tube amps provide gracious clipping (tube front-ends provide some of this) that results from an overdriven output stage into vintage speakers with stiff surrounds and uneven, peaky frequency response.

Since the goals of guiatrists and hi-fi enthusiasts are completely different, maybe your energies are better spent elsewhere.

There are a lot of good ideas on the web with repositories of tube power and preamps, even some of the botique and high-end heads and combos (Like the VOX AC30 and the Marshall JCD heads) circuits are available, in case you want to pursue that route.

Like leadbelly said there is more merit in putting in a 'proper' guitar preamp with overdrive, but a tube output into a vintage speaker cannot be replicated very easily.

The circuit you linked to is a full guitar amp, about 5 watts output AFAIK, perfectly fine for gigging. The preamp section cuts from the point after the second half of the AX7, or if you also want the tone control, then you'll need a buffer as that section needs to look into a highish impedance.

The P1 is a pretty good amp, but it won't be fully dirty, as the article says. More of the warm 60s blues tone...
I was intending to use only the preamp and tone control. This will likely be a bass guitar amp, so perhaps that won't be a bad thing. (And the fact that it's only a single tube is not a problem,either).

Would this preamp work if it were fed directly into a Gainclone, or would it work better with a buffer of some type? Or, if the preamp has enough gain, could the LM3875 be configured as a simple buffer with no gain?

EDIT:
Some preamps are designed to be used with standard power amps. Would any of these work?
http://users.aol.com/portaflex/schems/v3preamp.gif
And original power amp section
http://users.aol.com/portaflex/schems/v3powera.gif
-A simple, simple, simple tube preamp. The power amp stage uses tubes, so I'm guessing that a GC, if tube-buffered, should work.

http://www.kbapps.com/audio/tubeamps/preamp.html
This looks like exactly what I needed. It's a guitar preamp with line-out. And it only uses two tubes. (Of course, building it is likely to be a nightmare.)
Thoughts?


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Old 4th May 2006, 05:21 PM   #6
Khron is offline Khron  Romania
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I SERIOUSLY doubt that an LM3875 is gonna be stable at unity gain (i presume that's what you mean by "simple buffer with no gain"). At least the LM3886 datasheet clearly states that it's stable only at gains over 10.
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Old 4th May 2006, 06:42 PM   #7
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Quote:
Originally posted by Khron
I SERIOUSLY doubt that an LM3875 is gonna be stable at unity gain (i presume that's what you mean by "simple buffer with no gain"). At least the LM3886 datasheet clearly states that it's stable only at gains over 10.
Please search around a bit before posting such things. There are various examples of the LM chips being used in buffer service, dating back a long time ago:

http://www.diyaudio.com/forums/showt...061#post112061
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