| Spasticteapot |
I'm working on a guitar amplifier for a school project, and was wondering how well this would work.
I'm hoping to use a simple class-A tube amp, but with a very small, cheap tube in place of the usually massive output tubes. (In other words, a headphone amplifier). This will let me overdrive the amp, and get all the "tube-y" distortion guitar players love. (NOTE: I do not play guitar. I do, however, know a lot of people who do.)
This high-impeadance would then be fed into a gainclone through a bias-blocking capacitor, and then sent to a conventional guitar speaker cabinet.
So, how wild is this idea? I'm hoping that a "pre-clipped" input won't turn on the anti-clipping feature, and a bridged Gainclone connected to an 8-ohm speaker should work nicely. |
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| poobah |
That should work just fine.
Your gainclone wont regard it as clipped per se... it will just respond to all the harmonics present.
This will give the soft fuzz, "overdrive" type sound rather than "crunch"... but it's all good.
:) |
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| Spasticteapot |
| quote: | Originally posted by poobah
That should work just fine.
Your gainclone wont regard it as clipped per se... it will just respond to all the harmonics present.
This will give the soft fuzz, "overdrive" type sound rather than "crunch"... but it's all good.
:) |
I was hoping that I could get "tube sound" by simply amplifying the output of the tubes. Would the output from the gainclone be similar to that of the output of the tube amp? |
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| Tim__x |
| The output of the GC will be virtually identical to the output of the tube amp, but the output of the tube amp driving another amplifier will be different than the output of a tube amp driving a speaker. |
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| poobah |
No... transistors clip hard... while tubes soft... different sound.
I thought you were talking about clipping a preamp stage and then just using the gainclone for faithful amplication?
You will get tube sound from overdriving a preamp stage... very usuable and truly part of the "tube sound". The rest of the tube sound comes from from overloaded OPTs, output tubes, and sagging power supplies.
:) |
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| poobah |
12AX7 is probably THE most popular "pre" tube for guitar amps...
A good choice... easier to overload (clip) by virtue ot its high gain.
Keep in mind, you use both triodes to accomplish your task. Use the first to amplify and overdrive the second triode.
Member "tubelab" would be an excellent guy for some insight on this... tube guru... but also guitar guy.
:) |
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| Spasticteapot |
| quote: | Originally posted by poobah
12AX7 is probably THE most popular "pre" tube for guitar amps...
A good choice... easier to overload (clip) by virtue ot its high gain.
Keep in mind, you use both triodes to accomplish your task. Use the first to amplify and overdrive the second triode.
Member "tubelab" would be an excellent guy for some insight on this... tube guru... but also guitar guy.
:) |
I'm going for a "KISS" amp, as I've yet to get a gainclone to work properly yet. I'm willing to bet that I could make a kit without much trouble, but I figure a single-tube amp might be the way to go. |
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| poobah |
Just so we are on the same page here... A single 12AX7 has 2 triodes in the bottle. So a 2 stage "overdrive" amp is simple.
There should be plenty of circuits out there done up already. If you're a student, do it the hard way.
Maybe google [tube + distrotion + preamp + guitar]
:) |
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| Spasticteapot |
| I'm well aware that the 12AX7 is a dual-triode tube. That way, I can have a tube-overdrive amp with a single 10$ tube! |
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| poobah |
OK... but stick with the gainclone for a follower/power section. The 12AX7 would be pretty useless as a headphone amp.
:) |
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| Spasticteapot |
| quote: | Originally posted by poobah
OK... but stick with the gainclone for a follower/power section. The 12AX7 would be pretty useless as a headphone amp.
:) |
Will do.
I assume that it's safe to feed the output from the tube amp directly into the GC, with only a cap to prevent voltage bias? |
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| poobah |
Yes Sir,
And you will want to divide down the voltage considerably before feeding the gainclone. You don't want to clip it... sound nasty. |
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| Nordic |
| You probably want to introduce a time delay between switching on the valve stage and the GC, as the valves tend to put out alot of DC until the heater is warm. |
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| Spasticteapot |
I can see why this is a good idea.
Would you reccomend not using a cap with this amplifier? |
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| whatsnext |
I would find a Fender Champ circuit or Gibson Skylark. 12AX7 driving a SE 6BQ5. SS or tube rectifier. Then you have a 3-4 watt practice amp but if you drive a resistor instead of a speaker you have a cool guitar pre. The sound of a clipped 12AX7 is pretty lame compared to the sound of a overdriven EL84 with a saturated OPT.
JMO, John |
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| Spasticteapot |
| quote: | Originally posted by whatsnext
I would find a Fender Champ circuit or Gibson Skylark. 12AX7 driving a SE 6BQ5. SS or tube rectifier. Then you have a 3-4 watt practice amp but if you drive a resistor instead of a speaker you have a cool guitar pre. The sound of a clipped 12AX7 is pretty lame compared to the sound of a overdriven EL84 with a saturated OPT.
JMO, John |
Interesting idea. However, the output transformer is a large part of the cost.
What if I were to use a (cheapie) 1:1 signal transformer? That would fix the DC offset, if nothing else
You mean this?
http://www.harpamps.com/gibson/pg_0089.jpg
EL84's are usually output tubes. I've seen one or two amps with them in the preamps, though I can't find schematics on them.
Any suggestions on good ways to build this? |
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| Spasticteapot |
I was wondering if it might make more sense to simply take a popular amplifier (like the EL84 or Skylark) and couple it to a Gainclone? I'd have all the halmarks of "guitar sound", including the insanity with output tubes, without the nasty price of the output transformer. (Plus, it's about four more parts, and an addition of 12$ to the cost. Tubes are cheaper than I thought!)
Furthermore, the power use of the amplifier (minus tube heaters) will be quite small; the high impeadance of the Gainclone will mean that the whole amp will be pushing about half a watt, tops. This means that I can use a very small tube diode in place of the solid-state diodes, and use a less expensive power transformer.
Also, from what I understand, the output from a tube amp would be very high. What if I were to reduce the gain on the Gainclone? Or would I need to use a second resistor or voltage divider? |
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| whatsnext |
That schematic you posted is a push pull design that would probably put out 12 watts and so is a real amp. The resistor you would need to turn it into a preamp would be huge and expensive. If you build a single ended output design with a 6aq5, 6v6, or 6bq5 as the output your OPT with only be a five watt model so will be very inexpensive, maybe $10. I've lost all my links but I'll look around. You want a SE design. I have an amp buried somewhere that doesn't even use a power transformer, just rectified 120vac and then a little filament tranny. I think it's a Wards Airline. Look for an amp that has three tubes, 1x12ax7, 1x6bq5, and one rectifier tube. Then you'll be closer to the correct schematic.
John........ |
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| whatsnext |
| if you replace the word schem with layout at the end of the link it will show you where all the wires go. This is an all octal amp. Once I find the Gibson schematic that uses 9 pin tubes I'll post it. |
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| Spasticteapot |
| quote: | Originally posted by whatsnext
if you replace the word schem with layout at the end of the link it will show you where all the wires go. This is an all octal amp. Once I find the Gibson schematic that uses 9 pin tubes I'll post it. |
Sweet!
However, I think I found the solution: A 1/2 watt push-pull amp.
Firebottel Moonlight
What if this were converted to single-ended use? If the output were biased properly, I could build two seperate amplifiers each using half of the three tubes.
People have noted success with 6800 ohm primaries on the output transformer, so an input impeadance of 10k should be about right.
Any thoughts? |
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| whatsnext |
Why would you convert this to SE? You'd have like 1/10th of a watt. I see no point in something like this or the Firefly because it is no simpler than a SE 6v6(or other tube) but is way less usefull because it makes less power. I would use both halves of a 12AX7 in cascade driving one output tube. What could be simpler? If you want to get fancy pilfer the tone stack out of an early Plexi.
John........ |
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| whatsnext |
I guess I should mention that if you do include the tone stack you will likely need another gain stage after it making it a three tube amp, unless you tube rectify.
John...... |
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| Spasticteapot |
| The reason I was looking at a 1/10 watt amplifier is because it's not actually going to have to power anything. The Gainclone takes care of that. |
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