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Old 30th September 2006, 06:18 PM   #1
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Default Help wanted for a truly loco project. (A Tube--gainclone hybrid guitar amp!)

I'm working on a tube-hybrid gainclone guitar amp. I figure that by using a tube guitar amp fed into a Gainclone instead of an output transformer, I can get "tube" sound without most of the cost.

You can se the progress in the Gainclone forum here:
What happens if I put a clipped INPUT into a gainclone?

Helpful as the GC people are, most of the people in the forum are (not surprisingly) trying to avoid distortion. With guitar amps, "tube" distortion is what makes them desireable.

Any suggestions on how to do this? The AX84 looks like a good starting point; hopefully, with a voltage divider on the input, I can feed it in directly. If I scrap most of the parts from another amp, I can keep the cost of the tube stage to under 40$. (20$ for tubes, 20$ for the rest - and it's not a complex amp!)
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Old 30th September 2006, 06:36 PM   #2
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Have you considered the obvious/easiest, building a tube distorion/overdrive like the McTube and just putting it in front of the GC stage?

Your question is too broad otherwise; what do you want the tube stage to do? Just add tube clipping sound? Tone controls, presence, etc., are needed too?
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Old 30th September 2006, 08:18 PM   #3
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Quote:
Originally posted by leadbelly
Have you considered the obvious/easiest, building a tube distorion/overdrive like the McTube and just putting it in front of the GC stage?

Your question is too broad otherwise; what do you want the tube stage to do? Just add tube clipping sound? Tone controls, presence, etc., are needed too?

The tube amplifier stage I'd like to use is a complete tube amplifier, with both halves of a 12AX7 and an EL84 or 6V6 being used as an output tube.

http://annex.ax84.com/media/ax84_m311.pdf

Now, here's what I would hope you could clear up.

(Please refrain from laughing.)

From what I understand, the output transformer is to drop the super-high-impeadance output of a tube to the low-impeadance ouput needed for a common 4- or 8-ohm speaker.

A stock Gainclone is pretty high-gain, and would likely be driven into clipping by the output of the EL84. Gainclone clipping sounds nasty. I'd like to try preventing this by using a high-impeadance (Ground<-2k ohms->input<-8k ohms>Gainclone) voltage divider to drop the input far enough that it would'nt clip. Although I might lose some power, 30 quality watts is all I'm really looking for. (Besides, bridging Gainclones is easy!)
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Old 30th September 2006, 08:26 PM   #4
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OK, but then what are you asking; you have already decided on a very specific design, where's the question?
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Old 30th September 2006, 09:01 PM   #5
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Quote:
Originally posted by leadbelly
OK, but then what are you asking; you have already decided on a very specific design, where's the question?

Actually, I have'nt. From what I've heard, the output from this amp will blow the gainclone. This is bad.


However, I think I found the solution: A 1/2 watt push-pull amp.



http://www.firebottle.com/ampage/schems/moonlight.gif

It's a 1/2 watt push-pull amplifier. Could it be converted to a class-A amplifier? If so, I could build two discrete amps, each using half of two tubes. (Woot!)

Also, can anyone reccomend some (preferably cheap) tube diodes? A 1/8 watt amp likely does'nt need much power, so a tube diode is economical.

As an added bonus, it likely won't blow up the gainclone.
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Old 30th September 2006, 09:32 PM   #6
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You're throwing out a lot of things there.

Why would the amp blow the GC? Keep the input voltage below 40V by the required voltage divider, set up the GC as a unity gain buffer, and you're done.

Why do you care about Class A? You are trying to add distortion.

Why do you care about a tube rectifier? Economical? No way you can buy even a used tube and used socket for less than a few SS diodes.
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Old 30th September 2006, 10:04 PM   #7
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Hi,

If I understand you correct, you want a full tube amp, minus output transformer, driving a gainclone?

If this assesment is correct, it's not only not-loco, but has been done. Carver used an EL34 to drive a MOSFET stage, through a tranny though.

You want to keep it simple, economical and no tranny.... no problem!

Parafeed the final tube with one of them diyAudio HV PNP CCS's and cap-couple it to your resistor divider and drive your gainclone
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Old 1st October 2006, 12:53 AM   #8
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Quote:
Originally posted by leadbelly
You're throwing out a lot of things there.

Why would the amp blow the GC? Keep the input voltage below 40V by the required voltage divider, set up the GC as a unity gain buffer, and you're done.
I don't think that the most commonly used LM series chips are unity gain stable. You'd have to make sure to find one that is.


Quote:
Originally posted by Geek
Carver used an EL34 to drive a MOSFET stage, through a tranny though.
This should be just as easy as using the chip amps, but as Geek said, without the tranny. The chips have lots of front end stuff you don't need. On the plus side, they do have some built in protection. Check www.tubecad.com for some simple tube fed Mosfet output stages.

Sheldon

See if you can get member Tubelab to weigh in here. He does audio and guitar amps, with and without sand. Bet he has some ideas.
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Old 1st October 2006, 01:33 AM   #9
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Quote:
Originally posted by Geek
Hi,

If I understand you correct, you want a full tube amp, minus output transformer, driving a gainclone?

If this assesment is correct, it's not only not-loco, but has been done. Carver used an EL34 to drive a MOSFET stage, through a tranny though.

You want to keep it simple, economical and no tranny.... no problem!

Parafeed the final tube with one of them diyAudio HV PNP CCS's and cap-couple it to your resistor divider and drive your gainclone

Can you explain the last sentence in more easily understood english?


The reason I suggested the tube rectifier for the tube stage is that tube rectifiers do odd things when overloaded. Because the amplifier is pushing a ridiculously small amount of power, I can use a cheap-0 tube diode.


As for the Class-A on the amp, I was hoping to reduce the power even further, and build two amps into one. Adding gain to a Gainclone is easy, and adding a second Gainclone is even easier. Using each side of the amp seperately with two different Gainclones and drivers would allow for a stereo amp. That said, you're right; I think I'll leave it as a push-pull amp.
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Old 1st October 2006, 01:50 AM   #10
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Hi,

Ok, sorry about that.

There is a thread here for SS constant current sources/sinks
CCS for tubes/valves: PCBs

I suggest this for the parafeed final stage because it eliminates the need for an expensive inductor, or transformer. An example of parafeed amp can be found in an application note I wrote for this board, found here:
http://homepage.mac.com/tlinespeaker...HV_PNP_CCS.pdf

But instead of a transformer, use a resistor load equal to the required load from a transformer. You can use two resistors in series for the load, setup as a divider so you don't overload the gainclone.
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