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 Post subject: Advise on FireFly amp.
PostPosted: 22 Nov 2010, 21:59 
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Hi-ho..

I'm settling in on a first project. I've been looking, reading, "studying" and sorting to alot of ideas posted in the DIY AudioProject Forum, but I haven't seen much regarding the FireFly. Looks like a nice, simple first build - something to cut my tube building teeth on. I do have a question and I'm hoping one of you fine accomplished folks can give me a direction. This schematic is using 12AU7(A). I know these are medium mu tubes. I am wondering if 12AX7 will work well in this design? I know the 12AX7 is higher gain capable, but would it work. What changes could I make to get them to work if possible. I would like to be able to use 12AX7 for one simple reason. I have some 6C10 compactrons (basically, it's 1 1/2 12AX7 in one package) that I would love to use.
B+ voltages are;
B1 = 265V
B2 = 219V
B3 = 210V
Attachment:
firefly-r3.gif

Comments? Suggestions? Advice? All are welcome!
Thanks in advance! :D


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We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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PostPosted: 24 Nov 2010, 13:39 
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Hi Les,
The firefly is a guitar amp with a good following. Is that what you are looking for?
Cheers

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PostPosted: 24 Nov 2010, 14:32 
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Hi Gio!

Yes, it is a guitar amp. That I was aware of. It hadn't occured whether it had a good following, but I guess that's good. I'm going to build one from scratch. I figure it's a nice, simple, not too expensive and useful way to start my building projects. Unfortunately, since I haven't worked with tubes for many years, there's a whole lot of information I no longer am able to recall. I know in certain applications, the 12AX7 would sub in for 12AU7, that much I can remember doing, with little or no adverse affect. But I can't remember what situations those were. I really just need some advice as to whether a 12AX7 will drop in to the FireFly in place of 12AU7s, and what, if any, modification I may need to consider to avoid adverse affects. I have lots of 12AX7 and no 12AU7s.

:headphones: :yumcoffee:

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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PostPosted: 24 Nov 2010, 15:52 
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One would definitely have to make changes to get the circuit suited for the higher gain, but I have no experience with such.

I also have always read that the sound quality of both these tubes is very dependent on the circuitry. Some people will prefer the 12AU7 in some amps, but in others the 12AX7 is better.

Cheers,
Greg


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PostPosted: 24 Nov 2010, 23:50 
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Bluesman wrote:
One would definitely have to make changes to get the circuit suited for the higher gain, but I have no experience with such.

Yep, I'm kinda thinking that, which is why I would like some guidance. There's a big difference in servicing amplifiers and actually knowing what to modify to make a higher gain tube work "happily" in an audio stage.

In double checking the schematic I have I realized it is in fact utilizing 2 12AX7s (V1 & V2) and a single 12AU7 (V3), which is the OPT driver. (The parts list was not included in the image, it was farther down the page with the PSU drawings, not displayed.) How did I miss that? I was thinking they were all 12AU7. :blush: However, I would really like to us all 12AX7s. So, I wonder what could be changed for the driver stage? Hmmm. The OPT is a multi-tap secondary which is how the primary impedance is set. ( http://www.hammondmfg.com/125.htm ). Might that be where I need to make changes?
Attachment:
thinker.GIF

PS. Here's a smiley for ya Gio! I noticed your lacking the "thinker".


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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 27 Nov 2010, 12:22 
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It's an interesting idea but I do have a concern about the Firefly output stage as a general purpose amp. The output stage is seriously biased into class AB operation. Even with the 22.5kΩ plate to plate primary impedance the second and fourth order harmonic distortion is going to be very high (I'm guessing around 30%) with the 12AU7's plate curve compression near zero current.

Now, please don't misunderstand me, for a guitar amp this is a good thing. Guitars are so rich in even harmonics that this level of harmonic distortion will not color the sound much at all. But when overdriven it will allow some nice fuzz and compression. This is why the Firefly has such a following. However, for a general purpose amp, the goal is really true reproduction and a touch of 2nd harmonic distortion to add fullness and presence (i.e. the characteristic "tube sound"). For this type of performance you're really better off starting from scratch and choosing better operating points for the tubes.

Just my opinion. :soapbox:

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PostPosted: 28 Nov 2010, 21:25 
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suncalc wrote:
Now, please don't misunderstand me, for a guitar amp this is a good thing.

No worries, Matt. I was just pondering the possibility. You, sir, have far more knowledge and experience than I in this arena and I yield to your knowledge. While I have experience in solid state electronics and understand transistors, IC chips and the like, my knowledge in glassware is very rusty and I find myself trying to relearn alot of what I've forgotten over the years. I value the insite of those who have that current experience.

In this case, I don't want to try and make the FireFly a GP amp. I would like to keep it as a good guitar practice amp. If there is a way to make it totally 12AX7, that is what I would like to accomplish.

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
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PostPosted: 02 Dec 2010, 12:59 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Forgive me to go into a tubes 101 mode here. :soapbox: While the pins and max voltage ratings are similar for the 12AU7 and 12AX7 they share little else. There are two big differences. Gain (rather obvious) and power handling. The 12AU7 when used as an output tube as in this design will be able to deliver around a watt I would expect. It can handle up to about 30 ma per triode. The 12AX7 is a voltage amplifier and is maxed out at 2.5ma - roughly 1/10th the power. If all you need is a few milliwatts fine, the circuit values could be adjusted. I personally like the simplicity of the FireFly and would not mess with it except to possibly replace the output tube with a 6CG7.

Good listening
Bruce

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PostPosted: 02 Dec 2010, 13:37 
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Bruce, you're input is highly valued!
Mind you, I am really just starting back into tubes after over 25 years of not having any association with that venerable old technology. Needless to say, I have forgotten much of what I knew. So PLEASE don't hesitate to go "Tubes 101" anytime :mrgreen: . You bring up an interesting point..
I look at the table of gains for the two and the AX has a higher gain, therefore I assumed the AX may be too much for the OT. But what you're telling me is that the AX may not have enough push to even drive the OT. Makes sense, gain is not necessarily an indicator of i, but rather E. Do you have the tube data for the 6GC7? I don't see it in Frank's Electron Tube database ( http://tubedata.milbert.com/index.html ).

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
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PostPosted: 02 Dec 2010, 14:36 
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Sorry folks my eyes were on backwards. I intended to say 6CG7. :bawling:

Welcome back to tube audio.

Good listening
Bruce

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