Firefly 1.5W Tube Amp Build

duhvoodooman*** · February 4th, 2008, 10:06 PM

Well, after spending a bit more time with this little dynamo, I must admit I'm pretty impressed. It can generate an unbelievable amount of volume considering it's power rating (I suspect that 1.5W is very conservative), and when you crank the boost gain, it has plenty of raunch on tap. And I haven't even really "opened it up" yet, because I haven't had the house to myself since I built it. But I will on Wed. night, and I'll try to record a demo clip then, too.

The only thing I don't particularly like is that it's a little dark sounding, at least at low volumes. The good news is that John Calhoun included an optional "brightness" capacitor spot on his PCB design, so I'm going to get two different value silver-mica caps and wire them to the board via an on-off-on DPDT switch. That'll give me three settings: normal, bright and brighter. That should cover it....

**oldguy** · February 4th, 2008, 10:31 PM

Vood, that looks fantastic! You do nice work.
Thanks for the pics and updates, you are 'da MAN!!!

**Jimi75** · February 5th, 2008, 05:33 AM

Why didn't GOD give the talent to assemble small amps and pedals on my own?

This amp looks gergeous and the sound in the clip is killer. I can only look up to people like you - respect!

duhvoodooman*** · February 5th, 2008, 06:28 AM

Originally Posted by Jimi75

I can only look up to people like you - respect!

Funny, that's how I feel about you when I hear your guitar playing! Thanks for your generous comments, Jimi. Really--the building is easier than the playing!

**tot_Ou_tard** · February 5th, 2008, 07:29 AM

I waited a bit to make sure that you really have built it now

.

Looks beautiful as usual Vood!

Technically speaking, what makes the amp dark. Could you go into a little more detail about how a bright switch works?

You duh_man voodoo!

duhvoodooman*** · February 5th, 2008, 09:32 AM

Originally Posted by tot_Ou_tard

Technically speaking, what makes the amp dark. Could you go into a little more detail about how a bright switch works?

As I understand these things, and amp will sound dark if the circuit is laid out such that a large portion of the high frequencies are lost/bled off to ground during the course of amplifying the signal. This tends to be most pronounced at low volumes. Some amps are intrinsically dark sounding, while others are bright, or somewhere in between. Dark sounding amps are often modified in some way to give them more top end, with the addition of the famous "Top Boost" stage to the Vox AC30 being a well-known example. (Interestingly, that circuit was copied directly from an old 50's Gibson amp, right down to an error in the bass pot design!)

However, this can be done in other ways, notably by adding judiciously chosen capacitors to locations in the circuit where they can act as a bypass around points where high frequencies would otherwise be bled off. This works because of the the fact that the impedance of a capacitor varies inversely with frequency, so that at very high frequencies, a capacitor becomes a short circuit, and at very low frequencies, an open circuit. Now, exactly how you decide what capacitance rating to use in what circuit is still a mystery to me, but John Calhoun suggests something in the 200 - 450 picofarad range for the optional brightness cap in the Firefly. So I've ordered several silver mica caps in that capacitance ballpark and will see which I like the sound of best.

Clear as mud?

**Jimi75** · February 5th, 2008, 09:42 AM

Originally Posted by duhvoodooman

Funny, that's how I feel about you when I hear your guitar playing! Thanks for your generous comments, Jimi. Really--the building is easier than the playing!

Thank you :-)

**Tone2TheBone** · February 5th, 2008, 10:59 AM

Holy Moley Voo that's the coolest thing I've ever seen! Let us know how the cap mods do.

**aeolian** · February 5th, 2008, 02:35 PM

Very nicely done. It must be cool to play through an amp you build yourself.

**tot_Ou_tard** · February 5th, 2008, 05:02 PM

Originally Posted by duhvoodooman

As I understand these things, and amp will sound dark if the circuit is laid out such that a large portion of the high frequencies are lost/bled off to ground during the course of amplifying the signal. This tends to be most pronounced at low volumes. Some amps are intrinsically dark sounding, while others are bright, or somewhere in between. Dark sounding amps are often modified in some way to give them more top end, with the addition of the famous "Top Boost" stage to the Vox AC30 being a well-known example. (Interestingly, that circuit was copied directly from an old 50's Gibson amp, right down to an error in the bass pot design!)

However, this can be done in other ways, notably by adding judiciously chosen capacitors to locations in the circuit where they can act as a bypass around points where high frequencies would otherwise be bled off. This works because of the the fact that the impedance of a capacitor varies inversely with frequency, so that at very high frequencies, a capacitor becomes a short circuit, and at very low frequencies, an open circuit. Now, exactly how you decide what capacitance rating to use in what circuit is still a mystery to me, but John Calhoun suggests something in the 200 - 450 picofarad range for the optional brightness cap in the Firefly. So I've ordered several silver mica caps in that capacitance ballpark and will see which I like the sound of best.

Clear as mud?

I think you need to mod the above with a clarity boost

Thanks Vood, that did help some.

duhvoodooman*** · February 5th, 2008, 05:16 PM

Originally Posted by tot_Ou_tard

I think you need to mod the above with a clarity boost....

Got a schematic? I could use one of those!

duhvoodooman*** · February 6th, 2008, 08:04 PM

OK, here's a little sample of what the Firefly can do. I came across a posting at the AX84.com Firefly forum that said if you swap a 12AT7 for the 12AU7 power tube, it drops the max. power output but brings on an earlier breakup with a huge bottom end. Since I'm not using close to the max. output of this little monster, that sounded like a pretty good tradeoff to me. So I tried it through my 1x12 cab using my LP Std. and--WHAM--instant mini-Marshall stack sounds! Couldn't think of anything better to ram through it than some vintage Zep. Now, I'll never be confused for Jimmy Page, but playing aside, this sounded pretty good to me. Mind you, this is with NO effects other than a bit of plate reverb from my DigiVerb pedal. All the crunch comes purely from the amp. Here's the link:

http://www.box.net/shared/12u35riuck

**stingx** · February 6th, 2008, 08:12 PM

SWEET!

I have GOT to build me one of these!

**ted s** · February 6th, 2008, 08:14 PM

Crunchy Vood !, nice playing !

**F_BSurfer** · February 6th, 2008, 08:16 PM

oh yea I like that mini marshall!!!!

**tot_Ou_tard** · February 6th, 2008, 09:14 PM

Sweet is right Vood!!!!

**Adrian30** · February 6th, 2008, 09:27 PM

That's a truly sweet sounding amp!

**sunvalleylaw** · February 6th, 2008, 11:07 PM

Wowza!

duhvoodooman*** · February 7th, 2008, 08:11 AM

Two additional notes:

Though the Firefly is really about tube distortion, it's not purely a one-trick pony. Nobody's ever gonna confuse it with a Fender Twin, but if I put the 12AU7 tube back in and run just on the main preamp with the boost stage off, it will do a pretty nice clean tone, too. I'll record another clip with my Strat when I have some time, showing the cleaner side of the amp.
If you liked the sound of that 1x12 cab, it's got one of those $45 Warehouse Guitar Speakers "Veteran 30" models in it, a Celestion Vintage 30 clone. Unbelievable value at that price, IMO.

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