Firefly Rev. 5 Questions - Low output, high distortion - diyAudio
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Old 25th November 2015, 05:52 AM   #1
diyAudio Member
Join Date: Nov 2015
Default Firefly Rev. 5 Questions - Low output, high distortion

Hi everyone, newbie here! Please be kind!

So a long time ago in a house far, far away, I decided to build a Firefly amp. As I tell the good people at Guitar Center, I'm really more of a guitar owner than a player, lol, but I still like to pick one up every so often and try to teach myself, hoping that one day some different training technique will stick.

Anyway, I'm a general electronics geek, but not a particularly good one, that is, I probably won't understand you if you start launching into full EE speak, and I don't have any advanced tools like an oscilloscope (sadly), just a really good multimeter. I'm not so very good at figuring out what is wrong when things don't work without any obvious clues. So, with that in mind, here's my situation.

I have this Revision 5 Firefly amp board by John Calhoun that I put together a long time ago, and when I couldn't get it to work, tried to figure it all out with some help on another forum, and couldn't ultimately get anywhere. It didn't help that my soldering skills weren't up to par then, nor was my gear. Since then, I've upgraded both, with a nice Hakko FX-888 and a great Fluke multimeter (though my hands still shake, always have).

After moving, I finally got some room and some time to mess with it, cleaned up the board thoroughly from all the dust it'd gathered, and have taken a second look at it. My soldering really sucked, and despite not having any bridged connections, most of my connections didn't go all the way through very well, and some were borderline dry joints.

I cleaned those up, and now, where I wasn't getting any sound out when it was set to clean before, I am now, and I also get some sound when I flip to boost. Still, though, both sound outputs are very low and very distorted. This is, sadly, a very common mode of failure with the Firefly, so it doesn't seem to give any indication of what could really be wrong here.

Everything seems secure, and I've gone over all the solder joints again and again, nothing seems loose anymore or improperly soldered, so I'm at a loss. These are the assembly instructions for it, for anyone not familiar with it who'd like to look it over and give me some thoughts if you're not familiar with it.

I can upload pics of my final product if you think it'll help.
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Old 25th November 2015, 07:54 PM   #2
diyAudio Member
Join Date: Jun 2012
Default Firefly Rev. 5 Questions - Low output, high distortion

and Welcome I am also new in this forum, but I post for a long time in other places, let's see if I can help you until a more experienced guy shows up.

As you sad, You have a multimeter, so could you measure the voltage at the tubes pin?

They are all double triodes, so probably you shoul expect high voltages at pins 1 and 6 (the plates) of all of them, and at the first tube cathode V1a.

Check also cathode voltages, pins 3 and 8.

Post the voltages, that would be helpfull.

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Old 25th November 2015, 08:26 PM   #3
diyAudio Member
Join Date: Dec 2008
Location: Victoria, BC
I agree with Thomas' suggestion - measure voltages next.
Just to clarify - that would be the DC voltages between a ground point on the circuit board and the tube pins.
If you have problems with shaky hands, it can be nerve-wracking to measure voltages when the amp is powered up. I'd advise buying a pair of 'grabber' test leads to fit your Fluke (or cheap multimeter - having more than one meter is very handy and even a cheap meter will make good enough measurements for most guitar amp work, IMO). You can get grabber leads from China via 'the bay' for not much money. Then you can hook up the meter, sit back and throw the power switch.
If you eyes are like mine, a set of magnifying glasses can be a big help when checking solder joints. Either the flip-down headset (Optivisor or clone) or a cheap pair of (powerful) reading glasses from the dollar store will work. Another good check of the solder work and connections might turn up something.
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