FireFly mini-amp build report

[djepaine]

Hi!
I m going to build this amp but i have a little question.
Didi i have to to solder the lugs named "2" together in order to grouned it or not?
thanks a lot and sorry for my poor english

[klooon]

Hi!
I m going to build this amp but i have a little question.
Didi i have to to solder the lugs named "2" together in order to grouned it or not?
thanks a lot and sorry for my poor english

Yes! You're right. Please see here the new layout. Further, for completeness, you need to have the 6.3V fillament windings connected as well from the transformer to the pin 4/5 and 9 of both tubes.

I have chosen to have it ground it to 'point 2' because this part of the circuit is filtered by the filter capacitor at node 2... People say that it shouldn't do much in case of small wattage amps, but just to be 'safe' I always do this. And the amp is dead quiet! Hardly any hum, noise, etc. at all...



Which output tube and output transformer are you going to use? For a ECC99, it's better to use a larger output transformer than the 125A. So for instance a 125C is a better choice then. For a 12AU7, you can still use the 125A. But if you still have to order, just go for the larger one, because the price are roughly the same.

Furthermore, people say that this amp with the ECC99 is a 1 watt amp, but I think it's a bit bigger. I was able to hold up well with the drummer in a small bar gig...

[djepaine]

Thank you !
i have ordered a 125a so i will use a 12au7 but it 's in order to use with some tube preamp.
I will build it with high grade capacitor and carbon resistor.
High grade tubes are usefull for class a circuit.(no hum no buzz.......)
thank you verry much for your quick answer!!

[djepaine]

1/2 watt resistor is it enough ?
thank you

[klooon]

1/2 watt resistor is it enough ?
thank you

Yes, 1/2 watt is fine! I am using Carbon Film as they are supposed to be a good compromise between noise and 'mojo' and they are only available in 1W where I usually stock components. The last few amps that I built, I have used CF and they are really quiet and I don't miss anything in terms of mojo... They are just as big as the 1/2W CC resisitors...

Something nice to read: Resistor Types - Does It Matter?

The only larger resistors in power rating that I used are:
- the 4k7 dropping resistor between "A" and "B". 1W should be enough, but I have chosen 2W to be on the safe side.
- the 390R cathode bias resistor. I used a 5W version to be safe, but I think 0.5 Watt or even lower should be fine as well in this mini-amp. In my case: P=8.9*8.9/390=0.2W...

So, you're planning to make a line-out? What 'schematic' do you use for this?

Thank you !
i have ordered a 125a so i will use a 12au7 but it 's in order to use with some tube preamp.
I will build it with high grade capacitor and carbon resistor.
High grade tubes are usefull for class a circuit.(no hum no buzz.......)
thank you verry much for your quick answer!!

About the high grade tubes, I am using in general new JJ tubes and for V1 I am ordering a 'special' low noise selected version and for the phase inverter also a matched triode version, but I also did a check with 'normal' versions and could not hear a difference then. I used to go and search for NOS tubes a while back, but very often I got microphonic ones or rattling tubes. I must say I am quite happy with putting new JJ's in my recent builds...
About hum and buzz, I found that the way I do the grounding and lead dress layout is giving me excellent results.

Further, I found that the amp starts to overdrive almost immediately with my humbuckers, so you might want to chose a lower gain pre-amp tube, e.g. 12AY7 or 12AU7 to have a bit more control.

[djepaine]

hi sorry for the late answer .
sure choice of the resister doesn t matter it just for the fun!!!!!!!!!!!!!
The choice of the preamp tubes is important for the max volume in order to have no hi treble sound that break all my familly ears lol.
i choose hammond box 1590 DD in order to put a turret board like my mini 5E3!
I choose your layout and put an headphone out instead of attenuator.
Thanks a lot

[klooon]

hi sorry for the late answer .
sure choice of the resister doesn t matter it just for the fun!!!!!!!!!!!!!
The choice of the preamp tubes is important for the max volume in order to have no hi treble sound that break all my familly ears lol.
i choose hammond box 1590 DD in order to put a turret board like my mini 5E3!
I choose your layout and put an headphone out instead of attenuator.
Thanks a lot

Hi, choice of the resistor does matter! At least for noise and hiss... Please read the link above carefully: carbon composite produce quite some noise/hiss, the carbon film and metal oxide/film much less...

1590DD: great! I should have done that as well. Works much easier ;-)

What 'circuit' do you use for the headphone out? I have never done this.

[djepaine]

[djepaine]

I use this layout for my micro 5e3 but with toroidal PT and no v3 but 1n4007 rectifier and 2x100R resistor to virtualize center tap of 6,3v fill.I change 27k dropping resistor to 22k because my PT toroidal is the same as your firefly :220v 200v 6,3
in the layout jack are chassis grounded
Thanks