OT- FireFly Observation

[Paul Marossy]

For those of you who have recently built a  FireFly, here is a few of things that you might find interesting:

If you like a looser feel similar to a rectifier tube, try installing a "sag resistor". I used a 124 ohm, 1 watt resistor in series with the B+ supply which can be switched in or out of the circuit via a DPDT switch. This works just about as well as a real rectifier tube.

I recently built a closed back 2x8 speaker cab for my FireFly. It sounds really good with this amp. My cabinet measures 19.5" high, 18" wide and 9.5" deep. I wired them in parallel for a 4 ohm load. (I'm not sure where it peaks, though. Anyone know how to determine that? I don't have full specs on those speakers...)

Here is a couple of pictures:
http://www.diyguitarist.com/Images/FFSC-Front.jpg
http://www.diyguitarist.com/Images/FFSC-Speakers.JPG
I still need to finish it up with a speaker grille and corners...

Since the std. FireFly is designed for an 8 ohm load, I installed an impedance switch which allows me to switch between a 4 ohm or 8 ohm speaker cabinet. This is easy to do with a transformer that has a multi-tapped secondary, like the Hammond 125A, for example.

Also, using different preamp tubes can give you some interesting tones. I thought that a 12AU7 in the regular gain channel sounded kind of nice. It gives a softer distortion, with a little less volume. There's all sorts of preamp tube combinations one could try.

Hope it helps.  

[Joep]

Hi Paul,

The cabinet looks nice, what speakers did you used?

Thank for sharing all your information on the Firefly.

Bye,

Joep

[Paul Marossy]

You're not going to believe this, but the speakers are some thrift store treasures, $8.00 for the pair. They are from some ceiling mounted speakers that were apparently removed from a building. They are 8" Atlas "Soundoliers", which have an aluminum spider, are rated for 15 watts, are 8 ohms and have a frequency response of 50Hz to 18kHz. Believe it or not, they sound really good in this cabinet with the FireFly!

[Mark Hammer]

I have a bunch of 8" full-range speakers for ceiling P.A.'s and I love them.

Before he became known for his tuning system, Buzz Feiten was marketing a speaker cab that used one front mounted and one rear mounted speaker (mounting referring to whether it was attached to the baffle from in front or behind the baffle).  The cab was favourably reviewed.  Admittedly, there might be some audible difference between your "Soundoliers" and the Celestion Greenbacks that feiten used, but give it a shot and see what you think.

[Doug H]

I like your 2x8 cab! I think am going to do the same thing with my weber and electar "rio grande mud";-) speakers I had for the electar.

I wouldn't think a sag resistor (or tube rec) would do much in this amp since it is Class A.

Doug

[Paul Marossy]

"Before he became known for his tuning system, Buzz Feiten was marketing a speaker cab that used one front mounted and one rear mounted speaker (mounting referring to whether it was attached to the baffle from in front or behind the baffle)."

So, both speakers faced forward, but one was setback approx. 1" from the other one? That is supposed to make an audible difference? Interesting.


"I wouldn't think a sag resistor (or tube rec) would do much in this amp since it is Class A. "

It seems to make a difference to my ear, it seems to soften it up and make it sound sweeter. Maybe it's a power of suggestion thing.... but I can hear a difference, subtle as it is. I thought the Firefly was a push-pull amp, according to your page Doug. Is that not true? Just trying to understand.  

[ODwan]

It seems to make a difference to my ear, it seems to soften it up and make it sound sweeter. Maybe it's a power of suggestion thing.... but I can hear a difference, subtle as it is. I thought the Firefly was a push-pull amp, according to your page Doug. Is that not true? Just trying to understand.

Yep, it is! But that doesn't make it Class AB. Whether an amp is class A or class AB or class B just depends on the extension of current swing on each tubes plate over a full cycle if the sine waveform gets clipped on one end of the  cycle it isn't class A anymore.
Check out Randall Aikens page (www.aikenamps.com) for some great tubetech info on this ( "Is the VoxAC30 really class A") and many more issues. I think his site is great and highly recommend it.

[petemoore]

It should sound quite full, I prefer 'large' volume enclosure / per speaker cone face area...up to a point it increases bass...the sizing of the cab to those speakers looks to be a great choice.

[Doug H]

It seems to make a difference to my ear, it seems to soften it up and make it sound sweeter. Maybe it's a power of suggestion thing.... but I can hear a difference, subtle as it is. I thought the Firefly was a push-pull amp, according to your page Doug. Is that not true? Just trying to understand.

Yep, it is! But that doesn't make it Class AB. Whether an amp is class A or class AB or class B just depends on the extension of current swing on each tubes plate over a full cycle if the sine waveform gets clipped on one end of the  cycle it isn't class A anymore.
Check out Randall Aikens page (www.aikenamps.com) for some great tubetech info on this ( "Is the VoxAC30 really class A") and many more issues. I think his site is great and highly recommend it.

In a Class A amp the output tubes will demand the same avg DC current for the whole cycle. In Class AB the avg DC current demand will change and will depend somewhat on the amplitude of the signal on the grid.  This changing current demand is what allows a tube rec or sag resistor to provide compression. If the current demand doesn't change, then there isn't really any dynamic interaction with the sag resistor or rectifier to cause this effect.

Doug

[Paul Marossy]

"In a Class A amp the output tubes will demand the same avg DC current for the whole cycle. In Class AB the avg DC current demand will change and will depend somewhat on the amplitude of the signal on the grid. This changing current demand is what allows a tube rec or sag resistor to provide compression. If the current demand doesn't change, then there isn't really any dynamic interaction with the sag resistor or rectifier to cause this effect."


OK, I understand that. I think what I hear is a slight reduction of volume which gives the impression that the amp sounds a little "saggier". So, strike the sag resistor comment above from the record.  :oops:

An interesting observation on my part, I sure thought that I heard a distinct difference when I first put it in, but it really is barely perceptible...
Ugh! I've been struck by the pyschoacoustic bug! How embarassing.  :shock:

[Doug H]

I think what I hear is a slight reduction of volume which gives the impression that the amp sounds a little "saggier". So, strike the sag resistor comment above from the record.  :oops:

An interesting observation on my part, I sure thought that I heard a distinct difference when I first put it in, but it really is barely perceptible...
Ugh! I've been struck by the pyschoacoustic bug! How embarassing.  :shock:

Well, you did lower the B+ voltage a little which may change the sound in a way that you like. Nothing wrong with that...

Doug

[Paul Marossy]

Thanks for the reassurance, Doug.  

[Mark Hammer]

http://www.buzzfeiten.com/Merchandise/Buzz_Feiten_Speaker_Cabinets/buzz_feiten_speaker_cabinets.html

[Doug H]

Thanks for the reassurance, Doug.  

A few yrs ago when I first started experimenting with amps, I had modded my Electar 10 preamp with an extra tube for a "high gain" sound. One of the things I had was a "brown switch". It switched in a string of zeners on the power xformer center tap to lower the B+ by about 50V. It gave it a "browner" sound, more compression, drive, harmonics etc. It was smoother too. I imagine what you are doing is somewhat similar. How much does the B+ drop with the resistor switched in?

Doug

[Joep]

Paul used a 124 ohm resistor, so actually the effect should limited.....It will not drop more that 1 or 2 volts.....

bye,

Joep

[smoguzbenjamin]

Maybe 2 volts makes a hell of a difference. It only takes one drop to make a full bucket overflow right? Take away the drop and the water in the bucket will remain the same. I think you all know what I'm getting at :?

[Paul Marossy]

I haven't measured B+ with the resistor switched in, but I am curious about that now....

This being Class A, perhaps it would need to be quite a bit bigger for a really noticeable effect. Maybe something in the neighborhood of 1K to 4.7K ohm or so? With a 124 ohm resistor, I think it would drop B+ by a couple of volts or so.

In theory, a Class A amp is always conducting, but does it really work that way in the real world?