Firefly tweaks?

[Rp3703]

From what I've read, there is always a compromise when voicing a tube amp to sound good in both clean and overdrive. I feel like this is the case with the Firefly amp. While the clean does sound pretty nice, the overdrive sounds a tad heavy or loose in the low frequencies. Since I plan to only use this amp in boost mode, I'm willing to sacrifice the clean for sake of the drive. I’m hoping to get some help trying to achieve this.
First, all of the coupling caps used in the amp are Fender style orange drop caps. Would swapping these for metal film, like Marshall uses, make any difference or is it more a value thing.
Second, the bypass cap on the third gain stage. Marshall doesn't even use caps in these locations. This is again, a Fender thing.
Third, the path the signal takes through the second gain stage is something I've never seen before. It looks like B+ goes through this tube to reach the first gain stage. Any improvements anyone sees to be made there, let me know. I will say, the boost control can only really go to 3/4 before the amp starts sounding terrible, so more gain is not a requirement for this amp.
Last, is there any reason an amp with this type of power amp could not be coupled with any marshall style preamp? This low watt amps always seem to get little to no tone controls. Why is this?

[tubenit]

It would help us answer your questions IF you provided a schematic that is current and accurate to your amp.

I have no idea of the topology or components of your amp?  And I am not familiar at all with Firefly amps, so I suspect others may not be either?

with respect, Tubenit

[labb]

The only "Firefly" amp that I know of was developed by one Doug Hammond over on the old AX84 site. Don't know if any of those folks are around anymore. Know that Chris Hurley who started the site intended to shut it down. Might do a search for AX84 and see if they can help you. That amp was meant to be played at low volume and the lower the better the tone, if I remember right. Been a long time.

[Rp3703]

Sorry, it would help to post a schematic. Duh!

[Rp3703]

I should also state that there is an optional place on the PCB to install a bright cap across VR2. I have tried a few(280P, 470P and 1nf) and 1nf seems to be my favorite.

[SILVERGUN]

Some of your questions, separated for clarity:

1) Since I plan to only use this amp in boost mode, I'm willing to sacrifice the clean for sake of the drive. I’m hoping to get some help trying to achieve this.

2) First, all of the coupling caps used in the amp are Fender style orange drop caps.
Would swapping these for metal film, like Marshall uses, make any difference or is it more a value thing.
3) Second, the bypass cap on the third gain stage. Marshall doesn't even use caps in these locations. This is again, a Fender thing.
4) Third, the path the signal takes through the second gain stage is something I've never seen before. It looks like B+ goes through this tube to reach the first gain stage. -Any improvements anyone sees to be made there, let me know. I will say, the boost control can only really go to 3/4 before the amp starts sounding terrible, so more gain is not a requirement for this amp.
5) Last, is there any reason an amp with this type of power amp could not be coupled with any marshall style preamp? This low watt amps always seem to get little to no tone controls. Why is this?

1) Good, makes things easier when you eliminate variables
2) Not really somewhere I would start. More of an icing on the cake voicing thing. Kinda like trying different tube manufacturers after the amp is voiced.
3) Try not to see things either Marshall or Fender or Etc.....when you're dealing with a custom design like this those elements take a back seat
4) That feature is the cascode gain stage and it's not very common (which is why you haven't seen it). It was design element that was used for this amp and you can use it as is or reconfigure to get you closer to my next response below
5) No, the only thing stopping you from using a different preamp is not knowing how. The low watt amps get little to no tone control because they are usually designed to be minimalistic. Some people think they want an amp with one knob and then when they try it they wind up looking for more control.


Considering everything you have said, MY first experiment would be to bypass C4. You could do this by unsoldering one end or just use a jumper to bypass it. This will reduce overall gain and some bass.
Then answer these questions:
1) What happened when you bypassed C4?
2) Did it get you closer or further from your dream?


Good luck and have fun. Try little things one at a time and report back. Anything is possible.

[PRR]

Firefly is cute. Many people think it is fun. But it is NOT mainstream guitar-amp guts.

It shows that a good player can play anything. But if you are interested in the *amplifier*, why not build a g-amp, with proper bollzy tubes, only smaller.

IMHO if that cascode isn't totally satisfying, take it out. It is a very strange thing.

No real guitar amp FDR has used triode power tubes. A couple AC/DC-class pentodes in different OT will give 4W. A couple 6AU6 in the OT you have will give 1W. Lower G2 voltage for part-Watt.

[Rp3703]

Not sure if it was obvious or not but this amp is a PCB amp, so altering the signal path drastically could be difficult.
My main goal for this amp is to use it for recording. I work in a one room studio and trying to dial in a recording tone and determine a good mic position with a 100 watt amp playing next to you is somewhat of a challenge.
At this point, I cannot confirm that the power amp or cascode circuit is having any negative effect on the amp's tone. The amp sounds good but a little wooly. Since VR2 seems to contribute to this woolliness, it makes sense to me that the clean channel is the cause of this. I'll start with the bypass cap for now.

[SnickSound]

There's also the self split power section. I don't think you can get where you want without drastically changing the architecture

My first build was a Firefly, it was a fun project, but I never really liked playing through it. I tore it apart and built something else with the transformer. Now I have a high gain channel switcher that can sound pretty massive at low volume, only took one extra tube. One triode is used as a cathodyne so no self splitting here. Full tone stack with cathode follower, negative feedback. Power tube has been replaced for an ECC99 which allows more headroom. Power transformer is a 269GX instead of EX too for higher voltage (315VDC, the JX would be even better).

The Firefly is what it is. Any attempt to make it less "wooly" will likely also make it thinner.

[Rp3703]

UPDATE: I removed the bypass cap(C4) and it did reduce a bit of the low end and seems to have lowered the gain to where I can crank the boost volume past 3/4 without negative effects. It's not perfect but better than it was. To me it sounds like the amp has been high passed at around 400HZ. Ideally, I'd prefer to keep some 200HZ and just attenuate 400HZ but that may be above my pay grade so I will probably just leave the amp as is.

[SnickSound]

UPDATE: I removed the bypass cap(C4) and it did reduce a bit of the low end and seems to have lowered the gain to where I can crank the boost volume past 3/4 without negative effects. It's not perfect but better than it was. To me it sounds like the amp has been high passed at around 400HZ. Ideally, I'd prefer to keep some 200HZ and just attenuate 400HZ but that may be above my pay grade so I will probably just leave the amp as is.

You're not wrong. C2 at 2.2nF into a 250k volume control means it high passed at almost 300Hz.

I would try increasing this to as much as 10nF. Given the 250k pot, this is a similar response to 2.2nF into a 1M pot (a common Marshall value for tight distortion).

[Rp3703]

A quick update for anyone who comes across this thread while trying to fix their own Firefly amp, I tried using lower cap values for C1 and C3 but that seemed to reduce the drive and did not clear up the cloudiness I am hearing. I then removed C12 altogether which didn't seem to do anything at all. I ended up putting back C4 because it seems to add to the drive and did not really add too much low end. The amp still lacks the chime or top end that I'm used to hearing from my Marshall amps. It's real shame because the drive this amp puts out when cranked sounds amazing. I tried it with a V30, a Blue Alnico and a Greenback. The greenback was my favorite but all three were still not there in the top end.
It looks like changing this amp from cascode to cascade shouldn't be too hard. I may try that next.