Thanks again.

R35 disconnected.

R320
With speaker 1.2 millivolts
Without speaker 0.0 millivolts

R321
With speaker 0.0 millivolts
Without speaker 0.0 millivolts

Try removing or disconnecting one end of R35 (10K), then repeat those last measurements.

Thank you for getting back to me.
R320
With speaker
1.5 millivolts
Without speaker
0.0 millivolts

R321
With speaker
0.0 millivolts
Without speaker
0.0 millivolts

Measure voltage across R320, with and without speaker.  It will be millivolts DC, so use whatever range is most accurate.
Then do same for R321.

I did some voltage checks today.

20 millivolts at speaker output with or without speaker connected.


Q315 voltages referenced to ground

With speaker connected
Base -0.915 volts
Collector -30.4 volts
Emitter -0.377 volts

With speaker disconnected
Base -0.974 volts
Collector -30.4 volts
Emitter -0.435

Q310 voltages referenced to ground
With speaker connected
Base +1.07 volts
Collector +30.4 volts
Emitter +0.535 volts

With speaker disconnected
Base +1.00 volts
Collector +30.4 volts
Emitter +0.467 volts

I becomes more balanced when the speaker is disconnected.
Would that indicate the problem is the output transistors are out of balance even though I did change them?

I would appreciate any help.
Thanks.

Thank you for replying. I will do some voltage readings tomorrow.

Quote from: DrLev on May 26, 2026, 05:38:14 AMAlso I can balance the outputs and get rid of the hum if I change R323 from 330R to 470R. I was wondering why I would have to do that and if I leave that change would it be stressing any of the components.

You tell us where you came up with the idea to change that resistor. 
Likely what is happening is that you are compensating for a damaged or out-of-spec component, which is not how you want to do the repair.
2 things happen when you adjust R323, you alter the balance of the differential pair (Q312,Q313), and you change the bias of Q39, which affects the conduction of the output devices.

You did not mention whether there was any DC on the output, or the voltages of the drivers (Q310,Q315).

Differential circuits can be very picky about out of spec parts.  In some modern circuits, replacing the differential pair with new parts from the same batch will cure issues like this.
But it may also be a fault somewhere else in the circuit. 
In any case, modifying component values is not a good fix, if it was a poor design, all amps of this model would have complaints about the issue.

Anyone here ever worked on an Epiphone Firefly 30 DSP ?
I have one in for repair. The problem is hum.
It's caused by an imbalance of the idle current at the output transistors.
Q34 base to emitter -0.38 volts
Q36 base to emitter +0.52 volts
I have checked every component and replaced all the transistors to no avail.
I noted the schematic shows the voltage rails as +42V and -42V whereas the rails on this read +30V and -30V. The transformer is 22V-0-22V.
I see a schematic for an Epiphone Triggerman 100 DSP on this forum.
It is the exact same schematic as the Firefly 30 DSP.
I can only assume Gibson used the same board and used different transformers for different output power.
I powered up this board with a variable transformer and brought the rails up to +42V and -42V. The outputs became more balanced and the hum disappeared.
Also I can balance the outputs and get rid of the hum if I change R323 from 330R to 470R. I was wondering why I would have to do that and if I leave that change would it be stressing any of the components.
I would appreciate any thoughts or ideas on this.
I don't know how to upload the schematic here but it is exactly the same as the Triggerman 100 DSP here on forum except the name.

An opinion is just a snapshot in time. Dependent of what one has encountered, his opinion is formed.
Different encounters may motivate somebody to revise his/her opinion.
An opinion is always right for a given moment and given person, but being just a facette of a much bigger total, it does not claim universal validity. If somebody does so, they are on a questionable path.

Quote from: aquataur on May 21, 2026, 03:35:43 PM

Quote from: Kaz Kylheku on May 21, 2026, 12:23:02 AM...car audio and PA type speakers...
I'd love to experience what these types of speakers are like for guitar, for sure.

Guitar speakers are usually stiffly suspended, made for open cabinets. Those speakers will be totally unsuitable.
There is nothing new under the sun.

Not exactly.
It's a gamble as any speaker change is with any audio system of any kind. I have a 4x10 cabinet with a couple car subwoofers and some Radioshack woofers that have enough high frequency response and dB output that they sound great for guitar, also car speakers.

I must have 20 different amps, 50 to 60 speakers, a dozen stand alone cabinets.

Cone materials have damping factors which is what plays the primary role in dB and rated sensitivity at 1 watt from 1 meter with a 1 Khz sine wave.

Pro audio speakers are rigid but still move and can move a lot of air, guitar woofers included. I have some guitar woofers with zero movement and you can't put the bass beyond 4 or 5, but they still thump and are in a sealed cabinet. I have quite a few sealed cabinets, open back cabinets and cabinets that open and close.

I have woofers with *s!!t* for upper midrange let alone treble, I have woofers that are so tinselly and grindy they're useless for guitar even though they're guitar wooders.

I have polymer cone PA woofers that are some of the best sounding guitar speakers I own. They smooth over the high end and get rid of the excessive solid-state high, even order harmonics that gave solid-state the "harsh" reputation.

When I improvise a speaker from whatever intended purpose, I look at the specs to see if it's at least possible to use it for what I went, then just as obviously, start plugging it into different devices to see what comes out of it. To datge, I think polymer cones even for guitar sound superior to paper, but no one mainstream is producing polymer cone PA/musical instrument speakers that I'm aware of.

I also just got some aluminum cone 10's, also guitar speakers with of course, the fabric accordion suspension. They'll pose to be interesting.
The guitar 12's I had that were the first plastic guitar woofers I used were sold by Dayton Audio out of Ohio as "aluminum", and instead were polymer with aluminum powder in the plastic; I was not happy at all.

As it turned out, they were a blessing in disguise and because they have fabric accordion suspension, I painted them flat black and they're in my custom built 4' x 4' x 18" 4x12 death stack.

Nothing is so definitive, it's why I'm an electronics engineer, not just a symphonic shred guitarist. If I thought the way you do, I never would have wandered my way into college for a science degree by my early 30's