Solid State Guitar Amp Forum | DIY Guitar Amplifiers

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.

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.

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