Epiphone Galaxie 10 amp mods
I picked up one of these a few years ago- I can't get enough of these small powered tube amps! LOL
Here is the schematic I found (with a few errors):
I decided to fix it up a bit, starting out with replacing the coupling and tone caps with polyester Orange Drops (.022uF), the 470pF ceramic treble cap with a 390pF mica cap, also upped the series resistor ahead of the Gain control from 100k to 1M (I may try undoing that last item- more on that later).
So far so good- nothing very radical because there isn't a lot of panel real estate to add switches or pots. I wanted to add a Kendrick Climax "Texas Tea" style control to bypass the tone stack and it finally occurred to me to add the 2M pot right next to the preamp tube where you can get to the control by reaching up the opening at the bottom of the amp- not exactly very easy to get to, but a great location for a stealth control! LOL
It worked so well that I decided to move it up to the main panel, replacing the Standby Switch with a progressive off-standby-on switch I had gotten from Dave West, freeing up the hole for the main power switch for my "Tweed" (AKA "Texas Tea") control- which is actually labeled "Power"! :eek:
Before I did anything I had noticed that the amp would get very unstable if I set the Gain control up to 10, which was why I tried replacing the 100k series resistor ahead of the Gain control with a 1M resistor. It didn't solve the problem so I tried a few other tricks with no success. It finally occurred to me to check lead dress and bingo! when you moved the shielded cable from the input jack to the left it would howl. The input had a 68k grid stopper on the circuit board so I split it up with a 22k resistor on the board and a 22k resistor on the tube socket. Plus added a 50pF ceramic cap from the plate to the cathode of the 2nd stage. So it is much more stable now and I may try replacing the 1M series resistor with the original 100k resistor.
Okay and now for my questions! This amp uses a single ended 5881 as the power tube, with a 470 ohm cathode resistor. I checked the schematics for the old Fender amps and they would typically use a 250 ohm cathode resistor for a pair of 5881 or 6L6GC tubes, so 470 ohms is right in line.
However I would like to get a little more "oomph!" out of this amp. Could I replace the 470 ohm cathode resistor with something smaller? What about the NFB loop which goes through a 10k resistor to the cathode of the second stage? I notice that the Champs with a 6V6 power tube use a 22k resistor. Can I eliminate the NFB altogether? I noticed that certain amps of a similar design do not use NFB. (I would imagine that you do get a tighter bass response with the NFB loop.)
The amp hung in there at a low volume blues set last night, but I ended up hooking up my Fulldrive 2 Mosfet stomp box to get a bit more volume for my solos (that is a great pedal- I certainly have to hand it to Mike Fuller for producing the "iPods" of the MI world! LOL)
P.S. Errors on the schematic- the Gain control is 1M audio and the Volume control is 1M linear. There was an extra 0.1uF/630v poly cap not on the schematic (labeled C13).