Epiphone Galaxie 10 amp mods

[Steve A.]

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):

http://www.blueguitar.org/new/schem/...Galaxie_10.jpg

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"!

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)

Thanks!

Steve Ahola

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).

[Amp Kat]

Sup Steve, This is very similar to what I was going thru with the BF/SF Vibrochamp but I was going the other way worried about idle power frying stuff. Looking at your schemo your line voltage is showing 115 just like the V-champ and I can't see the two back numbers but looks like around 450vdc on the plates so that puts you with 122 to 126vac wall voltage at around 500+ and the heaters at @7.5. Being SS rectoed you don't have that sag or voltage drop but one thing that's cool is you don't have to worry about the plate voltage as much with that 5881 (Sovtek) so IMO you can idle much higher than I like in the Vibrochamp. What is your voltage across the Cathode resistor right now or idle power ? I agree with you that removing the feedback 10k would help and you'll find a point on that Cathode resistor where when you go any lower will start degrading the tone but with that 5881 I'm not sure you could damage it even as a mu amp so I'd go as low as you can to get somemore uhmp until the tone starts muffling on you. I would think with 500 volts your probably getting a stout number across that cathode resistor cranking up the milliamps to around 100 ? That must also be a pretty stout OT for SE. 10 watts ? hmmm

[Steve A.]

Update: I just replaced the 10" Celestion speaker with a big honker that just barely clears the chassis box. Lots of OOMPH! now but I need to turn the treble control up pretty high so I might try putting in a smaller treble cap (stock was 470pF ceramic, I put in a 390pf mica cap- I was thinking of going down to 330pF or 270pF- maybe even go Vox with a 50pF cap.

I am still unclear on how the NFB loop works connected to the cathode of V1b (similar to a Princeton Reverb). The hot lead going to the speaker is connected to a 10k resistor that connects to the cathode, which has a 1.5k Rk resistor and a 10uF Ck cap. If you were to connect this to a pot, would you use something like 25k or 50k? I've only done that with the NFB connected to a LTP phase inverter so I have no feeling for the numbers here.

I was also thinking about putting in a relay to switch in and out my "Texas Tea" control spelled out in my earlier post here- I was just wondering if it might be easier to do some variation of FET switching that taps right off the Hi-VAC (since it is just a simple SPST switching function that I want to implement).

Thanks!

Steve Ahola

[MWJB]

Yes, you could use a 25K to 50K pot for variable NFB. The preamp cathode resistor sets the load for the NFB voltage divider, whilst the resistor to the OT secondary is the dropper. The limiting factors on how big you can go are the point at which the tone gets brittle, or a point where you may start getting some instability.

The old Tube 10's used to run the 5881 at 70mA witha 470ohm cathode resistor, if your voltages are on par with that I wouldn't go lower on the cathode resistor. How about switching to fixed bias? It would mean having to add a coupling cap & load resistor upstream of the 1.5K grid stopper if you have room, it would help put some front back on the notes?

[cbarrow7625]

Steve, I agree that the NFB is a little unorthodox. It would appear to me that the entire AC NFB is shunted to ground throught the 10UF cap. Hmmm...maybe that was the intention? Is it actually wired up like that in your amp or is there another "secret" resistor in the actual layout that lifts the cap somewhat out of the NFB loop? I would guess that removing the 10k NFB resistor would make almost no difference except in the very low end of the amp where the 10uF cap is no longer shunting the NFB to ground.

To give the amp more "growl" (is that the same as oomph?) you could simply lower the value of that 10uF cap (maybe half or 1/4 the original value). That would give you less low frequency output but what is there would have better damping due to the increased effect of the NFB on the lower frequencies. The upper frequencies would be basically unchanged. I think the biggest affect on tone you could have would be to put the 10uF cap (or wahtever value you change it to) on a 25-50k pot like a presence control. It would have the same effect except it would cover the entire frequency response of the amp. True, it would lower the amp gain but it would have a big effect on tone. How about using a dual gang pot to isolate the cap at the same time you increase the 10K NFB resistor? That should give you a pretty interesting result.

Why is that .1uF cap there before the tone controls? I would pull that out just on principle.

BTW, what is a "texas tea" control?? I have absolutely no idea what you were talking about there. Thanks!

Chris

[Steve A.]

Steve, I agree that the NFB is a little unorthodox. It would appear to me that the entire AC NFB is shunted to ground throught the 10UF cap. Hmmm...maybe that was the intention? Is it actually wired up like that in your amp or is there another "secret" resistor in the actual layout that lifts the cap somewhat out of the NFB loop? I would guess that removing the 10k NFB resistor would make almost no difference except in the very low end of the amp where the 10uF cap is no longer shunting the NFB to ground.

To give the amp more "growl" (is that the same as oomph?) you could simply lower the value of that 10uF cap (maybe half or 1/4 the original value). That would give you less low frequency output but what is there would have better damping due to the increased effect of the NFB on the lower frequencies. The upper frequencies would be basically unchanged. I think the biggest affect on tone you could have would be to put the 10uF cap (or wahtever value you change it to) on a 25-50k pot like a presence control. It would have the same effect except it would cover the entire frequency response of the amp. True, it would lower the amp gain but it would have a big effect on tone. How about using a dual gang pot to isolate the cap at the same time you increase the 10K NFB resistor? That should give you a pretty interesting result.

Why is that .1uF cap there before the tone controls? I would pull that out just on principle.

BTW, what is a "texas tea" control?? I have absolutely no idea what you were talking about there. Thanks!

Chris

That control is a 2M pot wired to something like a 0.01uF cap which bypasses the whole tone stack.

I've been playing this amp a lot lately and it seems to get a muddy tone when the initial volume control is set any higher than 5- I've tried swapping out different tubes and speakers which doesn't really help. I'm thinking it might be a lousy output transformer- so what OT would be good for a single ended 6L6/5881? Like a Hammond number that I could cross-reference...

But I ought to try disconnecting the NFB and one or both of the Ck caps (maybe replace them with something smaller) before doing an OT transplant...

Thanks

Steve Ahola