Fixing up a Gibson Atlas

[Jonny toetags]

Picked up a real clean '64 Gibson Atlas this weekend.
So far I've done the following:
-replaced the resistors (circled green) with 510ohm cermets
-someone had removed one leg of the original 0.047 caps (red x) and installed 0.1's (circled red)
i removed all that and put new 0.047 Mallorys.
-new tubes



I was just wondering what that random cap is in this second pic?
It's just floating there and I can't see where it was connected before.



Also...are they any known mods for this thing? While it does sound great on it's own clean and cranked, it doesn't take fuzz pedals all that well. Something int the tonestack just sounds 'off'

[MarkusBass]

I was just wondering what that random cap is in this second pic?
It's just floating there and I can't see where it was connected before.
[ATTACH=CONFIG]32814[/ATTACH]

It would help if you had told what is the tube. Is it an input tube -what is the type? I see it has heaters on pins #1 and #2. So the capacitor is connected to the grid. Now it depends on what is the value of the cap. If this is in a range of nanofarads, this would be the input capacitor. If it is in range of picofarads, this is most probably feedback loop and it should be connected to anode of the tube (most probably pin #7). What is the value of the cap?

Mark

[Jonny toetags]

Sorry...it's the first preamp tube next to the input, 6EU7.
I forgot to mark down the value...I know it's 50...but nano or pico
I will get that info when i get home.

[Chuck H]

That floating cap looks to be on grid pin 8 and shouldn't be there according to the schematic.

[MarkusBass]

That floating cap looks to be on grid pin 8 and shouldn't be there according to the schematic.

If this is Gibson Atlas, it's the first tube in the amp, it is 6EU7, then on the schematic there is 50pF capacitor (C2) between the grid of the tube and the anode. This is negative feedback loop.
Here is the schematic: gibson_atlas_medalist_schematic.pdf

Mark

[Chuck H]

If this is Gibson Atlas, it's the first tube in the amp, it is 6EU7, then on the schematic there is 50pF capacitor (C2) between the grid of the tube and the anode. This is negative feedback loop.
Here is the schematic: [ATTACH]32818[/ATTACH]

Mark

I stand (sit?) corrected. But I didn't think that would be a 50pf cap as it looks too large and burly. Could be though. I would have clipped it out too. 50pf for a circuit like that is HUGE and would turn anything to mud. I've used this sort of circuit a lot and IME anything over 10pf is digging much too deep into the useful frequency spectrum for an early gain stage and squelches harmonics badly.

JT (can I call you JT?), you could test it. If it's 50ish pf you could try reconnecting it just to see how it works out. I would only do it if the amp was unstable and even then I would use a smaller value. As small as I could get away with.

[Jonny toetags]

Thanks guys....will check it tonight and get back to you.
So it goes to pin 7 right?

[Chuck H]

Thanks guys....will check it tonight and get back to you.
So it goes to pin 7 right?

If it measures 50pf then that loose end would go to pin 7 in the stock circuit, correct. There is likely a loose piece of lead floating in the pin 7 solder lug. So watch for that. I'd suck out all the solder, reinsert the cap lead and then solder it fresh (If I were to reconnect it, which I probably wouldn't ).

[Jonny toetags]

OK, so it's a 50pf. (clear marking 50MMF)
I reconnected it for now. After powering it back up, it definitively sounds better with the 0.047's on the power tubes than the 0.1's that were there.
I'm debating removing them all-together since I've read a few posts about that.
There are also two .047 coupling caps that some people have switched to .068 and said it sounds more 'jtm-like'.
I'm looking to get a bit more mids on this thing.

[g1]

Which caps were mentioned about changing to .068 ?

[52 Bill]

Which caps were mentioned about changing to .068 ?

g- I think that he's talking about the coupling caps being changed to 0.068uF and the 0.047uF cap are connected from the output plates to ground.

This thing was a bass amp and they probably didn't care about high end in the design.

[g1]

Thanks Bill, I removed the part of my post regarding the .047 plate caps (which I had confused for coupling caps).

[Chuck H]

Changing the coupling caps from .047 to .068 shouldn't have any notable effect on useful electric guitar frequencies and certainly wouldn't improve mids.

The tone controls on that design are overcomplicated and both the bass and treble circuits create a mid scoop effect. rewiring them to something more tweed or brown Fender-ish would definitely get you closer to what you say you want.

On another note... Some guys might like that amp for it's vintage/funky factor. If you don't like it and just want an amp to use it might be more efficient to sell it and buy something more suitable to start with.