Author Topic: Gibson atlas IV bass amp  (Read 2011 times)

EvilLordJuju

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Gibson atlas IV bass amp
« on: July 11, 2011, 11:05:50 AM »
I have an old Gibson Atlas IV amp - the cab is in great condition, but the head is knackered.

So I am looking to import a new head to the UK. They just don't come up here, and when they do, they are invariably modded a lot. I mint one would be great, but a non working project may also work if I can combine it with my existing head.

So i'm looking for someone to help me import one - naturally i'll pay all costs and an extra few dollars for your time....

Can any of my friends here act as a middleman for me if one comes up? Anyone know of one for sale?

Thanks all

Psycho Bass Guy

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Re: Gibson atlas IV bass amp
« Reply #1 on: July 11, 2011, 06:38:11 PM »
I'll start keeping an eye out for you.

Dave W

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Re: Gibson atlas IV bass amp
« Reply #2 on: July 11, 2011, 09:37:57 PM »
Same here.

godofthunder

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Re: Gibson atlas IV bass amp
« Reply #3 on: July 12, 2011, 04:21:39 AM »
How about a picture ? I'll check the House of Guitars for you they have tons of vintage gear, they usually want top dollar though.
Maker of the Badbird Bridge, "intonation without modification" for your vintage Gibson Thunderbird

Highlander

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Re: Gibson atlas IV bass amp
« Reply #4 on: July 13, 2011, 12:55:16 PM »
Staring at the event horizon is a dirty job, but someone has to do it...
Time is relative; lunch time, doubly so...
How many grasp that Thunder is the sound of the Lightning bolt, but in reverse, to the point of creation...?

EvilLordJuju

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Re: Gibson atlas IV bass amp
« Reply #5 on: July 15, 2011, 04:17:28 AM »
Thanks guys, really appreciate the help. I'm not especially looking for a bargain - rather a clean unmodded example. My head sounds great, but it has a lot of mods (a lot) - I suspect to make it more of a lead amp. I'm also finding out that it's a pretty dangerous amp - what with no back cover and a lot of big voltages inside

Jules

Psycho Bass Guy

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Re: Gibson atlas IV bass amp
« Reply #6 on: July 15, 2011, 06:06:19 AM »
Covers are for sissies! You only need to worry about proximity if your amp has its power tube plate connections on top of the output tubes. The Atlas IV has a duet of 6L6's running 283 volts on the plates and 186 volts on the screens. That's barely above most preamp tubes' plate lines (no grids in most of those). A typical Fender Bassman usually doubles both of those figures.  An SVT puts over 700 on its output tubes' plates and around 480 on the screen grids. Proximity arcing isn't a problem until you pass the 1kv mark, and I can show you a nice decomissioned Harris vhf transmitter that puts a few thousand volts at a few thousand amps across its HT lines. The output section is in two cabinets that look kinda like a pair of Acoustic 360 speaker cabs, and the tubes look like lawnmower engines.

Highlander

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Re: Gibson atlas IV bass amp
« Reply #7 on: July 16, 2011, 01:42:40 AM »
I used to run my Hiwatt without the front or the back on - I've misslaid the back and need to make a new one; mind you, I also still need to re-cap the beastie too... :sad:
Staring at the event horizon is a dirty job, but someone has to do it...
Time is relative; lunch time, doubly so...
How many grasp that Thunder is the sound of the Lightning bolt, but in reverse, to the point of creation...?

EvilLordJuju

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Re: Gibson atlas IV bass amp
« Reply #8 on: July 16, 2011, 04:42:05 AM »
Covers are for sissies!

thanks guys! I have two young children that like putting hands in holes, so I am forced to be over cautious really. Unfortunately, my amps are out on display in a family area, so I can't ensure they will never touvh them. I've been told that even once turned off these things still have charges floating around them.

My usual amp tech took one look at it, and said he wasn't going anywhere near it. (He doesn't cut corners - and I suspect there are legal saftey requirements that he insists on adhering to).

That being said, i've got another good tech looking at it now who is a little more optimistic about what can be done.

It is looking that we can reverse most of the mods, if we can find some period correct transformers. I'm still after another completely stock one though if possible.

This amp sounded superb when I got - with mismatched tubes aetc. I can't wait to hear it after a good servicing.

Psycho Bass Guy

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Re: Gibson atlas IV bass amp
« Reply #9 on: July 16, 2011, 05:11:19 AM »
It's a simple enough fix to install bleeder resistors on the high voltage bus line and it won't effect the sound of the amp one bit. It's actually good for the caps, as full duty-cycle discharge will keep them in good shape longer. The amp will be completely touch-safe, inside and out, in less than 10 seconds after use.

 I can't understand why anyone would shy away from even an extremely modded Atlas. It's a pretty simple circuit.  There are only two transformers in it, and if either has been changed out, the odds are that your amp probably sounds nothing like a stock Atlas IV, which has a pretty lackluster rep as sounding weak and thin, but probably mates well with a mudbucker output.

EvilLordJuju

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Re: Gibson atlas IV bass amp
« Reply #10 on: July 16, 2011, 07:23:23 AM »
There are only two transformers in it, and if either has been changed out, the odds are that your amp probably sounds nothing like a stock Atlas IV, which has a pretty lackluster rep as sounding weak and thin, but probably mates well with a mudbucker output.

Yeah, transformers changed out unfortunately - and yes, that's exactly what I want to explore - I haven't played it exhaustively yet through many basses, but I did test it out with an Epiphone Newport - and briefly with a 60s tbird - with some tone knob tweaking got some great sounds. I am after stock sounds, whether they are good or bad  :-\

my tech said the following

Quote
The circuitry up to the power output stage is pretty much untouched.
This amp has an unusual tone circuit using a triple gang bass control pot, and all of that is original and functioning. So, whatever signature characteristics this amp had in that area should be preserved.

So I don't know whether the great sounds I got from it will still be there after the work has been done, but i'm pretty hopeful.

I wonder how easy it will be to get a new transformer (or a used one) that will get the sound even more stock

Psycho Bass Guy

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Re: Gibson atlas IV bass amp
« Reply #11 on: July 18, 2011, 08:26:34 PM »
Yeah, transformers changed out unfortunately - and yes, that's exactly what I want to explore - I haven't played it exhaustively yet through many basses, but I did test it out with an Epiphone Newport - and briefly with a 60s tbird - with some tone knob tweaking got some great sounds. I am after stock sounds, whether they are good or bad  :-\

My feeling with amps is that even though I could easliy be considered a 'collector,' EVERY one of my stable is able to earn its keep and if it doesn't sound good, it doesn't stay. Honestly, since you're already looking for another Atlas, I'd leave yours as-is. I can tell you by looking at the schematic that a stock Atlas IV will perform equivalent to its sales, VERY lackluster. The circuit is far too conservative in many areas, and needlessly so for most of them. Basically what you have now IS a true representation of the amp's tone, just with punch.

 I can tell you right now it's going to be easier to find a whole different amp than to locate functional to-spec trannies, for the simple fact that they are the amp's biggest weakness. I'll venture that MOST surviving examples of the amp have probably had their trannies changed just to make then useful. I can appreciate a desire to have a stock Gibson from a collector's standpoint, but IMO, don't take a step backwards with the one you have now. Vintage Gibson bass amps have well earned their bad reputation. I have an old Saturn 2x12 guitar combo that's a better bass amp than the Atlas IV.

Pilgrim

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Re: Gibson atlas IV bass amp
« Reply #12 on: July 20, 2011, 11:54:41 AM »
Am I correct in thinking that Atlas is comparable to the old 50W blackface Bassman amps?  I see in some online specs that it had two 6L6 tubes, so I suspect it's about the same in output. 
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Psycho Bass Guy

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Re: Gibson atlas IV bass amp
« Reply #13 on: July 21, 2011, 12:47:55 AM »
Am I correct in thinking that Atlas is comparable to the old 50W blackface Bassman amps?  I see in some online specs that it had two 6L6 tubes, so I suspect it's about the same in output.

You would think that from its tube complement, but because it runs at such low operating voltages, you'd be lucky to get 25 watts out of one cleanly. This is the amp that can't keep up with a B-15!
« Last Edit: July 21, 2011, 02:13:21 AM by Psycho Bass Guy »

rahock

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Re: Gibson atlas IV bass amp
« Reply #14 on: July 23, 2011, 07:12:55 AM »
I've got an old Atlas combo that I brought back to life a few years ago that I think is now in need of a cap job. The head actually sounds great when I run it through a well designed single 15 Seymour Duncan cab with an Eminence speaker. The speaker is capable of handling about ten times the power that the amp puts out and I think that this may be the secret. When it runs  on the CTS 15, that is a proper match, and the Gibson cab , it is very weak and doesn't deliver a whole lot of bottom. It ain't no B-15 and it ain't no Bassman >:(. Attach a good cab and it sounds like a junior version of an old Sunn 200S. Very strange ???.
Rick