Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Coldhand75 on March 06, 2023, 07:38:14 pm
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I recently bought a Gibson Atlas amp head (but with no speaker cabinet), mid-1960s vintage, 50 watts. The amp was re-capped with Sprague filter caps when I got it, so no hum and the amp is fully functional. The schematic calls for a 15", 16-ohm speaker so I bought a new Celestion Fullback 15", 16-ohm speaker and built a Gibson-like cabinet for it. The problem is that the amp sounds thin and flat for guitar and bass alike. I was stunned how poor the bass sounds since this amp was originally sold as a bass amp. I tried other speakers and got similar results with only one exception: a G10-30 Celestion 8 ohm speaker that sounded rich and warm. (I have a 4X10 Epiphone amp of the same vintage as the Atlas, both built by Gibson at its Kalamazoo plant. The EPI sounds noticeably better.) Any ideas as to why this fully functional, recapped Atlas has relatively poor tone, and what if anything I can do about it? Thanks.
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You mean this one? https://el34world.com/charts/Schematics/files/Gibson/Gibson_atlas_iv.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_atlas_iv.pdf)
The design is what we could call "unconventional" today in a number of ways. It may well sound like what they thought an electric guitar/bass was supposed to sound like in 1961 or so when they designed it.
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.....anything I can do about it? ..."
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Yes.
Recap is only one of many thing to do on a very old amp.
Amp may need to see a good amp tech.
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Try a 10uf cathode cap across R12 cathode resistor on the 2nd gain stage. And you could consider changing the 100k input resistors to 68k similar to the Fender design.
With respect, Tubenit
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If you do a search here, you will find multiple discussions regarding 60s era Gibson amps. They are "different." The most recent one regarding the 1963 Gibson GA30 Invader relates. Hot Blue Plates has knowledge of these Gibson circuits way beyond mine, and his posts in that thread are interesting. From my perspective, with that tube compliment and transformers, there is no reason a good sounding amp cannot be obtained. But, it may take a bit of work. Do you know a good tech in your area?
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Thanks for the replies. They are helpful and are appreciated. Unfortunately, I am not aware of a 'good amp tech' where I live (Anchorage, Alaska). As far as I know, its just guys like me who like to tinker with tube amps and have modest success. I may try changing some of the caps and resistors as suggested. I don't have much to lose since the tone of this amp is so very disappointing.
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Back to Titanium Valhala: Yes, that is the Atlas IV schematic for my amp. By the way, plenty of volume, the controls work correctly and very little hum. Just poor tone.
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What model Epiphone is that 4x10 you have that sounds good? I'm guessing that it has a similar preamp circuit to the Atlas.
Also "functional" does not mean that there is not something wrong. I would certainly try swapping tubes, and then also recording voltages, posting them here.
I just did a curiosity search and found that many bass players like this amp - so something may be off. If so, best to find that before you start modding things.
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My 4X10 Epiphone amp is a mid-1960s Futura RVT. Two channels, first almost identical to the Atlas and the second with tremolo and reverb. Not as powerful (30 watts I think) because the Futura has two 7591 power tubes vs. the Atlas' two 6L6s. Preamp tubes are the same: 6EU7. I'll start with changing the tubes when I get back to town in a week.
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Yep, I looked at that Epi schematic and see the similarities. If you do not find a problem with the amp, at least you will have similar circuit to give you some ideas.