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Interesting amp build

MapleFlame

Well-known member
Joined
Jul 3, 2005
Messages
14,044
One of my best friends is an amp tech named Terry Dobbs. He recently restored an old Gibson GA75 for a customer and was very impressed with certain aspects of its design. He decided to build an amp that incorporated some of the GA75s features and added some Valco elements (their parahpase phase-inverter design) as well. Terry used some old iron, chassis and parts that he had lying around. He called the amp the Smoothie and gave it to a friend of his, but he had told me about it and I found it intriguing. So Terry gave me the schematic and I designed a tagboard-type layout and just put together my take of the Smoothie.

Man, it is a great sounding amp! It's cathode-biased with a 5U4 and pair of 5881s on the back end. The Gibson tone stack has active treble and bass controls; it's got that '50s darkish GA40 kind of voice...but you can really feel/here the Valco phase inverter's personality in the way the amp overdrives. It's got a pentode up front in the pre-amp, giving it a really responsive character as well. It's ultra-sensitive to pick attack and guitar volume changes.

Here are some photos of my build and a quick and dirty clip.

Chassis

Add a few more holes

Filaments and PT added

Pretty much done

Faceplate added

I still haven't build a cab. I'll be building one over the next couple weeks; will update thread. Here's the clip.


watch

I have a 55 and 56 GA amps with a single 12" jensen, they're fantastic amps. I call them Deluxe Killers.
Wonderful looking build. I too look forward to some sound clips
 

PaulD

Active member
Joined
Jun 25, 2007
Messages
671
Nicely done.

Just to clarify, the tone stack in the Gibson GA75 is not active, it is a purely passive tone stack (a passive tone control can only cut frequencies whereas an active one can also boost them). The tone stack in the GA75 is what is known as a James tone stack (sometimes called a passive Baxandall). I think the confusion probably arises because the original design of the Baxandall circuit was used in a feedback loop and was active. The version that is sometimes used in guitar amps (Gibson, Orange, Ampeg have all used it) is not in a feedback loop and cannot boost any frequencies.
 

PaulD

Active member
Joined
Jun 25, 2007
Messages
671
Yes it definitely responds very differently to the more common Fender/Marshall/Vox style tone stack. With the controls set to the mid-point the frequency response is essentially flat (albeit with quite a lot of attenuation). Turning both controls up from the mid-point gives a mid-scooped sound and turning them down gives a mid-peak. If you don't already have it there is a very handy free utility called Duncan Tone Stack Calculator, it enables you to model different types of tone stack and change component values to see what effect it has on the frequency response. http://www.duncanamps.com/tsc/
 

jrgtr42

Well-known member
Joined
Mar 24, 2005
Messages
2,307
Very nice.
It's amazing how a fairly simple circuit, with relatively few components, can create such a sound.
 

straightblues

Member
Joined
Mar 17, 2004
Messages
954
Love love love that first amp with the Strat. If it were me, I would have a second input adjusted to give the Les Paul a little more bite. You have obviously made those adjustments in your second build, because it kills with the Les Paul. I really like what you do.

Any thoughts of offering these builds to a few of us?
 

Cogswell

The Duke of Dumbassery
Joined
Mar 19, 2002
Messages
15,717
Nice and & pretty good picking, too. I like your style
 

Minibucker

Well-known member
Joined
Jan 12, 2003
Messages
6,372
Nice...kind of reminds me of my 5C5 build with octal preamp tubes (early 50's Tweed Pro). Could only get maybe 20 watts out of it, but just a glorious warm/juicy drive to it pretty much right out of the gate.
 

Thundermtn

Member
Joined
Dec 30, 2016
Messages
548
Sounds great Leon!

I like how open it sounds with that level of gain. Big, poweful, rock guitar.
 

K_L

Well-known member
Joined
Sep 11, 2014
Messages
566
Nice tones & love the feedback from the amp!
 
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