# All original ....in for it's first scheduled maintenance!



## loudtubeamps (Feb 2, 2012)

A little rough but bone stock .


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## Jim Jones (Sep 18, 2006)

Love it! I can sense the hantavirus from here!


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## loudtubeamps (Feb 2, 2012)

GIBSON GA6 SCHEMATIC DIAGRAM Pdf Download


View and Download Gibson GA6 schematic diagram online. GA6 musical instrument amplifier pdf manual download.




www.manualslib.com


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## loudtubeamps (Feb 2, 2012)

Jim Jones said:


> Love it! I can sense the hantavirus from here!


Not one poop in this guy!


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## greco (Jul 15, 2007)

Very prudent to be on a schedule for routine maintenance.

How does it sound?


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## loudtubeamps (Feb 2, 2012)

Sounds much better now..
Some may like it......me ?
No gas whatsoever.


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## Paul Running (Apr 12, 2020)

I like the way the schematic is drawn. I was wondering what the plate voltages are on your service model versus the Lancer? Here's the voltage chart for the Lancer:


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> I like the way the schematic is drawn. I was wondering what the plate voltages are on your service model versus the Lancer? Here's the voltage chart for the Lancer:
> View attachment 381952


Hey @Paul Running ...how's it going?
B+ At the 5Y3 with 121 vac coming out of the wall is very close to your chart.
Plates on the AY ...right around 71 vdc, same on the 6SL7.
The plate resistors had drifted a tad over the decades.
220k's on the L7 measuring around 500 to 600 k.
The 100k's on the AY ? One was wide open , the other coming in at a very respectable.......1 meg 
The remainder of the carbon's were surprisingly within spec.
Poor old Jensen sounds quite poorly and is really not up to the task any longer.
I foresee a re-cone on the bench soon...me thinks.
Cheers.


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## Paul Running (Apr 12, 2020)

He should be good for another lifetime with that design...they went by the book with that model.


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## Always12AM (Sep 2, 2018)

Do you make any adjustments to accommodate modern wall voltages or do you keep it to spec?


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## DavidP (Mar 7, 2006)

Now that's what I call real point-to-point wiring!!!


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## 2manyGuitars (Jul 6, 2009)

DavidP said:


> Now that's what I call real point-to-point wiring!!!


I think that's actually "what's the point" wiring.


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## loudtubeamps (Feb 2, 2012)

Always12AM said:


> Do you make any adjustments to accommodate modern wall voltages or do you keep it to spec?


As is...... l try to bring it back to factory as best I can...as you know, tube amps are pretty forgiving.


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## epis (Feb 25, 2012)

It looks to me cap job was done some time ago.
Or you did it already.


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## greco (Jul 15, 2007)

loudtubeamps said:


> Poor old Jensen sounds quite poorly and is really not up to the task any longer.


Question from someone with an interest in electronics:

Could a regular 8 ohm speaker, with some electronic changes to the amp circuit, be used to replace the field coil (correct?) speaker? 

Thanks in advance!


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## loudtubeamps (Feb 2, 2012)

epis said:


> It looks to me cap job was done some time ago.
> Or you did it already.


Caps were original...pic shows recap with what I had on hand.


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## loudtubeamps (Feb 2, 2012)

greco said:


> Question from someone with an interest in electronics:
> 
> Could a regular 8 ohm speaker, with some electronic changes to the amp circuit, be used to replace the field coil (correct?) speaker?
> 
> Thanks in advance!


No field coil on this one...What you are seeing is the output transformer.


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## greco (Jul 15, 2007)

loudtubeamps said:


> No field coil on this one...What you are seeing is the output transformer.


Thanks for the response and education. Interesting!


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> He should be good for another lifetime with that design...they went by the book with that model.


Yeah...simple and gets the job done. 
I can't help wondering why they used the values they did on the cathodes of the 12ay.🤔
Certainly no distortion will be present at the phase inverter but with only 70 volts on the plates, I'm not feeling any love for the overall performance.
Maybe you can enlighten me @Paul Running ..
Cheers.


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## nonreverb (Sep 19, 2006)

loudtubeamps said:


> Yeah...simple and gets the job done.
> I can't help wondering why they used the values they did on the cathodes of the 12ay.🤔
> Certainly no distortion will be present at the phase inverter but with only 70 volts on the plates, I'm not feeling any love for the overall performance.
> Maybe you can enlighten me @Paul Running ..
> Cheers.


Fender played mad scientist at this time. The Tweed amps went through many changes from their inception to the end of the Tweed era.....sorta like the revision abuse the Bassman took from the beginning of the BF era to the end of the SF.


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## loudtubeamps (Feb 2, 2012)

nonreverb said:


> Fender played mad scientist at this time. The Tweed amps went through many changes from their inception to the end of the Tweed era.....sorta like the revision abuse the Bassman took from the beginning of the BF era to the end of the SF.


Hey there Richard... @nonreverb . Camping outings winding down?
Re experimentation.....Yeah, trial and error to be sure....but 4.7 meg at the cathode of the AY?
Never would have imagined using a value anywhere close to that.
I say again.."what were they thinking at the Gibson factory "?
Cheers.


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## nonreverb (Sep 19, 2006)

loudtubeamps said:


> Hey there Richard... @nonreverb . Camping outings winding down?
> Re experimentation.....Yeah, trial and error to be sure....but 4.7 meg at the cathode of the AY?
> Never would have imagined using a value anywhere close to that.
> I say again.."what were they thinking at the Gibson factory "?
> Cheers.


True...they were even worse. I have the master schematic book from Gibson from around '65/'66. II've actually had Gibson amps whose schematic is located in the Epiphone part of the book! Gibson was all over the map...


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## jb welder (Sep 14, 2010)

loudtubeamps said:


> I can't help wondering why they used the values they did on the cathodes of the 12ay.🤔





loudtubeamps said:


> 4.7 meg at the cathode of the AY?


Cathodes of 12AY are grounded. 
They are using 'grid leak bias' and the 4M7's are the grid leak resistors. You need a high value grid leak resistor and there must be a cap before it.
You will only see it in real old stuff. Many get converted to the usual cathode bias arrangement. With grid leak bias you get too much variation between tubes of the same type, as the grid current can vary quite a bit from tube to tube.
Here's another example and a circuit description: https://robrobinette.com/SixShooter_Build.htm


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## Paul Running (Apr 12, 2020)

nonreverb said:


> True...they were even worse. I have the master schematic book from Gibson from around '65/'66. II've actually had Gibson amps whose schematic is located in the Epiphone part of the book! Gibson was all over the map...


I agree Rich...and it's a challenge to track down the designers for most of their mid-era amps. many of the early designs are taken from The RCA application notes.


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## loudtubeamps (Feb 2, 2012)

jb welder said:


> Cathodes of 12AY are grounded.
> They are using 'grid leak bias' and the 4M7's are the grid leak resistors. You need a high value grid leak resistor and there must be a cap before it.
> You will only see it in real old stuff. Many get converted to the usual cathode bias arrangement. With grid leak bias you get too much variation between tubes of the same type, as the grid current can vary quite a bit from tube to tube.
> Here's another example and a circuit description: https://robrobinette.com/SixShooter_Build.htm


Oops...my bad. Of course, cathodes grounded directly.
Thanks.. @jb welder
My eyes saw the ground point, my brain wouldn't let me relate.


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