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 Post subject: PA Amplifier Info request
PostPosted: Jul Thu 11, 2019 8:31 pm 
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Joined: Jan Mon 05, 2009 4:09 am
Posts: 1395
Location: Hudson, MA
I bought a Power Amp at the local flea market but I cannot find much info on it. I would like to find a schematic.
Here is what I know:
Marked as Marco but I don’t know if this is the distributor or manufacturer.
Model number GPA-45
Tube lineup 7581 (Qty2), 6AV6 (Qty1), 6L6 (Qty1), 12AX7 (Qty 3), GZ34 (Qty 1)

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Any help appreciated

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Eddy


Last edited by Eddy on Jul Thu 11, 2019 11:34 pm, edited 1 time in total.

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 Post subject: Re: Power Amplifier Info request
PostPosted: Jul Thu 11, 2019 10:00 pm 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 3427
Location: Tucson, Arizona U.S.A.
First of all, it isn't a basic power amplifier. It's a PA amplifier. And a rather unusual one since it doesn't have a master volume control like just about every other one does.

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Jim Mueller


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 Post subject: Re: Power Amplifier Info request
PostPosted: Jul Thu 11, 2019 10:43 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
Posts: 319
Location: Lawrenceville, Illinois 62439
Agree - that's a Public Address amplifier. Unique that it uses 1/4" instead of Amphenol Hi Z mic inputs.
I had a Bell which did not have a master volume.
You might look at Bell, Dukane, Bogen, etc. examples on the web and see if one of theirs looks anything like yours. Ebay sellers may not call them PA, they may use "tube, guitar, harp" wording.
Dan


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 Post subject: Re: Power Amplifier Info request
PostPosted: Jul Thu 11, 2019 10:52 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26395
Location: Annapolis, MD
Any chance it is these people?:
https://marcoelectronics-corpuschristi- ... irect.com/

If not, there is at least SOME chance that they would know something....

5 cookies says you may have to make your own schematic. With two 7581s (presumably P-P), I'm really curious what the 6L6 is for....

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: PA Amplifier Info request
PostPosted: Jul Fri 12, 2019 4:00 pm 
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Joined: Apr Mon 24, 2006 1:05 am
Posts: 2363
Location: Ellington CT
Interesting amp. 6L6 as a driver for the 7581's? Or perhaps some kind of voltage regulator circuit for the output screens.

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 Post subject: Re: PA Amplifier Info request
PostPosted: Jul Sat 13, 2019 2:29 am 
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Joined: Dec Sat 24, 2011 9:17 pm
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Location: Vancouver Island, Canada
stromberg6 wrote:
Interesting amp. 6L6 as a driver for the 7581's?
Yes, I have seen that design in a schematic but can't remember details.

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 Post subject: Re: PA Amplifier Info request
PostPosted: Jul Mon 15, 2019 5:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Freehold NJ 07728
It probably would make a nice guitar amp....

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 Post subject: Re: PA Amplifier Info request
PostPosted: Jul Wed 17, 2019 7:13 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9083
Location: Beaver Falls, PA. USA
Those white porcelain capacitors are suspect; many of them are paper inside.

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