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 Post subject: Need help with VERY old amplifier
PostPosted: Oct Thu 15, 2020 3:07 am 
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Joined: Oct Thu 15, 2020 2:53 am
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I scored this today. It is at least as old as 1937 but most likely older. There was a piece of tape on one of the tubes marked 11/27/37, which tells me it was a tube that replaced a bad one on that date. So the date of the amp should be older than that. Otherwise I cannot find a single thing about it anywhere. I did clean it up. It was covered in dust when I found it. One of the tubes was cracked but I found a replacement and ordered it. Then the power cord needs to be replaced. After I take care of those things I will ramp up the voltage with a variac and see if the amp works.

I need some help identifying this amp. I have never seen one like it and have not been able to find anything. Thank you for reading.


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Thu 15, 2020 4:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 424
Location: Olympia, Washington
Looks like maybe an amplifier made for schools?


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Thu 15, 2020 5:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Seattle WA US
Please, tell us the tube types !! and where is the patent license plate ?

Best guess for date: 1936. Circuit probably similar to "Microphone and Phonograph Amplifier" in RCA RC-14 tube manual.
-Chuck K7MCG


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Thu 15, 2020 2:11 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
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Location: Smithfield, Pa. 15478
Looks somewhat similar to one I have here that came out of a roller skating rink. Mine has no manufacturer name on it; likely was custom made for the speaker company. Are the filter caps located in the box seen behind the tubes? Mine is intended for a field coil type speaker, originally had Rola Speakers installed. (These were removed for another project several years ago.)


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Thu 15, 2020 7:10 pm 
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Joined: Feb Wed 17, 2016 6:51 pm
Posts: 141
What are the output tubes? Public address was a new and big market in the 1930s, and many small companies were making amplifiers. You can tell it is most likely not late 30s because by then most companies had adopted the 6L6 and 6V6. Think about how many small companies assembled computers in the 1990s for a comparison. The 1930s were an exciting time for tube development, with the preferred tube types changing almost yearly. First the 71, then the 45, 2A3, 6B4, 6B5, 42, 47, 6F6 and finally the 6L6 which has more or less stood the test of time ever since.

Also, this one is old enough I would not power it up without some strategic capacitor changes, it is just too risky.


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Fri 16, 2020 10:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beaver Falls, PA. USA
Yes, give us the tube lineup, and a picture of the underside of the chassis.

The early theatre/PA/jukebox amplifiers are easy to get going, with new electrolytic and coupling capacitors. Watch out for open driver transformer windings.

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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Sat 17, 2020 11:09 pm 
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The tubes are as follows: 79, 83-V, 6A6, 6C6, and (2) 6B5. The 83-V is physically broken inside, it looks like a small piece of glass is in it. I bought another one on eBay to replace it. The broken 83-V has a sticker on it that says 11/27/37.

I will get a picture of the bottom. I looked at it today, it looks like everything is original. Old Motorola caps etc. There are three more transformers underneath. Overall it is clean.

I was originally going to make a guitar amp out of it but this one is out of my league. People have been throwing numbers at me so I decided to put it on eBay. I am not asking you guys for values. Whatever it fetches is what it is worth.

Thanks for reading.


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Tue 20, 2020 5:57 pm 
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Here you go.


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Tue 20, 2020 7:50 pm 
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Joined: Jan Sat 12, 2008 12:25 am
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Location: Kingwood, Texas
http://uv201.com/Audio_Pages/pa_amplifier.htm


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Tue 20, 2020 8:24 pm 
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Location: Beaver Falls, PA. USA
With that tube lineup, and that style of resistor, it's definitely from 1935-38. If it was much newer, it would have octal tubes. Power output would be about 12-15 watts.

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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Tue 20, 2020 10:21 pm 
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With all due respect, the capacitors with the yellow stars in the picture below should all be replaced prior to application of power. Failure to do so could risk damage to other parts that may not be readily replaced. The cost of the replacement parts is probably less than $50. This includes the two chassis mounted capacitors that are under the top shield.
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The tricky parts here is that you will have to get all values from the existing parts because there is no schematic or other documentation. The one that might be the hardest is the one buried in the upper left corner of the picture. The chassis mounted capacitor cans will be hard to determine until the top shield is removed.

Best of luck.

Curtis Eickerman

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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Tue 20, 2020 11:34 pm 
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The 6B5 is a bit of a curiosity which was all the rage for a short while in the mid 1930s. You can look up typical applications in old magazines like radio news or radio craft on american radio history, it will get you very close.


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 Post subject: Re: Need help with VERY old amplifier
PostPosted: Oct Tue 20, 2020 11:57 pm 
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Location: Beaver Falls, PA. USA
He's already said that it is heading for EBay.....

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