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Topic: 1948 Oahu Model 403K Amplifier |
William Clark
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Posted 26 Jan 2011 7:11 pm |
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I just finished restoring this little Oahu gem for a friend of mine. He brought it to me with a blown power transformer and in bad need of servicing. It had been stored in a damp Michigan basement for a long time. I thought I would post some pictures because I have never seen anything like it. It even has a light up name panel in the front so that everyone will know that "Dolores" is on stage even if the lights are dimmed!!!
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J Fletcher
From:
London,Ont,Canada
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Posted 27 Jan 2011 5:27 am
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| Very nice! That amp would be the star of the show. What output tubes are in that amp?...Jerry |
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Blake Hawkins
From:
Florida
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Posted 27 Jan 2011 6:34 am
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A fine restoration! I'm sure you enjoyed doing
that amplifier.
Blake |
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John Billings
From:
Ohio, USA
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Posted 27 Jan 2011 9:17 am
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| Power tubes appear to be 6B4G tubes. |
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William Clark
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Posted 27 Jan 2011 9:57 am
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| The tubes are a 5U4G Rectifier, 2 X 684G Power Tubes, 6N7, 6SL7, 2 X 7A4 Preamp tubes. The speaker is a 12 inch Field Coil. |
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John Billings
From:
Ohio, USA
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Posted 27 Jan 2011 10:01 am
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| I couldn't find a 684 tube in my manual, so I assumed what looked like an "8" must have been a "B." Kinda de-rezzed when I zoomed in. |
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John Billings
From:
Ohio, USA
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Posted 27 Jan 2011 10:21 am
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| Well,,,, just did a web search for that tube, and went to several tube seller sites. No 684G tube info anywhere. But 6B4G tubes are available. Are you sure that's an "8," and not a "B?" I'll keep lookin', cuz now I'm curious! |
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Blake Hawkins
From:
Florida
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Posted 27 Jan 2011 1:17 pm
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RCA Tube Manual to the rescue!
Those are, indeed 6B4-G power tubes.
We are talking early 1930's for these.
The tube is a triode power amplifier and has a
filament (As opposed to the heater-cathode arrangement in a 6L6.)
A pair of them will put out 10 to 15 watts depending
on the way they are biased.
They are in the same "family" as the 2A3-G.
These power triodes were used in high quality audio
equipment through the '30's and into the '40's.
That amplifier, like many made just after the second
world war is a mixture of odd tubes. Probably
built out of what was available at the time.
That does not keep it from being a quality amp
with good tone.
I think you have a real gem there and the owner should be quite proud of it.
Blake |
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John Billings
From:
Ohio, USA
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Posted 27 Jan 2011 2:01 pm
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" Probably built out of what was available at the time. "
Blake,
One of the sites I went to gave a good history of the tube. They were widely used by our military during WWll, and after the war vast quantities were dumped on the market fer cheap. Right up Oahu's alley!
And,,, they are apparently abdo-lutely loved by the super audio geeks! They are known for their wonderful sound. Very, very cool amp!
From one of the sites;
"THe "250 Volt" rating for the 6B4G is the voltage difference between the plate and the filament / cathode. (V P-K) With the 305V on the plate, and the "cathode" sitting at around 50 to 55V, the ratings aren't being excessively "stretched". There is also a few volt's drop from the two "center tap" resistors, s there is a tad more safety margin.
The "classic" RCA tube manual design runs the 2A3 (and 6B4G) at 250V P-K with 60 mA plate current, just "nailing the" 15 watt PD rating. I'm sure, that in real-life situations, this often got exceeded by 5-10%, with no ill efects. " |
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John Billings
From:
Ohio, USA
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Posted 27 Jan 2011 2:07 pm
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| Also, Sovtek is making a new version that is supposedly even more powerful! But they ain't cheap! New Sensor wants $112 a piece! I saw other dealers for less. |
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William Clark
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Posted 27 Jan 2011 8:18 pm
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| I did look at the tubes again. The power tubes are in fact 6B4G's. I guess my antique eyes can't read my own writing! I also contacted Michael Lee Allen. He told me that this amp was the mate to the top of the line Iolana double neck steel guitar. The dealer could order the owner's name on the light up panel when they ordered the amp from Oahu. I know that the amp was purchased from the original owner with a double neck Oahu steel guitar. He also got the cover for the amp which is form fitting with a zipper. The material is a plaid color. All in all it is a cool set. I am really glad that the current owner decided to go through with the restoration. By the time he put the money out for the amp, the servicing and a new power transformer from Mercury, he is going to have a lot of money wrapped up into it. Kudo's to him for saving this cool piece from the scrap pile! |
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