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Strange homebrew amplifier found




 
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WA1LGQ
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« on: July 11, 2020, 10:12:25 AM »

One of you learned gentlemen should know something about this amplifier that I picked up. For final tubes it has 4 6AG7 tubes, with an empty socket for a fifth. My research brought in a reference or 2 to this amp being capable of 150W PEP output. Hmmm?

.....Larry



* 6AG7 amp1 smaller.jpg (1940.14 KB, 2080x1560 - viewed 294 times.)

* 6AG7 amp2 smaller.jpg (1416.93 KB, 2080x1560 - viewed 251 times.)

* 6AG7 amp3 smaller.jpg (1429.98 KB, 1560x2080 - viewed 252 times.)
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WD5JKO
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WD5JKO


« Reply #1 on: July 11, 2020, 10:47:50 AM »

Larry,

   That is a piece of history!

My bet is that this 5 times 6AG7 Grounded Grid amplifier is 80m only, and meant to be driven from a Central Electronics 20A. This might do 100 watts PEP input, or if over driven, 100 watts PEP output.

Notice there is no filament choke. That means the builder depends on a low capacitance to ground and low capacitance to the 115v primary filament transformer. That shortcut only works at the lower frequencies, such as 160 and 80m.

That dual tapped HV transformer is a nice find should you part this thing out.

Take a look here:
https://forums.qrz.com/index.php?threads/4x-6ag7-amplifier.646207/page-5

Jim
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WA1LGQ
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« Reply #2 on: July 13, 2020, 03:00:39 PM »

The grids are all tied to ground, the drive goes directly to the cathodes shunted to gnd with what looks like a 2.5mH choke, and  a pi net out using a roller inductor, probably 80 and 40 at least. It must have beat the crap out of those tubes. Yeah, there are some good parts there, the HV transformer is a S-45 rated at 200mA. Nice plate choke too.
LB
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #3 on: July 13, 2020, 06:51:17 PM »

Mmmmm... looks like an Arc-5 tuning coil?

Maybe an attempt at an 11M amp?
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_-_- bear WB2GCR                   http://www.bearlabs.com
AJ1G
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« Reply #4 on: July 17, 2020, 06:26:17 PM »

Grounded grid 4x6AG7s would not provide that much more strap than the barefoot 20A, you can actually use it as a low power transmitter.  It uses a pair of grid driven 6AG7s in the output stage, on a good day you might see  20W PEP out of it.  Iíve never seen more than about 18 watts out from mine key down on CW.  Perhaps the  4x6AG7 amp might have been used with the earlier 10A exciter, which only made about 10 watts PEP out from a single grid driven 6AG7.

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Chris, AJ1G
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Carl WA1KPD
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« Reply #5 on: July 18, 2020, 09:48:54 AM »

CQ MAG Sept 1956, page 19. 4 6ag7's with about 800 volts and 150 watts out.  


* 5x 6AG7 Amplifier 3.JPG (305.95 KB, 1216x848 - viewed 118 times.)

* 6x 6AG7.jpg (341.72 KB, 1591x560 - viewed 111 times.)
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
Carl WA1KPD
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« Reply #6 on: July 18, 2020, 10:17:48 AM »

https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1955/06/page21/index.html
Looks like this amp with 6AG7's
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
WA1LGQ
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« Reply #7 on: July 18, 2020, 01:43:05 PM »

Interesting. I suppose it could be used with a "piss-weaker" transmitter or a 10A or similar. The 1625 amp does look like it, I could not find the CQ article online for free. Maybe that is the original source of this project. It would make a good amp for my "BC-3.75" low power tuning unit transmitter. Maybe this winter.
Larry
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W3RSW
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Rick & "Roosevelt"


« Reply #8 on: August 19, 2020, 02:25:18 PM »

Good comments. Yes these sure use a minimum of parts and with One dollar 1625ís (modified tube bases) , or 6AG7ís for early low output SSB rigs; made home brew sense.

Think of that, an all 6AG7 rig, from Osc., followers, balanced Pair modulator, phase filters, drivers and finals for RF and a couple more for audio Amp section.
Get out ur soldering irons!

...And then along came 6LQ6ís and the like at these plate voltages.  Grin

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RICK  *W3RSW*
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