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Author Topic: HF amp using 1625 tubes  (Read 13792 times)
KC5HMC
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Posts: 90




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« on: October 15, 2008, 08:50:40 AM »

Hey everyone,
 I would like to build an HF amp using 1625 tubes, which a have a lot of for some reason. I can't find a schematic for one. Does anyone have anything on this? From what I understand it is basically a 807, but has a 12.6V heater and a different pinout.

Thanks
Herb   KC5HMC
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WB2WIK
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« Reply #1 on: October 15, 2008, 09:18:52 AM »

You're right, other than the pin connections, socket and heater voltage, they're basically the same tube so most "807" amp designs will work with 1625s provided you make the base wiring changes.

Some stuff here:

http://www.flashwebhost.com/circuit/807_and_1625_valves.php

This is a 65 year-old tube design and they're very cheap because at the end of WW2 there were millions of surplus 1625s available for just about nothing.  When I was a kid visiting NYC's "Radio Row" (Cortlandt Street), before they tore it down to build the World Trade Center starting about 1966-67, 1625s were available everywhere in brand new sealed cardboard cartons for $0.25/each.  I think you could buy 100 for $20, though ($5 discount for taking a bunch of them).

Each tube can only produce 25-30W output power so to make an HF "linear," this would be best suited for a QRP rig (say, 5W) to drive one or two 1625s to 25 to 50W output.  It would take at least ten of them in parallel to run any serious power, and that presents some of its own problems.

WB2WIK/6
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KC5HMC
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« Reply #2 on: October 15, 2008, 09:47:54 AM »

Thanks!! It would make a nice amp for my FT-817. I will look for some 807 amp designs and go from there. This will be fun!!
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WB6BYU
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« Reply #3 on: October 15, 2008, 12:27:04 PM »

I remember seeing a kw amp that used something like 12
1625s in parallel.  With a combined plate impedance close
to 75 ohms it didn't need an output matching network.  
I don't know if this would be practical for the higher
bands, though.

If I recall, the 807/1625 are not particularly good as a
grounded grid amp, but with a passive grid input it could
do fairly well.
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G3RZP
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« Reply #4 on: October 17, 2008, 03:46:47 AM »

The 1956 ARRL handbook has a design using 4 of them. It appeears in a number of ARRL handbooks of about that period.
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N3OX
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« Reply #5 on: October 17, 2008, 06:40:46 PM »

"HF amp using 1625 tubes"

Boy, that's going to take a long time to wire up ;-)

... Sorry...

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K4DPK
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« Reply #6 on: October 17, 2008, 06:53:44 PM »

You can make a GG amp out of them, but I recall (going back more than 50 yrs, so bear with me) there were some mfrs that had the suppressor grids tied to the cathode, and you had to take the bakelite bottom off the tube and unwrap those two wires in order to ground all the grids and drive the cathode.

Phil C. Sr.
k4dpk
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N2EY
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« Reply #7 on: October 18, 2008, 06:28:49 AM »

K4DPK writes: "there were some mfrs that had the suppressor grids tied to the cathode, and you had to take the bakelite bottom off the tube and unwrap those two wires in order to ground all the grids and drive the cathode."

That's true, and there was even a commercially manufactured amplifier (the P&H LA-400) that used the modified tubes.

btw, the 1625 has beam-forming plates, not a suppressor grid, but it's the same situation. You can't use unmodified 1625s or 807s in grounded-grid.

HOWEVER,

Not all 1625s can be modified that way. Some manufacturers, like RCA, connected the beam forming plates to the cathode inside the tube, so they can't be modified.

An ingenious fellow worked out a way to modify the tubes without removing the bases. He'd drill a small hole above the cathode pin, heat that pin, fish out the two wires, and separate them.


73 de Jim, N2EY

(who runs a pair of 807s in class C grounded cathode)

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KC5HMC
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« Reply #8 on: October 18, 2008, 08:30:33 AM »

Can you use a VOM to check if the tubes are the modded type?
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G3RZP
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« Reply #9 on: October 19, 2008, 12:31:42 AM »

No
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WB6BYU
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« Reply #10 on: October 19, 2008, 12:23:57 PM »

That is why I think a grounded cathode (grid-driven) amp is a better
choice.  You should be able to get the same output power.  With a passive
grid (perhaps 300 ohms to RF ground, with the drive power applied
across the resistor through a 4:1 stepup transformer?) it should be
quite stable and you should be able to get a reasonable input SWR.
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KC5HMC
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« Reply #11 on: October 19, 2008, 01:28:58 PM »

Sounds like the 1625 isn't the best choise for an amp. What is a good choise for an all tube amp in the 800 watt range? How would the 572's or 811's be for a fisrt timer?
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K4DPK
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« Reply #12 on: October 19, 2008, 03:21:02 PM »

What kinds of other parts do you already have?  In particular, do you have a plate transformer you plan to use?  If you do, that dictates the plate voltage and in some ways, the choice of tube(s).

Phil C. Sr.
k4dpk
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KC5HMC
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« Reply #13 on: October 19, 2008, 05:10:22 PM »

I don't have anything except aabout 10 1625 tubes.
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KC5HMC
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« Reply #14 on: October 24, 2008, 02:05:23 PM »

I recieved a 1958 ARRL Handbook ( thanks for the tip G3RZP ) and it has just what I am looking for!! In fact there are several to choose from.
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