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Author Topic: Been offered 3cx2500  (Read 2878 times)
ZL1BBW
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« on: June 03, 2013, 09:50:09 PM »

Hi,  Just been offered a 3cx2500 for next to nix, is it worth getting?

Objective to build a really decent HF amplifier for mainly CW that will run n run for contests.

The power limit here is 1000 watts out, so it would be loafing along.

Have a 100 watts to drive it with, but would like to rein the diver back a bit.

Is the socket common to anything else?

Thanks all.

p.s. what could it do with say 3700 on the plate and plenty of I available?
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
K2DC
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« Reply #1 on: June 04, 2013, 02:52:18 AM »

It should work just fine.  Here's the data sheet:

http://www.rell.com/filebase/en/src/Datasheets/8161R.pdf

At that plate voltage, from the operating curves it looks like you'll need about -140V on the grid to keep it to a KW output, and you may only need 15 or 20W of drive to get there.  If it's cheap and in known good condition, go for it.

73,

Don, K2DC


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AD4U
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« Reply #2 on: June 04, 2013, 05:51:02 AM »

The 3CX2500A7 and the entire 3CX series of tubes up to the 3CX20000A7 can be operated in a grounded gird configuration.  In GG amps the gain of this family of tubes is around 12-15 dB .  IMO it would make a great HF amp with LOTS of headroom.

Dick  AD4U
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WX7G
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« Reply #3 on: June 04, 2013, 06:35:11 AM »

The 3CX2500 has a mu of only 22. Tubes used for amateur grounded grid service have a mu of 100-200. So this amp will be a bit different and won't be a zero-bias design.

Looking at the characteristic curves shows that for a plate load of 10k ohms the input impedance is 500 ohms for Class-A operation. A tuned input is in order.

Useful links:

http://www.w8ji.com/grounded_grid_amplifiers.htm

http://www.somis.org/D-amplifiers1.html







« Last Edit: June 04, 2013, 06:45:52 AM by WX7G » Logged
WX7G
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« Reply #4 on: June 04, 2013, 08:44:35 AM »

If I'm doing the math right the RF in to DC plate power gain is about 20. Given a DC-RF conversion efficiency of 60% the RF gain of this tube is 12. 83 watts in for 1000 watts out. Given 0.8 dB of input matching loss and we have 100 watts in for 1000 watts out.

As to tube life, the cathode has plenty of emission and it will have to deteriorate quite a bit to render the tube dead. Additionally, being a thoriated tungsten filament-cathode the filament power can be reduced with a large increase in cathode life. This tube could last a lifetime. See the EIMAC Care and Feeding of Power Grid Tubes for the method to set reduced filament power.

As to the anode dissipation rating, reduced air flow can be used at the 1 kW level. 10 CFM will be plenty of air flow.

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ZL1BBW
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« Reply #5 on: June 04, 2013, 01:18:11 PM »

Thanks to everyone for the info.

I have been offered the tube for the cost of post n pack from another ham in NZ, it was a station pull, and he tried it on 6M but it did not perform well, I would think it required too high a drive level to be efficient.

Now the filament transformer.  5volts at 51 amps, am thinking of rewinding a big toroid I have with some copper tape, and use the rick i read somewhere of putting the filament choke in the primary leads.

I will upscale the Ht supply, the transformer I have weighs around 35kgs for HT, so that should allow a reasonable supply, will probably upgrade the diodes i was thinking of using.

Was going to use a Roller coaster and a couple of vac variables in the PA, and go to vacuum relays for RF switching.

Anything else I should must do.

Oh, better have HT interlocks on this baby I think.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
KD0REQ
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« Reply #6 on: June 04, 2013, 01:54:08 PM »

with the close spacing on this tube, a page I was re-reading on w8ji.com would suggest you try as best you can to run filaments spot on the Eimac recommanded voltage.  you could run a couple little dinky wires out to a pair of tip jacks from the socket itself to measure that fil voltage.

http://www.w8ji.com/filament_voltage_life.htm  theory and practice

http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm  things that go bang in the night
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WX7G
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« Reply #7 on: June 04, 2013, 02:11:44 PM »

With the high input impedance of the cathode the filament choke should be placed at the filament and not on the AC side of the filament transformer. The choking impedance should be well over 1k ohm. If placed on the AC side of the filament transformer the capacitance of the windings to ground must be less than 5 pF to meet the >1k ohm criterion. This will be difficult to achieve.

Winding the 240 VAC primary on a big toroid by hand is no fun at all. If you can find an old Variac core with the primary intact it is a simple matter to wind a 5 volt secondary.

Are you alright with the amp having only 10 dB of gain?

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ZL1BBW
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Posts: 346




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« Reply #8 on: June 04, 2013, 02:41:10 PM »

Yep 10db is fine, if its a genuine 10db, if I have to I can wind the input up a bit,.  I do recall visiting a ham many years ago who ran a SB220 as the driver to his linear, he also had a 3 ele 40m beam up, often wondered why he was the only one I could hear on 40.

That was not stateside either :-)

For me the ability to run 1kw for hours on end is the prime mover.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
K6AER
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« Reply #9 on: June 04, 2013, 02:42:45 PM »

With almost 400 watts of filament dissipation I would run at least a 25 CFM fan. The tube will be quite the room heater even in standby. You might consider a switchmode supply for the filaments. With transformer efficiency, you will have up to 600 watts of total heat in the filament supply.
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AD4U
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« Reply #10 on: June 04, 2013, 02:57:45 PM »

When I posted earlier that the 3CX family of tubes can be operated in GG configuration, I did not say but I assumed you know that you MAY need some cathode bias from a zener to keep idling IC and heat reasonable.

Dick  AD4U
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W8JX
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« Reply #11 on: June 04, 2013, 04:27:41 PM »

With almost 400 watts of filament dissipation I would run at least a 25 CFM fan. The tube will be quite the room heater even in standby. You might consider a switchmode supply for the filaments. With transformer efficiency, you will have up to 600 watts of total heat in the filament supply.

Interesting point. Throw in power for cooling fan and HV power supply and it could draw close to a 1000 watts on standby. Does not make for a very efficient 1kw amp huh?
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W6MTF
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« Reply #12 on: June 04, 2013, 06:15:55 PM »

Guys, this mediu-Mu triode was intended for low freq applications, primarily in grid driven neutralized service for AM broadcast or in PDM (pulse duration modulation) transmitters (again, AM service) or in the old days as push-pull audio amplifiers in a modulator for high level AM.

In a grid-driven circuit this tube needs to be neutralized, even if the grid is swamped or fed by a balun. The feedback capacitance is so high it begs for an oscillation to occur, that plus the requirement of a very high grid bias voltage obtained from a stable low impedance source) makes this tube not even on the list of preferred ham tubes. 

Granted, a few hams I know have built good working amplifiers around medium Mu triodes (a low Mu triode would be a 3CX3000A1) but they are experienced and most of them are (or were) broadcast engineers who are (were) familiar with this tube in commercial equipment.
The 3CX2500A3/H3 will be the last tube I would recommend a beginner use for a linear amplifier.

At one time a customer could request the tube rebuilders, Econco, in Woodland CA, or Freeland, in New Orleans (now moved to Woodland after CPI bought them out), to rebuild a medium Mu triode and in the process install a high Mu grid. Occasionally I believe they simply swapped the customers tube for a high Mu tube in their large inventory of duds, essentially the customer would receive a high Mu triode like the 3CX3000A7 in return (for a modest fee). The 3000A7 is in some circles the king of QRO and a great choice for a cathode driven linear amplifier. It has essentially the same filament voltage with instant warm-up,  no neutralization required, no bias or at worst case 10 or 15 volts of bias easily obtained from rectifier diodes in forward conduction. 
As someone noted you do have to pay the power bill for all that filament power but if you find that a problem then maybe you should be using an oxide cathode triode and live with the inherent long warm up time required - 2 to 6 minutes depending on the tube.
I suggest swapping the Med Mu triode for something "better, easier, etc" unless you are bold and want to adventure forth into a more challenging pursuit.
W6MTF
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ZL1BBW
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Posts: 346




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« Reply #13 on: June 04, 2013, 10:06:37 PM »

Well the heater will help keep the shack warm.

Yep had I figured I would need to some cut off bias etc.

Luckily the shack is on the south end of the house, and honestly its freezing down this end during the winter, so some extra heat will not be a huge disadvantage.

Also quite a lot of the contests are in my winter so that again helps the heater bill :-)

They way I will build the amp is with the socket in a removable die cast box, so it does not go as per plans, its not a major to deconstruct.  If it does not go well then I will use a pair of GS35B tubes, they are easy enough to get and will give somewhat  similar anode dissipation.

I do appreciate that it may or may not give me the performance I want, but as it is for a CW amp, if all else fails I will move it into class C and sort out the drive etc.

I have built amps before and have had to keep similar 2Kw transmitters going under some fairly arduous circumstances.

Am looking into using the Triode board from Ian White to provide the control features, so that will move onto GS35B'S quite easily.

Thanks to aqll, keep the ideas coming.


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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
ZL1BBW
Member

Posts: 346




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« Reply #14 on: June 04, 2013, 11:13:41 PM »

Re the Toroid transformer.  Being a horder, ok who isnt :-)  I have acquired a nice toroid already wound for 240 v in and 2 x 12v at 15 amps  I also got for buttons a very large 3kw switched 240 - 100v toroid transformer, that is one big baby, so was going to look at the winding/s and dis the 110v part and run a few turns on it and see what comes out, its about 8 inches diameter and 3 or 4 inches high weighs a good few pounds.

Anyway better go, am off to YJ1 for weeks holiday in a couple of days. yeah.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
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