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Author Topic: 3cx100 (aka 2c39) for 6m  (Read 6702 times)

Posts: 1824

« on: July 19, 2006, 07:03:03 PM »

Any reason why a pair of 3cx100/2c39 as grounded grid
(say two in parallel) wouldn't make a decent 200-250W
amp for 6m?

Someone offered me them and I happen to have a decent
1000V supply.  Also with max available drive of 20W
it could be a a good (less than 500W) amp if done right.
I figure two or three in parallel (not push pull) running
AB1 would do decently.

Last time I played with these type it was at 450mhz (old Moto '44 chassis).


Posts: 21837

« Reply #1 on: July 20, 2006, 08:21:46 AM »

The reason 2C39s aren't normally used at lower frequencies is because they're SHF planar triodes with a lot of gain up through a few GHz; so, it's kind of a "waste" to use them at a frequency as low as 50 MHz.  They also require a special coaxial socket, which is expensive and fairly rare unless you pull some out of surplus gear and can re-use them.  No reason they shouldn't work okay, though.

You shouldn't get more than about 10-13 dB gain, however, as that's about all they can make.  With 10W input, you should have 100-200W output.  Paralleling tubes doesn't increase gain, only dissipation, so you should get the same output power if you use two tubes, three tubes or four tubes -- unless you can come up with more drive.  With 20W drive, three tubes is probably a good choice (if you have three tubes) and might make between 200W and 400W output power, with proper cooling.

My last homebrew 23cm amp used two 3CX100A5s and made about 200W PEP output with 10-12W drive, GG class AB2.

If you use an axial ("muffin") fan for cooling, you need to build some ductwork to steer the airstream for best effectiveness, otherwise a lot of the air can be wasted cooling the enclosure and not the tubes...


Posts: 1824

« Reply #2 on: July 21, 2006, 01:32:59 PM »

Thanks for the info.  Last I'd used them was 30 years
ago for class C UHF FM (motorola and LINK).  I know
them for the upper VHF and UHF use but didn't see any reason why lower VHF was not possible.

Actually ~200-300W is reasonable for the PS I have and
I'm not above scroungeing sockets. It also fits with
the amount of power I seek (200W carrier).

What I was worrying was would it be a clean amp down low.

Cooling is not an issue as there is always water,
and ducts for air cooling are not that hard using fiberglass sheet.

The ideal tube for what I'd like to do is a 3cx300 if it existed (cheap that is).


Posts: 21837

« Reply #3 on: July 21, 2006, 03:24:06 PM »

If a 4CX300A would do the job, then so would a 4CX250B or even a 4X150A because at 50 MHz, these are all the same tube (essentially).  The 300A is more expensive because it has a fancy base that works better at 500 MHz.  Internally, these tubes are about identical (I've seen them sectioned).

A lowly 4X150A will produce 300W PEP on 50 MHz *easily* with about 5W drive.  The problem is, it should not be used grounded grid (terrible IMD performance), and really needs a grid bias and screen supply.  The screen voltage can be derived from the B+, using a dropping resistor and regulator system.  Almost everybody does that; but the grid bias supply still needs to exist.  It needn't be big or fancy, though, since the bias CURRENT is extremely low: In Class AB1, it is about zero -- so the only current drawn is the bleeder.

A 4X150A can operate with as little as 900V on the plate (as Collins did in their 62S1) and still run 150W PEP output power.  The *ratio* of Ep/Eg2 must be maintained, though, to prevent excessive screen dissipation.  So, if B+ is only 900V, you need to run the screen voltage very low, like 150V (instead of the usual 350V for Eg2 in AB1 class operation per the data sheets).  If you ran 900V B+ and 350V on the screen, the tube will fail immediately.

Very forgiving tube, more powerful than a pair of 2C39s, very inexensive, and on 6m, you can even use an old loctal kidding.  Socket's not very critical at such a low frequency.


Posts: 1824

« Reply #4 on: July 22, 2006, 06:26:57 PM »

I have a 4cx250B with socket and chimney but was looking before I lept into that.  Where a single
4CX250 would easily do 500W input at 2kv but screen
supply adds hardware.  I'd need at least a 6db pad
at the input for that as 1-2W grid driven will be
more than enough.  Running a 4CX250 at lower voltage
is somewhat appealing but once I'm down around
150-200W solid state is far easier for me.  Still
it's tempting.  

Cathode driven triodes are appealing for simplicity
but most really do not perform at best till the HV
gets up there. (2kv or more).  Besides the only
triodes I do have available are 3cx100s. This is
why a pair of 3xc100s caught my interest as at
they are near optimum for drive and voltages
available.  Experience with them says they stand
abuse well and spares are cheap(free!).

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