Single 3-500 Linear Amplifier Project

[ka1tdq]

I've decided to build a linear amplifier using a single 3-500 tube after I picked up a plate transformer from a local ham which was removed from an early model BC-610 transmitter.  It uses 120 on the primary and 6200 volts on the secondary, center tapped for 3100 volts out.  It is shown mounted on the bottom of the rack.

The next transformer up will be the line voltage reducer in order to use a 6.3 vct transformer on the 3-500 filaments.  The electrolytic capacitor bank is in the next row up. 

The whole rack, including the RF deck on top will be enclosed with plexiglass so that you can see the insides.  I will mount a cold-cathode blue light on the underside of the RF deck to light up the power supply section and the 3-500 tube will light up the RF deck.  It'll make the shack glow nicely in the evening to sit back with a glass of milk and cookies for an evening of ragchewing.

Jon
KA1TDQ

[W7TFO]

Lookin' good, OM.

May the only problem you have is termites!

73DG

[K1JJ]

It will be a cool looking rig with the Plexiglas viewing.

Since there is no metal ground plane in the RF deck, (all wood)  be sure to use wide (1/2" - 1")  copper connections between all hot connections and ground connection components.  Most every connection in that RF deck needs this, especially the connections within the tank circuit.  This will enhance stability and efficiency.  Also, think about safety and grounds.

You can get some copper flashing at any local scrap yard for about $1/ pound, cheap. Cut it in strips with metal shears or a bandsaw.


BTW, paint all the wood flat black and it will make that rig look very stealthy with the Plexi panels.   As a final touch, add a white trim around the front panel using  aluminum angle from Home Depot or the scrap yard.

If you can find some lightly smoked Plexi, whatever color, that wud be quite slick.


T

[ka1tdq]

By the way, thanks again Dennis for the iron. I love the history aspect of it too.

I was planning on using some sort of ground plane for the RF deck. Copper flashing sounds good though too. I might elevate the tube socket an inch or so and use a large diameter hole below it in the wood. That way i would have a path for air cooling and some of the blue light would make it into the RF deck. I put the blue light in a couple hours ago and it looks pretty cool. I'll post a picture tonight when the sun goes down. There's light seeping through the window blinds and I can't see the full effect.

Anyway, I'm willing to bet that this will be the heaviest 3-500 linear in the world, ever. To make a mosfet rig that heavy you'd have to go into completely unnecessary engineering, like adding kitchen sinks or Chevy big block engines as gate swamping bias resistors or something.

Jon
KA1TDQ


Jon

[ka1tdq]

This will be the night view without the tube (haven't built the RF deck yet). I might add another cold cathode light toward the front of the underside RF deck to make things brighter. 

[W2PFY]

This will be the night view without the tube (haven't built the RF deck yet). I might add another cold cathode light toward the front of the underside RF deck to make things brighter. 

A black light would help you to know when dusting is necessary

[W3GMS]

Jon,

I like the stack-up! 

I would just use an old aluminum panel or such for the base plate for the RF deck.  Nothing wrong with copper but aluminum will work just fine.   I have built many HV supplies on wood and it works well.  Just be aware that you could have a silent failure in which a HV choke, cap or transformer would short to the case and you would never know it until you touched it.  A wire from each of those components daisy chained together tied back to the electrical safety ground connection would allow you to know that ahead of time!  The safety ground is tied back to the neutral bar at the service entrance panel which would offer good protection. 

Joe, GMS     

[ka1tdq]

Aluminum might just do the trick.  Back in Springfield, I knew a metal shop that would give me new, 1/8" sheet steel for just a few bucks about that size.  Steel comes much more expensive in the desert, so I'll just find a scrap piece of aluminum (easier to drill too). 

The safety ground is something to consider too.  The center tap (or chassis) of the BC-610 transformer needs to be grounded and I'll run a copper bus bar or something up the back for all that stuff.

Another good thing about using a tube amp is that I don't need to be too concerned about antenna SWR.  I'm going to switch from my current dipole (I haven't been able to get the SWR below 3:1 for some reason) and connect a delta loop outside directly with coax.  The amplifier's Pi network will be able to tune to resonance no problem.  It should be around 100 ohms for the center frequency.

I'm going to corner feed on the ground with the coax with the apex on the house roof. 

Jon

[K1JJ]

 I'm going to switch from my current dipole (I haven't been able to get the SWR below 3:1 for some reason) and connect a delta loop outside directly with coax.  The amplifier's Pi network will be able to tune to resonance no problem.  It should be around 100 ohms for the center frequency.

I'm going to corner feed on the ground with the coax with the apex on the house roof.  

Jon

Jon,

The problem with a delta loop is you need to get it up at twice the height of a dipole for the same vertical take-off angle.   The take-off angle is based on the average height of an antenna. A delta's average height is much lower than a straight dipole.  

If you have plenty of height to work with, then a delta config is fine.  But on 80M just getting a flat dipole at 65' high can be a chore. (the desired 1/4 wave height above gnd)

In addition, your house and surrounding objects will tend to couple in more interaction with a low delta config.    In your case, if fed in the corner on the ground, it will have vertically polarized radiation, which will make surrounding interactions / attenuation even worse, (the house) added pattern distortion, RFI and might require a good ground screen.... more hassle.

If you have these height limitations, I would stick with a horizontally polarized dipole flat top config and you will see a much better signal both locally and farther out.  Put the center and ends up as high and flat as possible.  Try to figure out why you now have a poor match on the existing dipole.

T

[ka1tdq]

I could do that too.  The dipole right now is an inverted V with the apex at the peak of the house.  The ends are about 5 feet off the ground at the fence.  I do have 3 fairly tall trees at the corners of the property so that the antenna would be an "L" but a flat-top and further away from the house. 

I made up a nice large coil for an balanced line tuner and I'll probably feed it with ladder line (cheaper than coax). I also picked up a big plastic dog house at Goodwill for $10 to house the tuner. A 1:1 match shouldn't be a problem and I'll pass on more power to the antenna.  Getting the line up the tree is the next hassle.  It's not like throwing a line up a maple... there's really naster critters down here that sit in trees (snakes, scorpions, etc).

Jon
KA1TDQ
 

[KD6VXI]

6200 volts.......  Just SCREAMS 4-1000   

Shane
KD6VXI

[ka1tdq]

Yeah, but 6200 volts on a 120 line would probably burn up some copper really quick.  The voltage would need to be 3100 volts.  As it is, I'm going to run a 220 line into the shack and run the plate voltage from one phase and everything else from the other.  I'm guessing that a 15 amp line won't cut it.  I'd have to remove the blue light... :-)

[W2PFY]

6200 volts.......  Just SCREAMS 4-1000  

Shane
KD6VXI

The 610 transformer is supposedly rated at 1/2 amp for 2000/2500 VDC so it may be a little lite for a 4-1000? I guess it could scream on slop bucket, but it is an old style center tap transformer that may not handle that  6000 breakdown voltage.

[K1JJ]

I could do that too.  The dipole right now is an inverted V with the apex at the peak of the house.  The ends are about 5 feet off the ground at the fence.  I do have 3 fairly tall trees at the corners of the property so that the antenna would be an "L" but a flat-top and further away from the house.  

Jon
KA1TDQ

That's the way to go.  You will increase your signal MORE just raising that dipole up to 65' compared to  adding the 3-500Z and your exisiting inv vee with the ends at 5'.

* Add both the linear and new dipole and you should see an easy 15dB improvement over what you presently have.  That's huge.

Plus you will hear better... house / computer noise may disappear as well as neighbourhood RFI and TX interference to them.  

The dipole in the clear will tune better without all the close house interactions. Just imagine what a house skeleton looks like to an antenna.... plumbing, wiring, metal appliances, aluminium siding? , etc.

A bow and arrow or slingshot can put the dipole up in fine style. Think it out. If you can't make it all flat, maybe you can keep the first 60' of the dipole flat and drop the ends down towards the ground. Or 90' flat top and 15' dropped legs, etc.   That first center part of the dipole should be as flat and high as possible.


BTW, two blue lights put in series = 240V....  

T

[ka1tdq]

Ah, yes. Modern 240 standards.  Two blue lights!

[W2VW]

6200 volts.......  Just SCREAMS 4-1000  

Shane
KD6VXI

The 610 transformer is supposedly rated at 1/2 amp for 2000/2500 VDC so it may be a little lite for a 4-1000? I guess it could scream on slop bucket, but it is an old style center tap transformer that may not handle that  6000 breakdown voltage.

A full wave bridge configuration will exceed the center tap hi-pot rating of that iron. Must use full wave center tap.

Not too sure how it will hold up without a choke input filter either.

Late BC-610 and T-368 iron do not have that limitation even though they have a center tap.

They can be paired across the mains with secondaries paralleled as most have 110 volt primary. Use connection to neutral.

[ka1tdq]

Good point. I could use the two microwave oven transformers that I welded to a steel plate as a filter choke. I would just short the secondaries together and run the primaries in series. That should have enough inductance value to make a difference. It will add weight to the whole unit though. Luckily my floor is built on a concrete slab.

[W3RSW]

You can also put the choke in the center tap of the HV secondary thence to ground.  You will then keep most of the HV off the choke windings as well as make the delta V from the secondary windings outer taps to center tap less.  Voltage from the outer taps to grounded case will be the same, of course.

This idea as well as many others is covered in any older issue of ARRL's handbooks, etc.
BTW at the CT point to choke another DC voltage is then available for intermediate transmitter stages if desired.

I admire you for making do with components on hand, but in many cases (pun intended) getting quality iron will make your build a lot more compact and safer.

Some on this board may be able to help you get the good stuff.

[ka1tdq]

Sounds good. I've seen the circuit with the choke in the center tap. Makes sense.

Anyone in the Phoenix are have a strapping piece of iron for a choke?

Jon
KA1TDQ
480-427-7622

[W2VW]

Probably best not to short the unused windings for your choke. The whole choke thing can be tried at low voltage to find out if there's enough critical bleeder load.

I've used the choke in the negative lead thing a few times on old potted iron xfmrs and it worked without launching anything FWIW.

[W7TFO]

His plate iron (from me) has the CT permanently grounded to the case.

Best not to mess with that.  They knew something about it...

73DG

[KA2DZT]

Jon,

Try to find a proper choke for your HV supply.  Put the choke in the positive lead where it belongs.  These chokes are around, you should be able to find one.  You need one that has a high enough voltage rating.

Fred

[KF7WWW]

If W7TFO doesn't have a choke, I probably do... I have one from a Henry 2k somewhere around here.

[W2VW]

I just looked at the pictures again. That plate iron is not from a BC-610. It looks like the iron from the amplifier designed to follow the 610. Terrance Puffy has the specs for it. I have a couple of those around here. Very stout iron. 

[W7TFO]

I had 5 of the one like Jon got from me, all from the same guy, and they were harvested from some 610 variant.  Many were done, lots of changes over the war years.  I've seen three distinct types in them.

I also had one of those 833A X 2 linears, and the plate iron in that was over twice the size of the above iron.  37-Amp, 110-Volt primary, over 4 KVa rated.  Opcom has that one now.

73DG