The AM Forum

Warning: newbie question ahead

So let's say ONE has a fine working example of a 100 watt class Plate Modulated vintage tube transmitter.  I don't know, call it an E.F. Johnson Viking I if you must label it.  Let's say ONE wants MORE than 100 watts and maybe closer to the legal limit to really be able to play on 160 and 80 with the big boys. 

ONE HAS a linnyear amplioficator capable of handling some continuous carrier mode operation although the drive level required for said linnyear is much less than the 100 watts the vintage equipment likes to dish out.....

Proceed with the proposed solutions... (please don't get too expensive as COST is a major factor in ONE's proceedings!)

KX5JT

You can build an attenuator with noninductive resistors mounted on a heat sink, or buy one. A 100 watt pad might run some money.
You might consider wiring a variac into the HV plate transformr primary so you could reduce the voltage of the modulator and final so the plate Z doesn't increase.

Build an external DC supply for just the final, not the mudulator. Use a variac. Turn up the B+ to tune the amplifier. Turn the DC down to adjust drive for what your amp can handle. Enjoy the extra dynamic headroom of your loafing modulator.

 An R.F. attenuator throws away the option for modulator headroom.

This 50 ohm load made of 110 pcs of 56 Ohm 2W resistors handles 220W and is good to about 70MHz. You might want to screen it.. haha. It is very cheap to make and could be made as any kind of pad you wish, just about.

I use a light bulb in series with the output.   Got a regular octagon box, mounted a light bulb fixture and two coax receptacles.    Also have a shorting switch across the light bulb fixture when not using the linear amp.     You have to experiment with the type of bulb you need.    I am using an AF-67 and a Gonset GSB-101 linear.    On 40M I use a 200W bulb.   On 80M I am using a 60W bulb.     The type of bulb will be a function of the input circuit of your linear.    With the bulb in the circuit, I can tune up the AF-67 to full power.    The linear is heavily loaded so that you get full peak power while it is running at its ideal AM plate current.
Not an elegant solution but it works.

I would like to know if anyone has a Idea who makes RF attinuators that can be used between a AM rig and a amplifier. I did hear of someone but it has been a year ago.

Glen K9STH is making them.

http://k9sth.com/

He has three sites, they are on one of them.

Search for a Bird Tenuline attenuator on eBay or a hamfest.  They look like a dummy load but aren't too popular (ie expensive) as they aren't a complete dummy load.  They come in 3 dB, 10 dB, 20 dB, 30 dB etc models.

If one has a rig with multiply parallel tubes in the final, one might consider just pulling one or two of the finals and running reduced power that way. A lot of the older rigs got "Novice" power that way...

from a Johnson Thunderbolt manual, here is some information in there about building an attenuator for a Viking II that should fit the bill.

Yet another way.  I like Ken's approach as it gives a variable output without AM modulation quality compromise.  I don't think it has to be as involved as Ken did things but certainly varying the screen and plate voltage does the trick.

http://w2dtc.com/w2dtc-hi-fi-viking-ii-page.htm

Al

PS:  I will admit that gutting a V2 like that almost seems sacrilegious ;D

Never realized what a PITA it is to get vintage equipment DOWN in power.  Seems like a waste to generate 100W only to burn it up in a dummy load.

(Score one for ricebox low level modulation. Simply adjust the RF Power control.)

At the bottom of this page on my web site http://mysite.verizon.net/sdp2/id11.html, I show how you can use a ferrite core (the same type that some of us use to make transformers for our class E rigs) to make a simple adapter to convert any ordinary dummy load into a power divider.

No changes are required in the dummy load.
The dummy load plugs into the adaptor's SO-239 output socket #1
The transceiver plugs into the adaptor's SO-239 input socket
The amplifier you want to drive plugs into the adaptor's SO-239 output socket #2

The splitting ratio (the portion of the transceiver's power that goes into the amplifier, versus the portion that goes into the dummy load) is determined by the turns ratio.

For example, if you want 11% of the transceiver's power to go into the amplifier and 89% into the dummy load, use 3 turns on the primary, and 9 turns on the secondary (which goes to amplifier output connector).

The transformer and the dummy load are in series. The amplifier will appear as a 50/9 ohm load in series with the dummy load. The total load on the transceiver will be 50 ( 1 + 1/9) = 56 ohms.

As another example, if you want 20% of the power to go to the amplifier, and 80% to go to the dummy load... then use a transformer with 3 turns on the primary and 6 turns on the secondary (which goes to the amplifier output connector). The amplifier will appear as a 50/4 = 12.5 ohm load in series with the dummy load. The transceiver will see a load of 50 (1 + 0.25) ohms = 62.5 ohms.

I use the above approach when coupling my 40 watt Class E transmitter to the input of my 3-500Z amplifier (which wants to see around 10 watts of input power).

The diagram below shows a 50-50 power divider... with a bypass switch that allows you to send full power to the non-dummy load output,  if you want to.


Stu

This is what I'm talking about!  Great responses.  Quite a bit to chew on.  Thanks for playing along everyone!  I love AM Fone dot net!

KX5JT

What a great idea!!    I am going to build one of those!

What kind of ferrite core do you use?

Rich

Rich et. al.

The cores I use are type 43 material 1.125 inches long, 1 inch outside diameter, and 0.5 inches inside diameter.

These are the cores that Steve and others recommend for building transformers for Class E transmitters.

Naturally, they are very low loss at these frequencies (which is important for building an efficient Class E transmitter) as long as you don't saturate them.

For this application, the only reason we would care about loss is in the context of heating. By using multiple turns, (e.g. 3 turns : 6 turns) I find that a single one of these cores works fine with a 50 watt transmitter. If you are using a higher power, you can just use more turns (e.g. 6 turns : 12 turns) or use several cores to make the transformer.

I buy these by the dozen, and I use them to make transformers, safety chokes, and to block common mode rf in audio cables, power cables, speaker cables, etc.

I buy mine from these guys:

http://www.cwsbytemark.com/

Part number SB-1020-43  ($1.50 each)

Stu

Thanks Stu

Rich