The AM Forum

Looking for a easy and quick 300-500 watt PEP linear amp for use at 70 KHz. Something at 1500-1700 volts on the plate  or so to make the match to 50 ohms a little easier. Needs to be linear for SSB so a tube seems best. I was thinking maybe a quad of 811A's but a single tube may be easier. I have 1 watt of drive but can get 20 or so pretty easy withh a FET  driver.  Any ideas?

Why not use a 3-500z in any of the well-published grounded grid configurations?

Given the relatively low RF frequency of operation, you might even be able to add some negative feedback to further improve the linearity

Stu

That would work as well as a 4-400 would. I'd like to keep the voltage a bit lower and I see the 3-500 works well at 1500 volts. I have a few new 1500 volt plate transformers rated at 750 VA or so. Trying to keep it very simple. I also may look at a 4-65 A as they work well at 1750 volts in class B linear. Looking for stability and simplicity. Something I can get up and running fast. The 3-500 may be the easiest to get going. I have sockets and a couple of new Eimac tubes. I will need 50 watts of drive and I'd have to build a driver.

Pat

Pat

Considering the low RF frequency, you might want to use a grid driven design, with a 2:1 step up transformer at the input... instead of using a grounded grid design and a 50 watt driver.

I.e. even with grid current on RF peaks, the input impedance should be pretty high (I.e. greater then 300 ohms) compared to the 50 ohm output impedance of your existing RF source.

Stu

SSB at 70khz?  What are you doing?

There isn't any Amateur radio going on there.

http://www.jneuhaus.com/fccindex/10_khz.html
Unless he is planning on controlling the power grid

Not strictly ham radio, but very legit. Go to:
https://apps.fcc.gov/oetcf/els/reports/CallsignSearch.cfm
and enter WG2XRS for the call sign.

John Andrews, W1TAG / WG2XRS/3

Or my call sign WG2XLP which is just recently authorized for SSB.

A very tough task but but I am going to try. A big Hi Fi amp will work also with proper matching at the input and output.
One that will do 500 watts would cost more than a tube since I have most of the parts  for a tube amp. I also need a linear amp for 500 KHz. Possibly an interchangable output network might work.

A grid driven 3-500Z should be close to what I am looking for. At a low frequency like 70 KHz, I would think neutralization would not be required? How much higher gain would the tube have?

Pat
N4LTA
WG2XCT
WG2XLP

I would use the same short-connection practices and good quality parts as with any RF HF amp. Parasitic oscillations can occur just as easily with a LF amp.  Just like when we sometimes need to add parasitic suppressor chokes in the grid and plate leads of audio modulator amps.

You will not need neutralization at  <500 Khz for stability, but it will help to make a cleaner amp due to NFB.   Considering at 70 kHz a 3KHz ssb signal (4%)  is like running  280 KHz wide bandwidth on 40 meters, it will pay to make the amp as clean as possible for your band neighbors.

Grid driven may be best - GG might be difficult cuz how wud you make a bifilar choke that is 500 ohms at 70Khz to carry 14A?

Take a look at the gain vs: freq curves for the tube.  I would guess that gain would flatten out and be similar anywhere below 2 MHz or so, but never checked...


Have fun!

T

The link stinks ;D ;D ;D Won't work here for me?

And on the third try, it now works but when I put in the call, it won't find anything? Maybe later it will work?

I tried your link and the W1TAG link. Yours didn't work but the one posted by W1TAG does - Not sure why.

Yep - the bandwidth will be very wide in percentage but the ERP will be very low and neighbors will be few.  Doing a SSB QSO at 20 miles would probably be a real feat, even at 1KW. I'll use it mostly for WSPR and other weak signal modes.

I am going to try to put an amp together with a 3-500 and grid drive it. I will transformer couple the output. I can use a 4" fiberglass tube that I have that is 12 " long and wind with #14 magnet wire. About 3500 pF will resonate it. We will see what happens. The plate choke will have to be a pretty good one. I can use a ferrite toroid for the input transformer.

Thanks for the helpful comments  - more would be welcome.


pat

Here's some info:
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnx3ZDJ4Z2p8Z3g6MWY0OWZlMTk0YjIwMmFjMA

And Google:
https://www.google.com/search?q=wg2xrs&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

Looks like I can wind a plate choke with 20 ga wire on a FT240-77 core - about 150 turns will give me 50mH approximately 20,000 ohms or 10X the plate impedance. Probably use teflon insulated wire.

I hope to use a 5KV 1uF oil filled capacitor as the plate blocking capacitor and plate choke bypass capacitor.

Several 5KV mica caps and a 1000pF vacuum variable in parallel to tune the plate transformer primary.

I have a variable 1.5-2.5 KV power supply good for 500 mA already built.

200 turns of #16 enameled wire on the plate transformer primary which should be about 2K ohms
Z ratio of 40:1 - V ratio 6.3:1    about 31 turns

Pat