I've chosen 813s

[N9NEO]

Thanks for the response last month for helping with tube selection for amplifier that I will be building.  Dave scared me away from the big Tetrode project I had initially envisioned.

 I think a few of the 813s will be a good project.  I don't know so much about tubes so this may be a good project to start with.  Rene has offered to send some information and I have asked that he send.  Frank says they are a cheap and safe way to go.

I only have two questions  for now.

First off I have a Yaesu FC-301 antenna tuner that I think might play ok for the output section of amplifier.  It has beefy bandswitch and does 160m-10m.  The power switch says 500w so I'm thinking it will handle the load.  Will this be ok?

Also, I am wondering if a microwave oven transformer might be ok for the plate transformer.  I have one kicking around workshop that I could use.  If not then maybe I can afford to spring for a new one if the tuner saves me some money

73
Bob

[Jim, W5JO]

The load cap of the FC 301 is not spaced wide enough for the application.  With high reflected power it will arc. 

[n2bc]

Hi Bob, I think you will find that the antenna tuner, as it stands, will not do the job.  It is designed to match 50 ohms on it's input to a 50 - 300 ohm load on it's output (I guessing on the output impedance, maybe up to 600 ohms).

For a PI network stage on an amplifier the PI net must match the tube's load impedance (for a pair of 813s at 2500V and 500W that would be upwards of 6000 ohms) to a 50 ohm load.

I don't know much about microwave power transformers, I think they usually have one side of the HV winding grounded to the iron.  I guess people use a pair of them for a classic full wave circuit.

You need to start with the power supply in order to design the tank circuit. You need to know the plate voltage, then determine the current you intend to draw (within the limits of the tubes). From that you can calculate the plate load impedance. From there you can calculate the values for C1 (tune), L, and C2 (load).

Look around in any of the recent (1985 or later) ARRL handbooks under "PI Network", you will find the equations and some charts for common plate loads - unfortunately the charts are pretty much aimed at legal limit amp calculations. 

If you have lotus 1-2-3 I can send you a spreadsheet I cooked up.  You plug in the voltage, current, class of operation and a couple of other things and it will crank out the C1, L, C2 values you will need.

There are also a couple of programs available on the ARRL website to do the same thing, these are mentioned in the handbook later editions (they are noted in my 2004 handbook).

Have fun!

73, Bill  N2BC

[K1JJ]

Bob,

Glad to see you're gonna build some 813's, OM!

Too bad HossTraders wasn't a little sooner. But Marlborough and Framingham are coming up in Feb/Mar.

If I see you there, I can take you around and get all the parts you need for the rig including a strapping power transformer  -  except perhaps for the mod iron.  I did that for a guy a few years ago and he built his 813 rig based upon one day's flea scavenging.

For a pair of 813's modulated by a pair, you need one of those standard multi-section 365 pf per section variables [1200-2000 pf total] for C2 loading. The C1 plate tuning cap can be a 4kV 150pf unit or whatever you find.

For the antenna tuner, just find one big variable cap like C1 above, or better yet, find a 500pf at >5kV vacuum variable and build my floating antenna tuner design. You need a strapping tuner to handle a plate modulated pair of 813's . Forget that FC 301 tuner...

T

[WA1GFZ]

Bob,
Let me know if you need anything. I plan to be up for Marlboro. I think a pair of 813s will do about 1 kw peak.  Yes one side of the microwave transformer is usually connected to the case and I don't know if the insulation will hold up if you float it.
Class AB you will be running a lower plate Z. the 813 has a lot of internal plate c so the layout is critical at 15 and 10. Older handbooks from the 60s had a bunch of 813 projects. 2500 to 3000 volts will motivate it as a nice linear. Hey how about an off line supply. cool tube  fc

[K1JJ]

Oh.... this is gonna be a 813 linear, not a plate modulated rig?

Should be a snap for you, Bob.   

Later on, it's not that hard to convert it over to class C, plate modulated, as long as the C1/ plate tuning cap has adequate spacing for 4-5 kV or so..

T

[WA1GFZ]

They will work fine grounded grids and cathode driven. Yup 4.5 kV plate tuning would be perfect.  fc

[The Slab Bacon]

813's would be great for plate mod service, a pair by a pair would be fine at somewhere around legal limit. But a pair in leenyour service would be somewhat lacking for big power. If you're gonna go through the trouble to build something from scratch, it isn't any more work to build something even more strapping. For leenyour service I would much more tempted to build up a pair of 4-400s or even a single 4-1000. Used pulls are still cheap and plentifull, and will make a lot more smoke than a pair of 813's. A pair of 813's in linear service would be working pretty hard at 200-250w, where a pair of 400s would be a lot more comfortableat that power level or even more. (remember the low plate efficiency of linear service) 813's by 813s would kick ass in plate modulated service, but If you're thinking linear, I'd do a little more head scratching.
         Here is a thought, (I'm actually thinking about building this) Build an RF amp deck with a pair of large 5-pin sockets, a 5v-30a fil transformer and a 3000v plate supply, and assemble it to operate grounded grid. Then you can stab in a pair of 4-125's, 4-250's, 4-400's, or 3-500s. This way it will run with whatever tubes you can come up with pairs of.(or have laying around). Just my take on things.

                                                                      The Slab Bacon

[KL7OF]

813's x 813's  rock with 2 to 2.5 kv.....600+ watts of unmodulated carrier from a class c deck  and the 813s triode connected in the modulator provide more than adequate audio power for LOUDNESS...   If you are building a Leenyar...consider using 3 or 4 tubes for that max scrote signal..I also favor the grounded grid 4x1 ...Good luck... Steve

[W2VW]

(4) 813s will work on the lower bands 40/75/160.

4-1000 opens up a whole new can of worms power supply wise.

[N9NEO]

Thanks again,

It's pretty much settled on the 813s.  A bunch of good reasons for me.  Easy to get up and running and rugged are major for me.  Rene is building one and he has helped me out before with other project.  Frank suggested too, so good enuff for me.  As Dave said, I think a big tetrode would be a big PIA.  Better for gain considering I don't have much to drive it with, but I am a little green when it comes to tubes.  I don't really care about really big power out.  If I can get 100w of carrier then that is fine for now.  Glenn's suggestion of 3-500Z or Russian tubes didn't go unnoticed, but 813s just got more votes.

Ok, on the Antenna Tuner.  Yes, it's obviously not going to be able to match the tube loadline.  I think butchering it would be a mistake anyway as it does a nice job of matching my mosfet rigs to antenna.

I don't have Lotus 123.  There is a Motorola app note around here someplace that goes through the calculations for a few different matching type output filters.  Worse case if ARRL site doesn't have what I need I can stick the Pi filter eqs into mathcad and spit out some values

Yes, microwave transformer is grounded to case.  I've used them before for some high voltage pulse generators, but you have to mount them on a glastic chassis.

Tom, Frank,

I'll look forward to seeing you guys at Marlborough and Framingham.  I'll bring a little cash for parts.  Maybe you guys can wander around with me and help haggle price down.

73
Bob

[WA1GFZ]

Bob,
Throw out a list of stuff you need. I'll see what I can do in the junk box.
BTW you can always add more tubes for more strap.  fc

[WD8BIL]

What did you say Tom ?? 

[Glenn NY4NC]

Bob,

Don't forget a nice viewing window on that big mawl  813's look great lit-up!