Help a newcomer with AM project

[SA2CLC]

Hello everyone.
New to this site, but have been reading along here for quite some time now, so I´ll figured I better start a thread to ventilate some thoughts.
Last winter I built a 2x813 linear amp set up for 80 thru 20 meters, and I had a lot of fun and learning getting it going, and with that project done, I was thinking about what to do with the other stuff I´ve gathered. Along came the the AM transmissions from finland and opened my ears.
I got an old BC-348 to listen with, but still wanted to transmit with some more style than the IC-735. Bottom line is, I need to put together a plate modulated rig.
Someone told me that the modulation transformer determines the rest of the rig, and recently I acquired an UTC S-22 mod transformer (hard to find mod iron here in SM). Rated at 250w it sure feels lightweight, but from reading I understand that this is not commercial ratings. I was thinking about an 813 modulated by a pair of 811´s.
useful stuff I have on hand is as follows:
S-22 mod xformer.
5 NOS 813 tubes. (philips brand, QB2-250)
Filament transformers. Both 10v and 6.3v.
Plate transformers. One 0-900-1000v at 1A. One which is roughly 1500v at 1A.
Two large variacs.
A couple of widespaced plate tuning capacitors, and some loading caps.
A box of doorknob capacitors.
A mind eager to learn a tinker with this kind of stuff

Will the S-22 handle 2kv on the primary, or should I put the rf amp voltage around 1.5kv?
Use two power supplies?
Reactor choke? looking at the scarce supply of modulation transformers, i´ll bet a decent reactor choke would be even harder to find. Maybe winding my own would be an idea. Needed at all at moderate voltages? Suggestions?
Speech amp? solid state or tube gear?
One thing I have decided for sure is that a dds with a buffer will be used as rf exciter.
The questions are many, maybe you guys can guide me in the right direction.

//Tommy, SA2CLC

here´s a youtube link to the 2x813 linear amp I built (top cover have been fitted by now)

https://www.youtube.com/watch?v=d1XJSgttlko

[KB2WIG]

T,

Welcome aboard.

There's always the famous Tesla 300......

http://www.amwindow.org/tech/htm/813/813.htm

That will spank the ionosphere.

klc

[KA0HCP]

Welcome aboard Tommy!

Your amplifier is beautiful.  Good luck on your AM project. Bill

[N2DTS]

Go easy on the S22 and it will sound better and last longer.
Nothing wrong with an 813 in the rf deck and a pair of 811's.
Those tubes will work over a wide range of currents and voltages, 1500 volts on both
would be ok, 1500 volts at 200 ma is 300 watts input, 250 out, about 1000 watts pep.
You can take it down to 1000 volts no problem.

And don't let the lack of a mod transformer stop you, many tubes work well with screen modulation.
The 813 is not one of them, at least they do not seem to work well for me, but others work very well.
4-125, 4-400, 4cx250, 4x150, easy to get a VERY hifi signal of 300 watts carrier using screen modulation.

Its all fun, I have home brew plate modulated rigs, screen modulated rigs using solid state circuits and tube circuits and they all work very well.

[SA2CLC]

Tnx guys for the replies.
N2DTS, yeah, i´ll treat the s-22 like a baby.
I think I will order some russian 811´s and sockets next week, and run the whole set at around 1200v.
I will probably build this as I built the linear, install the big components and start filling the blanks in between
Hey, I´m no rf engineer or a master with electronics (just your normal John Deere technician) but I am having so much fun around
all these  projects.
Wish I had some of the older arrl books with a similar rig and schematic. Guess this was pretty much a "standard" circuit back in the days.
Maybe someone can point me to a site with a decent schematic of an 813 being modulated by a par of 811´s?

//Tommy,SA2CLC

[KB2WIG]

T,

Go here                        http://www.tubebooks.org/technical_books_online.htm

The Radio Handbook 15th  by William Orr (ed).       Its also called the  'west coast handbook' by some.

Nice site, with lots 'o stuff.

klc

[SA2CLC]

So after some thought, I have decided to drive the 811´s with the WA5BXO circuit, seems simple and saves me some effort to track down some hard to get iron. I have a couple of old tv tubes rattling around in a box in the schack, PL36, so I thought that would make a nice "driver for the driver". EU style.
Found this schematic of an PP audio amp, so I figure i´ll use this as a starting point
http://www.vk6fh.com/vk6fh/2009amplifier_euversion_1.htm
What say?

[SA2CLC]

Hello again folks.
I am slowly making progress on the build here, most things for the rf deck is finding their place on the chassis.
On to the modulator, as I stated in the above post, I was thinking of using the WA5BXO circuit as driver, but as you all know during a build, things tend to change.
I found the Altec Lansing 1570b circuit, and it seems to be the driver circuit I´m looking for, no driver transformers or any hard-to-find parts here in Sweden.
I´m thinking of just making a "clone" of this as a modulator/driver.
Experiences using the 1570B circuit from you guys out there??
//Tommy, SA2CLC

[W1RKW]

Tommy,
I have no experience with the 1570b circuit but built my own audio driver for my 813 AM rig.  Instead of going hollow state I went solid state using a LM1875 audio amp IC that drives a pair of 813 grids in the modulator using a hi-fi push-pull audio transformer in reverse to the 813 modulator grids. Like you I thought about using a tube/valve driver but space limitations changed the equation. The audio driver covers a 6 inch x 9 inch area on the chassis which includes the power supply to excite the LM1875 and the hi-fi transformer. You could do something similar to the 811 grids with plenty of headroom.

[N2DTS]

I have thought many times about building a tube audio driver, but the parts count, the nice iron needed, and the space it would take up does not contrast well with the 1 rack unit high $50.00 symetrix dual channel audio amp I have been using for the last 20 years.

Still, it would be a fun project to build a 30 watt audio amp with mic and line level inputs and tone controls.
I built a few tube based screen modulators that go from a D104 to the screens of the RF output tubes and they work very well so a power output stage would not be that hard...

Power transformer (around 700 volts), big choke, lots of filter caps, a hifi output transformer, AB1 power output tubes, 807's, kt88, kt90, 6l6gc, whatever gives a clean 30 watts output in AB1 would work.


Maybe this winter...

[WBear2GCR]

Tommy,

I have worked with the 1570b circuit, and the 1570.

Excellent circuit. The benefit is that you can use almost any output transformer of 5-15w class as the choke.
Size this iron to meet the drive requirements, but larger is always fine. Use the primary.  

There is no need whatsoever to use 6W4s as driver tubes, or 811a as output tubes. Although that is fine, and I prefer 6V6 as equivalent drop-in driver tubes.

Need be you can put some bias on the output tubes via the CT of the driver choke.

I would leave extra room for the mod iron, and keep ur eyes open for something better than the S-22.
Using the S-22 I'd give serious consideration to modified Heising - better bass is potentially available that way.

Btw my guesstimate is that you could run up to about 1250vdc on the 811s.
Keep in mind that one can use 813s as modulator tubes too.
The specs are shown in the tube's sheet... main benefit I think is that you can run more B+ than with the
811a.

572s are always an option, IF you can find any.

                       _-_-

[W4RFM]

Tommy, I am also building a very similar set up.  Mine will be a pair of 813's modulated by a pair of 811A's.  I also have an S-22 to use, and have acquired a broadcast transmitter modulation reactor of 72 Henry's. My audio driver is an Altec A-334 design, using the Russian 6L6 tubes, but replacing the output transformer with a Thordarsen driver transformer to drive the grids of the 811A's.

Here is the RF section - so far.

[K1JJ]

Tommy, I am also building a very similar set up.  Mine will be a pair of 813's modulated by a pair of 811A's.  
Here is the RF section - so far.

Very nice, clean layout and metalwork, Bob!

Suggestions - Add parasitic suppressors with wide strap from the 813 plate caps to the plate choke. Also use direct wide strap to inter-connect the loading cap to the rest of the pi-network if you haven't already.

Looks like you have plenty of room to mount a band switch right above the coil with a long insulated shaft, no?

T

[WBear2GCR]

Quite retro/buzzardly and deco!!

How did you do that neat bezel treatment??

For RF shielding, put wire mesh behind the glass...

                 _-_-

[W4RFM]

This one is for 3880 and 3885 only, so no band switch, also no parasitic suppression.  According to ARRL and
K6JCA who built one that I am shamelessly copying, they are not necessary. The secret of the bezel is: cut out the hole with a jig saw, use chrome tape up the left and right sides, and use auto parts house stick-on chrome molding for the top middle pieces and bottom. Plexiglas in the hole. Looks kinda retro doesn't it? Fact is, I have started building this transmitter 10 times, and learned a little every time. Here it is, 99% finished.

[WBear2GCR]

Ok, auto molding it is!

Now you all you need is one of those 1950's Pontiac hood ornaments!!  

I'd use glass myself, and again put some fine wire mesh sandwiched between two bits
of glass, ground the wire mesh... but maybe it doesn't really matter.

               _-_-bear

btw, when you post if you use the "Attach" button, it will put a full rez image in, now
you have an "inline" image, which does not expand...

                     

[Steve - K4HX]

Beautiful!

[N2DTS]

I would use braid or strap, its just a good idea, plate, screen and grid.
Ground the 813 bases with spring clips.
Loading cap looks a bit small, you may need to pad it with a door knob.
I would also use some sort of screen over the large hole...

Its also nice to have a cathode current meter on the RF deck, that way you can tune and look at the meter with it close, its odd tuning knobs when looking at meters someplace else.

With a zener bias setup you can run 811's at 1500 or even 1750 volts to get the 813's up where they like.

[Steve - K4HX]

Most BC rigs had the meters at the top. No need to have the meter on the same deck.

Many BC rigs had viewing windows. No need for ugly screen.

[W4RFM]

My intent was not to hijack Tommys' thread, but I appreciate the comments everyone.
The loading cap measures 1900 puff, the vacuum variable is 500 puff,
the 80 meter only tank coil is just a smidge under 11 uH. The meters will be 5 across on a 2RU panel with grid current, screen current switchable between V1-V2 or both (just like the ARRL design from the early 60's) Plate voltage (the broadcast guy in me will not go away) Plate current, and RF Amps.

[N2DTS]

As far as the meters go, I have voltage and current meters on the power supplies at the bottom of the rack, and if I do not put a current meter on the RF deck its odd to tune for a dip with the meter way down on the bottom of the rack.

[SA2CLC]

Tnx for the replies.
W4RFM, that rf deck sure look good. I use the same plate choke in mine. Main difference is that I'm only running one 813 in the final, together with the exciter on the same deck.
And as far as the modulator goes, the last tubes for the 1570 speech amp should be arriving next week, so the lineup will be one 813 modulated by two 811's, driven by the 1570b circuit.
Thanks for the guidance and hope to meet you on the air.
//Tommy, SA2CLC

[SA2CLC]

W4RFM, just have to ask, which route did you take with the screen supply for the 813's ?
Dropping resistor from the modulated B+ ?
Series choke from a fixed supply?
Clamp tube in case you loose drive?
//Tommy, SA2CLC

[N2DTS]

All my homebrew rigs run a separate supply with a dropping resistor and a choke, plus an overload circuit.
The dropping resistor (big pot) drops 100 or more volts and allows a wide range of tuning without excessive screen current, plus you can short it (turn it down) to run the deck as an amplifier.
The choke (and the resistor) allow the screen to self modulate, I can short the choke with a switch on the back.

Dropping from the modulated side is not good in my book, better to drop from the unmodulated side and let it self modulate, although some rigs (tubes) work better with a combo of modulated and unmodulated voltage.

The overload should always be used, part of the screen current flows through a relay coil, part bypassed with a pot, excessive current pulls the relay in which connects the screen voltage to a resistor that holds it closed untill you reset it.