All Things Wireless & Letterpress

All Things Wireless & Letterpress

Thursday, April 30, 2015

Update: The "Mini Bandmaster"



I suppose the first thing I should write about is the blue transformer.  Many of you who build audio equipment will recognise it immediately as a power transformer from Edcor.  And you would be spot on.  Two months ago I ordered a 360-0-360v / 440mA power transformer from them to serve as part of a larger power supply for the Mini Bandmaster.  The reason was that I was having a very difficult time getting more than five watts from it.  I was using a much lower current transformer, which I initially figured was the problem.  And to be sure, it was part of the problem, but only a part.  I will explain.

I built this "MOPA" (Master Oscillator Power Amplifier) type xtal controlled transmitter many times over the years, without a hint of problem.  Always with surplus or "junque box" parts.  This project was no different.  The original circuit comes from the 1954 ARRL handbook.

One of the unique features of this circuit was the oscillator output to the 6V6 final.  The author of the original article has the builder utilising a 100uH RF choke to serve both as a Radio Frequency choke, and also to tune the oscillator output to 5 MHz. (hereafter referred to as "Mc".  Hertz rents cars!).   This was to tune the oscillator output to 5mc, which is halfway between 3.7mc and 7.1mc, the then current Novice frequencies.  He states that this provides sufficient input to the final to drive it properly. It is a compromise. This eliminates the need to tune the grid input on the final, reducing the parts count, cost, and complexity of construction and operation for the beginner.  And to be sure, it did, and it does. When it works.

The only problem was that in this case, it didn't.  I purchased two 100uH chokes from my favourite Military Surplus shop down in Orlando (Skycraft Military Surplus), installing one, as per usual.  When the construction was finished, the power output was very low, only five watts.  This rig should do at least fifteen watts.  

I removed the 6V6G, and inserted a 6L6G, only to find the results being the same.  Five, maybe six watts.  The best dip I could get was 60mA on the 6L6G, and 49mA on the 6V6G.  

I rebuilt the pi-output, thinking this was the culprit.  No change.  I concluded that the power supply just did not have the moxy, so I set about rebuilding it with a transformer which I knew could handle whatever was needed.

Here it is, in mid construction.  I figured that I could use this for future projects handling 807s and 6146s, so it was a great investment.  I built the chassis from angle aluminium, which provides a very, very sturdy foundation to build upon, and allowed a space in the center to both mount an octal socket, and feed the transformer wiring through.  It came out nicer than I had allowed myself to hope.

But when it was finished, I realised only a scant increase using either the 6V6 or the 6L6!  This had me going back over the schematic.  What did I forget?  What was that one thing missing?  I went over all my bypasses, all my tolerances, measured all voltages, everything was there and functioning.  WTH??  (What The Heck?....No, the F-bomb will not work it's way into my blog!)

Bry, AF4K, wondered about that 100uH RFC.  I told him I used these all the time, no problem.  He said that he always used a standard 2.5mH RFC, and also had no problems.  So, I pulled it, and stuck a 2.5mH RFC in there.

Wonder of wonders.  10 watts on the 6V6G!!  That 100uH choke was the problem.

The only thing I can think is that these 100uH that I bought were used for something other than RF.  Maybe audio.  I don't know.  But one thing sure: it was the problem!

So, as long as I had the rig open, I thought why not just go all the way and actually place a tuned circuit in there?  The original design concept was to save money and simplify operation, but I figured that since I built this transmitter specifically for use on forty meters, why not optimise it accordingly?  So.....I did.

There went my hopes for simplicity.  But I was glad I had room to make the change without seriously overcrowding things.  Here, you can see on the lower left a coil, wrapped around a 5/8" nylon form, that just happened to snug into a hole in the chassis.  On the lower right, you see a 100pF pilot variable, which again, fit into a convenient hole that already existed.  I only had to drill the two screw holes.  The coil as a computed value of 11uH.  The capacitor peaks the oscillator output at around 45-50pF.  

With the tight spaces getting tighter, I decided to take some parasitic precaution and added a 2.5mH RFC in the 6AG7 cathode line as well.  So now, my simple little Mini Bandmaster has three, count 'em, three RFCs.

That coax you see was problematic.  It added stray capacitance to the output.  It threw tuning off.  I removed it after this photo, replacing it with a single 20g. insulated wire, which the loading cap thinks is just dandy.  

When you build in tight spaces, especially with point to point wiring, be prepared for weird things like this.

So, I buttoned the little "Mini" up, and attached a fifteen watt light bulb to the output, and here is the result:

  video

I thought it prudent to make a notation for my own records the changes I made to my "tried and true" little Novice MOPA.  This notation took the form of a revised schematic, which I share below.  A tip of the hat to the RSGB (Radio Society of Great Britain) regarding the schematic style.  These types of schematic stylings were commonly used in their publications of the 1950s and 60s, and I love them.  To me, they are ultra clear and easy to read.


I should add that I do not feel the RFC in the cathode of the oscillator is critical in value.  I would suggest anything from 500uH to 5mH.  If you really want to choke off parasitics, you could also place a 40 - 50 ohm one watt resistor in series just after the amplifier plate pin, winding about five turns of wire around it. This is actually a standard item in most higher power transmitters, but you rarely see it in these little rigs.  But it wouldn't hurt to include it if you have the room. 

So here it is, the completed NRR station project, as seen in the top photo.  I get a clean 15 watts output using the 6V6G, and 20 or more watts using the 6L6G.  The 6V6 dips at 70mA, the 6L6 dips at 90mA.  The signals are very clean.

So far, I have been pleasantly surprised to find that making contacts with this station is easier than I had thought.  The receiver is regenerative, and requires a spotting oscillator to locate the transmit frequency.  This is accomplished by that little box on the right side of the receiver.  It is a 2n2222a xtal colpitts oscillator.  I simply insert the transmitter xtal into it, turn it on, locate the heterodyne on the receiver, then remove the xtal and re-insert it into the transmitter xtal socket.  This places me well within proximity of anyone calling for me, on either side of zero beat.  The swamping of the receiver by the transmitter serves as a sort of "side-tone".  I also use one of the other receivers as a side tone, depending on my mood.  Nice to have choices, no?

This project has been completed for about a week since this writing.  I did use this set-up for NRR in February,  using the prior supply, generating five watts.  I still managed 360 points for NRR, and scored Sweden on 7045.  With all the improvements, I've scored both coasts of the US, a lot of Texas, but after midnight, I am getting calls from Europe!  Really!  Dx on forty meters has been really great lately.  F6HKA reported 559, and OK1FIM reported 579.   Huh.  Fifteen watt Mopa.  Regenerative Receiver.  Really.

This has been a most rewarding project.

I want to give special thanks to WB4WHH for his article, which was actually based upon my original article! His article can be found here.  I helped him with this project over the e-mails originally, several years ago.  I used his improvements on my original design as a guide.  Talk about a serendipity!  We've come full circle, OM!  :)


Well, here we are, the parting shot of the current operating position.  Left to right, Power Supply, Antenna Switch, Mini Bandmaster with SWR bridge atop, Regenerodyne, and spotting oscillator.  The key is a Royal Air Force 8 Amp from 1942, and the headphones are US Military low-Z from the early 1950s.  A Novice Station in 1930s clothing.

Now, all I need to do is print some new QSL cards!!

If anyone has further question, check out the Novice Rig Round Up Facebook Page.  I'll be there.  Also, I can be reached at wd4nka@aim.com.

This station can be heard on the following frequencies usually around midnight EST on: 7035, 7037, 7045, and 7050.

'73, de wd4nka.







Wednesday, April 1, 2015

The "Mini Band Master".

Before I go too far, I thought first that I would show the dear Reader just what a "Bandmaster" is.  This was an Amateur Radio Short Wave transmitter, which saw production in the mid to late 1950s.  It was built by the then well known Harvey Wells people.  The Band Master in it's essence is a radio telegraph and radio telephone transmitter, the frequency of which is controlled by crystals.  It used, if I recall, a single 807 power tetrode final, capable of about 75 watts input "CW", or "continuous wave", which is sent by hand key.  In other words, International Morse Code.  It was probably realistically capable of about 40 watts output using A3 controlled carrier radio telephony (an AM telephone transmitter).  It was designed for use on the Amateur Radio Service short wave frequencies, between 3.5 mHz, and on up to approximately 29mHz, although I doubt seriously many folks worked these rigs much above the forty meter band.  Power drops considerably with these rigs much higher in frequency.

Many an excited Novice Class license holder used these transmitters, and a fair share of seasoned vets as well.  Many are still on the air today!  Mostly doing code work.  They required an external power supply, and some models came with a VFO, or Variable Frequency Oscillator, expanding the versatility of these black wrinkle-finished boxes.

Were they a "great" transmitter?  Well . . . I thought they were ok when I had one, but the exposed power connections on the back left much to be desired.  Ouch!  But, yes, they were fun.  Eventually we Novices  upgraded to that HT-32 Hallicrafter or that Drake TR3 or that National, (bigger transmitters) and the rest is history.

But oh, these rigs were Iconic.  No other radio looked like the Bandmaster!  They harkened back to the 1930s and 40s with that vertical posture, the black wrinkle finish, and oh, the knobs, knobs, knobs...!  What heart does not melt in the bosom of the wistful Geek just looking at this baby.

Yes, any Op (operator) who's been around the block for a while will know the Harvey Wells TBS-50 Bandmaster at an instant.

*          *          *          *          *          *          *          *          *         *

I went to this year's Orlando Hamcation February last, and came home with what I hoped was a working VTVM (vacuum tube volt meter), which I planned to use to tune up my HQ-170 receiver with.  But when I finally had the time to look it over and fire it up, I discovered that the meter was compromised, requiring a replacement, which is nigh unto impossible.  These VTVMs ceased production sometime in the 1970s, and my particular specimen dated to the mid 1960s.  What to do?

Why . . . what any other red blooded Ham does with a piece of junk!  Build a transmitter inside it!

And that's just what I did.  

Now, rather than go through the blow by blow detail, sufficient to relate that I simply duplicated the last MOPA which I have already described in detail in a prior installment, which used a modified Pierce 6AG7 xtal oscillator to drive a 6V6G in class C amplification mode.  The difference is that this one has Pi output tuning. But when I was done, this former VTVM really reminded my of none other than the Harvey Wells Bandmaster, only in miniature, and with a lot fewer knobs.  What cinched it was when I applied wrinkle finish black paint to the front panel and cabinet!  Oh, my goodness!  I was in love.

Here is a quick Show and Tell of my little "Mini Bandmaster".  This was one of the rigs I used on Novice Rig Round-Up in February.

Building a radio is never a tidy affair with me.  Much less so since my radio shack happens also to be my  Letterpress Shop!  I try to keep my tools over there.... and my wire over here.... my solder gun.... ooops.....over there.  Somewhere in the middle of all this is the object of my attention.





Here are a couple side shots during mid construction.  I just happened to purchase a 0-150mA meter at the Orlando Hamcation, which fit the spot vacated by the damaged meter perfectly.  You can see the Pi tuning variables which I purchased off eBay.  The tuning coil is hand wound around a plastic 35mm film canister, my go-to coil form for MOPAs (Master Oscillator Power Amplifier).  I had just enough room to re-punch the tube socket holes around the existing mini-tube holes.  Because I was able to get away with minimal punching, the chassis integrity remains intact.  That choke you see in the upper photo under-deck is the 2.5 mH RFC (radio frequency choke: helps to keep high frequency signals off the high voltage DC power line powering the tubes.)

The chassis is a simple "L" shaped piece of aluminium which bolts at one side to the front panel, and on the other to the cabinet.  As such, it is held quite rigidly in place.  You can just see the original VTVM front panel beneath the metal sub-chassis here in this mid-wiring photo.

This is a shot below deck.  The variable on the upper left is a 150pF tuning cap, for tuning the output to resonance, and the one on the upper right is a 365 broadcast tuning cap, which tunes the transmission line to the resonant circuit once tuned.  My power receptacle came from a computer power supply.  The wiring is, of course, point to point.  I used bare 22g tinned buss, sliding insulation over it after each piece is cut to length.  For tight spaces, for me, this works better than using pre, or factory-insulated wire.

The rear view.  The rear apron had this convenient wedge cut out of it, so I took advantage of it.  That wedge originally provided a "tongue" of metal to hold a battery for the VTVM.  I cut that off, and utilized the space left by it.


 The top view.  Why are the wires to the mA meter criss-crossed?  Oh . . . because.  Heh, it wasn't until after I hooked it up the first time that I realized that I had the polarity crossed, so I flipped the wire around.  I just didn't re-bend them.  I kinda like it that way, don't you?  No?   Awww.....

Ta-dah!!  There she is, xtal in place.  Code key plugs in front between the tuning knobs.  Lights show DC power (so I can tell the transmitter is on!) and RF output.  With my current power supply (described in an earlier post) I can just squeak 5 watts out of this little rig.  BUT....with the proper power supply it is capable of 25 watts output with the 6V6G, and 40 watts output with a suitable 6L6G.

Aint she pretty?  Yeah, I'm still pretty taken by her.

At first, I wasn't sure if I should put the original brushed aluminium VTVM handle back on the top.  I did, and I liked it so much I left it there.  Maybe somewhat less "Bandmaster-ish", but it sure does lend to that 1930s "portable" look!

For NRR, I paired her with my Regenerodyne receiver, which I converted down to forty meters.  They are quite fascinated with each other, I can tell you.  Oh, and not to be left out, the J-36 keyer, which I also used, along with my RAF 8-amp straight key.

Here is the actual operating position for NRR.  Left to right: Power supply, antenna switch with my little spotting oscillator atop, the "Mini Band Master" (see the light?  I love lights.) and finally the Regenerodyne, which is a regenerative based single conversion superhet of my own design.  The phones (head sets)?  U.S. military, c. 1952.  low impedance. The key shown is the 8-amp'er.


One last shot:  my 33 foot vertical on my roof.  All home made from tubing I had laying around the back yard.  That little five watt transmitter made it across the Mississippi and into Missouri and New Mexico coupled to this antenna.  Also, I worked up into New England!  Keep in mind, five watts, during an event, with solar activity beating the snot out of forty meters.  Not too shabby, methinks.


Well, that's all for now!  Stay tuned for our next installment where some how, some way, in some universe, at least at WD4NKA, Wireless meets . . . eventually . . . Letterpress!!

"And where...", you may ask, "....did that happen here?"

Ahh, yes.  Novice Rig Round-up QSL cards.  But that's another story for another time.

Best 73's!

-gary, wd4nka