Most MV rectifiers in a homebrew rig?

[WQ9E]

I came across this interesting design while perusing some recently acquired Radio and Television News magazines.  It was designed by an engineer from Hammarlund and uses four 872A rectifiers in a voltage doubler circuit along with a "pole pig" transformer.  Four 866A's are used in another pair of HV supplies.  There are a pair of 833A's in the final and the modulators are 810's.  I don't plan on recreating this but it is an interesting example of a homebrew transmitter designed by a radio engineer.

Rodger WQ9E

[KE6DF]

It looks to me like the 872A's are in a bridge circuit rather than a voltage doubler.

[WD8BIL]

Yup....... the 872s are definately in a full wave bridge config.

Interesting schtuff though.

[WA1GFZ]

I think back then a bridge was considered a doubler. I bet some buzzards thought this was cheating.

[k4kyv]

One precaution when using the full wave bridge (that's what the circuit in that transmitter is).  If the plate transformer was designed for the regular full wave rectifier circuit, the midtap in the HV winding may not be insulated well enough to withstand the voltage that will appear at that point using the bridge rectifier circuit,  since it was designed to operate with the midtap grounded.  The same goes for putting the filter choke in the negative,  from midtap to ground.  Even though the DC voltage across the choke is likely to be no more than a few volts, the AC voltage across  the choke may peak high enough to destroy the transformer.

There should be no worry when using a pole pig, since the midtap, if any, usually operates at half the full output voltage above the grounded end.

For a given total transformer secondary voltage, the peak inverse voltage across the rectifiers is the same with the conventional full wave circuit and the bridge circuit.  So with a given PIV rectifier limit (with 866A's and 872A's it is 10 kv), you can safely get twice the voltage output with the bridge than with the full-wave.

With a quad of 866A's or 3B28's in a bridge circuit with a 7200 volt 3 kva pole pig, you should be able to get 6 KV at a half-amp out of the supply.

[w8khk]

Well my grandfather, W8YNG, did not beat this rig for number of MV rectifiers, but at least he tied it.  I guess it would be called "Buzzardly", in fact he used to participate in the "Old Buzzard's Net".  You can see the old buzzard qsl card on the front of the center rack transmitter, to the left of his face in the first pic.  Second pic is a scan of the old buzzard card.

The transmitter on the right, with the aluminum panels, is a pair of 250THs modded by a pair of 810s.  I am restoring this rig, but using 450THs, because one of the 250s has an open filament.  In the bottom of the rig, you can see two horizontal windows.  Behind the bottom window is a string of four 866As in a bridge configuration for the final.  The second window up is another bridge of four for the modder.  Both supplies were older, small pole pigs; I still have one of them.  Puts out about 1900 volts RMS with 240 in. No center tap in the HV winding of either of my pole pigs. So to get full-wave rectification, they must use a bridge. The original secondary, now the 120/240 primary, does have a center tap. 

[w8khk]

I forgot to mention that my grandfather purchased this 250TH rig in 1937.  It has been in the family for 71 years!  Dad used it in the late 60s, first as an AM rig, fed RF and audio from the 32V3 now in use by my brother W2WM.  Later, when he got an HW100, he used the 250TH final as a linear amplifier for Sideband.  But I can guarantee this rig will never run sideband again, it will only be plate modulated with the original Thordarson CHT 500 watt tranny.   

To the right of the aluminum paneled 250TH rig is the original 304TL rig built by W2DU when he was W2FCY in New Jersey.  Built in 1949 when I was only two, it had only four 866As, both plate transformers had center taps.  Under the final panel, is a panel with two tuning dials; these are ARC-5 transmitters used as exciters for 75 and 40.   Under that was a 300 watt UTC mod tranny tickled by four Taylor TZ-40s in push-pull parallel, or should I say Shove Yank in Unison?

This rig is under restoration, the final is completed, but the mod tranny never made it from NJ.  It will get a Stancor 3899 and 304TL modders for the 304TL finals.

[W3SLK]

Now that's my idea of a shack! Right down to the operator wearing a tie Damn nice and I bet that rig will really stroke the aether, especially when being used with 450T's!!

[k4kyv]

How many hams would you see to-day wearing a neck tie while operating?

[Ed/KB1HYS]

Nice shack!

A lot of things have gone away.  Ties for technicians/everday Joes... Wearing a suit with vest was it.
I think we had a lot more class back then.

Who remembers when you could Smoke at your desk/workbench in the shop??   The assemblers/techs on the line would more often than not drop their cigarette buts on the floor, with a quick twist step.  At the end of the day they'd get swept up with what ever else ended up on the floor. 

I remember sitting in meetings with a thick hazy layer of smoke about 2 feet above the table, big glass ashtrays always nearly full..

Today they freak about the smoke from the rosin core solder... 

[W4EWH]

The difference is the current of the full wave bridge circuit will be about 1/2 of the center tapped circuit.

Please tell us why that is so. Thanks!

Bill W1AC

[KE6DF]

I think that the current you can pull from a full wave bridge circuit is about 70% of the current you can pull from a full wave center tapped circuit.

If you figure that the limitation on current from a transformer is determined mostly by the i squared r losses in the coils resulting in heating, then you can figure that if the average current you pull from a full wave center tapped design is I(FWCT), then the power dissipated in the windings is:

I(FWCT) * I(FWCT) * (R/2)

Where R is the DC resistance of the whole secondary. You use R/2 in the above equation because at any given time the current is flowing through only 1/2 of the secondary.

If I(FWB) is the current from a full wave bridge power supply, then the power dissipated in the windings of the transformer is:

I(FWB) * I(FWB) * R

In this case, the current is flowing through the whole secondary during both halves of the cycle so you use R instead of R/2

Now to have the same I squared R losses in the secondard in both cases you get:

I(FWB) * I(FWB) * R = I(FWCT) * I(FWCT) * R/2

Taking the square root of both sides of the above equation you get.

I(FWB) = I(FWCT) / SQUAREROOT (2)

or

I(FWB) = I(FWCT) * .7  (approximately)

So a transformer can actually deliver more power in a full wave bridge circuit than in a full wave ct circuit.

For example, if a transformer can deliver 1000 V at 1 amp in a full wave ct power supply, then the same transformer can deliver 2000V at 700 ma in a full wave bridge power supply design.

This is why full wave bridge circuits are often used in large power supplies -- they deliver more power for the same sized iron.

BTW, if you look in an old UTC catalog you will see that both FWB and FWCT current ratings are given for some of the CG (commercial grade) transformers and this 70% factor is reflected in the ratings.

[N0WEK]

Really heavy iron!

By my count there are 21 pieces of iron in that transmitter, counting the filter chokes and the variac! Lots of filament transformers for the rectifiers.

How about scans of the parts lists and such too?

[ka3zlr]

Looking on YNG's Station man, to drool, is to drool, those receivers in like new condition..wooooo..a very effective desk i'd say...

..the mechanism at the left of the operators center is a multi buttoned manually activated device of some sort, with a roller element mounted a mid top...a type of paper inking mechanism comes to mind...i'll have to study this further...

 

[Ed/KB1HYS]

I like the desk in the older photo, the rigs are actually tilted backwards abit and elevated above the work surface to make for easier access.  Very good design there. Obviously a lot of thought went into that setup.

For the transformer question, think of it like this.  If a tranny is rated for 1kW of energy thats all you will get.
So if you double the voltage you will must halve the current.  Strictly conservation of energy. 

[KE6DF]

For the transformer question, think of it like this.  If a tranny is rated for 1kW of energy thats all you will get.
So if you double the voltage you will must halve the current.  Strictly conservation of energy. 

IMHO, I don't think this is really quite right.

The reason you get more power from a FWB circuit than from a FWCT circuit is because with a FWCT circuit only 1/2 of the secondary is conducting at a time. If power dissipated in the windings were proportial to the current, then you would indeed see 1/2 the current. BUT, power is dissipated in the resistence of the windings is proportional to the square of the current.

So using a FWCT circuit is less efficient because it utilizes only 1/2 of the secondard in each full 60Hz cycle but at twice the current. So during the time each half secondard is conducting, it't generating 4 times the heat. This is where the fractor of the square root of two comes from in my calculations above.

See below page from the 1970 UTC catalog:

http://www.bunkerofdoom.com/xfm/UTC_1970_WEB/UTC700044.html

See the ratings for the CG-300W transformer for as an example.

It is rated to deliver 500v at 200 MA with a FWCT design and 1000V at 140 MA with a FWB design.

See any of the other UTC plate transformers on this page.

(Note most CG transformers in the older 1950s and 1940s catalogs are not intended for use in FWB circuits because the center tap is not insulated well enough and needs to be gounded per Don's point. But the part numbers ending in W are).

The current maximum in the FBW circuit is  equal to the max current in the FWCT circult divided by the square root of 2 -- or 1.41. This works out to the equivalent of multiplying the FWCT current by 0.7.

Dave

[KC4KFC]

Maybe its my computer settings, but this photo seemed dark so I photoshopped it some to see the equipment better.

[WQ9E]

Really heavy iron!

By my count there are 21 pieces of iron in that transmitter, counting the filter chokes and the variac! Lots of filament transformers for the rectifiers.

How about scans of the parts lists and such too?

As requested, the parts list and for further inspiration a flock of Super Skyriders from another issue.

Rodger WQ9E

[W2DU]

Hello Mark

Thanks for lightening the picture of my Dad and his 1956 rig. I don't know how it became so dark when Rick posted it, because I lightened it before adding the caption.

Incidentally, I still have, and use, the RME-69 shown in the pic, and Rick has the SX-28, still in mint condition. Rick also has the Signal Shifter shown in the pic, as well as the xmtr with the aluminum panels that he's refurbishing. As Rick said earlier, I used that xmtr during the  60s and 70s.

Walt, W2DU

[WQ9E]

By the way, the rationale for the interference reduction technique in the article using the SX-28 receivers is interesting.  It referred to the need for a passband extending out to 5 khz. for the Japanese language to be understandable by our intercept ops.  Unless I am missing something this doesn't seem to fit with the modern Japanese SSB equipment where the cutoff is generally fairly sharp somewhere at the 2400 to 2700 hz point.  Don (K4KYV) as I recall you are a linguist (or linkster in old mountain vernacular)-what is your take on this?

Rodger WQ9E

[KE6DF]

As a EE, but one who worked mostly on computer software with a little hardware thrown in, I have to say that sometimes I think miniturization and design for high volume manufacturing is a mixed blessing.

The modern all-mode rice box transceivers are in many ways a miracle of engineering with receivers with DSPs etc and fairly powerful SSB and AM in a small box.

But there is something about a big rack with lots of glowing tubes that the modern rigs can never equal.

The exact same thing happened in the computer field. When I started on software I worked with big mainfames which made lots of noise, were kept in raised floor rooms, and had control panels several feet wide with lots of blinking lights and tape drives spinning and big printers grinding away.

You really got a sense of power and priviledge using those big computers costing millions of dollars and supported by a big staff of gurus.

Now days your notebook has more power than any fo the old mainframes from the 70's and 80's plus is also has more memory.

And Windows and the Macintosh have user interfaces are lightyears ahead of punch cards and dumb terminals.

But I still feel like I'm missing something in terms of tangeability of these highly engineered and miniturized systems -- both radios and computers.

[Tom WA3KLR]

The transformer is not the whole situation in a power supply.

Consider a low voltage high current power supply.  The FWCT avoids the loss of 2 additional diodes.  Which supply is more efficient? It depends on whether you are in the engineering department of a transformer sales company or an engineer in the power supply or product engineering company.

Bridge = 1 secondary winding.
FWCT = 2 secondary windings.

[Opcom]

Very nice rig, and fine pictures of good hams!

Can't top the number of MV rectifiers, but there are two pole pigs in there and the four MV's are huge 673's. The picture is of Durward Tucker, W5VU, the builder of the rig. Looking at the rig with all the MVR's, it seems like Mr. Tucker might have got some ideas about the front panel, lots of meters and the use of variacs, and heavy steel panels, from that article. Check the desktop gear. There's not a huge amount, but it is first rate stuff, right down to the highly radioactive radium dial antenna position indicator. Notice the bow tie! One does not get any more respectable than that.

[N0WEK]

Really heavy iron!

By my count there are 21 pieces of iron in that transmitter, counting the filter chokes and the variac! Lots of filament transformers for the rectifiers.

How about scans of the parts lists and such too?

As requested, the parts list and for further inspiration a flock of Super Skyriders from another issue.

Rodger WQ9E

Thanks, I wonder what all that would weigh?

 

[Opcom]

One transformer is listed as 5KVA but I think it is 0.5 KVA?  -the modulator power iron. otherwise that would be one very heavy ingot. I have to respect the rig, it uses MV for bias rectification. I am guessing 1500 lbs. I wonder if the rig still exists?