813 filament supply

[VK7ZL]

I am going to build a linear using a pair of 813'swhich require 10V@10A for the filaments. I have contemplated using a pair of computer power supplies, each one being able to supply 5V@18A.
Has anyone used this method, if so are there any pitfalls?

Bob  VK7ZL

[KL7OF]

Center tap? Put two 5 v together and make your own center tap?...I haven't tried it

[KA2DZT]

Sometimes regulated supplies are disrupted by the presents of RF.  Probably not a good idea.

What filament xfmrs do you have to work with, anything??

Fred

[k4kyv]

It is best to avoid using DC on directly-heated transmitting tube filaments.

You might want to read over this thread if you haven't already done so:

http://amfone.net/Amforum/index.php?topic=29039.0

[VK7ZL]

It is best to avoid using DC on directly-heated transmitting tube filaments.

The more one reads the more confusing the issue becomes.

The RCA data sheet for an 813 states: Voltage (AC or DC) 10.0 +/- 5%

A fellow ham gave me a transformer yesterday. It has two centre tapped secondary windings each one is 10V@5A but it is quite large,  4" x 4.25" x 5"H and weighs 9lbs. (pic below)
Another alternative is a toroidal transformer. A new one with multi tapped primary and centre tapped secondary rated @ 100VA is $50 delivered to my door or I could rewind an existing unit.

As computer power supplies were not around in the hay day of hams building their own equipment is it just convention now that states we must use iron core transformers or as Fred suggests is RF going to be a problem in a switched mode filament supply?

Bob

[KM1H]

Since its only a 100W requirement Id look for an old house radio or TV that has a transformer of sufficient wattage, strip the secondaries which are usually on the outside, and rewind. There should be plenty of tube gear collectors that can help.

Carl

[KA2DZT]

I rewound a 6.3V 10A for 10V 5A.  Worked fine.  Carl's idea of using an old TV xfmr and rewinding it for 10 volts is not that hard.  You need to count the turns on the 6.3V or the 5V windings.  Then from that number of turns you can calculate the turns needed for 10 volts.  You'll need at least 14ga wire.  I used 16ga wire on my 10V 5A rewind.

OTOH you could maybe bridge the two windings on the xfmr you have.  Just phase the windings correctly.

Fred

[Ed/KB1HYS]

One a computer switching supply,  The center tap issue remains to be solved, since the 813 uses the filament as the cathode you need to figure out how to keep the B+ out of the switchers. Be ready to add additional filtering to the output and shielding to the case as well. In theory you should be able to parallel them for more current. 

Rewinding isn't a bad option.  I don't know how easy they are to find in VK land but I've rewound transformers from microwave ovens to provide filament transformers.  Work quite well and only took a couple hours to remove the secondary, add a few turns to the primary, wind a new secondary and trim the windings to get the exact voltage I wanted. 

[The Slab Bacon]

Nice transformer!! Remember that overkill is always youe friend!

[KC9LKE]

Bob:

Life is hard enough without adding the headache of a PC switcher to the mix. A power transformer is a very robust device and will outlast a recycle bin full of switchers. Use a transformer and you will “rest easy” like your avatar.

“A fellow ham gave me a transformer yesterday. It has two centre tapped secondary windings each one is 10V@5A but it is quite large,  4" x 4.25" x 5"H and weighs 9lbs”

Size and weight of a transformer are the children of power handling capability. You are not going to cheat it much. A 10V 10A open frame transformer is not much smaller, physically, than the one you have. Plus when HB hollowstate, too heavy is a prerequisite.

Rewinding a transformer is a very rewarding exercise and at 10V you won’t have to split it apart, just thread it like a needle.

However it looks like you have a very nice potted transformer there. If you can parallel the windings, and from the picture its looks like you can, I say “Run what ya brung”

Just my 2˘

Best regards

Ted / KC9LKE

[WA1GFZ]

How about a small variac core with a secondary wound over the winding.
You don't want to overkill a heater transformer to protect the tube when it is cold from inrush power. Rewinding low voltage transformers is quite easy.

[K1JJ]

"Rewinding a transformer is a very rewarding exercise and at 10V you won’t have to split it apart, just thread it like a needle."


Good idea.

I've made many a filament transformer for 7.5V at 21A using a simple Variac core.  Remove the contactor arm leaving just the 120V primary winding. Wind heavy insulated wire onto the donut for a voltage ratio of  ~ 10:1 step down. Add a center tap.   Add or subtract a few turns with it under load connected to the tubes to zero in 10V.  Works like a charm.

T

[VK7ZL]

The concensus seems to be 'rewind' so that is what I will do.
Re the microwave oven transformers - I was intending to use two of those in series for the HV supply (after removing the magnetic shunts).

Thanks for all the input.

Bob

[Opcom]

Jacob has removed the magnetic shunts and rewound microwave transformers for 4-400's filaments. It is entirely do-able and very cheap.

[WB3JOK]

The concensus seems to be 'rewind' so that is what I will do.
Re the microwave oven transformers - I was intending to use two of those in series for the HV supply (after removing the magnetic shunts).

Thanks for all the input.

Bob

Check the secondaries carefully... I think on some microwave oven transformers, one side of the secondary is connected to ground (and also not insulated for the full HV).

This won't be a problem if you use two transformers in a full-wave circuit with the CT grounded (e.g. tie the grounded "low" ends together and phase the primaries properly). But in a full-wave bridge, you may encounter breakdown problems with one or both. Just my .02, I have a couple in the junkbox but never used them.

[VK7ZL]

Check the secondaries carefully... I think on some microwave oven transformers, one side of the secondary is connected to ground (and also not insulated for the full HV).

I picked up two MOT's yesterday from the local repairer and indeed one side of the HV winding is connected to the frame and the output is 2Kv, not 1Kv as I thought, so putting them in series is out of the question.
My next thought was to rewind the secondary for use as a filament transformer and although the footprint is slighty smaller than the transformer I already have they weigh the same so nothing really to be gained by that excercise.

Bob

[The Slab Bacon]

Why not put the primarys in series? ? ? ?

[VK7ZL]

Why not put the primarys in series? ? ? ?

A good idea.
The problem is I would then have 3 transformers weighing a total of 26lbs. I will look for a lighter option.

[KE6DF]

Brand new Hammond 10v 10a transformers cost about $46.

I've gotten NOS transformers with those ratings for less than 1/2 that amount on epay.

By the time you mess around rewinding microwave transformers, are you sure it's worth it?

If you spent the same time working for only $5 per hour you would probably be able to pay for a new transformer.

But maybe that isn't as much fun.   :-)

Dave

[The Slab Bacon]

Why not put the primarys in series? ? ? ?

A good idea.
The problem is I would then have 3 transformers weighing a total of 26lbs. I will look for a lighter option.

the filament transformer for my 4X1 rig weighs around 10lbs, the plate transformer weighs 100lbs, and HV filter choke weighs around 50lbs. Not to mention the LV power transformer weighs around 12lbs and the LV filter choke weighs 5 or 6 lbs.

So what's a few pounds amoung friends.................................

[Ed/KB1HYS]

Jacob has removed the magnetic shunts and rewound microwave transformers for 4-400's filaments. It is entirely do-able and very cheap.

Exactly what I did for my Dual 4-400 amp.  I also series paralleled a pair of MOTs (Microwave Oven Tranny) for the Plate tranny.  Dumped the whole thing into a 866 based full wave bridge, whose filaments are powered by yet another rewound MOT.  The whole powersupply fits nicely on a 3/4 inch thick plywood panel with heavy duty casters on it which forms the bottom plate of my home brew rack for the amp. Whole thing is about 60 inches tall, cause I left a big ole space for a future upgrade to a stand alone QRO rig.

[W2PFY]

What is the typical output voltage of microwave xformers?

Are they quiet?

[KE6DF]

What is the typical output voltage of microwave xformers?

Are they quiet?

I was under the impression that microwave transformers are designed for very low duty cycles -- only on for a few minutes at a time. At least, that would apply to home microwave ovens.

So there are barely enough turns on the primary, they do tend to hum, and draw a fairly high (reactive) current even with no load and are therefore prone to heating through i squared r losses in the primary.

But the above comes mostly from hearsay, so I would defer to people who have actually used them in transmitters that are used extensively on the air.

[W2PFY]

was under the impression that microwave transformers are designed for very low duty cycles -- only on for a few minutes at a time. At least, that would apply to home microwave ovens.

Nope, you can run some models for extended time for cooking. I often run mine at the camp for an hour when cooking frozen macaroni. The few transformers that I have seen were wound with Teflon wire but I don't know if that was the lead out or if the whole thing was wound that way?

[cx2ua]

I do not think that the source PC are the best, for me the ideal is to use a filament transformer designed for this case, we must remember that the order for  813 volts 10 at 5 amps and the regulation of filament is the life of the valve + / - 5% or less or more.
 Then there is the issue of the midpoint for the return of the cathode and the other is that the transformer must have an insulation from 2500 to 3000 volts for when the midpoint this case floating standby or act the overload relay.
 A microwave transformer can be useful but never used.
 Well so is the experience and homebrew amateur radio