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Author Topic: 813 Lew McCoy Linear Amp.  (Read 26044 times)
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W2PFY
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« Reply #25 on: April 26, 2015, 11:29:09 PM »

Another way you can get a center tap is by connecting another filament transformers secondary
across the 813 filament transformer that has a center tap. Anything above 10 volts will work. That will give you an additional voltage source from the primary of the new transformer to provide bias or some type of control voltage for a future purpose. Looks like a GG circuit and seems to be built nicely. If your going to use it on AM, the expected output will be about one third of the plate dissipation plus the drive power feed through.
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« Reply #26 on: April 27, 2015, 02:46:28 AM »

No harm in running a lower voltage, especially for checking the amp out. Later if needed you can put in a bigger tune cap. The loading cap should be enough voltage rating for a single 813 at full voltage if attached to a decent load.

The drive especially G1 current will be quite higher when the tube reaches max. power at a low plate voltage, and less drive is needed at a high plate voltage. So watch you don't burn the control grid.

For a simple HV probe that can be made in 30 minutes, figure out the input resistance of your meter. Might be 1 meg ohm on all ranges, as some digital meters are, or like an analog meter it might not be one value, but differing depending on the selected range.

Analog meters are usually rated in Ohms per Volt, that is, a 20,000 ohms per volt meter: on the 500V range it would be 10 Megs, on the 200V range it would be 4,000,000 Ohms, and on the 50V range it would be 1 Megohm, and so forth.

You can make a X10 probe for it that will be safe to use with care.
Multiply the input resistance on the selected range by 9.
Make the HV resistor string: Put 10 resistors in series to achieve this value.
Be conservative with wattage. Don't overlook that 1KV on a 1Meg resistance pulls 1mA current, and is 1 Watt dissipated.
Put the resistor string inside a PVC or glass tube for insulation. One end is the probe tip, the other end accepts the meter's probe.

So for a 1 Megohm input voltmeter on the 200V scale, putting a 9 Meg resistance in series with it makes it a 2000V meter.
And for a 20,000 Ohms per volt meter on the 500V scale, putting a 90 Meg resistor in series makes it a 5000V meter.

At this point many people may prefer to solder an alligator clip to the high voltage end of the HV resistor/probe and clip it to the 'B+ test point', and avoid handling the resistor while it is live. That's a good idea, and best not to rely on cheap PVC or unknown glass tubing for keeping such high voltage off your hand. Also note be sure the meter's GND or negative lead is very definitely grounded, otherwise the meter will float at the high voltage.

A similar design is in the picture attached. It is a 20:1 probe for use with a 1 meg input resistance meter or audio scope for checking modulated B+.
It's wired different, so disregard that, but just to show the resistors in a tube idea.



* HV probe.png (28.48 KB, 972x561 - viewed 396 times.)
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« Reply #27 on: April 27, 2015, 01:01:46 PM »

It probably would be advisable if future discussions on "breadboarding HV supplies" be continued in a separate thread. Let's keep this thread focused on the 813 Lew McCoy Linear Amplifier.

A "breadboarding HV supplies" thread can definitely stand on its own with probably very useful information especially on safety and design.

A new thread has been started: http://amfone.net/Amforum/index.php?topic=38298.new#new
W9ZSL and K1JJ threads have been moved.
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« Reply #28 on: April 27, 2015, 01:27:21 PM »

Thanks Guys for all your great Ideas, I think I will rewind the Filament transformer
and I certainly will put a CT on this time!
I do have a winding machine, Homebrew of course, but it works.
This transformer isn't to hard to rewind, because it has two different bobbins
one ,the primary the other secondary. I have a good supply of #18,16, and 14 enameled
wire. So give me a few days and I'll see if I can get this puppy to come alive.
Yes, I am careful about the HV, I can see the caps bleeding off when I shut the rig down.
No I won't be using this amp for phone, just CW.
Thanks again, I'll be getting back soon.
Gary
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KB3WYZ
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« Reply #29 on: May 19, 2015, 01:42:25 PM »

Ok guys, Here is where I stand on the 813 Linear Amp. First off I put a 2.5uh choke
from one side of the 10v fil supply. That worked...At least I am now getting FR to the
Ant. tuner. However, with key down the plate voltage drops from 2300vc down to
about 900vdc. The set is now showing some plate and grid current. I have been told by others that
my plate transformer is way to small wattage wise. Before I hooked the choke to the filament
transformer I got jack squat nothing. At least I think this project may still have a chance.
On CW I am getting about 40W out put with the amp maxed out. Just the DX-60, with out
the amp will do about 75 W out. Oh yes, I burnt out the motor for my transformer winding
machine  Cry Guess I'll have to use a vari-speed drill. I have some iron for a bigger tranny
about 14 lbs. Now to get it rewound, any ideas are welcomed.
Gary/KB3WYZ
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« Reply #30 on: May 19, 2015, 02:19:43 PM »

Ok guys, Here is where I stand on the 813 Linear Amp. First off I put a 2.5uh choke
from one side of the 10v fil supply. That worked...At least I am now getting FR to the
Ant. tuner. However, with key down the plate voltage drops from 2300vc down to
about 900vdc. The set is now showing some plate and grid current. I have been told by others that
my plate transformer is way to small wattage wise. Before I hooked the choke to the filament
transformer I got jack squat nothing. At least I think this project may still have a chance.
On CW I am getting about 40W out put with the amp maxed out. Just the DX-60, with out
the amp will do about 75 W out. Oh yes, I burnt out the motor for my transformer winding
machine  Cry Guess I'll have to use a vari-speed drill. I have some iron for a bigger tranny
about 14 lbs. Now to get it rewound, any ideas are welcomed.
Gary/KB3WYZ

Gary,
What is the plate current while the HV supply drops to 900V?   When your bringing things up, its a good idea to have the proper metering in place so you can see some of the key parameters.  If the drop is being caused by the very small plate transformer, the transformer should be getting very hot in a short period of time.  Once you know the current you have to supply from the power supply you should then be able to size the transformer correctly based on the rectifier topology you pick.  I assume this is one of your first projects and no better way to learn than by doing!  Keep up the good work.

Joe-W3GMS     
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« Reply #31 on: May 19, 2015, 05:32:41 PM »

A good source for a filament transformer base unit is a microwave oven transformer. 
Remove all the wire on the secondary or hv side, leaving the primary or 120 volt winding intact then rewind the secondary with about #14 or #12 neopreme covered wire to get the desired voltage for the filaments in use.  I believe the the turns ratio for most microwave transformer is one turn per
volt on the secondary, that being the case, only about 10-12 turns should do the trick for 10 volts out and don't forget to put in a center tap.

I have one made this way running dual 813's with no problem and very little heating.
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« Reply #32 on: May 19, 2015, 05:33:31 PM »

No correctly designed power supply is going to drop 60% of its no load voltage with a moderate load of a 813.  You need to do a serious review of your power supply.

Fred
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« Reply #33 on: May 20, 2015, 02:21:15 PM »

Do have ratings for the B+ HV xfmr??

Pix of the PS?
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« Reply #34 on: May 20, 2015, 11:01:02 PM »

 "You can make an artificial CT with two resistors." A resitor Y 10 ohms each leg.
This works very well BTW. But use carbons which have no L factor. I use a DX60b on a HB amp and ran into a problem at first using wirewounds. (would occasionally mysteriously go open)Guessed it was because they had L and I didnt want that so made them up from paralleled carbons instead. End of problem completely. Mine is a twin 811A but the problem was exactlly as you state when there was a problem in that circuit. Voltage present and correct but no current and no performance. If you are just using the DX60B you dont need the relay since you aren't listening on it.
I also use that trick in my directly heated 1929 one tube transmitters (45 , 01A , 171A)
don
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« Reply #35 on: May 21, 2015, 12:20:46 PM »

Hi guys, here is the DC power supply.



* 001.JPG (3518.44 KB, 3648x2736 - viewed 373 times.)
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KB3WYZ
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« Reply #36 on: May 21, 2015, 12:29:05 PM »

The Plate transformer is about 100ma, the AC secondary is delivering 862 vac to
the rectifier board, I would estimate about 2350VDC no load.
I am going to start rewinding the Fil. tranny today CT this time!!
More later,
Gary
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« Reply #37 on: May 21, 2015, 02:40:41 PM »

Yeah well, multiply 862v x 0.100, tell me what you get?

That's the number of watts you have available.

You need MORE watts than the expected power output.
Check the Bill Orr Handbook for better info on how to design and rate HV power supplies.

Best to get one at a hamfest, online, or used via Amazon. They were published for each year,
so you want one from some time in the late 60s I expect. You'd need to read up more, before
proceeding any further.

The power supply transformer ought to weigh at least 10lbs, or more. More is better.
Rule of thumb here.

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« Reply #38 on: May 21, 2015, 03:27:41 PM »

airc the Tron sez 10 Watts per pound for usual EI core transformer .... somewhat more for toroids or hypersil cores
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« Reply #39 on: May 21, 2015, 06:02:36 PM »

How about this...About 13 lbs.
This is from a 1960's Zenith TV.
Gary


* 020.JPG (3269.9 KB, 3648x2736 - viewed 329 times.)

* 021.JPG (3502.38 KB, 3648x2736 - viewed 451 times.)
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« Reply #40 on: May 21, 2015, 07:10:07 PM »

What is the the part number on the Zenith xfmr??  Should start with 92-.  I can cross reference the piece to a Stancor or Thordarson, from that we can find out more about what current the xfmr was good for.

Fred
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Steve - K4HX
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« Reply #41 on: May 21, 2015, 09:48:03 PM »

Someone had a BC-610 HV/plate transformer in the For Sale section. Get it and quit messing around. TANSTAAFL applies. There ain't no such thing as a free lunch. Get a real power transformer and your problems will go away. This may seem harsh, but the power supply of an amp is like the foundation of your house. If you don't get that right, the rest doesn't matter.
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« Reply #42 on: May 21, 2015, 10:21:25 PM »

Yeah well, multiply 862v x 0.100, tell me what you get?

That's the number of watts you have available.

You need MORE watts than the expected power output.

I wasn't going to say anything, but I'm starting to feel guilty to see a project that is surely doomed for failure unless there are some changes.

Yes, Bear pretty well summed it up.  A voltage doubler, tripler or quadrupler  multiplies the voltage, but it also multiplies the poor regulation sag proportionally.   It's a cool ham thing to want to use what ya got, but using a small power-rated transformer AND using a multiplier too is like building your own tubes. Why bother when the results will suck?

In my opinion, a linear amplifier with good IMD figures (cleanliness) is the most important thang. A 2KV power supply that sagged even to 1600 volts under modulation (20%) will have your ham neighbors in a tizzy about splatter.

As Steve suggested, look for a husky transformer in the ham ads. Get something that weighs at least 40 pounds and preferably bigger..  There are a lot of older hams with cellars filled with iron that they will never use and don't care to move anymore. Put a few "wanted" ads out there and you will be flooded with offers.

You won't believe how solid a big transformer using  a choke input and 30 uf or more filter capacity can be. You might see only 50-100 volts of sag -  which is needed to be super clean.  Much will depend upon where you idle the amplifier vs: peak power -  and your syllabic rate on ssb or AM.

Good luck.

T

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« Reply #43 on: May 21, 2015, 11:02:24 PM »

Thanks Tom, glad you told him and not me.  Voltage doublers have no place in ham xmtrs.

You're right about hams having basements full of iron they'll never use.  I have some plate xfmrs that I've had for for over 50 years, still waiting to use them in something.

Fred
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« Reply #44 on: May 22, 2015, 10:41:49 AM »

Another way to make a suitable filament transformer is to use an easy to get 12V/6A Toroidal transformer.

I measured the voltage per winding, and calculated the amount of windings for 7.5V and 5V (For Taylor T40 and 100TH tube  use)

Used a knife to scratch of the laquer and soldered the leads. In the center I soldered a CT for both voltages.

I'll attache a picture to give an idea....

Martin, PA4WM


* Toroidal filament transformer.jpg (245.6 KB, 926x542 - viewed 346 times.)
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« Reply #45 on: May 22, 2015, 11:01:50 AM »

Thanks Tom, glad you told him and not me.  Voltage doublers have no place in ham xmtrs.

You're right about hams having basements full of iron they'll never use.  I have some plate xfmrs that I've had for for over 50 years, still waiting to use them in something.

Fred

Maybe we should have a HV iron section on this forum.where we could trade plate XFMRs.  I must have a ton of iron (I think I'm just kidding).

 Grin
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« Reply #46 on: May 22, 2015, 11:12:09 AM »

Fred, the numbers on this Transformer is...

95-1311
138236
In was a 13 pounder...
Guess that's still pretty small.
This tranny, the secondary was open.
I have never seen wound like this...Looks like the primary had a CT on it???
Thanks guys, about ready to cut bait.
Gary
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John K5PRO
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« Reply #47 on: May 22, 2015, 11:26:49 AM »

I have a lot of transformers that would fit that sized amplifier, Dahl and Chicago/Stancor mostly. Let me know what you really want and I can look, and give you an option that way.

The series capacitors look to be different makes and sizes, maybe same rating. I advise against mixing electrolytic brands and ages in the same series circuit, as the leakage current of one may be far different than another. This would result in dangerous differences in transient, ripple and DC voltages across those caps. In essence, one might be exceeding its 450 volt rating during some portion of the waveform or turn on step. You have to put a stiff low enough resistor across each one to try and balance that, it ain't worth it in power loss to do that.
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« Reply #48 on: May 22, 2015, 01:52:48 PM »

Zenith 95-1311 crosses to a Thordarson 26R59 and Stancor P-8353.  P-8353 is rated 265-0-265 vac at 300ma, 5vac at 6amp, 12.6vac at 6amp.  Primary 117vac 60Hz.  The xfmr is rated for cap input filter (300ma).  With a choke input filter the current can be increased 30%.  The xfmr has two 5U4 sockets built into the top plate.  Wt 12.5Lbs.

The 138236 is a manufacturer and date code.  138 indicates the xfmr was made by Chicago Xfmr Co which was bought by Stancor in the 1960s.  The 236 indicates the piece was made in the 36th week of 1962 or maybe 1952 (IIRC).  The date code didn't indicate which decade, you had to guess that. (The two 5U4 sockets in the top plate was more of a 1950s thing).  Once burnt my forearm on one of those top mounted 5U4s reaching around it to pull some other tube out.

You can add up all the VoltAmps the xfmr was designed for and put all of it into the HV winding and most old big TV xfmrs can supply more than that (easily 50% more) without getting too hot.

IMO the xfmr is still too small for any type of 813 xmtr.  You need a bigger xfmr.

One last thing,  the primary unlikely had a CT or any tap,  very often the HV winding is first on the core and the primary is over it with the filament windings last.

Fred
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« Reply #49 on: May 23, 2015, 01:39:12 PM »

Thanks to everyone for your kind suggestions, The McCoy "JUNKER" Linear Amplifier
is now HISTORY...I started to part it out. I wanted to keep it all onboard like described
in the article. I have used voltage multipliers before very successfully in CW homebrew
transmitters. I guess it's just a matter of choice. I have been having health issues and my
eyes are failing me. I love to wind coils, but just can't see all that well to do a neat job.
I see another gentleman on this forum is going to build this McCoy amplifier. Sure do wish him
the best. Looks like he has the right Transformer. I'll be watching this forum for his progress.
Best wishes to all!!
Gary
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