813 Lew McCoy Linear Amp.

[KB3WYZ]

Hi Guys, Just discovered this forum, and this is my first post.
First of all, I'm 70 years old and retired as an electronics tech.
All of my work was mostly solid state circuit boards.
I have had my Tech.class license for about 3 years now.
I rebuilt a Heathkit DX-60A and it works fine, however my code skills are
terrible...
My hobby is building stuff and restoring old tube radios. I have also built 3 QRP
CW transmitters.
Now to the point, I ran across the "JUNKER" linear amplifier, It is almost complete.
However I can't seem to get it to work with the DX-60. The 813 glows nice and bright
and is getting 9.75vac to the filaments. The separate
 plate transformer is delivering
about 2300vdc. I posted a picture...I think?
When tuning up into a dummy load, it shows no plate or grid current what so ever.
I have talked to some other guys on the ARF and they think my Transformer is to small current wise
and I agree. But wouldn't you think I would get something out?
I hope someone can help me out, by the way, I just started rewinding a new power transformer,
this one is about 450ma. weighs about 15 lbs.
I was wondering if the 813 tube is bad? and how can I test it?
Thanks for any help,
Gary
KB3WYZ

[Pete, WA2CWA]

After reviewing your picture, your problem seems to be that you don't have any wires connected to anything.  

If you're an ARRL member, you can review the article here:
http://p1k.arrl.org/pubs_archive/63846

[w1vtp]

Gary

Congrats on your upgrade from Tech.  What did you upgrade to?

http://www.qrz.com/db/kb3wyz

You may have posted the wrong pic.  No 813, meter etc.  We'll need more specifics such as a schematic of what you have, grid drive (ma).  Are you using cutoff bias with gridleak etc.  The more pics the better but that seems to be a very early pic. the 813 socket is still in its poly bag

Recheck and repost please

Again, congrats es gl, Al

[KA2DZT]

Is this a trick question??

Welcome to the Forum.  Please don't start out by trying to confuse us.  Most of us here are already pretty much confused.

Seems you put up the wrong picture as I'm not seeing any 813 or as Pete said any wires.

Anyway, too small of a xfmr would not cause you to have no plate and no grid currents.  Something else is the cause.

Fred

[Opcom]

An 813 can be tested in circuit, or by applying the various book voltages to it and checking a few points on the curves.
Care should be used with the HV of course. The idea is to cover sitations of high current, and situations of high voltage.
I test the odd high power tubes like that and it seems good.

Just take the values from the AB1 modulator data and also use the curves:
high current test examples: eB, Eg3, Eg2, eG1, plate-mA
2500, 0, 750, -95, 25
1000, 0, 300, 0, 150
1000, 0, 300, +100, 850 (very short pulse)  and Ig1=35mA

now, it might not make the 850mA if it is worn and old, so be the judge of it.

high voltage test example, series resistances advised to limit fault current:
eB, Eg3, Eg2, eG1, plate-mA
2500, 0, 400, -300 check if it is cut off. class C data
4000, 0, 400, and bias it to cutoff - see that it is possible. Peak modulated plate voltage withstanding test. A half wave rectified ac source can be used for this.

Tube should be gettered if it is old before doing tests with high voltage.

[KB3WYZ]

Can't seem to reply?

[KB3WYZ]

I'll Try again..
http://www.w7ekb.com/glowbugs/projects/JunkerAmp.pdf
Here is the schematic.
Gary
KB3WYZ

[KB3WYZ]

Here is some more Pic's

[KB3WYZ]

Sorry I had to use up so many reply's, when I posted everything I wanted
It kept coming up ERROR.
But anyhow, I was wondering if any body would test my 813 in one of there
working projects? I would pay shipping both ways...
I got quite a bit of $$ in this project, and would hate to part it out.
Has anyone here ever built this project?
Thank's, Gary
KB3WYZ

[WBear2GCR]

Gary, welcome to AMfone.

First suggestion: slow down.

A couple of random thoughts to start.
Check the tube manual to see if the 813 is rated for horizontal operation - many tubes are not and some need a specific
orientation of the grids.
I suspect that your loading & plate tuning caps are on the edge as far as the voltages and power expected. So, probably a good idea to leave some extra real estate for putting in physically bigger ones.

Patrick (opcom) posted the specs from the tube manual.
Do you see PLATE current when the amp is on and keyed, no signal??
Is the Plate Meter connected, connected properly?
Do you have HV on the plate?
You can see the filaments lit.

If you do not see quiescent plate current, find out why first.
You can measure the plate current indirectly via a small resistor in series with the cathode.
Ohms law will let you convert the measured voltage across a cathode resistor to amperes.

Before you send the tube anywhere (which could break it) make 100% certain that you have wired everything
properly.

MAKE 200% CERTAIN THAT THERE IS NO HV PRESENT ANYWHERE AND THAT THE PS IS UNPLUGGED WHEN WORKING INSIDE THE CHASSIS ONCE IT HAS EVER BEEN PLUGGED IN. PERIOD.

[Donnie SWL]

Just looking at your pictures was wondering if you are grounding the center tap of the filament trany in transmit??An yes the 813 will fine laying down as long as the plate structure is in a vertical plane as you have it......
Your layout of parts may need to change some.....And pin 3 needs to have a shorter ground ....I would ground it right at the mounting screw closes to the pin .....

[WBear2GCR]

Looking at the schematic (had to open it with Reader 9.0 to see the schematic, tnx Adobe!) you can put a 1 or 10 ohm resistor between the CT of the filament xfmr and ground and measure the vdrop across it to determine plate current.

I assume you can directly measure with a HV probe the B+?
In which case you should monitor the B+ no load, and key down, no input signal (no RF). It ought not drop much at all
with quiescent idle current. That would be the first set of tests.

Double check ur wiring, make sure you have the inputs and outputs connected?

Btw, what is the little cube relay doing??

                 _-_-

PS, if ur rewinding the power iron, suggest you change it so that the secondary can use a simple full wave, not a doubler.
Much better.

[K1JJ]

Hi Gary,

What they said.

To measure plate current, you can also break the filament transformer CT and place a 500 ma meter directly in series with this lead to see plate current.  Later, once you determine what plate voltage you will be using, you can add a string of diodes or a zener in series with the fil CT to get the proper idling current.

Under full load, you may have problems getting a high enough plate voltage and good regulation with the voltage doubler method using a small "TV-type" transformer. Though, one 813 in linear service is a pretty mild load.  

An 813 in linear service can go up to about 3500 volts. I know some guys who have run 4KV. But  at 2000 volts, you may be running out of voltage headroom before the tube puts out all the power it should.   This is not to be confused with the lower ~2KV voltages that are normally run in class C plate modulated service.  I would look around for a more robust plate transformer that can do  1800V-2200V using capacitor input into a bridge diode at 250-400 ma. This will give you what you need for peak performance.

Why do I worry so much about HV plate regulation? It's because poor regulation will produce poor IMD and splatter. Right now in GG, the grid and screens are tied to ground, so they are rock solid. The only thing susceptible to modulation current sag is the plate supply. Get that covered and you have a solid, clean amplifier.  

Be sure to load the stage heavily, thus, peak C1 and C2 for max power out and then unmesh C2 a little to produce about 7% less power out. This is the most linear way to run the amplifier.


This may then require you to replace the plate tuning cap with a higher voltage rated unit, but the rest of the circuit may be fine.  A big Variac on the high voltage supply is a good idea to use for testing and later finding the amplifier maximum limits.  Do you have a big Variac?

Also, replace the red HV thin wire lead that runs from the 813 plate to the pi-network plate coupling cap. This is an RF path and needs to be of copper strap or at least copper tubing.  Grounded grid is a pretty stable config and will probably be OK as-is below 40M, but might as well do it right.

If you find the amplifier has instability problems, especially on the higher bands, consider shielding the filament and 813 input circuitry from the  plate circuitry. Right now they can see each other, though in GG you may be OK.

As the guys have suggested, the first thang is to get plate idling current and go from there. That is an interesting metering circuit the way Lew is able to monitor plate current, voltage and grid current all on one meter. Hopefully the meter is still working and it is wired correctly.

There's a few more suggestions, but get it working first and we'll chime in later on.  Be super careful with the HV, OM.

Tom, K1JJ

[KB3WYZ]

Thanks for your support and Ideas.. First of all, the filament transformer that I
rewound don't have a CT...Could this be the problem?
I would like to stick to the original plan as much as possible.
I have already started to rewind a bigger Transformer, the primary is done, and
the wire for the secondarys is on order.
Here is how I have been trying to make it work. I tune up the DX-60 on 40m to 7.120
and get the best SWR on the tuner going to the Ant. The output of the exciter, DX-60
to imput of the amp, the amp output to input of the ant. tuner.
The cube you see is a relay which bypasses the amp, and when in the transmit mode
it switches it through the amplifier. Going into a dummy load, with key down I get no output
at all through the amp. When I bypass the amp, the output is just fine.
I don't like testing like this because it's going to take out the final on the DX-60.
I get nothing for plate or grid current, I do get a constant 2300vdc on the plate of the 813, key up or down.
I do not have a HV probe.
I hope you guys can read the project write up..I sent the link to.
Thanks for your kind help and patience.
Gary
KB3WYZ

[Donnie SWL]

Gary the tube will not work unless the cathode is at ground which is the fil on an 813......No center tap then try a 2.5 mh choke from one side of the fil to ground an you should start drawing some current then...

[WD5JKO]

 The filament transformer in the image is a Signal 241-8-24. This is a 24vct @ 4Amp transformer. Apparently with a split bobbin design, the coupling primary to secondary allows the secondary voltage to droop to 9.75V. Does that transformer get hot?

The correct transformer in the family is a Signal 241-7-10 which is 10vct at 5 amps.

Either way your transformer has a CT. Put a 50K 2W CT to ground for standby bias, and then short CT to ground to operate. If you key the B+, then just hard ground the CT all the time.

Jim
Wd5JKO

[KB3WYZ]

Jim, Your right about the transformer. But I removed the 24V secondary
and rewound it to put out 10vac which under load comes out to 9.75vac.
No the transformer don't get hot.
Sure wished I would have wound it with a CT.
I think I'll try the choke idea running one side of the
Filament to ground.
Thanks, Gary
KB3WYZ

[WD5JKO]

Gary,

  You can make an artificial CT with two resistors. For example, two 10 ohm resistors could be used (will dump 2.5 watts each, so use 5-10w resistors). One from each side to ground. The cathode return though will see 5 ohms to ground. At 200ma cathode current, that will add 1 volt of cathode bias. If this bias fluctuates with excitation, the amplifier IMD goes up. Dropping to 4.7 ohms each side might be better except that each one would dissipate 5 watts heat from the filament voltage.

   My suggestion would be to add the CT since your an experienced transformer rewinder.

Jim
Wd5JKO

[WBear2GCR]

also, pull the old incorrect label from the transformer and mark it appropriately.

It might be possible to bypass the resistors with caps that function above the 60hz freq, so that the modulation
is essentially bypassed (it looks like ground at AC).

Easiest to do this by converting the filaments to regulated DC first...

[W9ZSL]

I'm going to follow this with interest since I'm about to start a build using a single 813 from the ground up though my project won't be for a linear amp but for an AM/CW rig.

[Donnie SWL]

I think your tune capacitor might do some arc welding.....but maybe i'm wrong.....be careful

[KA2DZT]

Just for test purposes you can ground one side of your filament xfmr to see if the amp and the 813 are working.  813s are a directly heated cathode tube meaning the filament is also the cathode.  You need to ground the cathode to have the tube do anything.

You can ground through a small resistor (10 ohms) to measure the plate current by way of the voltage drop across the resistor (as others have mentioned).

Fred

[WBear2GCR]

Hey Gary, this is half the fun!

as far as I know, everyone has had to go through a similar "learning curve". And the truth of the matter is that with a very
few exceptions all of the true masters of these things are SK now.   We all follow behind them, in their shadows...

                                    _-_-bear

[W9ZSL]

I find the prospect of messing around with 2KV for the RF and 1500 volts or so for my modulator REALLY SCARY!   I've bench-tested the power supplies and no problem, but the application is another story.  I've built a few transmitters and a lot of audio gear in my time but if nothing else, I'm building this 813 amp at a slow crawl.  Slow, SCARY crawl.  At least the guys here are patient and willing to help.

Of course there are a few slightly "off center" folks here, but they have to be because we believe in Ancient Modulation!  Oh, one more thing, when you use a linear for AM, you run into a whole 'nether world of efficiency problems provided you can get the beast working properly.

You might want to check out "Don's Tech Talks" here and the article from 2/18/2011 dealing with linears VS plate modulation.  Good luck!

[KC4VWU]

Respect it, but don't be scared of it.