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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #25 on: March 03, 2009, 02:23:46 PM »

Rich,

To just follow up a little more...

The following examples are just rough examples not related to your 813 amp:

Let's say you have an input power of 1 KW = 3200v X  312 ma.

Using a class A amp at 30% efficiency, you would dissipate 700 watts in the tube's plate and put out 300w to the dummy load.

Using a class B amp at 65% efficiency that becomes 350 watts in the tube and 650 watts output

Using a class C amp at 80% eff = 200 watts dissipation in the tube  and 800w to the dummy load.

So, you can see how your output power is limited strictly by the class of operation and the tube's ability to absorb the wasted power as heat.  Each class has its own advantages and disadvantages.


For AM, in your case, the two tubes' dissipation is pretty much set up when you key the carrier on. If your carrier output is 125 watts, then you may be dissipating  250 watts in the tubes at 33% efficiency. When you modulate 100%, the efficiency swings up to 65%, and the power output to 500 watts. This means 175 watts of dissipation and 500 watts out pep. So, your tube color (plate dissipation) should change very little under modulation and even go down slightly depending on your loading.

Think about it for a while and then experiment.

Later -

Tom, K1JJ
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« Reply #26 on: March 03, 2009, 02:33:24 PM »

Check the voltage rating of your scope probe. It sucks to blow them out.
A pair of 813s may do more than 500 W PEP but I think you want to blow a little air past them. 200 watts carrier is pushing them a bit but might work if the tubes are good and you don't let them get too hot.
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« Reply #27 on: March 03, 2009, 03:23:39 PM »

My scope hooks to my TR switch box which has a connection for the scope.  Simply 2" of stiff wire that runs parallel to to one carrying RF.  So there is no direct connection but the readings are only relative, not absolute.

Rich
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« Reply #28 on: March 03, 2009, 05:28:02 PM »


<Snip>
We have a young cat that I had to ban from the basement while doing this work, he keeps trying to use up all his nine lives.....

Yep!  When I was  JN, I had my cat jump down from an unfinished ceiling right into 400 VDC + RF on my operating bench.  He jumped down, yowled and straight up from whence he came quicker than an eyeblink!  Oh yeah, in spite of this death-defying jump, he still lived to over 20 years old!!
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« Reply #29 on: March 03, 2009, 09:19:34 PM »

speaking of filament sag, you need to remount your 813's to put the plane of the filaments to vertical when you use them mounted hortizontally ... the amp shows a lot of promise ...73 ...John
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K1JJ
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« Reply #30 on: March 04, 2009, 12:09:52 AM »

speaking of filament sag, you need to remount your 813's to put the plane of the filaments to vertical when you use them mounted hortizontally ... the amp shows a lot of promise ...73 ...John


Yep, you're correct, John - I didn't notice it myself.

Rotating the tubes 90 degrees will also put the plate structure in a vertical plane. Right now it's horizontal.  I understand it can sag too and create shorts for the screen, etc.

Haven't thought about this for years cuz all my amps have the tubes standing up.

T
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« Reply #31 on: March 04, 2009, 10:20:09 AM »

I saw the note about orientation in the spec a few days ago but was in Deeeeeniiial.

 Wink

Rich
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KM1H
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« Reply #32 on: March 04, 2009, 10:22:10 AM »

Play it safe and mount that transformer above the chassis. 3/4" plywood and nylon hardware will hold it in place. You could cobble up a little alarm circuit to light a bulb to tell you if the case becomes voltage active.

Decades ago I repaired an AL-80 for a customer who had a hot transformer case. That transformer is still running fine today in a bench test PS mounted as I mentioned above. It even has a warning label on it Cheesy

Carl
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W7SOE
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« Reply #33 on: March 04, 2009, 01:40:28 PM »

Play it safe and mount that transformer above the chassis. 3/4" plywood and nylon hardware will hold it in place. You could cobble up a little alarm circuit to light a bulb to tell you if the case becomes voltage active.

Unfortunately the power supply is mounted in a Heathkit speaker chassis and the transformer is an "interference" fit.  This precludes insulating it from the current chassis.  So I will have to replace the transformer,  covert it to a voltage doubler circuit as Patrick suggested, or remount it in a bigger chassis.
The last option would be the easiest.  I also want to add a better bleed resistor setup.  The current one does not work well, it takes several minutes to bleed down.

In the mean time I will make sure the chassis is grounded well.


Rich
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« Reply #34 on: March 04, 2009, 05:47:17 PM »

I have a better understanding of the operating point of the 813's in this amp but I an not as clear as to what I should do from practical point of view as far as where to set the cathode bias.

When I got the amp there was no bias and the plate glowed a bit.  Can I assume that it was operating as a class A amp?  (and exceeding max plate dissipation)  Maybe this was ok for the builder as in class A the dissipation is constant, I think...

As I add bias I approach class B operation, I am in the AB class now correct?  Is this type of amp ever operated in class B or is there too much distortion?

Ok so anytime I set the bias so that the tube idles at some plate current that is lower than in class A I am in class AB, correct?   I suppose the exciter signal strength would determine where class A becomes class AB, correct?

 I still don't know where to set the bias.  Is there an optimum bias where this amp could be used for both AM and SSB?  Or do I need to add a bias control to switch in more/less diodes?

Rich
 
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K1JJ
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« Reply #35 on: March 04, 2009, 06:29:23 PM »

Rich,

Set the diode bias (by adding or shorting out diodes in the CT string) where the amp keys up (with no signal, idle) and draws about 10ma per tube. That's it, done.   The amp will be set at a compromise between efficiency, cleanliness and ability to drive easily. This is for BOTH AM and ssb.

The amp will be idling  and running in class AB.

When you drive it, the class does not change except for the amount of grid/screen current flowing. If grid current flows, then you are in AB2. If not, then it's called AB1.  Since this is a GG amplifier, it will always draw grid current with drive, so your amp is in class AB2.

The plates shud show no color cuz you are keying up with only 3200v X 10 ma = 32 w per tube. They are rated to dissipate 125 watts.

When you drive it hard in ssb, you may see some plate color at times . In AM, the plates shud not glow during dead carrier conditions or with full audio. A little bit from time to time is OK if you try to squeeze ever last watt out of it before saturation, but keep the air flowing well, if so.

Yes, if the bias is set so the tube idles high and the plate current doesn't change during full ssb drive, it is class A.  Idle it lower and the plate current changes under drive and you are in class AB. Class B is when the tube theoretically draws no current at idle - it's exactly at cutoff. Most amps are run somewhere in AB for best cleanliness and least crossover distortion.

Once you start running tests with tones, watching the scope, you will see the subtle differences of class operation if you add or subtract diodes.  But use class AB2 as set by the 10ma idle per tube setting for this GG linear rig.

Hope this helps.

Tom



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There's nothing like an old dog.
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« Reply #36 on: March 05, 2009, 05:48:44 PM »

Tom,
  I have the diode board installed, set for 20mA for both 813s. 
I applied an AM carrier (using the ricebox) and set the power in for 125W of carrier out.  Unfortunately the power can not be turned down enough to get 1/5 of that.  ( I will double check this, perhaps I was looking for 1/5 on the scope (voltage) instead of the wattmeter(power) )

With the exciter turned all the way down I sent in the 1KHz signal and increased the audio gain.  I never saw the signal flat top even way beyond 100% modulation.  Only when I turned the power up did I see the tops flatten.

This morning I tried using the Viking II to drive it.  First I tried driving it directly, using the "drive" control to attenuate the output of the Viking.  This did not work.  I can see the signal Viking carrier on the scope but there is no increase in the amp plate current and the the roller inductor in the amp tank arcs over!  No power at all on the wattmeter.

I quickly cobbled together an attenuator as you described in another post.  Two turns on the primary of a binoc core in series with a dummy load.  Two turns on the secondary going to the amp.

This worked as far as tuning up the VII and the signal through the amp was clean but still nothing on the plate current meter.  (No arcing this time)

Rich
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« Reply #37 on: March 05, 2009, 08:08:50 PM »

 Tongue

All the problems with the JVII in the previous post were due to a simple wiring error.  (Brainfart)

When I have time I will try Tom's test for maximum power using the VII which gives me better control over the input power.


Rich
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« Reply #38 on: March 05, 2009, 11:04:48 PM »

speaking of filament sag, you need to remount your 813's to put the plane of the filaments to vertical when you use them mounted hortizontally ... the amp shows a lot of promise ...73 ...John

yep. "pins 2 and 6 in vertical plane" Amp looks great!
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« Reply #39 on: March 05, 2009, 11:12:56 PM »


<Snip>
We have a young cat that I had to ban from the basement while doing this work, he keeps trying to use up all his nine lives.....

Yep!  When I was  JN, I had my cat jump down from an unfinished ceiling right into 400 VDC + RF on my operating bench.  He jumped down, yowled and straight up from whence he came quicker than an eyeblink!  Oh yeah, in spite of this death-defying jump, he still lived to over 20 years old!!

Olde Kitty sometimes would sit on the dot matrix printer, printing or not. Once the tip of her tail go into the cog belt that ran the head shuttle, and caused a "print error". From that day on, the sound of that printer would send her under the bed! Kitty lived to 22 years. Half of my life at the time. You get attached to them. 1983 to 2005 RIP
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K1JJ
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« Reply #40 on: March 05, 2009, 11:36:59 PM »

Tom,
  I have the diode board installed, set for 20mA for both 813s. 
I applied an AM carrier (using the ricebox) and set the power in for 125W of carrier out.  Unfortunately the power can not be turned down enough to get 1/5 of that.  ( I will double check this, perhaps I was looking for 1/5 on the scope (voltage) instead of the wattmeter(power) )

Rich

Hi Rich,

Tune the amp up for as much as it will put out. (like 500 -700w?)

THEN reduce the carrier down to 1/5.   When you modulate 120%, you will hit the max pep power of whatever the limit of the amp is. (500-700w in this case)  Watch the scope to see when it flat tops and then back the audio and or drive down to just before that point. You want to be able to modulate 125% with whatever carrier you set it at. It's a juggling act - you have to go back and forth with your adjustments until you get it right.

Sounds like you drove it only to 125W out and reduced it to a carrier of 25w.   It will do much better than that.  With only a 25w carrier, you SHOULD see huge headroom.. :-)

But sounds like you're getting real close to final operations, OM.  Good show!

Tom, K1JJ

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There's nothing like an old dog.
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« Reply #41 on: March 06, 2009, 12:40:55 AM »

OK, I guess I am being timid.  I cranked up the power.  I got to about 350W, I can't remember for sure what was on the plates, something like 150-200mA.

At that point the output power did not go higher even as I increased the input.   After about 30 seconds I got fireworks from inside the roller inductor tube.  Tried it again, same result.

I am not sure why the power stopped going up and it looks like I should take that inductor apart.  Maybe it just can't hang with two 813s.....

Rich
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« Reply #42 on: March 06, 2009, 12:30:07 PM »

Here are some pictures of the inductor zorch.  It looks like the arcing is between elements of the center shaft (at ground potential) and the far end of the coil.  Blew the solder ball right off the "spoke" attachment.

Pretty darn sure that the tank was "dipped" properly.  I am going to clean up the interior roller contacts and give it another whirl.

Perhaps a better solution is to build my own coil with fixed taps.  Are there large benefits to the adjustable inductor vs fixed?  I suppose you can always get a perfect dip for any frequency....

Rich


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K1JJ
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« Reply #43 on: March 06, 2009, 12:52:10 PM »

Rich,

I didn't want to ctiticize the rig, but, yes, I've also had very poor luck running those small roller inductors in amplifiers.  Usually the small roller itself starts to make poor contact and starts pitting and goes intermittant.  It's all a matter of the power you run thru it.

Remember that a resonant tank circuit (L/C)  has circulating currents within the loop that can be extremely high - in the tens of RF amps.  I think the formula is Q times the plate current?

I use these type of rollers in my mini ant tuners for inputs to amps, but not as outputs.

Anyway, consider doing what you suggested - build up a fixed coil with taps and you'll be fine and bulletproof to move on to the next weak part. It's all a shakedown run.

T

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Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #44 on: March 06, 2009, 12:58:32 PM »

When it is all shaken out and running FB then the fun and learning are over.

:-(

Rich


PS Brace yourselves for 2134 postings regarding the proper construction of a multi-tap tank coil.   ;-)
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WA1GFZ
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« Reply #45 on: March 06, 2009, 05:58:48 PM »

I was taught, if you solder your taps you are not QRO.
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« Reply #46 on: March 06, 2009, 06:34:15 PM »

I was taught, if you solder your taps you are not QRO.

As opposed to alligator clips?

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« Reply #47 on: March 07, 2009, 09:37:03 AM »

Nuts and bolts with straps.
If you tune up and the output stops going up as input increases look at the plate current. If the plate current is still increasing there is a good chance the pi network L is too high meaning Q too low. A small inductor will run out of steam at some point as the series resistance becomes an issue.
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« Reply #48 on: March 07, 2009, 05:09:36 PM »

Nuts and bolts with straps.
If you tune up and the output stops going up as input increases look at the plate current. If the plate current is still increasing there is a good chance the pi network L is too high meaning Q too low. A small inductor will run out of steam at some point as the series resistance becomes an issue.

Straps = strapping?  ;-)

I put the roller inductor back in after cleaning it up.  It still arcs over...

I was able to get it up to 500W.  After that the output power did not increase and the plate current did not rise.  I would guess that the B+ transformer was out of poop.  200mA X 3200V = 640 W

The transformer is rated for 249mA at 2260 VAC

Looks like I need to get an old fashioned tapped coil in there.

Maybe next on the list will be a new power supply.


Rich

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« Reply #49 on: March 07, 2009, 11:32:01 PM »

The roller inductor probably has some carbon tracks on it now. In any case, if it is arcing, you need to lower the voltage  or get a bigger one.
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