The AM Forum

I though (perhaps incorrectly) that the Plate Current on a linear amp should remain stable when I'm transmitting through it.

What I've found on my home brew amp is that when I start transmitting through it the plate current will change quite a bit. 

It's a grounded grid amp, pair of 4-400's in parallel.  Seems to work OK on SSB & CW but my SSB rig has lower output than the rig I'm using as an AM driver for the amp.

am linear operation needs 4x carrier power for modulation (100%) headroom ... more if asymetrical ... a am rig to use with a linear amplifier needs to have this ability to ensure low distortion operation

Do you have enough air going past the tubes?
How is your tank Q? You should need around 300 pf plate tuning at 80 meters and a bit less at the top of the band. Try putting a bolt and a couple washers between two turns of the tank coil shorting two turns with a low resistance connection. Retune, it will take more C. If things improve then the Q is too low.

Ed,
     Soundz like you might not have it loaded heavily enough. A small amount of forward swing on high audio peaks is normal. But if you have excessive swing you either dont have it biased properly or (most likely) you dont have it loaded heavily enough. A leanyour must be loaded much more heavily than a plate modulated
class-C final.

What are ya driving it with on A.M., Ed?

If it's got any form of controlled carrier it'll look more like SSB.

Just run it pure Class A.  The plate current will hardly wiggle.  But run it only in the winter 'cause the thing will throw off LOTS of heat.

73   ;D
Ted  W8IXY

OUCH!
Most linears, even the healthy ones would melt in Class A mode......BTW any links on AMFONE or AMWINDOW how to tune a linear for AM mode?

I use an AL811H (with 572's) driven by a K3.  I tune the amp for maximum output with full carrier (CW) and max out the output power by futzing the tune and load controls.  Once I get about 700-800 watts CW carrier out (with about 60-70 watts from the K3), I reduce the drive from the K3 to about 125-150 watts carrier from the amplifier.  That should allow the sidebands to peak about 600-700 watts, and stay within the linear capability of the AL811H.  The carrier output level from the K3 at that point is about 12 to 15 watts.

So, do I fire up the Valiant 1 for 125 watts carrier out, or use the K3/AL811H for essentially the same power output?  The K3/AL811H combo probably uses about the same amount from the AC mains as the Valiant does.  Its nice to be able to use one or the other depending on how much boat anchor weight I feel like firing up.  Choice is good.

73

Ted  W8IXY

It doesn't make much difference, although the Valiant would probably be easier to tune up, adjust and get everything exactly right with the carrier and the audio.

A long time AM broadcast transmitter manufacturer once pointed out that there is very little difference, total efficiency wise, between high level plate modulation and low level grid modulation or linear amplification.  By total efficiency we mean watts pulled from the a.c. power mains compared to watts fed into the antenna.

ED: I run my ranger into my FL2100B about 12 Watts in 120 Watts out the 572B's plate current gose up about 15 ma as I talk. This is normal what is not normal is a large plate current swing (now we are talking AM here) not ssb or cw.

John W9BFO

Ok, well food for thought here. 

I am driving it with my valiant. I typically do not run the valiant at full output so let's say I'm hitting it with 90 watts or so into the cathode to drive the dual 4-400's.   BUT I do notice similar behavior when I am driving it in SSB mode with my KW TS-520.

I do load it up to 600mA Plate Current (no modulation). Resting current is~50-60 mA.  B+ is 2250 volts or so (best I could do).   I did notice that the Voltage regulation is lousy, occasionally the B+ will drop to 1900 volts or so under modualtion.  I'm sure this isn't helping either. 

Hi Ed,
Maybe  some of the builders of amplifiers and transmitters will drop by (K1JJ). I believe your B+ supply needs to be a strong 2500 supply.
And you are driving it with 90 watts from the Valiant??
Tuning in the CW mode, what is the max carrier out of the amplifier?? Power supply volts at this max carrier? How many watts of RF drive from the Valiant in this CW mode tune up??
Once you get to that max carrier then you reduce the DRIVING RF (Valiant) 1/4 of whatever the peak was in the CW mode.
The 90 watt drive doesn't seem right, Ed

Fred

Unless you have a very lossy input that is way too much power. I can drive a pair of 3-500's to a 350W carrier with 30W of drive which is about the safe max for those tubes as they do show considerable color of a dark orange without modulation where the efficiency is only about 30%. Its approaching 60% from that point up on voice peaks.

 

Thats too much Ip, those tubes are running right at their full PD. Load to 300-350W out and peak the grid current to about 125-150ma. You really need a monitor to fine tune it but you should see just a small Ip rise along with an upward kick on the Ig under modulation. Since the meter is slow and the max Ig should be 300ma or less I wouldnt drive or load any heavier.

Going from 2250 to 1900V is excellent regulation and much better than many commercial ham amps will do.

With my LK-500ZC with outboard Dahl transformer, which was an option, I go from 2900 to 2550V from standby to full bore key down at 1500W.  The Alpha 76CA (3 x 8874's) is a little bit stiffer and drops from about 2300 to 2000V key down at 1500W out but its a lot less on SSB and even AM at 400W carrier thanks to the energy storage of the electrolytics. Amps such as Dentrons, SB-220, and others in that price range drop more.

Carl

I operate a Ten Tec Centurion on AM.  it is a pair 3-500zg.  I tune it and load it driving it with ~ 100 w. in the form of cw dits while I look at a SB610 then I increase the loading a tad.   I run the 3-500s at around 300 w.; the Ip is 360 to 400 ma and the Ep is around 2.8 KV.  I modified the amp in a few minor ways, the most significant being an after market fan that moves a lot more air through the p.s. and RF deck.   I use about 20 w. drive.  almost forgot--I see the Ip meter wiggle just a bit with modulation. 

My non-glass tube linear AL1500 runs 3600volts and I drive it with 9 watts from the FLEX. Ceramic tubes are wicked!!! Pa current is around 800 mils
The munkey schwangs on that amp.

Youre correct about that Fred!

The one I like to put on 75 is an updated LK-800 series with the 2900V Dahl transformer and 3 of the pulse rated 3CPX800's. Whenever I get one in Im forced to test it on 75. Thats 2400W of available plate dissipation 8)

That was a nice amp Carl

http://www.youtube.com/watch?v=XzMMi1bhye4

http://www.youtube.com/watch?v=Vn7EYKGjdNE&feature=related

1900 volts on 4-400s will not make much power.

Yep I know.  I am going for a stiff 2500 V plate supply (under load) as soon as I can.  I also want to convert this into a grid modulated Class C Final at some point, but I think I should understand the principals of operation as it sits before I go and rebuild it as something even more complicated.

I think your problem is when the plate voltage goes down to 1900 volts the plate z drops and the tank Q drops hurting efficiency.

The modified ones do an easy 5 out with 120-150W drive and grid current well under max spec.
The 6M EME crowd loves them ::)

The Command Tech HF-2500E that Stan here on the Left Coast ran did almost 7500 watts PEP.  He took a LOT of heat on contesting.com stating that when it went up for sale.

I do believe he had an external supply, custom built, though.

3 times plate diss is pretty easy to get, ICAS...  (as Orr would say IVAS)

7500 come on man

There is one sitting on the table in front of my as I type this.  Not a chance at 7500.   
C

Not as built but with the 3CPX tubes and a 3500V PS its possible to get close.

Carl

Thanks Carl.

At that power level, 6800 can be interpreted as 75.....  10 percent.


--Shane

I dont expect long life at those levels, at 3000-3200 they stand a chance as long as they stay cool.

I had one on 75 a few months ago on AM....for testing purposes ya know ;D Anode temp stayed well under redline ::)

I can't remember where, but I did read an email put forth by an ex Engineer for Harris Comms, where he had stated the 3CX800 was a Harris tube (built by Eimac spec'ed by Harris".  The ratings, as most commonly know, where dropped.  There are two common data sheets floating around, and a third that I've seen, but do not have a copy of.  The third was Harris marketing...  We all know inflations on marketing materials, BUT, usually Mil-Spec isn't inflated NEARLY like consumer equipment.

I think most hams also realize that if they want something that will last forever at a given power level, that same amp better be able to do near or better than twice Pout desired.  As you stated, 3500 isn't that bad of expected Pout.  I know people that run them regularly at 5Kw.  Much more than 5, and you start having issues with parts being run at the ragged edge. 

Doesn't change the fact that you can pull 6500 to 7Kw out of a trifecta of them, if you have the correct power supply.  Stan's uses an outboard Dahl, 3700 volts (give or take) unloaded, 3500 key down.  Xformer is capable of 5 amps CCS.

Anywho.  Internet's back :)  Antenna is now a screwdriver on the eves...  BIG difference from a full size ladder line fed dipole!

--Shane
KD6VXI

What is "Good" voltage regulation for a linear amp?  10% more? less?  I CAN get 3300 volts on here, but it will drop to about 2800 under load. I thought that was poorly regulated so I went with a lower voltage option to get something that didn't show as much swing.  Looks like I should have just kept the original lash up. 

Commercial ham amps run in the 7-12% range, and a bit more with older ones with less output C.

I wouldnt worry about key down regulation as the filter C will provide energy on SSB and even a bit on CW.
Regulation on old iron with a high DC wiring resistance is poor with a C only filter unless it is bled into submission. For those a choke input is better and regulation can be close to 5% with a key down DC output approaching 90% of the AC.

Shane, I have the original 3CX800A7 spec sheet which uses 2500V, and sneaks in a 800ma SSB single tone rating under "typical" but uses 600ma under absolute maximum. Charley didnt appreciate that Ive been told.  ::)
Ive yet to see any 800 that wont load to beyond 800ma, even at 2200V,  while keeping all other parameters in the black. All show 3500V max for pulse. When the first LK-800A's were built they ran at 2200V or so and then Eimac blessed 2800V for SSB for the first custom models and then Denny went to 3100V with some of the 3CPX versions which was pushing the limit of the 7  450V caps in the PS. Ive gone to 8 caps in most and 9 with 3500V key down and 3CPX's. All I know is it buries a 5KW slug.
The 3CPX has a 4500V maximum in pulse service with a 8A peak BUT at a .005% duty cycle as does the 3CX at 3500V. The only difference is an extended ceramic barrier internally to allow the higher voltage.

Carl

I'm running a 866 Full wave Bridge config into a Choke input filter. 

Yea, but why beat the livin crap out of a tube when it can last a lifetime if treated well.
If you want big power build a big rig that can handle it.

That's kinda what I was getting at, with saying if you want an amplifier that can comfortably put out 3500 watts, it should be able to PEP at 6.5 to 7 grand output.

I didn't advocate that the amp should be run at 7 thousand.  I said his can do it.  The 800 is a 1200 watt tube, run sanely.

--Shane
KD6VXI

IMHO, the CPX version pulse rated V(anode) can be used on all the 'pulse rated" tubes.

At 3300 volts, at .8 amps....    That's 2600 input.  Times three....  Well, all of a sudden 6500 -7 grand PEP out doesn't seem ALL that out of reason.

RIP Peter Dahl.


--Shane
KD6VXI

Thats only 7800 INPUT, figure 5100 or so out on a GOOD day, more likely 4800. That should be fine for SSB without stressing the Pd as long as some good airflow is maintained for a few minutes after the last yakking and before powering off. That anode really holds the heat.

I'm talking 6.5 to 7 grand PEP.  The meters don't read peak A, Carl.


IE, the SB220 doesn't read 2800 volts at .75 amps during normal SSB, does it?  BUT, it will do 12-1400 watts PEP out, depending on band.

We can argue semantics all day long.  It's like Rich and his 68 lb xformer in a 14 kilowatt amp.  I'm agreeing with you, the amp IS a 3.5 to 5 kw pep output amp.  It WILL do 6500-7000 PEP.  Seen it.  Don't recommend it.


--Shane
KD6VXI

Yes!  And the biggest reason (at least to me) for running the amp way below its ratings is to achieve a super clean output. It's amazing how dirty the side products come up as we close in on maximum power output.  As Jay says, for every 3db in power reduction, the linear amplifier cleans up 9db in 3rd IMD. Now that's a lot and well worth the reason to build around a BIG tube and run it way cool.

T

darn tootin'. Rather than sell hams an amp with decent overhead, the powers that be prefer to restrict the models for sale leaving hams to push the amp and make rotten signals with it. So to build.

That's because the manufacturers can get away with 'truth in advertising'.

A crap generator is not a linear amplifier

PEP is a laugh to sell product Shane. Real power is the dead carrier and never mind the minimal intelligence in a few voice peaks, its still the average that counts.

Now if you build a PS like Jim, VE7DX,  with strings of 6800 or 10K uF caps then you can bring the average way up but the 3CX800's will be seriously hurting so now you need a 3x6.