Linear Amp Behaviour

[Ed/KB1HYS]

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.

[w4bfs]

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

[WA1GFZ]

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.

[The Slab Bacon]

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.

[WD8BIL]

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.

[W8IXY]

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   
Ted  W8IXY

[flintstone mop]

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   
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?

[W8IXY]

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

[k4kyv]

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.

[Knightt150]

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

[Ed/KB1HYS]

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. 

[flintstone mop]

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

[KM1H]

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.

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.

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.

 

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

[K5UJ]

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. 

[flintstone mop]

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.

[KM1H]

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

[ke7trp]

That was a nice amp Carl

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

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

[WA1GFZ]

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

[Ed/KB1HYS]

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.

[WA1GFZ]

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.

[KM1H]

That was a nice amp Carl

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

[KD6VXI]

That was a nice amp Carl

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)

[WA1GFZ]

7500 come on man

[ke7trp]

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

[KM1H]

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

Carl