The AM Forum

First off, I know this does work for AM, and AM only. But what I can't figure out is if there is if the layout has any effect on how it works. Here are the 4 major amp layouts I could think of:

Single ended grid driven
Parallel grid driven
Push-pull grid driven
Grounded grid (single ended or parallel)

I already know that a GG amp biased to class C will work as a linear for AM, but will a class C grid drven amp work too? I also know that the reason a class C amp can be driven with a modulated carrier and amplify it is because the carrier acts to reduce the bias voltage, so the tube is only actually in class C when it isn't being driven. When the carrier is applied, it reduces the bias voltage, which causes the tube to shift into operating in the linear portion of it's curve.

Class C amplifiers cannot be used to linearly amplify AM, but can be themselves modulated by injecting audio into cathode, grid, screen, or plate circuits.

Chris

Yes, from my own experiments, I believe you have it correct.  Any amplifier, whether it be grid driven or grounded grid is not linear when biased into class C. Put a ssb signal through and part of the signal will be missing when below cutoff. However, it is a different thing when an AM carrier is used to drive the amplifer. The carrier acts to shift the operating point back above cutoff and it becomes linear again, but now operating in the AB class.  No free lunches. The carrier acts like a substitute "positive" bias.

I wouldn't have noticed this unless I did it myself by experiment some years ago. I had a 4X1 GG linear that was biased by a string of diodes in the cathode. I could switch in any bias level from class A to class C.  When in class C, ssb was distorted, of course. But an AM carrier brought the idling current up to near class A and the audio was clean when modulated.

I THOUGHT the AM efficiency looked better compared to normal bias in class B just by looking at the plate glow, but not sure about that.  Not sure theoretically why the overall linear efficiency wud change, if at all, just becuz a carrier is used for bias vs: standard DC bias. Seems like the same thing in the end. But it sure feels cool to bias the amp into class C and then drive it with linear AM... ;D

There is a limit, whereas if the bias is really large so that the tube is biased close to class D, it just takes that much more AM carrier RF drive to bring the amplifier up to a normal carrier output to put it into the linear curve again.

T  

http://amfone.net/Amforum/index.php?topic=16488.msg113118#msg113118
http://amfone.net/Amforum/index.php?topic=29516.msg229169#msg229169

And this article:
amfone.net/Amforum/index.php?action=dlattach;topic=30787.0;attach=31340 (http://amfone.net/Amforum/index.php?action=dlattach;topic=30787.0;attach=31340)
That article discusses class BC linear amplifiers, which are pretty much what I am asking about (biased beyond class B but not deep into class C). Take note of figures 1 and 2. If you look closely, you can see how it works. The bias is set for class C. If you look how the input and output carriers are plotted against the curve, you will notice that when the input carrier is at rest, the output carrier falls in the middle of the curve. The modulation then is kept inside the linear portion of the curve. As long as the 100% positive and negative peaks are within the linear portion, a class C amp can act as a linear for AM ONLY.

Edit: thanks for chiming in Tom.

This is a modern "envelope replicator" class B - C linear amplifier
 
http://www.813am.qsl.br/artigos/py2ko/burrico/quaggi.pdf

Il looks very interesting. Too complex for most of us.

Giorgio

PS: by comparison this is a very compact grounded grid linear amplifier. Class B, less power.

http://www.ing.unitn.it/~fontana/liexnear.pdf

That first amp is basically a tube version of an EER amp, which are usually class E SS amp made linear.
The second amp could be biased into class C and still work for AM. I had a thought last night that might explain the increases efficiency with a class C linear. The thing is, the peaks are probably more effiecient with class C than Class AB. Class AB is about 66% efficient on peaks, where class C is probably about 75% efficient. Of course the carrier is still gonna be low to allow for headroom, but even that should be slightly higher with class C than class AB.

In the early SSB handbooks there are discussions of using standard AM/CW transmitters as a linear amplifier for SSB exciters.   One details the use of the Viking 1 by feeding the SSB signal into the VFO input with the transmitter set for CW mode and reducing drive to the point that no grid current is observed..  The author notes that this will not work on bands where the VFO signal is multiplied.

I used my homemade 20A knockoff to drive a Viking II straight through on 75 & 40 as described by Rodger. I seem to remember putting a 470-ohm resistor in the key jack to ground per reccomendation in QST. Worked very well as I recall.

Bob - NE

Doesn't the tank circuit play a key role in class C plate modulation.

Yes, in class C the tube really only amplifies the signal in pulses, the rest of the signal is formed in the tank circuit due to the flywheel effect.

 I sure did open a can of worms when I tried to explain this on the air tonight, that it is possible to use a class C biased amplifier to linearly amplify AM. An interesting point came up: sure you might get a result that sounds like it is linear, but is there any garbage that being generated from using class C bias that wouldn't be there if the amp was biased to class AB?

I heard some of that.

The amplifier is not class C while being driven.

Some of those guys stopped listening as soon as they heard class C.

If the thing were biased hard into class C it would not work the way you desire.

Yea, I tried explaining that to them that as soon as the thing gets drive, the carrier throws it out of class C and in to class AB, and they just didn't seem to follow. And I did tell them that it can't be biased too deeply into class C, since the more bias you have means the more carrier you need to overcome the bias, until you get to where you can't add any more carrier to pull the bias up.

It does work. I used to bias my Gnational NCL-2000 to give less idling current than I could get away with on ssb. Based on a couple years usage and plenty of on air reports from intelligent folk with panadaptors, I'd give it the green light.

Of course the concept can't be taken too far....

So why Hasn't this mode been the commercial standard all these years?
Relative efficiencies?  The power bill being the main factor?  Anyone actually superimpose this set of changing classes on tube curves, plotted differential biasing on real load lines, etc.?
Hmmm?

Does something like this help?

Yes, it does indeed work and is clean.  But it does fly in the face of conventional ham wisdom and beliefs until we specify what really is a class C amplifer.

The question still is, are we gaining anything or just trading off use of a carrier for less bias?   As I mentioned earlier, I never ran any precise tests to determine if the masquerading "class C" amplifier was running any more efficient when the carrier was providing the shift in position on the transfer curve  -  or when it was being shifted by real applied bias.  Is it technically still in class C?  Probably not.

It wud be an easy experiment to do.   The initial question is,  "Can we use a standard class A, AB or B linear on AM when biased into class C?"  The answer is yes.   But we must be sure this class C amplifier does not have grid leak or sloppy screen regulation, a screen choke, etc., as with most plate modulated finals, or this will cause distortion all its own.

This technique only applies to a normal ssb linear amplifer that has been biased into class C. The jury is still out whether it is more efficient to run it this way.    A simple test is to add a string of diodes in the cathode to a standard grounded grid pair of 3-500Z's in linear to bias it into class C. Then run AM and note the differences.

T

If you had a RF deck that was driven by an exciter and used some form of high level modulation, an interesting experiment to try would be to turn off the high level modulator and then modulate the exciter, without making any changes to the RF deck. Something else that kinda goes against conventional ham wisdom is having a modulated amplifier where the driver is modulated too. The same concept applies here, we have a class C final that is being driven with a modulated signal. This idea came up at one point last night too, where the input to a final was modulated, but the final was also getting some kind of high level modulation at the same time. I actually know of one example of a commercial transmitter that used this, the Harris VP-50. The purpose of modulating the driver of a high level modulated class C amplifier is to improve the linearity, which is the complete opposite of what was being told to me last night, which was that "class C" and "linear" don't belong in the same sentence.

In this example, severe distortion would be generated by the Heising choke, mod transformer, grid leak resistor (unregulated), screen choke and likely unregulated screen supply. If all of these parts were eliminated and the final simply had a higher class C bias (and regulated), then the AM carrier would cause the amplifier to work linearly due to the carrier generated bias point as we discussed.




A <180 degree plate modulated class C final's output actually becomes a linear signal when plate modulated and run with the proper class C grid and screen currents and plate operating point.  If it didn't, then there would be severe distortion.  Same way a mixer works in a receiver.  So, the resultant class C final's output actually should be linear...  ;D  (linear output when compared to the modulated audio input signal)

I think we are all talking the same thing - just different ways of thinking about it.

T

Ok, that makes sense. It would make more sense to try it with a triode final , then you could also run fixed bias (not that you couldn't do that with a tetrode or pentode) and have so way of switching the mod iron out of the circuit, which might be worth trying.

If I am understanding this correctly, we are talking about driving a grid driven Class C amp with an exciter.

I tried this a few moments ago with my ranger/ Thunderbolt combo.  I turned HV off on tbolt, Put Tbolt into Class C mode.  Still used Swamped input to Grids.  Loaded up ranger for 5 MA of Drive on Tbolt meter.  When I modulate ranger, Tbolt Grid meter goes to 15 MA.

I turned on High voltage and keyed up.  Loading of Tbolt changed slightly. I had to rotate the loading knob a bit. 

The result, Super clear audio, Less Plate current at carrier, MORE pep output. Seemed to have gained some efficiency?  Less heat blasting out of Tbolt.

Got great reports on air.

Hmm.. Class C linear.   Sounds like most cheap 11 meter amplifiers.  Transistors with no bias supply at all for AM only use.

C

Yep, you have it exactly right, Clark.

 It could be done with a triode, GG linear too.

I thought I saw less heat too, when I tried it with my 4X1 linear amp.  I have been told by others the same thing.

Try this if you can....  Load the amplifier up in normal linear fashion (normal bias) and set the output power at 200 watts AM carrier.  Put a thermometer over the final air flow and see what it reads after five minutes of constant carrier.

Then let the amp cool down and repeat the same test again with the class C bias setting, same power output, same duration.  

The second test could be to see how the efficiency changes under modulation.  Put thru a 1kc audio tone at 70% modulation and run the same test for 3 minutes each.

I'd be curious what you find.


Just to repeat for readers,  on ssb, this class C linear technique would create crossover distortion and bad splatter. It works on AM only. (with a constant carrier)

T

I will do that tonight Tom.  I am home sick with the flu and need something to do anyways :) 

I can vary the "class" with the ranger ANT Loading knob.  I played around with it. I can get 250 watts carrier out with less current and an upward swing on plate in Class C and more pep.

The guys said the rig was pounding into CA and way up North to Kingman AZ on 40 meters.  They said the radio ocupied the same Bandwidth and there was no splatter.

I will report back on heat and plate current tests.

C

Ok.  Here is the data Tom.

Class C mode:
250 watts carrier on Bird43
300MA plate current


104F output at rest.  This was stable.
After 5 minutes 140F. 
It hit 140 and stayed there at 3 min point

Class AB (linear mode)
250 watts carrier on Bird43
400MA plate current
After 5 minutes air temp was 170F.  Rising steady... Was going up when I let off.

In Class C,  The 4-400s have very little color.  Slight red spot the size of a dime or less.

In Class Ab,  The 4-400s are Red from top to bottom and white/yellow in middle

A HUGE decrease in Temp and current.  In Class C, the plate current swings UPward.  My Friend called to say that the Class C mode is Crystal clear, loud and is Clean on the air.

Hmmm, so I should add another bias zener in series with the existing one in the LK-500ZC and add a switch to bypass it for SSB?

Probably means a bit more CW efficiency also.  Gotta give it a try and if that works do it to the Alpha 76PA which has 1200W of PD available.

There is also a NCL-2000 that sometimes gets used on AM with the HT-32B and that could sure use less heat.

It also means that guys with the SB-200 and 220's can get some more oomph without melting tubes and transformers :)

Wow, that's pretty significant, Clark.

So you are getting more power output for a given plate current (more efficiency)  and a lower temperature for the same power output with the class C mode.  (again, more efficiency)   And it stays clean on the air.


Now the next logical test is to try it with an audio tone and see if the same trend holds up under modulation. The efficiency normally goes up from carrier to modulation for a standard linear amp, so let's see if it follows this pattern in "class C"  AM linear.


BTW, I used to own a T-Bolt linear myself. But can't remember how it gets placed into class C mode... is there a switch that selects  the regulator tube bias?

If these tests work out, I will certainly be placing my linears in a more heavily biased class for AM in the future. I use a big string of diodes in the cathode for bias with a rotary switch to go from AB1/2 to C.    Later I will run some IMD tests to see what effect there is. Apparantly it is clean, but there's usually no free lunches... ;D


T

Let me know exactly how to test it and I will.  I would guess you want a 1K audio tone injected at some % of mod for each mode?  Heat, current and power measured?

Carl, A popular mod for many years for the SB220 is to change out the Bias diode so lower the Class for AM only use.  Shane will jump in here I am sure. He told me about that modification. 

The Tbolt has a CW/Tune and Linear mode switch Tom.  In CW you are class C with your choice of RES or Tuned input. 

My Tbolt is factory except I used the new high curent hockey puck DC filter caps and double size Screen supply caps.  I also plugged in VR150s in all screen reg tubes to go from 450 to 600 screen.  I then put one of the VR125's I took out and put it in the Bias regulator spot which biased the amp down in curent.  I posted about it in my Tbolt thread.  I think johnson intended you to do the tube swap and shipped them the factory way to keep it "legal".
 
This is why alot of guys down talk the Tbolt.  In factory form they are so deballed they only do 150 to 200 watts Am out of a 125 lb amp!

In CW mode you must have the Blocking Bias contacts switched by a relay from the exciter.. Otherwise, Its FULL plate current and damage to amp.  Mine is switched from the ranger so when I unkey the ranger, the rig is in cutoff.

What all this means to me is that I can load this thing up higher in the Class C mode and still have less heat.


C

Yes, that's right.  Do it just like the last test but use as high a percentage of modulation as you dare.  100% for 3 minutes would be ideal.


OK on the regulator tubes. Yep, that is what it takes.

A tetrode amp can be a super clean stage if the grid draws zero current and the screen is electronically regulated and draws it's reccomended current under a given loading.   Some guys run them unregulated (both grid and screen) and that will create a mess.

I'm crazy about the prospect of building the cleanest amp chain I can.  Just finishing up a new 4CX-350FJ  linear amp right now to drive my 8877 amp. It is possibly the cleanest tube ever made for linear service, being at -45db 3rd according to Eimac.  The "J" is almost 15db cleaner than a standard 4CX-350F. I built an electronically regulated screen supply (+- 100mV)  and also working on a sensitive trip safety circuit for the screen. It's easy to make mistakes and pop a tube like this.  In fact, I like the trip circuit so much, I may build one for all my amps and place it in the grid circuit to shut down the amp when overdriven, mistuned, etc.

I'll post some pics of the amp and this trip circuit in another thread next week.

Looking forward to your next tests, Clark.

I'm seriously thinking of converting my 2000B to class C now. On that amp, I think it might actually be pretty simple, since it appears that they are using a 300 to 350 ohm cathode bias resistor to cut the tubes off on standby, then shorting it out on transmit. But they also have 37.5 ohms from each grid to ground, so don't know if that would need to be changed or removed or not.

And Tom, I was going to mention to test the IMD of AB vs C too see which was cleaner, but I see you already thought of that.

Shelby,

The PTT keying resistor shud stay as is. Also, those 37.5 ohm resistors in the grid are for neutralization. But I usually just strap the grids directly to ground with copper strap.  There is some contraversy of the value of using those grid resistors vs: a direct path to gnd.

To bias the stage into class C, make up a board of about 50  1A 1KV diodes  in series. You may need more or less, depending on plate voltage, tubes, how far into class C you want, etc.  Use whatever diodes you have that will handle the current.  You can use a rotary switch to select various points of idle by shorting some out.  Place it in series with the cathode filament center tap of the fil transformer.  Point the arrows down - the diode cathodes go towards ground and the anode points towards the tube cathode pin.  This will give you a very well regulated grid bias for whatever class you wish. 

This works for both tetrodes or power grid triodes, etc.  This makes the tube's cathode slightly positive in relation to the grid, which is actually a negative grid bias. Used is most GG amps these days.

Some use zeners, but the diodes offer a finer control method.

T

Ok, that's what I wanted to know. Of course, at the rate I'm going with things, it might be a while before I even attempt to tear that thing apart again, seeing that I had it apart twice within an hour last night (second time due to screwing up what I was trying to fix the first time I had it apart) because I might have to do some wire tracing before I start doing mods, I think a previous owner did some mods and of course there's no documentation.

Ok.  Time for bed. Not feeling to hot.   Tomorrow, I will drag out the SDR radio/scope and AF gen.  Then take some tests.  I dont know about 100% for 5 minutes on an old Tbolt.  But maybe 60% will do.  I am intested to find out if there is much gain when modulated.

I already noticed that there is a Peak on the Grid drive in Class C.  If you vary the exciter there is a point where current goes up and then with more drive, the current goes down.  However, There is one spot where the output is the greatest.   

Just like my 2x4 Class C transmitter.  Which I now realized, I could run as an amp if I wanted to :) 

c

Clark, you saw this years ago..  Look at that SB220 I did when I came out to help with the IT Security years ago.  LOTS of peak power, and great efficiency.

Most people won't believe what the SB220 can do, AM, when modified.  1600 PEP with modified input, output and bias.  New BIG C caps in the power supply.  This from a 300 watt carrier.

Another (needed) move is to add a large amount of C to the end of the bias string.  Keeps the dynamic bias source stable.  

I went over this on the telephone to Jim, VE7RF a few years ago.  You can search the yahoo amps archive and find a large writeup about bias, AM and SSB, and how surprised some people get.  He's done large amounts of bias on 3x3 and 3x6s and tested them.

Then, it's not impossible to understand some of those numbers thrown around on CB.  Relays click another half volt on the filaments on TX, and they are biased deep class C, then driven like dog snot to overcome it.  They take it to an extreme, and it's not clean, but done correctly, it can and does work.

--Shane
KD6VXI

Yeah. I remember that Shane.  Gotta be 20 years ago. 

kb3ouk-  How about an EBS-1 board for your hunter?  Its triodes.  You can mod the board to have adjustable bias and set it class C to Class A.

Probably work FB for the SB220 and other amps. 

C

I might try that sometime, for now I'll try the diode string in the center tap idea, plus I'm in the process of gathering parts to build a driver for the hunter, that way I can quit dealing with the stupid FT-901 audio issues. Basically use the 901 for an RF source and receiver, then have a 25 to 30 watt modulated amp that feeds the linear. Which that thing can use all the help it can get as far as eliminating heat is concerned. That's what I was working on last night, ripping out the old fan that somebody put in to replace the stock fans, then I just placed a quiet 12v fan on top of the cabinet to pull air out the top and that thing has never ran cooler. It is cooler running with the little fan than it was with the big noisy thing that was in it when I got it, and if I rebias it, it should run even cooler.

I have been watching this thread with great interest. Often wondered about this idea. Somewhere way back in my fadded memory I remember there was a way to use a diode to provide feedback to the grid to improve a class C for AM but I was too young then to pay attention.
I was told often too class C has too much distorion but then SSB came along. Too much distortion ?????????? How bad could class C AM be? Too much distortion or a small percentage compared to none? Then would the ear hear it? The ear that can only sense a change on signal strength of 1 DB . When you think about what 1 db is it makes one wonder. So during operation the signal goes around the corner a bit of the linear portion of the slope. Would it be discernable? In small doses probably not. How much would be too much. That would probably surprise us. For those playing Wolfman Jack with their AM transmitter (broadcast quality) probably they are not interested. I personally like the old antique carbon mic sound on AM and use one on my favourite homebrew. Minor distortion if mostly unperceptable would not bother me. I cant hear well anyway. ;>). Very interesting thread. Thanks to all. I run my small linear right up to the edge.Now I may try leting it sneak around the corner a bit and see what happens.
Don Ve3LYx

Back to reading today.
Kb3ouk, your illustration was simply an in/out projection based on a perfect slope, m, of 45 degrees.  It was not real tube characteristics.  Perhaps the tail of a pentode's constant current characteristics, but certainly not real.   Thank you for your class C vs. B idealization.

Still wondering about commercial practice all these years missing "the discovery of the century."  -Not counting recent development of class D and E, of course.  Yes, engineering practice can get stuck it ruts, designs can get ossified,etc.  and yes, a ham's definition of distortion free might not correspond with commercial AM's, so I'm wondering why class ab/C linear amplification hasn't been used, AT ALL.

Once again the amplifier is only biased class C without drive.

Nothing new here. It was about 10 years ago when I first noticed this. Mentioned it to W3DUQ and he was familiar with the concept.

Same amplifier used on ssb without resetting bias gave the expected bad signal reports.

Not too many full carrier M.W. or H.F. high power linear applications out there today.

Rewind to the days when AM M.W. broadcasters used linear finals before high level modulation was popular.

Maybe some info from 90 years ago.

Like anything else in amateur radio some seem to find a way to take the concept beyond its capability.

If one looks around long enough on the bands one can get any signal report they desire.

I understood it perfectly.  I and many of your readers understand that the AM carrier of a previous stage sets a pedestal bias voltage sufficient to allow linear amplification of a superimposed audio signal in what once was a class C stage is now at resting a class AB final stage.
I excepted class E and D..
It is a very neat circuit application and due to bias being applied thusly is no longer class C, but now AB within limits.

Further in your remarks, How does linear amplification of lower level stages into linear final as used in BC service some time ago have anything to do with it?   ...except trying to fluff through an argument.
Why do you feel that nobody "Gets it "except the annointed.?

My question was exactly historically that which you posited again.  What happened 90 years ago or whenever that it was missed if it is more efficient than C plate modulated?

I am digging into " electronic Designers Handbook, " Landee, et. al. , and other references to see if there's any mention.  

Sorry for all the "I's" but certain situations tend to draw them out.  ;D

Another thought. Perhaps 100 percent modulation being a limit didn't historically satisfy those seeking 125 percent plus.

Rick, you were the person asking why this revelation did not appear on your radar. Your q:
"So why Hasn't this mode been the commercial standard all these years?
Relative efficiencies"?

I offered a possible answer.

Who said this is more efficient than class C plate modulated service? It ay be marginally more efficient than B or AB.

Some are trying to take this 5 steps further referencing CB amplifiers. That is a far cry from what works on AM on the ham bands.

As far as I'm concerned this is not much more than a curiosity.

heheheh...  It's starting to get somewhat confusing to follow here.

I agree that this is just a curiousity - that needs further testing.

The test results seen by Clark are worth pursuing. Let's see what he shows when putting an audio tone through at full modulation.


What I don't fully understand is -   is this the result of simply running the amplifier in a lower class (class B vs: class AB) or is something else going on here?    What if the normally class AB linear is "being pushed" (or at least acting like efficiency-wise) being closer to pure class A when running the constant AM carrier? Poorer efficiency.  So by increasing the bias maybe we are simply bringing the operating point back down where it belongs - back to class AB or B again.  

But if lower class, then the tradeoff would certainly be poorer IMD between nearer class A vs: AB or B, which I plan to test myself once I get caught up on projects here.


Let's focus on Clark's results last night and continue with tests:


"Class C mode:
250 watts carrier on Bird43
300MA plate current


104F output at rest.  This was stable.
After 5 minutes 140F.  
It hit 140 and stayed there at 3 min point

Class AB (linear mode)
250 watts carrier on Bird43
400MA plate current
After 5 minutes air temp was 170F.  Rising steady... Was going up when I let off.

In Class C,  The 4-400s have very little color.  Slight red spot the size of a dime or less.

In Class Ab,  The 4-400s are Red from top to bottom and white/yellow in middle

A HUGE decrease in Temp and current.  In Class C, the plate current swings UPward.  My Friend called to say that the Class C mode is Crystal clear, loud and is Clean on the air. "


T

Not true at all.  I only pointed out that the claims made by some of the mfgs of those amps are always poopoo'ed as impossible, but then on the amateur bands they call the technology something else, and it's WOW, LOOK AT THIS!!!!!

You may think of this as a curiosity, but other people might actually want to learn what, if anything different, is going on.

I think Tom hits it on the head pretty well later, where we may be using our amplifiers in class A during carrier periods, resulting in the poor efficiency.

I've done some experiments with audio derived bias in both transistor and tube circuits.  It DOES work.  It also works with EBS designed to change the class of operation (although you need to set the EBS to act upon modulation peaks, rather than a carrier).  You run pure class C during the carrier, GREATLY increasing efficiency, and during any modulation (as detected at the microphone level, not RF in my circuit), the bias slides up to AB / B.

Saves old unobtanium based tubes a lot of Pdiss averaged over time.

Clark, about 15 years ago.  1998 maybe?  It was near Y2K, I remember that much.  Man, that is close to 20.  Getting old? :)


--Shane
KD6VXI

This EBS  (electronic bias system) stuff is very interesting to me. It was first written about in the 70's but may be a better design now. I've been reading about it and thinking of adding it to some of my GG linears.  I think it gets a bad rap cuz some circuits on ssb linear operation go from hard class C cutoff to class AB and generate splatter on the first word.  Some have too slow of an attack time. In addition, I think maybe an audio derived trigger using a downward expander trigger might be a better source -  instead of an RF derived trigger.  (100mW, etc)

That said, maybe a Schottky diode can give 20uS attack as the latest EBS-1 advertises. Also going from 50 ma idle to 150ma idle is still in the linear portion and shud not generate the crud of full cut-off to class AB with the first word.

The Ameritron EBS-1 circuit (a W8JI design?) - where does it connect to the cathode of the tube for bias - at the D3 / D4 area?  This can be modified for any GG amp, right?   I have an 8877 I'd like to try it on.  If it works cleanly (with mods) on ssb, I will try the carrier idea where it is running in a lower class with carrier only to idle higher under modulation. That's an interesting idea if it can be done audio transparently for gating - and done cleanly IMD-wise.

T

The class-shifting of the final is beginning to sound a lot like a Doherty circuit, albeit with one (or parallel) tube(s) and no phasing network.

It may indeed work, but TANSTAAFL.

73DG

I was thinking the same thing last night, and I think that's really what is happening. When the amp is biased into AB, it would stay in AB for ssb since there is no carrier, but with AM the carrier is actually pushing it into class A which is why the efficiency and power output sucks. On AM, if the resting bias is in class C, then the RF carrier voltage is pushing it into class AB or possibly even class B. So the limit on how deep you can bias it into class C would have to be based on how much bias and drive you have so that on key up the tube is running in at least class B or lower. If the tube would be in class C on transmit, then that will be when things start to distort. This brings up another point, if that's what is happening in a linear amplifier when it is being driven, that the bias voltage is actually lower than we think, then the same thing could possibly be happening in a high level modulated final, which makes sense why with some designs you can gain efficiency by putting heavy bias on the tubes. It could be possible that in a plate modulated final, if the bias isn't deep enough into class C, that the tube might actually be running closer to class B when it is being driven. Which would then prove something else that is against common ham knowledge that you cannot modulate a final that is in class B or lower.

My apologies to those for the previous.

I spoke with Robert this morning on 15 meter AM.  He said this was Class B.  That makes sense to me.  I think when modulated and the Grid drives forward, then you  are in Class AB2 again.   Slept for a few hours. I am going to Drag the gear out to take measurements.

C

Yep, I think that's it, Shelby.

And think about it....  there is really no practical limit to how much bias we can run...even biased into class E, as long as we have enuff RF drive to push the amp up out of the class C  into B - enuff to make a normal carrier. It's a diminishing return at some point.   So if we have enuff carrier, then we will always be above class C to make the linear operate linearly.  If there is no idle current (class C) this means we don't have enuff carrier to work with on AM. It's almost a self adjusting mechanism... ;D

So, the bottom line is when running AM linear, we may need to add more bias to bring the amplifier back to where it normally runs on ssb - in class AB or B rather than close to class A.  As a reward, there is less heat per given power output.   The IMD will probably not be a significant factor since we are talking about the small difference between AB and A.  Getting close to class B may begin to make a slightly bigger difference in IMD, however.

The thing to do is make power efficiency and IMD measurements while adjusting RF drive and bias looking for the compromised sweet spot between efficiency and best IMD.

T

I was wrong..  The modulated plate current is MUCh higher in AB1/2 then in Class C.


"Class C mode:
1000 watts modulated on Bird43
350MA plate current


104F output at rest.  This was stable.
After 3 minutes 145F. 
It hit 145 and stayed there at 3 min point
Seems to never run over 145F.

Class AB (linear mode)
1000 watts modulated on Bird43
450MA plate current
After 3 minutes air temp was 190F.  Rising steady... Was going up when I let off.


I cant see any difference on the Spec An. Its clean either way. 

For these tests I used the RESISTiVE input selector.  They suggest this for use with Ranger exciter

Things to note:

In class C mode, The Exciter output is MUCh higher.  I have to load the Ranger to 130MA (factory spec) to get the perfect Grid drive

In Class AB1 linear mode, I can only load the ranger to about 60 MA or I over drive the Grids on the Tbolt.

I do have a Z communications 50 ohm attenuator for use with the ranger and Tbolt but do not normaly use it. 

I decided to try it with the Tuned input Mode on the Tbolt.  I got a GREAT increase in signal quality on the spec An!  That tuned input REALLY cleans things up!  Its showing Second harmonic at 35 DB with RES input and 50DB with the Tuned input.  WOW.

So Tom, Can you explain why in Class AB1 carrier and Class AB2 modulated uses an extra 100 MA of plate current?  I tuned this thing to perfection in both modes.. No matter what I do, linear mode, Uses another 100 MA at 2400 volts DC!!!!

C

Looks like everything is following as we expected, Clark.

Yes, the Ranger will need more power out as the amplifier stage is biased harder into class B.  For example, in pure class A, no power is needed at all, just voltage.  (Well maybe 1 watt to generate voltage across an L/C circuit)


The T-bolt: The reason for 450 ma plate current in class AB to achieve 1 KW output  vs:  350 ma in class "C" to get 1 KW out (actually class B)  is simply the effects of better tube stage efficiency.  ie, Less power input to give the same power out when using a class closer to class B.  This extra current is showing up as more heat and a higher temp reading in your tests.


What is quite appealing is that your temperature in the last test (carrier at 250w)  is the same at 1KW modulated in the second test.  This tells us that the efficiency is actually going up under modulation, just like a conventional linear in class B.

So the next step wud be to try various bias and RF drive settings and compare the input power (I X E) to the output power in the dummy load. Look for the best efficiency along with a reasonable RF drive level and still have good IMD.  There is usually a sweet spot to be found.

Good job, OM.

T

Ok. I am going to read some of my old handbooks again to learn what I missed the first time.

This thing runs ice cold now and is really clean!

I think the EBS-1 with a selector pot (they mentiont that as an option) and or a simple switch for AM and SSB would really cool off alot of linears for the AM/CW mode.


Thanks for the lesson!

C

Why would the current be any different under modulation?

He's seeing about 50 ma difference if I read the tests A vs: B  correctly.   Maybe carrier shift?


Clark, tell me about your EBS-1.   Since it triggers on 100 mW of RF, the AM carrier must be a problem. Did you use audio triggering like Shane suggested?

It goes in series with the fil CT of the cathode, right? 

What mods were made for AM use?   I was thinking of picking one up from Ameritron and trying it out on the homebrew 8877 amp - with some serious mods.

T

There is absolutely no reason why the average plate current would increase with any level of modulation less than 100 percent.

I ran it fully modulated.  300 MA carrier and 350MA modulated.  Maybe the plate meter is off since I have 2400 volts vs 2000 listed in the manual.  It says to calculate the current if the B+ is higher on the meter.  

Does not matter though.  The delta between AB1/2 and ClassC/B is 100 MA.  At the same exact 1000 watts output.  Right on the bird and same Grid on scope.  

I ran the tests again with the tuned input in check. The numbers where the same.  Someone else needs to run tests and report in.

I was like you steve, I thought it would not matter modulated but it surely does.

I think its like Tom said.. In Class C mode when its modulated we are at Class B.  Vs linear mode which is AB1 and well into AB2 when modulated.  So under mod we compare Class B vs AB2 efficency. 

Right Tom?

C

I think once the class bias is set by that AM carrier, (and fixed bias combo)  it remains in that class.

I'm not sure if the class of operation itself is changing under modulation, but more the fact that the amp is acting more like an efficiency amplifer during modulation.  It's overall in to out efficiency goes up as the modulation increases.

 I'm still learning too... ;D

T

In linear mode A/AB, the plate current should not change at anything less than 100% modulation with sine wave modulation.. Anything else indicates distortion or non-sinusoidal modulation. Plate voltage is irrelevant.

It's not even clear to me how it could be more efficient. If the idling current is the same and the average plate current is the same, where is the extra efficiency coming from?

??

We're talking about the change in efficiency when the amplifier is biased into a lower class (class C) and then pulled up into class B by the carrier bias effect.  Originally, it was in class AB.  Is this what you mean?

T

In AB there should be no change in average plate current with sinusoidal modulation.

In "so called" C, where is the extra efficiency coming from?

That's great news! 8122 are expensive. What is the idling current on yours?

Heat now being converted into Bird watts - as measured by Clark's class AB vs: C/B   temperature tests?

This cutoff bias scheme has been done a lot by experimenter on tube CB amplifiers. A bias control knob was added and the result was talked about as less carrier and more 'swing'. Same PEP maybe, but generally they don't measure that and the meters in use are questionable. On CB it is taken too far e.g. the 4 watt carrier and 120W "swing", but there is less concern for a non-interfering signal among those users.

Just to be clear...

There's actually been two threads going here.  One is about the AM carrier bias effect that supposedly pulls the linear stage up one class, essentially creating a standard linear amplifer, that has been biased to class C  to operate in class B on AM.  Or, a class B stage pulled up into class AB, etc., due to the AM carrier.


The other thread is about EBS - (electronic bias system)   This is a circuit that is triggered by RF or audio and quickly adjusts the amplifier's bias up or down to cause it to idle with less heat when not talking, while returning the bias to normal when talking.

They are two different ideas, but can be closely related.

T

Its been 10 years since I unloaded that amp so I'm not sure. Maybe 190 mills. Increase the bias on ssb until it starts to get ratty then switch back to AM ;D

Tom, I dont think that its in Class B on Carrier. The reason is that I simply following the Class C tune up procedure for CW mode and johnson clearly states thats class C.

Also.  My 2x4 is biased solely by my FT450 transciever. It has no protective bias.  That is certainly Class C.

I guess the only question is what happens when its modulated? Well, that should be Class B as Robert said.  It makes sense to me.  The grid curent is increased during modulation.

The only thing missing to me is the Tbolts lack of Grid voltage.  If I could tap a choke leading up to my volt meter, I could see the actual - voltage and based on that look up the 4-400A tubes and confirm I am in Class C.

My point is that until you speak and modulate the grids, Its just Class C CW mode


C

Clark,

That would be a desirable thing if it rested with carrier in class C and then shifted to B under modulation. That wud be ideal.  But I'm not sure.

I think what's happening is whatever class the amplifer starts in, it gets shifted upwards a class by the constant carrier. This puts the amplifier into the linear operating point simply by the audio now able to swing around the carrier without cutoff. And by adding bias, we can shift the amplifier back down a class where it started and belongs for best efficiency.

But we can speculate all we want...  ;D  What we need is more guys testing and posting their results. I'd like to see several with 3-500Z GG amps trying a string of diodes in the cathode fil CT lead and running the temprature tests as you did.  It's so easy to try and the returns are great if successful.  

I already have diodes with a switch ready to go in all my amps as well as a digital air exit gauge built in, so I shud run the tests myself.   I already instinctively add more bias for linear AM - just a habit from the past.

If anyhing, we may get a group who now can run their amplifiers cooler with the same cleanliness with longer tube life - or run a little more power with the same temperature.   Or we may get some who say it didn't do anything at all...  whatever it takes.  I'm just a bloke looking for answers...  :-)

T

Agreed. We need more people testing and posting results.

I think the carrier is class C.  There is no difference from Class C/ CW mode with ranger and Tbolt when I am not modulating the rig.  When I modulate, the Grid goes positive.

Its simply Class C/CW tuneup... until you talk.   The johnson manual states its class C in CW mode and the amp has super efficiency in that mode.

More tests from others will show us alot!
C

Well, while we're awaiting more test results...

I shud review this, but it might be fun to get some more input first. I understand that a standard linear amplifier is really an "efficiency" amplifier. The bigger the input signal, the higher the amp efficiency and higher output. A linear amplifier's efficiency may shift from 30% at dead carrier to 65% under full modulation.

Now the question that applies to what we are doing here:  Assuming what I said is true, does the amplifier class shift along with the modulation to achieve this changing efficiency or does the class remain constant?  If not, what mechanism is causing the efficiency change in the linear amplifer?


T

Running 3-500s in GG for these tests?  Don't forget that input power is passed through and added to power derived from Ep*Ip.

Yep, dis is true.

Though when the two tests are done (class AB and B/C comparisons)  the same feedthru power will be present both times, so will not be a factor in temperature measurements or in/out power comparisons ...   Though it will certainly be a factor in calculating overall raw, stand-alone efficiency numbers.

Do you have a GG amp you cud try, Rick?


Once I finish up on this 4CX-350FJ project, I will be posting my 8877 results too.


** second thought.... Since the amp will be harder to drive in class B/C, then more feedthru power will show up during this side of the test - and shud be accounted for.   Maybe that's what you were warning about, yes?


T

This sounds similar to the bias shift circuits that were around in the late 40's and 50's. Time to dig out the old CQ's, etc. I remember one in particular using a 304TL.

Carl, I posted about the 304TL amp and the consensus was not do build it.  But I see now what the designer was thinking.

Tom,  I am worried that on a GG Tride amp, the drive power will have to go way up.  If it acts like this Tbolt, its gonna need more then the 100 watt rig most have.

I cant wait to see your results.
C

Yeah, good point, Clark.   Most riceboxes are at their limit when driving a pair of 3-500Z's anyway. And from what I am learning about building and tuning a clean amp system, it's a good practice to run each amp in the chain at 1/2 power or less to get super IMD results.   My new system chain uses amps that are running 1/4 of rated power, so it will be interesting to see what it does soon.

As far as being hard to drive an amp that has been biased harder for AM use, we can simply add some bias until it starts to hit our driver limits.  If it works, then even SOME additional  bias will be better than none.   Though, if we then need to run the exciter to the limit, I wud opt to keep things as they are with no additional bias so the overall chain runs cleaner.    

That's the advantage of using amplifers in the chain that are overrated for each job. They can exhibit that better IMD without stressing out.

Quick summary of what I've been doing lately:  I just bought an ElectroCraft two-tone generator kit board today. Plan to do some serious IMD testing and will post some pics and IMD results in another thread when the new amp system is complete.  Getting close. I will be using the 3 mW clean output of the FT-1000D driver stage into a lab ZLH-3A amp (100mW from 1 watt amp)  into the homebrew 4CX-350FJ (25 w from a 100w amp)  into the homebrew 8877 (500 w from a 2kW amp), etc.  I'm hoping for -45db 3rd IMD or even prossibly -50dB 3rd results.  The average ricebox is about -31dB third, so I'm shooting for at least a -15 to -20dB cleaner signal. (less splatter)   I've been trying to accomplish this goal for over 20 years now with rather fair to dismal results so far. It's not easy to break thru the -35dB IMD barrier while QRO.   Oughta be sporting... ;)  

T

And that's the problem I know I'll run into, driving the 4 572Bs in the hunter to 150 watts takes about 15 to 25 watts, but that's why I'm considering building a driver that consists of a 1625 modulated by a pair of 6CD6s (sweep tubes that are basically an economy version of an 807), I have most of the parts on hand, just need the mod iron (I'm thinking a TCS modulation transformer would be perfect). That thing with 600v on the plate should do about 40 watts, which should be more than enough drive. With GG amps, it's easy to have too little drive power. I also have to do some work on the input circuit, the current input circuit is showing a unusually high SWR, so a lot of the drive is reflecting back to the FT-901.

Which amp was this Clark?


Im thing more along the line of one of the vintage 100-150W AM rigs or a SB-200 style IPA ::)

Carl

Yes!  There's nothing wrong with an oversize IPA (intermediate power amplifier) that just loafs along.  The IMD results can be excellent, rather than pushing a single amp to its saturation limit.

For a coupla years I ran a pair of 3-500Z's with 1500 v and loaded very heavily (less C2) to put out about 50 watts to drive my main linear amp.  Eimac shows the 3-500Z's at 1500V to be upwards of  -45 dB 3rd order IMD.  This is stellar and an easy way to start with a beautifully clean signal to drive whatever we wish.

Nothing  more depressing than spending the time to build up a clean final amp and having a trash exciter to muck up the works.  Remember the rule - the final signal output will NEVER be any better than the worse signal in the chain. If our final amp is -45db 3rd and one of the earlier amplifiers is -25db 3rd, then we will get something less than -25db 3rd out of the final amp.  Quite the motivation to work on the whole chain as a unit and use very conservative power demands on each stage.

I am finding that just cutting the power in half, from near saturation to half, can change a signal from abomidable to very nice.  It may mean an extra stage to get where we want to go, but WTF.

Shelby, sounds like you have the right idea with your new plans for the 40w exciter. Work to make it as clean as possible as a stand-alone unit and it will then complement your system when added.

T

I have a friend that has a large amp, Its not on the air and is in storage.

The amp has Alpha 77 buttons, meters and controls.  It has two 3-500's Driving a single 3cx3000a7.  The power supply is common but I think the 3-500s are running at lower voltage somehow. The transformer is massive. Must be 300 lbs easy.  At first glance we thought, What the world would anyone go trouble all this trouble to build 3-500's driving a single 3cx3000a7 for! 

Tom, I think I now know what the builder had in mind....


Carl,  The amp I was thinking of was the 304TL or 304TH in GG (or how about a 450TL or TH) You could drive it with the 100 to 150 watt Ham transmitter.  I made a thread about it but everyone started talking about higher power/ lower drive amps which was I was not interested in.. So the thread just died.  I like running the low power rigs here and I thought it would be really neat to have a period correct 304 Amp that would take full drive from any of the typical ham transmitters, Even if the power output would only be 4 times.

I know a man that has a GG Class C, 450TH amplifier that is Driven by a DX100B.  That is then modulated by two 250TH's.  Its one of the loudest radios I have ever heard.  Massive audio.  Makes no sense, as the feed through drive power is not modulated but I have heard the thing and its very loud.
   

C

This thread has been very interesting. Anyway, while sipping a coffee this morning I played around with my AL-82 as it was being driven by my Flex 5K on AM. The AL-82 has two series connected 7.5v 10 watt zener diodes in the center-tap return lead of the filament transformer to set the bias for the pair of 3-500’s in the RF deck. On the front panel there is a switch labeled SSB/CW that shorts across one of the zener diodes when in SSB to produce around -7.5 vdc of bias. When it’s switched to CW it puts both zeners in series to produce around -15 vdc of bias. My observation is that there’s quite a bit of variation among these diodes when purchased from Ameritron and they admit that you can see quite a difference in resting current depending upon the charactersitics of the diodes you end up installing. My two zeners are certainly very different from what I can see by the idling current as I switch it between one or both zeners in series.

At any rate, I recorded a few parameters as the AL-82 idled and then recorded the same parameters when it was driven to 100 watts and then 200 watts of carrier power. The plate voltage with no current being drawn is 3700 vdc and in this test it dropped down to between 3600 vdc and 3500 vdc depending on the amount of current being drawn as the tubes were driven. As expected, and as others have noted, it took more power to drive the tubes when they were moved closer to cut-off.

I did not see any difference while watching the scope between either the SSB or the CW bias setting while modulating to 125% positive peaks. I ran it on 10 meter AM and made 2 contacts there and later tried it on 40 meters where I spoke with 2 stations for roughly 45 minutes. Everyone reported that my signal was perfectly clean. I did observe that the 3-500 plates were orange at the bottom and bright red at the top after a few minutes while running 150 watts of carrier with the bias set for SSB. With the bias set for CW after a few minutes of operating at 150 watts of carrier the plates were bright red at the bottom and dull red at the top. That certainly leads me to believe they are running significantly cooler while biased with –15 vdc.

With the chimneys you should easily be able to run 350-400W carrier. Are those Eimac or Chinese graphite tubes?

Time to change that CW zener to a higher voltage or a series string of reverse connected 1N5408 and select increasing voltage steps.

Carl

Thank you for running the tests and posting the data.   It seems you cut the plate current down by 100MA at idle and 25ma or so at 200 watts.  I wonder if you retuned the amp after switching to CW?   I noticed that if I did retune, the current was even lower and with CW mode.. 

I see you are at 75MA idle in CW mode.  On my Amp, There is no idle as the drive provides the bias so I think that would be removed also.  That makes sense as I was showing a 100 MA reduction across the board. 

That is a neat feature of the ameritron amp.  I am used to the old SB220 where the plate voltage is cut way down in the CW mode.

C

The NCL-2000 reduces input power from 2000W on SSB to 1000 on CW by reducing HV and screen voltage but bias remains the same. The idle current drops from 225ma to barely noticable and I think its time to give it a try in that position.

Carl

Another happy camper... ;D

That's cool to hear, Rob. (pun intended)

Yes, as Carl suggested, try some more diodes for additional bias and see where the diminishing return is... a compromise between drive, heat and IMD.    Add diodes or zeners to the existing zeners - doesn't matter which, except for finer selection.

T

Well, I haywired some diodes to a Clipperton L last evening and made several contacts on 3875 and all with clean signal reports. It took a few tries to get the proper balance of increased drive and mike gain from the TS-950SD. The end result was a 400W carrier and absolutely no plate color from 4 brand new Shuguan 572B's...amazing!  After about 90 minutes everything was still quite cool. Keying waveform on CW at 1300W out looked clean on the SA so the amp was not into Class C and most likely well into B; idle current was miniscule and Pout was up about 150W for about a 7% improvement.
 
I'll look at the curves, efficiency plots, and SA more closely on my own amp, the customer wants his back and I'll do a more permanent change to the LK-500ZC.

Cooler is the key.  Looks like you gots it working right.  400w carrier with no color sounds like something out of a Gotham vertical-type ad.... HA!


Just a general word about using riceboxes for drivers when increasing the bias in the final linear amp...  I got an email from a friend who brought up a good point.... Many of these Yaesus, Icoms and Kenwoods might be stressed out trying to keep up with the average amplifier that is biased heavily. I agree and want to emphacise again that we must check the overall drive, IMD and heat levels before deciding if it will work for us.  The harder we push the ricebox to full power, the more the IMD degrades.  The first priority is a clean signal, so we try to run our exciter as lightly as possible..   If our exciter is rated for 100w pep, running it at 75w pep can make a big difference in cleanliness. It's a matter of relativity and degree.

Yep, there's no free lunches and will always be a trade off. What else is new?  :-)

This learning exercise will give us more options.  In my case, I use an IPA, so I
have plenty of excess drive available.  Also, some guys use Rangers and
other drivers that are good for more power output than a ricebox.  (Or a grid-driven amp like Clark uses)  This
gives them the option to bias it up and run the final amp cooler at the same power - or the
same temp at more power.

It's just another technique to have in the arsenal.

That's why I keep stressing to find the sweet spot between IMD, heat and
drive requirements. In the end, it may not be the best decision to increase bias. Might be better to keep it as is - in some cases. It's not for every situation.

T

Clark - I did re-tune when switching the bias setting but did not actually see any difference in any of the parameters when I was done.

Carl - These are Chinese RF-Parts 3-500 tubes. They seem to have quite a bit more gain than Amperex or Eimac tubes that I have run in this amp. Agreed that the amp would not flinch at all with a +300 watt carrier. The Flex that I drive it with will produce a 20 watt AM signal with 125% positive peaks very cleanly. If I limit the positive peaks to 100% it will easily do 25-30 watts cleanly but I like the idea of slightly more audio and slightly less carrier rather than the inverse. As I move the tubes closer to cut-off the amount of power required to drive the amp increases so I would imagine the amount of drive available (with a clean AM signal) would be the biggest limitation.

Carl and Tom - Access to the zener diodes (they are stud mounts on the chassis divider) is really easy with the AL-82 so adding an additional series zener or a diode string to move the tube closer to cut-off would not be difficult. The next time I have the cover off I'll add what is necessary to drop the idling current closer to cut-off in the CW position. I like the idea of the 3-500's not glowing like orange popsicles while I'm babbling away.

Rob W1AEX

am I right in thinking that hb'ers who don't have the big bucks for huge transformers, etc., might do best by building a small self-contained 20-50 W carrier plate-modulated xmtr and a "linear" ?

Peter

Peter,

That's a great way to go if you want to keep things very simple. An excellent example is what Ken W2DTC does using this method. I have not heard him using it lately, but he has a huge rock-crushing signal with beautiful audio with his tiny rig running into a linear. Purists will point out that this is not an efficient way to operate, but the reality is that it works just fine. Ken detailed his rock crushing mini-rig at the link below.

Rob W1AEX

http://w2dtc.com/w2dtc-homebrew-hi-fi-am-driver-transmitter-page.htm

Clark,

The SB220 I did up for you, which did well over 550 watts and would hold it under modulation (did not start to downward modulate) used 40 to 50 6A10s in place of the zener diode.  That translates to roughly 22 to 34 volts of bias.  I also threw a LARGE (can't remember the value) uF cap across the output.  This kept the bias stable under the varying drive conditions.

I may try putting together a regulated pass transistor scheme on my SB220 to test different bias levels. 

I also have a low voltage class A modulator.  Designed to modulate about 10 watt carrier.  Any reason someone can see not to use this as a bias adjust or "assist" to slide the class of bias as we modulate?  IE, basically do grid AND plate modulation?


--Shane
KD6VXI

As others post up data, I like this more and more. 

Carl, Those are great results,  I am eager to see what your efficiency numbers are. I calculated mine but I think I am doing something wrong as I am showing 80% which I dont think is possible.  I think my plate current meter is off a touch because of the added plate voltage on my Tbolt.   In AB1/AB2, The calc is 55%.

Shane,  I would think there has to be a better way then to use 50! diodes.  I know my current SB220 that I rebuilt has the Harbach board with the string of zeners rather than the stud mount.   I built the board, but I dont remember nearly that many diodes.

I think the EBS-1 removes all need for the 50 diodes as it can be modified to be adjustable and costs just $39.

For reference with the harbach board,  My SB 220 has 100 MA of idle in CW and 150 MA in SSB mode.  Maybe its less than the old factory setup, what does yours run?

Rob, Another Stud mount zener would be cake to install on that amp.

Tom, Very good point about the rice box driving harder and IMD.  Drive power goes WAY up as you go into C or B.   Its more then double on the ranger. Now, the ranger does not care and runs better when loaded fully.  However, I see your point about solid state rigs.  My FT450 WILL overtemp at anything more then 25 watts if you are long winded and are very active.

C

I was shocked as I slowly brought up the drive Tom and the anodes remained dark. I just got the tubes direct from China and Im beginning to wonder if they increased the dissipation after all the complaints about their 125W rating. If they are now 160W as with Cetrons then the 640W Pd is a lot more. Plus the fan on the back helps. A 10000uF 50V lytic did the stabilizing, its what I had handy but at least 4700uF should be used; it depends upon the number of diodes to some degree.

The TS-950SD was used since its rated at 150W CW and 35W AM versus 100/25W for the TS-940. I didnt go over 25 Bird Watts on the input so it runs cool. 

Another issue about using ricers is that a QSY say from 3705 to 3885 requires the amp be retuned and even a small change in input VSWR requires a careful retune rather than just tweaking the Tune cap. I may try it a bit more tonight as the customer is coming tomorrow AM. With a Ranger, etc it wouldnt be as fussy....I suspect. Gotta get the Ranger and AF-54/AF-67 on the shelf here in shape to try low power drive. Id rather have a TS-830M that has AM built in 8)


Those are wimpy 1N4007's Clark, not zeners. I always replace them with just enough 1N5408's to get 200-220ma idle in SSB. Ive had several stock Harbach boards blow up after just an hour or so of OTA testing before getting it to the customers.

Carl

Clark,

No, it doesn't have that many diodes.  It doesn't, because it wants to keep the amplifier in class B.  You want it barely in class C, so when you apply drive, you get it to slide to class B again.

If you want to approximate it, throw your zener back in, and use the Harbach board in series.  And while your at it, remove those little diodes they put in, and use real 6A10s.  I just did a rework of an amplifier for someone that had a single 3-500Z in it.  It's now got an AM-FM / SSB / Getter switch in it.  In SSB, it's using a Henry diode from the 1kd.  In AM/FM, it throws 15 more diodes in series with the zener.  In "getter" mode, it's grounded, and throws a nice reddish glow.

Another method: a regulated power supply for the bias voltage, instead of the zener. 

Carl,

For the SB220 and other 3-500 amps that switch HV taps, I ensure that the HV tap runs about 100 mils TOTAL ZSAC, for amps that run app 2800 to 3300 V loaded.  When switched to CW, it's nearly cut off (I can't detect any current).  I've never had a complaint, even from the SSBers 6 to 8 kc away from me banging my Smeter at 50-60 over (I'm 3500 ft above them, line of sight, about 100 miles away).  This greatly increases efficiency of the amplifier, and lets you run it safely on the SSB tap, nearly giving a double in PEP output.


--Shane
KD6VXI

My SB220 has been runing biased down with the harbach board for many, many years now. Never a single problem.  No need to alter it or mod it.
 
Good idea on the switch!  That would be one way to go!

C

The switch is easily put in without messing with the cosmetics.  Replace the meter sensitivity pot with one with a push / pull switch on it, and there you are.

Also works for making a standby switch, if one ever wanted one.

The zener and Harbach still won't give you as much bias as you need, but will definitely help keep the amp cooler on AM.  50 diodes is nearly 32 volts, give or take.  Also, throw a few thousand uF cap across the bias board output.


--Shane
KD6VXI

Thats too many diodes to mess around with. In the LK-500ZC I'll just add another 50W zener.

I cant wait to get another LK-800 in for a rebuild, wonder how much carrier a trio of 3CPX800's can handle ;D

Now, OTOH, how safe is it to raise the grid flange on a YC-156 and switch between GG and some level of bias?

Carl

I know of one Commander HF series (the old, originals) export 3 holer in Nor Cal that does 8Kw on peaks.  He runs it at 1500 carrier.  It is NOT a stock power supply.  I believe it's one of the Dahl transformers for a 8 or 10 3-500Z amp.  It was for sale on  the amps reflector a few years ago, not sure if he sold it or not.

As far as raising the YC tube's grid flange, is it necessary?  If you're going to do it, I'd look at a Zsupp if you don't have one:  Most people with the 156 don't use one, but....  I'm a big fan of a grounded grid being just that.

I use a string of cathode diodes for the 156.  Works FB. 

--Shane
KD6VXI

Duh....I had a serious brain fart, never mind ;D :o

:o

Wonder how much carrier that would do?

If you care to try to do the math, the last 11 meter idiot told me it worked like this:

His 3CX3000 had the CAPABILITY of keying 14 thousand watts.  That was balls to the wall, as MUCH as it would key.  It would only "hold" (ie, backwards modulate) to 1500, maybe 2500 watts. 

According to him, this means he was getting at LEAST twice the CARRIER power, PLUS the "holding watts" on top, ie, 28,000 plus the 2500 watts.

So, according to this math, which is the math of the amp builder in Tennessee, the 10 3-500Z box would key 12 thousand, hold about 3000, meaning his 10 tube Z box did 27,000 watts PEP, from a 12 thousand watt carrier.

I'll wait for you to pick yourselves up off the floor.  Yeah, I literally laughed at this idiot on the phone.


The last 10 tube amp I saw with Z tubes in it would do an honest 2000 watts carrier, 100 percent modulated.  That was in another life, in a character I played on TV, though :)


--Shane
KD6VXI

And why do we care what some dumb CBer says?

Two 3-500s are good for 1800 watts (2k if you want to push it with 4kv on the plates). So 10 are good for 9000 watts. Of course you would need 500 watts drive.

C.W. McCoy cares. Whenever you need to impress hams just bring up CW.

C.W. McCall of Let Them Truckers Roll fame.

Because if you look past the "Us Vs.  Them" war, it's funny as hell, some of the things that are said and claimed.

Anywho.

--Shane
KD6VXI

Plus they must have money to burn (or more money than brains, that sounds more like it) if they can buy 10 3-500Zs which run about $190 each, then beat the crap out of them in a multi kilowatt amp. Of course, in linear service you can make sure you have enough tube power then wimp out a bit on the tank circuit, since you're not handling near the amount of voltage that you would with a plate modulated amp.

Instead of arguing with CBers who make crazy claims, ask some technical questions, like plate voltage and current..

I'm sure in some cases, like where the plate voltage has been cranked up way high, the claims could have some element of truth; otherwise it's more fun to see what answers come back. If it's bad math or all some big secret, then both of you know who the liar is, and that is worth a lot more than an argument.

The amazing thing is many of them actually seem to believe the power ratings they spout. It's the cruddy power/SWR meters CBers use.

Nothing you can say will change their mind, or their spiel when a sucker shows up to buy. Consider it a free show.

Of course they are funny. I've enjoyed the CB crowd for 35 years. If you've read any of my posts here over the years, you would know this. But let's be real. Telling a story about some CBer who is technically off base and then laughing is about the same as throwing a rock into a pond and then shouting, "Hey, look, ripples." Yea, it happened. Yea, you made ripples. So what?

Some Cbers are technically pretty sharp. Most are not even close. Given this is the Technical section, that should be a guide as to which ones to quote. A legitimate question was asked and you didn't answer it. Save the CB funnies for the QSO section.  ;)

"The last 10 tube amp I saw with Z tubes in it would do an honest 2000 watts carrier, 100 percent modulated."

Posted nearly an hour before you did.  Guess I DID answer the question posed.

I'm not going to get into a pissing match with you.  I did answer his question, honestly, and threw an anecdote in for a good laugh.  Guess you didn't read far enough down?

I'm done with the BS side of the conversation.  I answered both questions that where asked, about the 3 tube amp and a 10 tube amp.  The 10 tube was brought about as a question on the 3 tube, as the power supply had a bigger Dahl transformer in it, from what I remember (it was nearly a decade ago I saw the amp in question run)  was a 10 tube replacement, running 3CPX tubes.

Sorry if I posted my response in the wrong section.


--Shane
KD6VXI

It's a pissing contest only in your mind. I'm interesting in putting out factual answers to questions asked. I couldn't care less about any contest with you. It's not about you. It's about conveying good info. That's it. There's nothing else going on here. Silly stories about your CB buddies add zero information.

I've seen no mention of the CQ Magazine papers on "The Class C Linear Amplifier"  by D.O. Mann  W6HLY March (p32-)-April (p31-) 1964.  It referenced a CQ Sideband Handbook intro on the "the ZL Linear"  Its basic consideration was SSB Linear - high efficiency.

Essentially it was based around a CW (Class C) Amp with a Clamp tube on the tetrode screen grid.  It was with either a tetrode (preferred) or a twin triode clamp tube.    The circuit operated from a tap on the bias (var resistor adjustments of both the bias level and the tap) feeding the clamp tube, to drive the screen grid level.  The clamp tube was never allowed to get to cut-off.

The claim (I've not tried it - but maybe will) was 80-85% efficiency, Linear Drive Loading, Nominal Drive power ("20 db nominal power gain"), intermodulation distortion of 2 tone signal about 30 db below max pep.

I have the Heath DX-40 and DX-60 (resurrecting a DX-100b) - but may try "high power" this way, with an old home-brew amp (pair of 4-400s)  (and not use the modulator (pair of 811As), that I have from another old amp (in the basement) - amp is gone - 2500v power supply [866As] and modulator).