Finally - Some 4-1000A Grounded Grid Linear IMD Specs

[K1JJ]

Hi All,

I've never seen 4-1000A GG linear specs showing IMD figures. Eimac doesn't publish them nor have I seen a ham post them.  

Today I ran some tests using my 4X1 in GG, 3500 volts, idling at 150 ma,  and driving with a clean FT-1000D.  At lower powers, the FT-1000D  shows near -40db 3rd IMD, so it is a reasonable driver for this test.  (The FT-1000D is about -31db at full 200w power)  I was using a 2.8 kc ssb transmit signal and measuring 3kc or more away.

I used my HPSDR SDR receiver with a 3" wire antenna as a spectrum analyzer to sample RF and kept the receiver level as low as possible to eliminate overload, etc.

I ran three types of audio programming to see the worst results - a two tone, a pecker pulse and my own voice thru the audio to get an average of various IMD conditions.

* These results apply to either ssb or AM linear operation.

The results were surprising but expected in some areas too.  On average, putting out 2KW pep, the 3rd order IMD was about -31db.  At 1200 watts out it improved to -33db.  At 750 watts out, it was very close to -35db!  At 750 watts it is barely out of class A and certainly shows it in cleanliness.

Bottom line is the 4X1 in GG  is acceptable at 2KW pep out, being near -31db 3rd.  At 1200W it is a very clean amplifer compared to most modern SS transceivers on the market. At 500-750 watts out, it is a super star.

It was interesting to see how the general noise floor out 5khz or so would come up as the drive was increased - and how the main signal started to compress and become less linear.  

By increasing the voltage above 3500 volts may or may not have much effect on IMD, but it might be worth a try.  Maybe more plate voltage will produce less grid current and allow the amplifer to run at a higher power level for a given IMD.

I was also surprised that by loading the amplifer VERY heavily (less C2 loading) that this had little to no effect on IMD.  It just reduced the power and was the same IMD when brought back to the original power level.  I found that it was best to peak the power out using both C1 and C2 and then slightly decrease C2 (heavier loading) so the power dropped maybe 50 watts to be sure we are on the correct side of the loading and transfer curve.  Again, adding further loading past this point had little effect on IMD.  It is still a valid practice in most cases, however.

I want to try a 3-1000Z (when I find one) and see if a tube designed for linear service is better (IMD) than the 4X1 in GG.  But bottom line is the 4X1 in GG, run at a reasonable level, is an economical contender.

This again proves the old philosophy that to run a clean signal in linear operation, use a tube that is at LEAST twice as big as you intend to run it for power.  For example, for 1500-2KW  out pep, a 3CX-3000A7 loafing along would be a great choice.   Sure, we can run a 4X1 in GG at 2KW out, but we pay the price with more side splatter and this can be a problem when the band is quiet and crowded.


More tests later when I get a 3-1000Z to try.  Anyone have one to sell?  :-)

Tom, K1JJ

[Todd, KA1KAQ]

Cool stuff, Tom - thanks for posting it! 

[K1JJ]

Thanks Todd.

I think over time, general ham web searches may find this thread of use.


BTW, after running more tests on my other amplifiers, I am finding that the limitation in my setup is the FT-1000D driver!   Whenever I pull more than about 10 watts of drive from it, the IMD drops quickly from -37 down to -31db.   This probably has a lot to do with why the 4X1 IMD is dropping off so fast at higher power.   The 4X1 amplifier may be cleaner than I first thought at higher powers.  I will add a clean IPA and see what happens.

So, at least the figures I posted are minimums - and they show that the 4X1 is a pretty decent tube in GG.

T

[Opcom]

Despite the issue with the exciter's distortion, the 4-1000's distortion may also have to do with the plate voltage or indirectly the percentage of the amp's rated power that is actually used. The Elenco Commander had a GG 4-1000, 6 KV on the plate, and was rated 4KW PEP input (1500 AM), with some of their advertising literature saying the higher plate voltage in combination with running the amp at normal 'ham' levels, helped to keep distortion lower. It was made from 1958-1959, perhaps a magazine ad or article could be found to support this.

I don't recall where I saw the ad. Unfortunately some amateur mags got a little skittish when gear was able to blow past the limit and there are fewer ads for ham-purpose things like this. A few more though for commercial gear like TMC and other big stuff.

This aligns with your measurements at 3500V anyway, lower distortion when demanding a lesser percentage of available power, even if it was the exciter's amp.

[W2VW]

I'll bet you could get the 4-1000 distortion down to the level of a 3-1000 by applying screed and control grid potentials.

I heard some W6 dude going on and on explaining to a new ham that he should always run his transceiver at 100 watts and not less in order to avoid distortion............................

[kb3ouk]

I heard some W6 dude going on and on explaining to a new ham that he should always run his transceiver at 100 watts and not less in order to avoid distortion............................

I just saw something somewhere yesterday that claimed you shouldn't run a modern 100 watt rig at 50 watts or less because it could destroy the finals. 

[KB2WIG]

' I heard some W6 dude going on and on explaining to a new ham that he should always run his transceiver at 100 watts and not less in order to avoid distortion............................ '

ya shuda broke in and said that " It's O.K. if you use Bird Watts. "

KLC

thats the watts I use

[Steve - K4HX]

It's not all necessarily BS. I've heard some fairly reputable people claim the IMD on certain rigs was get worse as the power was lowered. This had something do to with how it was done in those rigs.

[K1JJ]

Interesting.

It's amazing to me that in this day and age, most modern solid state transceivers (riceboxes) still exhibit -31db 3rd order.  Most except for the rare exceptions that permit class A use at reduced powers.  The bottom line is if a transceiver is only -31db, then no matter how clean the next linear amplifier is, the cumulative IMD will always be LESS than -31 db. For example, if the linear amp is -40db and the driver is -31db, then the overall result may be -29.5db or whatever.

Yes, some rigs do degrade as we reduce power. I understand that bi-polar SS finals exhibit better cleanliness when reduced in power. I know my FT-1000D does. Whereas MOSFET finals appear to degrade or stay the same as we reduce power.  The 1000D uses bipolars. The Ft-1000D, running alone, is clean as a whistle, like almost -37db 3rd at times until we increase to that 10 watt level and it slowly degrades to -31 at 200w.   The whole noise floor starts coming up out +-5kc using a pecker pulse source.  This is normal.  So no matter how clean our 4X1 or 3X1 or 8877 amp may be, we are fooling ourselves with marginal drivers.

With MOSFETs and possibly some configurations, I think as we reduce power to very low levels  the cross over distortion become more prominent.

I am thinking of building up a class A linear to take the clean 10W 1000D level and go to 100 watts. Maybe a single 3-500Z in class A would do it. This wud give a clean 100W signal (-40db) to drive ANY amp in the shack - and let it shine.  A 3-500Z at 1500V at reduced power is said by Eimac to approach -45db+ third.

The 4X1 with higher voltage will exhibit more gain, thus will require less drive, which is a good thang. Though, I have been warned that anything over 5500V produces indirect electron bounce?  problems which can cause distortion.  The 4X1 sure is a robust tube and I have a good supply. Wish I cud find a 3-1000Z to try in its place.

I'm still toying with the idea of adding MORE  negative feedback to the GG stage. I understand that adding a small resistance and parallel cap to ground from the grid is the way to do it. I have the grids strapped to ground right now and prefer this for stability..  

But why won't a 100 ohm resistor in the cathode work as degenerative NFB, just like a single ended audio stage? It must have something to do with the GG configuration, no?  I once tried it and thought it didn't do much, but cud be wrong.

T

[KD6VXI]

Tom,

A cathode resistor in series does work.  Denny Had used it on the Dentrons, and I've heard of it on a few others.

10 to 25 ohms is the value that comes to mind.




On a different note, what type of mod xformer are you using on the 2X1?  I'm contemplating a pair of 4-400s modding my 4-1000....

--Shane
KD6VXI

[K1JJ]

Tom,

A cathode resistor in series does work.  Denny Had used it on the Dentrons, and I've heard of it on a few others.

10 to 25 ohms is the value that comes to mind.

On a different note, what type of mod xformer are you using on the 2X1?  I'm contemplating a pair of 4-400s modding my 4-1000....

--Shane
KD6VXI

Thanks for the info, Shane.

OK on Denny using it in his amps. Now that I have a spec analyzer set up, think I will try the cathode resistor again and see what happens.  Now, does it get bypassed with a capacitor of some value or does the resistor sit in series alone?  I also have a string of diodes in there as cathode bias.

On the 4X1 plate modulated by a pair of tetrode-connected 4X1's, I am using an RCA 1KW broadcash mod transformer. I think it originally used 4-400As.  It weighs about 140 pounds.

T

[KD6VXI]

Here's a link to Measures and a few others smarter than I disguisting it...

http://groups.yahoo.com/group/ham_amplifiers/message/6143?var=1



I KNOW I did a virgin MLA2500 here that had both R in series and shunt across the input. Don't recall, and don't have the old work orders to find out if it was an A or B. The shunt was the gain killer (to keep within rules and regs of the time I'm supposing) and the series served two purposes:  It increased input R while dropping distortion.....  Sounds like feedback to me..


--Shane
KD6VXI

[W2VW]

It's not all necessarily BS. I've heard some fairly reputable people claim the IMD on certain rigs was get worse as the power was lowered. This had something do to with how it was done in those rigs.

I've seen and heard the same thing. It could easily be true for certain circumstances. Problem was the "elmer" making it into a blanket statement.

Barring gain control loops and quirky bias schemes more headroom should always be cleaner.

[W2NBC]

Mr T,

With the amount of 4-1000's, iron, aluminum, and plexi for wide open viewing youz got ..

I say a PAIR of 4-1000's in GG.. Loaf with low 3rd order..

Yours in amplitude,
Jeff

[K1JJ]

Mr T,

With the amount of 4-1000's, iron, aluminum, and plexi for wide open viewing youz got ..

I say a PAIR of 4-1000's in GG.. Loaf with low 3rd order..

Yours in amplitude,
Jeff

That's actually a great idea, Jeff.  Running a pair at 1500w out wud be as good as it gets. The challenge wud be to find a driver as clean. A 32S3 uses negative feedback with its 6146's IIRC, so that wud work out to be an easy -40db 3rd driver..   

The problem with two tubes for me is that the amp is already built and can barely fit one tube. The fil xfmr is also just for one, etc.

The good news:  I just bought two used 3-1000Z pulls today with a money back guarantee if they are not good. I pick them up tmw locally.   Can't wait to see how they do.   Now I can take my time looking for a NOS 3-1000Z.


T

[W2NBC]

T,

NOS
http://www.ebay.com/itm/NEW-3-1000Z-in-EIMAC-factory-box-8164-NOS-boxed-HF-linear-transmitting-tube-/170954958669?pt=US_Radio_Comm_Tubes&hash=item27cdb5a74d

you could build a few amps up for that price!

Have fun, and great info BTW!

Jeff

[K1JJ]

Yeah, I've been looking at that one for a few days now, but it's in Greece- plus he wants a left BA for shipping.

These kind of things work the same way... suddenly 3 or 4 new ones will pop out of the woodwork for a good price all at once and I will regret buying the two pulls... :-)

T

[KD6VXI]

I looked for an inexpensive 3-1000 for a couple years for my Harris RF-103.

It's currently using a 4-1000.  The 3- tube is about worth it's weight in gold.


--Shane
KD6VXI

[K1JJ]

I guess they will eventually find their way to market as time goes by.

Well, I just ran some tests using a 50 ohm glowbar in series with the input to the cathode of the GG 4X1 linear. (after the tuned circuit) It should give about 3db of improvement in IMD.  I had a hard time seeing any difference.
I was told that it just adds attenuation to the input signal, no NFB.  My conclusion is that it's probably not worth it considering it's only 3db and we would need to double the drive power to make up for it. <aybe I'll try the resistor series cathode method later.

I expect the 3-1000Z to have another 3db of gain over the GG 4X1, so it might make sense to add the 50 ohm 3db IMD improvement later.

Using a big tube and reducing the power output to 1/2 it's capability is probably the easiest way to grab a quick  IMD improvement.  I will revisit this later once the levels are figgered out.

T

[K1JJ]

I'm trying some different audio inputs for the IMD tests.  I had been using the pecker pulse and voice for general measurements.  I find that these audio sources, especially the pulse pecker, tend to produce more generous IMD numbers.

I just ran some serious two tone, three tone and four tone tests from a computer generator and find the results are much harsher.  The two tone shows the 4X1 in GG is at least -31db 3rd, but rarely gets above -34db 3rd for most tone combinations.  I made some changes in my numbers for past posts to reflect this so the archives will be more accurate.

Still, it seems the limitation is still in the 1000D driver as usual.   So far, the 4X1 does not seem to be adding any meaninful IMD degradation to the overall signal, which is a good thang.  The 3-1000Z tests will be interesting to see tomorrow.

T

[KM1H]

The highest B+ Ive run 4X1 IMD tests on is 4500V and yes, it does improve IMD when tuned up for 2300W output and the Load cap set for a bit higher loading until the output power is reduced 50W or so.

The IMD 3rd runs in the -33 range at full bore and gets better with less drive until the TS-940 output is below its 75-80W out sweet spot where it is in the low -40's and drops to -36 at 120W. The 4X1 is then around -36 at 1500W. This was with an almost new tube that came from some scrapped industrial gear; the usual tired pull likely would not be as good. I do have a NIB 4PR1000A and a 7KV PS with a Variac for whenever I get another urge.

The TS-950SD is a bit better for IMD plus it has 150W specs but I didnt have it back when I tested that amp back in the late 80's and I dont like pushing the 940 past its usual 100W for more than some quick tests.

I believe I also used the generic ARRL 2 tones back then which is really cherry picking for best IMD.

[ke7trp]

My 4-1000 GG amp ran 6900 volts from a 2.5 amp PD transformer.  I would like to see more voltage and the results.

I see now why the top line rigs have the Class A option.

Tom, I wonder what kind of input the amp has?  Tuned input?  Fixed 50 ohm?

Cant wait to see the 3-1000 numbers.

Good to see you back Shane.

C

[K1JJ]

OK on the comments, Carl and Clark...

On a friend's suggestion, I did the glowbar in the input lead test again with the more stable two-tones, looking to add NFB - and am hard pressed to see any difference except for input attenuation. Just wanted to set the record straight. I will try the series resistor in the cathode and see what that does for NFB.  

Clark, I am using a standard pi-network input to the cathode. Variable C1 to ground, L1 roller in series and a second variable C2 to ground. Works on all bands for a 1:1 match and supposedly a better IMD via flywheel effect, if that really exists. Your 4X1's must really sing with that voltage.  A good layout needed, thats for sure.

Carl, looks like we are seeing similar IMD numbers - at least when I am using the two-tone test. I was getting fooled by the pecker pulse numbers which always looked better. Why is this so?  I would think a pulse is rich in harmonics and crud.

If a pecker pulse is hitting the same peak power at a quick 10% average power pulse, why would a tone-tone at a more constant 50% power average create worse IMD numbers?   I would think the tube would see the same internal dynamics of non-linearity whether the audio is a low average or high average. But that's what I am trying to learn...

Today I decided to pull out the 4-1000A GG and replace it with an 8877.  (external anode 3CPX1500A7)  The 8877 will be able to loaf along even better than a 3-1000Z with 1/2 the drive. It's a higher gain tube.  It probably has better IMD specs too. There are now Chinese 8877's called 3CPX1500A7's on ebay for <$250 these days. The word is they work as well as the Eimacs for both power  performance and IMD.  I already have an Eimac 8877, so will try that for now.

So, I will not be buying the two 3-1000Z pulls and instead going with the easily obtainable 8877.  I'm not gonna  play the unobtainium tube game with 3-1000Z's anymore.


More later -

T

[W2VW]

Limit the frequency response especially the low end to cheese out some lower IMD.

[KD6VXI]

Today I decided to pull out the 4-1000A GG and replace it with an 8877.  (external anode 3CPX1500A7)  The 8877 will be able to loaf along even better than a 3-1000Z with 1/2 the drive. It's a higher gain tube.  It probably has better IMD specs too. There are now Chinese 8877's called 3CPX1500A7's on ebay for <$250 these days. The word is they work as well as the Eimacs for both power  performance and IMD.  I already have an Eimac 8877, so will try that for now.

So, I will not be buying the two 3-1000Z pulls and instead going with the easily obtainable 8877.  I'm not gonna of play the unobtainium tube game with 3-1000Z's anymore.


More later -

T

Same issue I found.  I'd love to have the glow, and the  gain of the 3-1000, but at the same time, it's hard to spend more on a tube thats already absolete when for a lower cost you can run a ceramic.  I've got a 3cx1000 here that's going to go into the harris when the 4-1000 is a dud or gets converted to the plate mod'ed rig.  Not to mention, it takes me 120 watts to get the same Pout that 60-65 gave before.

If it's the Greenstone tubes, I've had good reports on them vs the Eimacs.

Clark, good to be back.  A full strapping 10 watts of carrier!  My antenna is good for DX, locally it sucks..  But, TimTron comes in during the daylight hours full quieting.  Listening to you on 40 right now, matter of fact  Just made a contact with you, a full 10 watts carrier!

--Shane
KD6VXI