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Recommend AM linear




 
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Author Topic: Recommend AM linear  (Read 57985 times)
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Burt
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« on: April 03, 2009, 09:58:50 PM »

I am looking to buy a tube type linear amp to be used on AM, 500 watts or so, recommendations?
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WB2YGF
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« Reply #1 on: April 03, 2009, 10:03:42 PM »

I am looking to buy a tube type linear amp to be used on AM, 500 watts or so, recommendations?
500W carrier or PEP?  500W carrier is over the legal limit when modulated.

I use a AL-80B (single 3-500ZG).  Good for about 800W PEP, 200W carrier. (1KW SSB)  An amp that would do AM legal limit from Ameritron is over twice the price of the AL-80B
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« Reply #2 on: April 03, 2009, 10:05:53 PM »

A pair of 3-500z's with a robust power supply will do 300 watts I think, at the limit, for 500 watts, you need a BIG amp I think.

I thought the rule of thumb was 4x the carrier power for plate disapation, you would need tube/tubes with 2000 watts of plate disapation and a heavy power supply.

Brett
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ka3zlr
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« Reply #3 on: April 03, 2009, 10:22:32 PM »

Burt,

 As I understand you to be a pretty thorough Fellow just from on here, you've left the Field pretty open on choice/Price.

 There is Alpha Amps that will do quite well for your needs but at a large sum of money an I like them alot... But.

 Ya might give these guys a Look See here they will build what you want.

http://www.qrotec.com/ an they open up here shortly.

73
Jack.
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Ralph W3GL
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« Reply #4 on: April 03, 2009, 10:26:48 PM »

Amp Supply made an amplifier with three 3-500z's...  (1500 watts dis. !)

LK-550ZC was the version with QSK built in.  It will sustain a 500 watt
carrier all day with enough head room to make big AM.

Expect to put out better than 1000 bux if you find one.  

The unit without the QSK addition should be a bit less.

That amp has the power transformer external in a seperate case.
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73,  Ralph  W3GL 

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« Reply #5 on: April 03, 2009, 10:46:28 PM »

Drake L4B is still a nice commerical amp.
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K1JJ
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« Reply #6 on: April 03, 2009, 10:54:47 PM »

The 1960's Henry 2K 10-80M amps are built like tanks and run a pair of 3-500Z's with about 3KV. Good looking amp too.  I have one and admire the quality work inside from time to time. No comparison to the ham amps built in the  1980's and beyond. (Except for maybe the Alphas)   They even use air chimneys installed correctly ... that's rare these days vs: the common homo-fan blowing air across the tubes.

You can pick up a 2K for about $600, or so, the last I checked.  It can be modified to cover 160 meters with some work.

T
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« Reply #7 on: April 03, 2009, 11:12:50 PM »

Burt,

Here is a nice amp for sale for AM. 

http://swap.qth.com/view_ad.php?counter=730855


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AMI#1684
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« Reply #8 on: April 03, 2009, 11:16:38 PM »

I am looking to buy a tube type linear amp to be used on AM, 500 watts or so, recommendations?
500W carrier or PEP?  500W carrier is over the legal limit when modulated.

Not necessarily 
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Patrick J. / KD5OEI
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« Reply #9 on: April 03, 2009, 11:20:58 PM »

For 500W carrier under continuous duty, the tube(s) must have at least 1500W plate dissipation, maybe 2000W.
Consider the linear amp is at worst 25% efficient at carrier level (sometimes more like 33%, but nothing is perfect)
2000W in, 1500W burns in the tube, 500W out.
Be sure the power supply can deliver 2000 watts continuously.
At peak, the amp might be only 50% efficient (it should be better like 65% but what's perfect?).
The power supply will have to therefore deliver 4000 watts at the peak of the modulation envelope.
The average DC power required at 100% modulation by a sinewave would be about 3000W.

All of it scales down for lower power levels such as the often-stated 375 watts of carrier.

For it to never blow up {famous last words / marketing babble}:
(plate dissipation)
= (4 * carrier power)
= (1.33 * carrier DC input)
= (0.5 * peak DC input)
= (1.5 * 100% modulation average DC input - the suitable power supply CCS rating)

That's why the big amp boys on there, and they know who they are, have 250LB transformers and more than one 4-1000.

The purpose of overbuilding is for proper and safe operation within the rules consistent with hopefully endless component life, not for breaking the rules. Ham brands of amps are mostly no good for legal limit AM, except the very few and costliest, then then they prescribe a "duty cycle" so the power supply does not boil. The power supply is a weak link.

If you want to know the true AM average power capacity of an amplifier, see the RTTY rating and duty cycle in the manual. That gives a good idea what the amp will do without burning up either the tube or the power supply.

The cost of replacing tubes is the most expensive periodic maintenance on an amplifier, as long as the power supply does not cook.

Find an amp with two 3-1000's or two 4-1000's, or a 3CX3000.
The amp should have a proper cooling system as called for in the tube manuals, not some cheap fan.
If the power supply can be lifted by two sissies, forget it.
If it takes three men to barely lift it, that is the PSU you want.

I really do beleive all this stuff. That is why I am working to repair an amp with a 4000V supply and a 3CX3000. I have the new rectifiers now!! It will idle and coast and keep the lab as warm as toast.

One thing also to mention - the difference, at the other end, between a 500W carrier and a 350W carrier does not translate to very much in the grand scheme. The point is to get a well designed and well built amp and know its limits.
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« Reply #10 on: April 03, 2009, 11:41:50 PM »

(Note: Patrick hit the "Post" button a little quicker than I did, so we said a few of the same things)

I think Burt knows what the legal limit is. No need to tell him the obvious, he took a license exam too. That 350W number that people like to throw around is not cast in stone anywhere.

In my opinion, no brand-name linear amplifier built in the last 30 years is worth squat. When the good-buddies started putting ham amps on channel 19, the FCC put ridiculously overprotective limits on stage gain in commercially built amps. They've relaxed a bit in the last few years, but all the manufacturers are still building panty-waist grounded-grid amplifiers that only serve to humiliate the tubes you put in them.

I'd keep an eye out for a homebrew grid-driven linear built by somebody who knew what the tube(s) was/were capable of. A good pair of grid-driven 4x1s (not some whimpy triode-connected grounded-grid 4x1s) will give you a good comfortable half-kilowatt of snot with ample headroom with modest drive.

Outside that, Tom mentioned the Henry 2k, which is probably your only real choice for a suitable commercially-built amp.

You're far better off with a non-linear amp. A single 304TL with about 2000 volts modulated and 50 watts of drive will put you well over the 500W level with only about 1/4 of the power supply needed for a linear.

Whoops... I see Patrick was posting at the same time, and even did most of the math.
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Don
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« Reply #11 on: April 04, 2009, 01:33:23 AM »

If a linear or grid modulated tube amplifier is set up to run 100% modulation on positive peaks, the tube should dissipate approximately twice the carrier power.

When properly set up, the efficiency is about 33% when running unmodulated carrier, or about 1/3.  That means that 2/3 the input power is dissipated as heat.  So if the amp is designed to run 500 watts carrier power out, 1000 watts will be dissipated in the plates of the tubes, for 1500 watts DC input.

When modulation is applied, the plate of the tube should run cooler.  The DC input should not vary under modulation, so at 100% modulation, you still have 1500 watts DC input, but 750 watts output (500 watts carrier power plus 250 watts sideband power).  Now, the tube is dissipating only 750 watts.  The tube runs most efficiently at the 100% positive modulation peak, about 67%, or 2/3 efficiency.

This is plate efficiency, and does not take into account tank circuit efficiency, feedline losses, losses in the tuner (if used), etc, so if the tube is running 500 watts output off the plate, you will be lucky to get 400 watts into the antenna, so the real efficiency of the amplifier will be less than what the tube charts say.

I never heard of a rule of thumb that the plate dissipation should be 4 X the carrier output power, but with tubes getting  scarcer by the day, it wouldn't hurt to run them conservatively.  Theoretically, the plate dissipation should be twice the carrier output, but again that doesn't take into account losses that occur between the plate of the tube and the antenna, so the tubes will need to be rated for substantially more than 2X the carrier output for continuous service.

A linear is just as efficient when running AM as it is when running SSB.  The difference is that the 100% duty cycle of the carrier combined with the intermittent nature (short duty cycle) of the modulation peaks makes the overall average efficiency just a little higher than the carrier efficiency, but the peak output efficiency is still the same if the pep output is equal.


 
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« Reply #12 on: April 04, 2009, 04:51:22 AM »

My post was from experience with dissecting military RF amplifiers, they are built not to fail and so use about twice the tubeage as is called for. The GRC-106's amp for example uses two 4CX350's, but does only a 100W carrier and 400W PEP on AM {carrier+USB}. The RT246 uses a 100W triode to make 35 watts FM.

To use the tubes to their maximum book ratings, sure it is more like (plate dissipation) = (2 * carrier power) =(0.5 * "SSB single tone" power). That's the way storebought linears generally run.

About the plate current variations, maybe I have to go back to the books about the DC input not changing. I could swear it does change with increased drive and power output, but maybe that math was for a CW signal at comparative levels of 25% power and 100% power, and not related to the average readings taken with a modulated signal.

The Henry 2K was mentioned, and it is a fine amp -most models anyway. Even the Henry 2K fully utilizes its tubes.
Out of my own curiosity I checked through the models since it is legion.
One positive note is there was no duty cycle mentioned except on one model, and only one model actually warns against AM use.
That is a big plus.

I decided to make these notes here for Burt since he is asking.

2K, 2KD, 2KR - 2*3-400Z - 1KW input on AM/CW. No warning against continuous duty. It is tuned & loaded at 2500V/800mA (which is a slight overload of the 3B28 rectifiers due to the PIV rating-vs-average current rating), then reduced to 400mA for the 1KW input on AM. Plate transformer originally rated 2800-0-2800V, Peter Dahl replacement is 3000-0-3000@500mA CCS - that is 1500W - quite good if the Dahl iron is there! There was a note from Dahl in the posted manual that his transformer would "last longer" than the original. Unknown if this signals a problem. The 2KR does not come with a power supply. The 2KD is a desk model with remote power supply. The 2K is a floor model

2K3 - 2* 3-500Z- looks like the same deal otherwise, solid state power supply and combined plate/fil transformer. Floor console.

2K Ultra - 2KW PEP inout on SSB, 1KW CW and RTTY, AM operation "not reccommended" (If AM used, DC input reduced to 600W). "cooling system is not designed for AM..", HV=2200V@400mA Uses 2 * 8873 400W conduction cooled triodes and large heatsink operating at up to 210 deg F on rear panel. This is not really an "Ultra" amp in any respect except that the conduction cooling was considered high tech in the days. Yet it still has a fan. Fine for SSB. Tubes might be scarce or costly. Self Contained desktop unit.

Classic series:
AM is claimed, but no tuning instructions are given for it. It is not clear whether the HV is to be set to "SSB" (high) or "CW" (low) for AM. All use a pair of 3-500Z's. The manual uses the word "barefoot". Maybe this is not a CB term after all.

2K Classic - 1200W PEP output, HV=3KV, 10M band an option, average speech input power 1000 watts, Continuous duty, 125 LBs shipping weight. Floor console.

2KD Classic - 1200W PEP output, HV=3KV, 10M band an option,  average speech input power 1000 watts, Desktop version (self contained power) Intermittent Amateur service, the ugly duty cycle has arrived with a warning that the desktop amplifier can be damaged by extended.. (use). 80 LBs  shipping weight

2K Clasic X (domestic) - 1200W PEP output, HV=3KV, 10M band an option,  average speech input power 1000 watts, Continuous duty, same RF deck as export version, 140 LBs shipping weight. Floor console.

2K Classic X (export) - 1800W PEP output, HV=3.7KV, average speech input power 1500 watts, Continuous duty, Larger power supply, 160LBs shipping weight. Floor console.

There are also the 3K (3CX1500 tube) and 8K (5KW), others.. but not likely to find easily, and might not be as inexpensive to own. edebris.com has all those manuals.

So the plain 2K is a real bargain. Can it be refitted with inexpensive 3-500Z's? They are a little taller than the 3-400's IIRC. If so, I think it would be my first choice, but in the 2KD variant since the power supply is remote.

The Warrior.. Hmm. four 811's will get you some watts, but they will burn at 100W carrier and 500W PEP unless you talk fast. I'd have to say, and I hope it does not piss anyone off, four 811's are good for a 65W carrier CCS. Put 572B's in and throw a 125W carrier/500W PEP till the power supply gets warm, which would take a while - that is a well made amp.

The QRO stuff looks real good. On the HF-2000 I'd ask about duty cycle first. Still, 800W input for RTTY would do for AM, if the 100% duty is there. It would be interesting to find a way to buy the HF-2500DX Mark III export version, and just not worry about it. 2000 watts continuous output - there is your 500W carrier capability Burt!
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« Reply #13 on: April 04, 2009, 06:29:04 AM »

4CX800's     .... GU74 ...has the supply situation for these Ruskie toobs gotten any better ? 

most of the guys I know with Alphas are nervous about this ..73 ...John
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« Reply #14 on: April 04, 2009, 06:35:00 AM »

Patrick said:
Quote
So the plain 2K is a real bargain. Can it be refitted with inexpensive 3-500Z's? They are a little taller than the 3-400's IIRC. If so, I think it would be my first choice, but in the 2KD variant since the power supply is remote.


A lot of them did and can most of the power supplies have been beefed up to 3200 VDC. I have one, (albeit in parts, different story), and it will accomodat the 3-500Z's. With a little effort, you can also modify it for 160 Meters.
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« Reply #15 on: April 04, 2009, 07:35:53 AM »

When properly set up, the efficiency is about 33% when running unmodulated carrier, or about 1/3.  That means that 2/3 the input power is dissipated as heat.  So if the amp is designed to run 500 watts carrier power out, 1000 watts will be dissipated in the plates of the tubes, for 1500 watts DC input.

When modulation is applied, the plate of the tube should run cooler.  The DC input should not vary under modulation, so at 100% modulation, you still have 1500 watts DC input, but 750 watts output (500 watts carrier power plus 250 watts sideband power).  Now, the tube is dissipating only 750 watts.  The tube runs most efficiently at the 100% positive modulation peak, about 67%, or 2/3 efficiency.
Don.  The AL80 claims 65% efficiency for SSB & CW due to their "dynamic bias"  circuit.  Would older commercial or homebrew linears have this feature? (Of course this is useless for AM.)

Quote
Conventional bias circuits force high power linear amplifiers to dissipate hundreds of watts during low or no signal periods. This creates needless heat, since virtually no dissipation is required unless the amplifier is being driven with large signal levels.

The AL-80B contains an exclusive bias circuit that reduces the idling (quiescent) current very close to the tube's cut-off region. The power amplifier tube in the AL-80B has a full resting period of very low dissipation between dots and dashes on CW and between words on SSB. The lower idling current reduces component temperature on both CW and SSB. If only a few milliwatts of RF power are applied to the amplifier, the quiescent current will increase. Linearity remains excellent with this circuit because the tube can remain biased for class AB operation without unnecessary standing dissipation.
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WB2YGF
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« Reply #16 on: April 04, 2009, 07:51:01 AM »

I am looking to buy a tube type linear amp to be used on AM, 500 watts or so, recommendations?
500W carrier or PEP?  500W carrier is over the legal limit when modulated.

Not necessarily 
Thanks for clearing that up. I stand corrected. Cheesy
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ka2zni
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« Reply #17 on: April 04, 2009, 08:47:36 AM »

Well, I can't run the 500 watt carrier but my Henry 2K(X) will run 350 watt carrier all day long... Works out to 600 watts of Plate dissipation +/- and the RF deck was hooked up with a Blower that mounts in stock configuration with no altering that moves about twice the CFM of air keeping the Pins of the 3-500's cool and saving from melt down.

Seems if you wanna go bigger... Next step for me would be dependant on the size of my wallet.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=160324772175

http://www.alpharadioproducts.com/8410.asp

Or.... Build something BIG.... I am sure you have already looked at all this, but I couldn't resist throwing the Commander in there.. Desktop amplifier with a pair of 8877's... Kinda interesting too see what they were using for a xmfr. in that thing since there were only a handful of them built.
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Burt
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« Reply #18 on: April 04, 2009, 09:10:43 AM »

I am looking to buy a tube type linear amp to be used on AM, 500 watts or so, recommendations?
500W carrier or PEP?  500W carrier is over the legal limit when modulated.

I use a AL-80B (single 3-500ZG).  Good for about 800W PEP, 200W carrier. (1KW SSB)  An amp that would do AM legal limit from Ameritron is over twice the price of the AL-80B

Wouldn't carrier be 750 watts if PEP limit is 1500 watts, with 100% modulation, pos peak is 1500, neg peak 0W?
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Burt
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« Reply #19 on: April 04, 2009, 09:19:46 AM »

Amp Supply made an amplifier with three 3-500z's...  (1500 watts dis. !)

LK-550ZC was the version with QSK built in.  It will sustain a 500 watt
carrier all day with enough head room to make big AM.

Expect to put out better than 1000 bux if you find one.  

The unit without the QSK addition should be a bit less.

That amp has the power transformer external in a seperate case.


I tried HRO and AES, who sells it?
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« Reply #20 on: April 04, 2009, 09:23:19 AM »

For AM, instaneous plate voltage doubles at 100% mod.. Thus
PEP is four times carrier.

Pete
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Burt
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« Reply #21 on: April 04, 2009, 09:23:38 AM »

Burt,

Here is a nice amp for sale for AM. 

http://swap.qth.com/view_ad.php?counter=730855

NO QSK

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« Reply #22 on: April 04, 2009, 09:52:20 AM »

I am using a Gonset GSB-101, four 811's, rectifiers replaced with diodes.    I run it continuously 175W carrier output on AM.   You have run the loading heavy so you get 600-8ooW PEP.    The amp is rated for this AM power, probably due to its hefty power supply.   I use new Russian 811A's.
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« Reply #23 on: April 04, 2009, 10:37:21 AM »


And for the same money, you can find an SB220...  2 500Zs, a slightly bigger power supply, and it doesn't weigh much more.

Its only good for about 200 to 300 watts AM carrier, though :/


--Shane
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« Reply #24 on: April 04, 2009, 11:22:52 AM »

What are you going to be driving it with? How about a plate modulated amp?
N8QPC
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