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Author Topic: Ameritron AL-80B on AM  (Read 28117 times)
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K5WLF
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« on: November 12, 2011, 05:52:41 PM »

I'm thinking about buying a commercial amp to 'get me by' until I can collect the parts to build one. A friend of mine has an Ameritron AL-80A and it's working real well for him. But you don't see many of those around for sale, so the current production 80B comes to mind. However, it has a couple of features that I'm wondering about the effects of on the quality of an AM signal.

Specifically, "Dynamic ALC", which is supposed to markedly improve SSB performance (not a real big deal with me) and, "Instantaneous RF Bias" which cuts off the tube (except for the filament) between words, according to their website.

Do any of you know if either of these features have an undesirable effect on the transmitted AM signal? The RF bias thing sounds a little weird to me, since it seems that if the tube is turned off between words, so is the carrier and that would seem to be counterproductive.

All words of wisdom appreciated.

ldb
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Steve - K4HX
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« Reply #1 on: November 12, 2011, 05:56:06 PM »

I'm pretty sure you can turn off or set the electronic bias so that it's not a factor on AM.

If you don't need 160 meters, and SB220 is probably a better deal, unless you are getting a really good price on the AL80.
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K1JJ
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« Reply #2 on: November 12, 2011, 06:26:50 PM »


Specifically, "Dynamic ALC", which is supposed to markedly improve SSB performance (not a real big deal with me) and, "Instantaneous RF Bias" which cuts off the tube (except for the filament) between words, according to their website.

ldb


Yes, that's an efficiency technique. Less idling heat generated.

I wonder if that bias circuit is fast enough (attack time) to prevent class C cut-off  splatter on the leading voice syllable without using a time delay?  Just like using external ALC with slow attack time, I have heard some of these circuits generate garbage.

If you get one, try it on ssb and tune your local receiver up the band. Say a quick "four" over and over and see if you get a popping splatter for an instant.  

One way to handle this problem is to use a delay that shuts off the RF drive for an instant while the amplfier comes up to class AB.  But the downside is this could cause VOX-like cutoff of the first syllable, depending on the delay.

I'd be curious how it works and the results.


I also agree with Steve that an SB-220 would be a great way to go.  The SSB-220 has two 3-500Z's while the AL-80B has only one. That's 200W out vs: only 100W carrier, if run very conservatively with reserve and big 150% audio peaks.


T
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« Reply #3 on: November 12, 2011, 06:33:14 PM »

not to hijack the thread, but has anyone tried an alpha 78 on am?
I've got one with the three 8874s. It's questionable whether i could push them to legal limit but running 250-275 watts carrier should be no problem.
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flintstone mop
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« Reply #4 on: November 12, 2011, 06:35:08 PM »

I guess it's going to depend on your antenna system. Is it a marginal system?? Resonant? OWL?? Good tuner? How high off the ground?
If the antenna is around average or a nice high up antenna and full length, you could get by with a 100W carrier on AM. 160M will be a challenge on the noisy nights.
If the antenna is not doing well, then 100W will be frustrating. 80M is a little easier and 40 gets better.
The 80B is an 800 W PEP amp. So a 200 Watt carrier for AM is about it. And that particular amplifier won't like old buzzard operation..........pushing that hard on it. Prolly a 125 W carrier might make it happy.
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Fred KC4MOP
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« Reply #5 on: November 12, 2011, 06:42:30 PM »

not to hijack the thread, but has anyone tried an alpha 78 on am?
I've got one with the three 8874s. It's questionable whether i could push them to legal limit but running 250-275 watts carrier should be no problem.

The Alpha is a 1200 W PEP rig so your calculations are very close to give the amp some breathing room if the tubes are getting a little age on them.
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Fred KC4MOP
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« Reply #6 on: November 12, 2011, 08:00:39 PM »

Not to be negative but from my experience, going the rf amp route you really need at least 1000 watts plate dissipation with your tube or tubes.  That plus some mods to make a modern day ssb amp AM sustainable.  The problem is anything less, gets you so little carrier power you are better off getting a DX100, Viking1 or 2, Valiant,  Globe Champ...something like that and skipping the hassle of running an amp.  the hassle is the mods, the little issues of driving the amp, cooling it, monitoring the exciter power, dealing with the exciter's ALC and/or folding back if the match to the amp's input is not good and so on.  If you already have all that stuff that's one thing.  You may as well put it to use.  But for the cost of the AL80, you can probably get a DX100 or Viking2.  I would not run a stock single 3-500z in AM service at more than ~125 w. output.  BTW, contrary to Ameritron advertising, it is not a [reliable] 1 KW amp.  All you have to do is study the power supply specs and do a little math and you'll see.   All this assumes you only want to operate AM.  If you want to operate SSB occasionally, the aforementioned transmitters are not useful.
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« Reply #7 on: November 12, 2011, 08:52:17 PM »

Ive run both my LK-500ZC (2X 3-500Z and with external Power Pak) and Alpha 76PA (3 x 8874's) on AM at around the "legal" limit with no difficulty or ill effect.

Carl
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« Reply #8 on: November 12, 2011, 09:28:26 PM »

Ive run both my LK-500ZC (2X 3-500Z and with external Power Pak) and Alpha 76PA (3 x 8874's) on AM at around the "legal" limit with no difficulty or ill effect.

Carl
Carl,

it is my assumption that the alpha 76pa and the alpha 78 are essentially the same amp except the 78 has the "no tune feature", correct?
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K5WLF
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« Reply #9 on: November 12, 2011, 10:57:46 PM »

Thanks to all of you for your insights and comments. It looks like the 80B would just be a waste of money. I've got a DX-100 that'll be on the air shortly. I was hoping to pick up an afterburner for it but, from the info y'all gave me, I'll be better off to hold out for the one I'll eventually build.

I'm real estate challenged where I am now (100' X 70' city lot), so I'm running a 97' HB coil-loaded dipole on 160, and it's not very high. Works quite well, but with very limited bandwidth. I have a 5-BTV on the metal roof that's done me real well, but it's no good inside of about 200 - 300 miles. I've got a folded dipole in progress for 80-10 that'll pretty much be a cloud-burner for the close-in stuff.

I've passed many SB-220s for good prices at the 'fests, but they're all missing that magic "160" position on the bandswitch. With good plate iron getting so hard to find, I'm almost considering building a solid state amp, but I just happen to love those tubes. The planning and thinking goes on.

Thanks again for all the great info.

ldb
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« Reply #10 on: November 13, 2011, 09:18:14 AM »

not to hijack the thread, but has anyone tried an alpha 78 on am?
I've got one with the three 8874s. It's questionable whether i could push them to legal limit but running 250-275 watts carrier should be no problem.

I had a 76 with 3 8874s. It had no problem making 400 watts carrier and legal limit peaks.
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« Reply #11 on: November 13, 2011, 10:48:11 AM »

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Carl,

it is my assumption that the alpha 76pa and the alpha 78 are essentially the same amp except the 78 has the "no tune feature", correct?

Correct. All the Alphas with equal tube counts from the 274 to 78 are esentially the same with the bands covered and no tune features being the differences.

If you want some AM smoke then go with a LK-800A or a later version and run 3CPX800A7's and a 3200V PS. Its an easy 5KW on SSB Grin Huh  Ive built many for customers and tested a few on 75 AM  Roll Eyes

When the 8874's in my 76 finally die I'll go with a pair 3CPX800's and a bigger PS.
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« Reply #12 on: November 13, 2011, 02:07:18 PM »

It looks like you have made the decision to look past the AL-80B for AM use and that's fine. I did use one with my TS-440S and Flex 5K for a few years and it had enough headroom to produce a clean AM signal with a 125 watt carrier. Much more than that and it would start to flat top. It was a very reliable amplifier and very easy to work on when needed. The only work mine needed was to clean the dust out now and then. I did replace the bleeder string and PS caps simply because I had the parts on hand and the ones that were in there were looking a little decrepit. A limiting factor with this amplifier for AM operation is that the 3-500 is cooled from the side. They did arrange this properly so air is always flowing past the pins underneath the socket and there is plenty of air moving around the tube envelope, but it's not quite the same as using a pressurized lower chassis with a chimney. The amp performed quite well with all the other usual modes.

I never used ALC with mine but I did find the dynamic bias to be annoying. It was very easy to eliminate, and there are several places in the circuit where it can be defeated. The way I did it was the most commonly used method when the amp first came out, but later many users simply forward biased a transistor one stage later. I did detail what I did here:

http://www.w1aex.com/al80b/al80b.html

At any rate, the 80B is a fine amp, but I moved on to an AL-82 e-bay special simply because it had a pair of 3-500's running at +3600 volts, with chimneys, and it covered 160 meters (like the AL80B).

Good luck with whatever you select!

Rob W1AEX
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K5WLF
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« Reply #13 on: November 13, 2011, 03:03:08 PM »

Thanks for the additional info, Rob. Your comments solidified my decision to pass on the 80B. There wouldn't be much point in spending $800 or so for only 125 watts of clean carrier, when I have a DX-100 that'll put out 120-ish. I think I'll just hold out for an amp, either store-bought or HB, that'll get me up around legal limit country.

As I mentioned, the lack of good (i.e. cheap) plate iron is going to be a drawback for any hollow state HB projects, so I've been thinking about going SS. The Class E stuff looks really promising, but from looking at steve_qix's site I've gotten the impression that you can't build just an amp that way. That the RF deck has to be driven by either the analog or digital mod deck he shows, eliminating the possibility of building just an amp for an existing xmtr. I may have the wrong impression there. Not sure yet. Plus, I really do like tubes.

A friend of mine locally has a Dentron MLA-2500 he's looking to sell, but the cost of the 8875s is just plain scary. And so, the quest goes on.

Thanks again,
ldb
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« Reply #14 on: November 13, 2011, 05:21:27 PM »

As I mentioned, the lack of good (i.e. cheap) plate iron is going to be a drawback for any hollow state HB projects, so I've been thinking about going SS.

   Don't overlook cathode modulation of one of those linear amplifiers. Chuck, WA0ZHH had a simple design that was matched to work with the SB-200. Here is the link:

http://www.kc3ol.dynip.com/downloads/wa0zhh.jpg

   So with cathode modulation, the efficiency is often ~ 50% (when grid and cathode modulation are about the same), so for a given set of tubes, and for a given power supply, the RF output will be considerably more then what you would get for linear amplifier mode on AM. Remember we go to class C with cathode modulation.

   This approach does away with a modulation transformer, and can be added on to a existing linear amplifier.

   It might be a little tricky to pull off the first time. I had a long talk with chuck over this and he was using it. Last I heard he had a stroke 3-4 years ago.

Jim
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« Reply #15 on: November 13, 2011, 05:32:02 PM »

As I mentioned, the lack of good (i.e. cheap) plate iron is going to be a drawback for any hollow state HB projects, so I've been thinking about going SS.

   Don't overlook cathode modulation of one of those linear amplifiers. Chuck, WA0ZHH had a simple design that was matched to work with the SB-200. Here is the link:

http://www.kc3ol.dynip.com/downloads/wa0zhh.jpg

   So with cathode modulation, the efficiency is often ~ 50% (when grid and cathode modulation are about the same), so for a given set of tubes, and for a given power supply, the RF output will be considerably more then what you would get for linear amplifier mode on AM. Remember we go to class C with cathode modulation.

   This approach does away with a modulation transformer, and can be added on to a existing linear amplifier.

   It might be a little tricky to pull off the first time. I had a long talk with chuck over this and he was using it. Last I heard he had a stroke 3-4 years ago.

Jim
WD5JKO

I often wondered about this circuit Jim.  "... can be added on to a existing linear amplifier"  No need to change the bias to class C?

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« Reply #16 on: November 13, 2011, 06:00:42 PM »

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No need to change the bias to class C?

It appears the class C bias comes from the 200K pot adjustment off the bias supply to the 6336 tube grids.



That's a cool looking circuit.


I would be curious to see some actual efficiency measurements with this circuit compared to a conventional linear amplifier that is biased into class C for AM use.  

Class C? for AM linear use someone might  axe? Yes.  I have done this:  With the lots of cathode diodes putting the linear way below cutoff, (class C) the AM carrier will bring the tube back into static class AB idle and the audio will ride on this with clean results.  IIRC I found a slightly better efficiency.   I was able to do this with the amplifier biased almost to class D, but it took more and more drive to get the carrier up to a normal level.  IE, the carrier takes the place of a reduced bias. This would never work on ssb, of course, where there is no carrier.
 

T
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« Reply #17 on: November 13, 2011, 10:11:23 PM »

I asked about class C because I know mixers need to be non-linear to do their algebraic magic and create a complex signal.  Modulating a stage is akin to a mixer. Smiley
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« Reply #18 on: November 14, 2011, 11:25:09 AM »

Don't worry about the auto bias thingee. It's RF activated. So even between words on AM there's RF drive with the carrier. It don't know the difference 'tween your words and yyour carrier.

It really helps heat control on SSB and CW where no words or dits = no RF!
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« Reply #19 on: November 14, 2011, 12:38:16 PM »

Thanks to all of you for your insights and comments. It looks like the 80B would just be a waste of money. I've got a DX-100 that'll be on the air shortly. I was hoping to pick up an afterburner for it but, from the info y'all gave me, I'll be better off to hold out for the one I'll eventually build.<snip>

ldb

That IS the way to go. The AL-80 really is meant for SSB operation.  AM operation is about the Viking 1, 2 DX 100 etc class
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« Reply #20 on: November 14, 2011, 01:31:01 PM »

Don't worry about the auto bias thingee. It's RF activated. So even between words on AM there's RF drive with the carrier. It don't know the difference 'tween your words and yyour carrier.

It really helps heat control on SSB and CW where no words or dits = no RF!


Hola Bud -

Yes, on AM it should be OK.  What I was wondering about is on ssb. How do they handle the first voice syllable? Is the attack time fast enough to keep the amp from an instantaneous splatter peak in class C cutoff mode or maybe they use a time delay circuit to let the amp come up to class AB first - but that might cut off part of the first voice syllable?

The test would be to tune up the band and say "pop -pop - pop"   and listen for peak garbage. Or if a delay is used listen for the first part of the "pop" missing on frequency due to the bias time delay. (much like a VOX)  That's what I'm wondering about.

T

 
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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« Reply #21 on: November 14, 2011, 01:58:52 PM »

Hi Tom,

I don't know exactly what the switching time is on the bias FET but it does pickoff the RF before the tuned input circuit so there is an inherent phase delay. My guess is the bias FET is switched long before the RF hits the cathodes.
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« Reply #22 on: November 14, 2011, 02:03:26 PM »

Here's Tom's W8JI explaination.

Elec. Bias

As I read it, it appears he encountered some problem with the pickoff at the RF input. The revision puts it after the tuned input at the cathodes. But with the very low level of RF it takes to switch it and the speed at which it switches there's ample attack time to minimize any destortion.
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« Reply #23 on: November 14, 2011, 03:12:08 PM »

Yes, on AM it should be OK.  What I was wondering about is on ssb. How do they handle the first voice syllable? Is the attack time fast enough to keep the amp from an instantaneous splatter peak in class C cutoff mode or maybe they use a time delay circuit to let the amp come up to class AB first - but that might cut off part of the first voice syllable?
 

Tom, if you are referring to the dynamic bias circuit found in some Ameritron amplifiers, I found that with SSB the first syllable had some grit to it. It probably would not bother most guys on the other end listening to it, but I didn't like it. It reminded me of what an AM transmitter sounds like when the modulators are biased just at the point of cut-off. I defeated the circuit by biasing the switching transistor used to sense RF on the amplifier's input to its "on" state. The dynamic bias design is a clever circuit, and it responds very quickly, but the word "instantaneous" used by Ameritron is a bit optimistic. Besides, I could care less how much power is consumed between syllables as the tube returns to its 50 milliamp idling current between words (50 milliamps at 3000 vdc = 150 watts). Since I usually don't leave a lot of "dead air" when I'm talking I doubt the power saved while the amplifier is keyed would be enough to heat a teaspoon of water during a normal conversation.

Of course that's just my own 2 cents on it...

Rob W1AEX


* al80schembias.jpg (40.76 KB, 958x364 - viewed 1318 times.)
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« Reply #24 on: November 14, 2011, 04:52:33 PM »

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Besides, I could care less how much power is consumed between syllables as the tube returns to its 50 milliamp idling current between words (50 milliamps at 3000 vdc = 150 watts).

I absolutely agree with ya there, Rob. If I don't care about efficiency on A.M. why would I consider it for SSB??

But, there again, we're just AMers. What do we know?
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