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Author Topic: How to choose a linear.....  (Read 8030 times)
N4CR
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Posts: 1672




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« Reply #15 on: July 23, 2012, 08:30:06 PM »

Chances of over driving/damaging the grids of four 572's in parallel with 100 watts are pretty slim. Nice window dressing though.

It also stops ALC overshoot from splattering up the bands by tossing it's cookies when it's over driven. This is a good thing.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KB8BAB
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Posts: 101




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« Reply #16 on: July 24, 2012, 06:24:07 AM »

OK...572's...811's...3-500?

With the 811HD (4x 572's) are they "under utilizing" the tubes? (Larger "safety factor"?)

The opposite on the 80B with a single 3-500?

 Huh Huh Huh Huh

Some older models (other brands of amps) use for instance 4x 572's for over 1kw output, or 2x the 3-500's for a 1kw output.
Is this just different design philosophies? ("Overbuild" to be safe?)

Bart 
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W8JX
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Posts: 6092




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« Reply #17 on: July 24, 2012, 06:53:33 AM »

OK...572's...811's...3-500?

With the 811HD (4x 572's) are they "under utilizing" the tubes? (Larger "safety factor"?)

A 811 is rated at 45 watts continuous dissipation, a 572 at 160 watts. A 572 can take close to 3kv on plate/anode and more grid and plate current too. To put it into perspective, if you built a amp with proper power supply and two 572's it would match output of four 811's and last longer too. Money is better spent than buying a new 811 amp with 572's using same low voltage power supply. Looks good on advertisements though.


The opposite on the 80B with a single 3-500?

 Huh Huh Huh Huh

This is a pretty solid amp and many run them at a kw but tube life can suffer. Run it at 800 watts or so and you will get a very long service life.


Some older models (other brands of amps) use for instance 4x 572's for over 1kw output, or 2x the 3-500's for a 1kw output.
Is this just different design philosophies? ("Overbuild" to be safe?)

Bart 

Long ago 572 and 3-500z were the "standard" to use. Twin 3-500z's make a solid legal limit amp if power supply is up to it. Most are not though in older amps. Old L4b and SB220 did not really have enough power supply to overload tubes as long as they were tuned properly and this is why you still see some of them with original tubes still doing well. The "bad" thing about a amp with twin 3-500's is they require a lot of drive power to make full output usually. Not a high gain tube.
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NA0AA
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Posts: 1042




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« Reply #18 on: July 24, 2012, 04:57:36 PM »

I ran this decision around for a while before I bought my AL-80B.  Reasons:

1.  Budget - OK, I blew HUGE bucks on the tranciever, but I decided that an entry-level amp would serve me better.

2.  Glass:  It's subject to a lot of debate but I think the 3-500Z's are a better tube than either the 811 or 572's - and you don't have to manage three or four of them.

3.  Station hardware:  Once you exceed a KW, your hardware starts to become larger - most of the antenna protection is up to a KW or legal limit and beyond at a higher price.  For me the cost to upgrade all my arrestors, etc. was not insignificant - with a KW you can still get by RG-8x if you want to.

I hope that in time, the cost of the solid state amplifiers will come down, and the overall quality/aviailiblity will increase, but let's face it, hams are not big hardware users, HF is not a big item so much anymore.

In the mean time, I'll enjoy the friendly glow of the 3-500, or even operate, gasp, barefoot!
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KB8BAB
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« Reply #19 on: July 24, 2012, 07:09:40 PM »

Thanks to all for the info...

It's going to be an AL-80B......

73's

Bart
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KC9TNH
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Posts: 304




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« Reply #20 on: July 25, 2012, 12:51:01 PM »

Thanks to all for the info...

It's going to be an AL-80B......

73's

Bart
Good choice imo, ran thru the same quest you've done. I really like mine. With 60-65w an honest KW on 40, 700-800 on 75, 20, 17 and 5-600 on the higher ones. Not overdriven, not under-loaded. The 3-500 is happy. All on a good residential 120V line. Can't recall if you said it before but if you don't have one already get a dummy load of the commensurate size, dry or wet, but something that'll handle you playing with your new amp. Everyone will be glad you did. (Settings on my -80B are pretty repeatable, honest a complete band change takes a few seconds.) I don't even run it full-tilt boogie if the band is cooperating, just something to "hold" a signal. Like someone said, you get an amp for your friends.

Have fun! Smiley
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
KC4MOP
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Posts: 743




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« Reply #21 on: July 31, 2012, 04:46:26 AM »

Interesting responses (with some heat)!
You will have to decide; legal limit or one of the contested amps mentioned here.
The guys are pretty accurate in saying that the difference between a middle-weight amp and legal limit is a couple of dB on the receive end.
Stick to something fairly recent instead of the older amps. Some excellent deals on eHam Classifieds and QTH dot Com. Almost like a Hamfester deal. eBay gets into hurting your wallet.
I like an amp that I can pop the lid (after unplugging from AC supply) and service. Those small table-top with power supply on the floor amps are a nightmare to service.

Fred
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NO2A
Member

Posts: 801




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« Reply #22 on: July 31, 2012, 01:04:23 PM »

Bart,You won`t be disappointed with the AL-80B. I have the older "A" version. The wattmeter on the "B" version is much better,very accurate. Much of a tube`s life depends on how well it was made. W8JI has stated that 50% of new 811`s and 572`s arriving from China are defective! Yes 50%. I wouldn`t touch them with a 10 foot pole. Even with a warrenty. It`s not just the power output that`s important,it`s that the 3-500Z is so much better. You really can`t overdrive them,the grids can take quite a bit. They`re not usually damaged by momentary power "spikes". Good luck with it.
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VE3FMC
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« Reply #23 on: July 31, 2012, 01:08:45 PM »

Thanks to all for the info...

It's going to be an AL-80B......

73's

Bart

Bart I read through the posts here, lots of good points made. I own an AL-80A, the older brother of the AL-80B. I am quite happy with it. Although I only use it about 2 hours a week on 75 and 40 it has done the job for me. This is the second one I have had on the desk.

I think you will be happy with the AL-80B. I drive mine so it produces about 600-700 W PEP on SSB and it never gets warm to the touch. One of those hours I use it during the week I am a NCS so it is being operated hard during that hour. Again no heat issues.

Currently I have mine on 110 V because I have not brought 220 into the shack. Yes folks, I know it would perform a little better on 220 V. That is the upcoming project this winter. Problem I have is my main breaker panel is just about full!
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KC4MOP
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Posts: 743




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« Reply #24 on: August 12, 2012, 06:06:27 AM »

A lot of guys "steal" 220 from the dryer circuit or electric stove. I hope the NEC cops are not reading this

Fred
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NA0AA
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Posts: 1042




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« Reply #25 on: August 13, 2012, 01:28:30 PM »

I know one guy unplugs the dryer and uses an extension cord. 
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W5DQ
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Posts: 1209


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« Reply #26 on: August 14, 2012, 01:15:59 PM »

Just to add some more fuel on this fire ... I have the AL-811 3 tube amp. I push it hard and as such, I have made a couple of changes:

1) replaced the 811a with 572B from RF Parts
2) reworked the original plate VHF oscillation circuitry by replacing the orignal with the new AL-811HD plate circuit board unit (after a meltdown of the original 811 tubes and RTTY use at 500W - I said I pushed her hard)
3) Added a beefier fan that pushes about 2 time the air through the amp. Air typically is warm after using for a while but nowhere near as hot as it was with stock fan.
4) Been thru the entire amp and checked every solder joint after a couple of failures of weak joints caused some headaches for me. All joints were resoldered and verified solid now.
5) Tweaked input tuned circuits for 80-40-20-15-10 since these are the contest bands I use this one most of the time.

I routinely push this little amp to the limits on SSB, CW and RTTY. It does get warm on RTTY at 500W in contesting (mainly Search and Pounce so some cool off time in a low duty cycle) but been a couple of years now after rebuild and still going strong. Wired on its own 230V Line. Drive around 75 watts for 600W out on SSB peak using ALC and ARB-704 Interface.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
K6AER
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Posts: 3525




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« Reply #27 on: August 14, 2012, 01:39:41 PM »

I know this is trivial but the Americas have not used 220 VAC for over 40 years. Out standard voltage from the pole is a minimum of 240 VAC. If you are measuring 220 VAC your volt meter is bad our you need to call the power company.
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