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Author Topic: What Tests for Used Amp?  (Read 2268 times)
KL3HY
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Posts: 117




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« on: April 16, 2012, 08:31:39 PM »

I may be buying my first amp soon, and I'm curious what things I should be looking for when evaluating a used amp for purchase.

I'm looking at an AL-80B for sale locally (this is a big deal because I'm in Alaska and shipping for big heavy things is lethal to the faint of heart). It's apparently wired for 110v from talking with the owner.  Unfortunately he's not a ham, but selling off his grandfather's equipment.  This means that trying the amp on the air before buying is almost certainly not going to be an option.

What things would you look on this amp if you were in my position?

Thanks,
Mike
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KA5N
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Posts: 4380




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« Reply #1 on: April 17, 2012, 02:57:13 AM »

Since neither you nor the seller know anything about the condition of the amp other than
the general condition of the cabinet and paint, it would be a good idea to get a local ham
with amp experience to give you an assist.
Can you plug it in and do a few simple checks like does the fan run and do the meters work.
What is the high voltage? etc.  Is there a burnt smell about the amp?  Does the tube light up?
(these checks can be made on standby). 
But the only way to really check out an amp would be to open it up and give it a good look for
burnt components, etc.  and then do an on air check out. 
Good Luck
Allen
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KC9TNH
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Posts: 304




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« Reply #2 on: April 17, 2012, 04:43:22 AM »

I strongly second Allen's suggestion to have someone WHO KNOWS their way around a tube amp to at least perform an onsite inspection & some perfunctory checks once powered up. As he says, with the OPR switch on standby it could at least be plugged in, check for the tube being lit, fan running, and what the HV (high voltage) meter setting is (i.e., is it making good voltage from house current to begin with?). Don't let arranging that deter you at all. But if buying something that might've also been sitting for awhile this could be important and should be done by someone that knows at least how not to hurt it while checking. If the distance isn't too great (I know, Alaska...) perhaps you could make a trip with a local ham & do it together. Hams can sometimes be had cheap just for the fun of assisting someone else. (where's the bar & grill that has the best bacon cheeseburger?)

Again, don't let that deter you. The AL-80B is a great amp; I love mine, bought used from another ham. Was made in Feb '95 and still trucking with its Eimac tube.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K8AXW
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Posts: 3860




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« Reply #3 on: April 17, 2012, 07:14:33 AM »

I would buy this amp under only one circumstance.  That is if the price was so good (low) that I could afford to re-tube or re-cap it and or do some major repairs. 

Since the manufacturer is still in business and is reputable you're in a good position to do any necessary repairs to it.

You should also consider the question of your ability to do necessary repairs or at the very least consider your motivation to learn. 

I feel the bottom line is the price of the amp.
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WB2WIK
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Posts: 20603




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« Reply #4 on: April 17, 2012, 08:31:18 AM »

If it's a local deal, why not just bring a rig and a dummy load to the seller's location and actually test the amp out "on the air" (well, sort of...)?

That's what I'd do.

Can't tell condition of the tube by looking at it or seeing if it draws idling current.  In the AL-80B, actually, it will NOT draw any idling current even if it's a perfectly good tube because that amp has a drive-activated bias circuit that keeps the tube cut off all the time even when it's "keyed;" so without RF drive applied, it draws nothing.

A new tube is $180 + shipping.
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K1ZJH
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Posts: 1005




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« Reply #5 on: April 17, 2012, 08:37:52 AM »

All of the posters have made extremely valid points. I agree with all of the comments so far...
Used amps and today's Chinese tubes add a lot of "ifs" to the equation.

Pete
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KL3HY
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Posts: 117




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« Reply #6 on: April 17, 2012, 08:57:23 AM »

Thanks guys, this is exactly what I was looking for.  I do have an experienced ham that lives nearby, and I might be able to bribe him into coming with me.

Is 3100v what I should see when I power it up in standby?  That's what the manual shows for "no load voltage".  What would be a normal variance from that value?  For example, should I be concerned if I only see 2900v?

Asking price is $1100, so I could definitely afford to retube it if necessary.

Thanks again!
Mike
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KC9TNH
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Posts: 304




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« Reply #7 on: April 17, 2012, 09:32:22 AM »

$1100 ?  Uff-da! Still, cheaper than new especially if you have to factor in shipping. In the FWIW Department, my '95's insides look a bit different than recent ones, but everything was clean as a whistle and not a bad looking connection in the bunch - good work done. Just perusing the fora it doesn't seem like the oft-screeched about QC issues with Ameritron 811 line & other MFJ stuff impacted the AL-80 series.  Just take your buddy with you & check out the amp as suggested. If it rides back safe in your backseat it'll be better than playing the com'l package carrier lottery.

Also, re your voltage question (and this is a sample of n=1) I have a pretty good (read "stiff") household line, something like 122+V idle, about 118+ with the entire room at full-tilt boogie. My eyeball perception of the width of a needle tells me the amp is showing 2950-2975V idle, and drops to 2600 + a hair at full load. Properly tuned (and loaded) at max drive, then drive backed down as practice has shown, it still does fine and gives great service across the bands, some more, some less, as would be expected.

I forget which of many posts it was in but W8JI or maybe WB2WIK mentioned that best bet on a new tube was just to get one of RF Parts' "best-bet" Chinese tubes - they stand behind them. No workee, they replace.

YMMV and that's almost a given.  Hope it works out for you, IF all is right. The AL-80B is a Clydesdale.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
WB2WIK
Member

Posts: 20603




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« Reply #8 on: April 17, 2012, 09:51:08 AM »

Thanks guys, this is exactly what I was looking for.  I do have an experienced ham that lives nearby, and I might be able to bribe him into coming with me.

Is 3100v what I should see when I power it up in standby?  That's what the manual shows for "no load voltage".  What would be a normal variance from that value?  For example, should I be concerned if I only see 2900v?

Asking price is $1100, so I could definitely afford to retube it if necessary.

Thanks again!
Mike

The HV can read almost anything, depending where the user had set the transformer primary taps and what the actual line voltage is.  Could be 3100, could be 2900.  Anything from about 2700 to 3400 is acceptable (they warn to not exceed 3500V).

$1100 doesn't sound like a great deal to me.  A brand new AL-80B with factory warranty is $1329 from HRO.  Shipping by UPS ground would not be expensive, it's not very heavy.

Looking at the B+ voltage won't reveal if anything's wrong.  One of the big "cockpit errors" people make with amplifiers is turning the bandswitch while transmitting, which can wipe out components including the switch; you'd have to transmit and test each band to determine if there's any problems.
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W8VVE
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Posts: 190




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« Reply #9 on: April 17, 2012, 11:27:13 AM »

I have been looking at this thread today and wasn't going to chime in till I saw the $1100 price.  I realize you are in Alaska...mabey that makes a diff...but I see them listed on a regular basis for $900 and down...and sometimes that includes shipping.
If I was thinking of paying a premium price I would take a rig, power supply, and dummy load to make sure it was 100% before buying.  If I could not test it before buying...I would only offer $700 tops.  73 for now...Sam W8VVE
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KL3HY
Member

Posts: 117




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« Reply #10 on: April 17, 2012, 12:24:25 PM »

Good points on the price.  I'd planned on offering somewhere between $800 - 1000 depending, but I may hold off and just buy a new one later on.  I don't even have a tuner yet that can handle the power, so it'd be a while before I could use it anyhow.
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K8AXW
Member

Posts: 3860




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« Reply #11 on: April 17, 2012, 04:42:36 PM »

Why wait until you get a tuner?  A resonant antenna will work fine with any amp.  If you can't afford a beam or an big fancy tuner then go with a wire antenna and apply the old "brute force and ignorance" policy!   Cheesy  Enjoy high power!
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