hunter 2000b question

[kb3ouk]

ok, i have one of these linears, and the manual says on 75 meters to have the aux load switch set at 3, which places a mica loading cap in the output circuit. placing the switch at 0 takes all fixed caps out of the circuit. i can only run this amp for about 3 minutes till it overheats.. can i run it on 75 without the micas in the circuit? the caps are all supposed to be 350 mmf. i should also mention that it does still put out power in 0 (i tried it) but the variable ant loading cap has to be closed the whole way

[ke7trp]

What do you mean by saying the amp overheats?  It has a failure and arcs over?  Blows breaker?  What is the AM carrier?  I used to have one of these years back.

C

[kb3ouk]

well, i'm guess that something is heating up because the power output suddenly goes downhill if i key it and let it sit for three minutes. after 3, it begins to drop. i ruled out the fixed micas that are the aux load caps, last night i tuned it up without them in the circuit, and it went for a little over 2 minutes, then started losing power. there's a couple small dorrknob caps beside the tubes, i think now maybe they are heating up from a combination of tube heat/dissipated heat and are changing value, causing the circuit to go out of tune. if i let it cool off for a few minutes it will run fine, but then after cooling off if i were to key up for 3 more minutes, it starts losing power again. i think what i might do is get my dads ifrared thermometer, one of those you just point it at something and it says how hot it is, and key it up for 3 minutes till it starts losing power, then shut it off, pop the lid open and start checking to see whats getting hot. i'm only running about 180 watts out of it. if i reduce drive so it only puts out 100 watts, it will go a little longer before it starts losing power.

[ke7trp]

Its a 200watt amp and it requires extra cooling. I used a fan over top hooked to a variac.  I think you are on the right path with the temp gun.  It could be the tubes dropping power. 

[kb3ouk]

i have another fan i can lay on top of the case and have it force air down into the case. someone at one point replaced the two fans that would have been it in new with one larger fan, that thing can really more some air. i just tried aiming a fan over those doorknobs in the output, the ones that are right beide the plate caps on the tubes, and was able to go a little over 4 minutes until i lost power, that was with the fan blowing directly on the caps. next i tried the fan over the tubes, that got me about 3 and a half minutes till power loss. so those dorrknobs mustn't be taking the heat too well. i would've used the temp gun, but as usual around here, no one seemed to know where it got to when i wanted it.

[ke7trp]

Do have an cap checker?  You can run it hot, Unplug, SHORT OUT B+.  Check cap very quickly before it cools.  Then you dont have to wonder. 

Is the DIP changing on the tuning when hot? 

C

[N2ZD]

200 watts?? I had the 2000a and it made 1200+ on 20m all day long. Best amp I had ever owned, never had to open the lid once except to clean it in 7 years. Like a dope I sold it.

[kb3ouk]

i'm running AM with it, so yes that is right. i have the voltage on the lowest setting, too. and no, i don't have a cap checker, but i thought of one other thing it MIGHT be, tohse resistors they used for parasetic supressors.

[ke7trp]

Yes. It is a 200 Watt amp.

Get a cheap cap checker off ebay. I got two of them for 30 bucks.  They are spot on.  Yellow, with a rotating dial and LCD readout.   You can check those resistors but you will have to unsolder them. Just buy replacements.

C

[Mike/W8BAC]

Shelby Wrote...

i have another fan i can lay on top of the case and have it force air down into the case.

My 2 cents worth, If heat is the problem you want it out of the case and not blown back at the tubes and other components. It's always best when using a non engineered fan to draw the heat out. Make sure the fan you add doesn't restrict the flow of the air being pushed through the amp by the on-board fans.

It sort of sounds like something other than tubes. If the tubes overheat and power drops like you say the tubes might do it once or twice and poof! Can you monitor your plate currant and voltage.

[WD8BIL]

A quad of 572Bs will do 200w carrier no sweat.

Key it up until the power begins to drop off. Watch the plate current. If it rises it means the tank is untuning. That will indicate your suspected capacitor is breaking down. Replace it with the same or higher voltage doorknob.
If the current drops it indicates something in the input circuit is sucking dive from the tubes.

Does it do this on the higher frequencies where no plate padders are used?

[kb3ouk]

A quad of 572Bs will do 200w carrier no sweat.

Key it up until the power begins to drop off. Watch the plate current. If it rises it means the tank is untuning. That will indicate your suspected capacitor is breaking down. Replace it with the same or higher voltage doorknob.
If the current drops it indicates something in the input circuit is sucking dive from the tubes.

Does it do this on the higher frequencies where no plate padders are used?

next time i'm down there i'll try that. i can watch the plate voltage and current. i haven't tried it on higher frequencies, but i did tune it up on 75 with the padders out of the circuit and it still lost power, a little quicker than it does tuned up the right way. blowing air on the doorknobs got me a minute longer till it lost power.

[Mike/W8BAC]

blowing air on the doorknobs got me a minute longer till it lost power.

Whatever blows up your skirt! 

[WD8BIL]

Another thing to check is any solder joints in the tank circuit.

[kb3ouk]

ok, i think i may be on to something here, would a not-so-good solder joint on the parasetic supressor of only one tube cause this?

[ke7trp]

I have a box of doorknobs. If you decide thats the issue and will take a chance on a used old doorknob, let me know.

I think it could be a solder joint!  I just had my tuner change tuning after about 3 minutes of keydown on AM.  I traced it down to one of the clips being loose on the link.  After soldering it again with my Hako iron on 900 degrees, This stopped.


C

[kb3ouk]

if the joint was heating up but still connected, would it have any effect? it's only one of the tubes that are like this, because i broke the clip off the end of the one supressor when i was installing the tubes and soldered it back on. it stays toget even when hot, but i wonder of something is changing as it heats up.

[WD8BIL]

Probably not. At 180W 3 572Bs could handle it.

[kb3ouk]

ok, then i'm gonna try watching the plate current next time i use it, to see what it does. i'm almost positive it's the doorknobs in the output.

[kb3ouk]

well, friday night i figured out what was causing the problem. i wasn't looking in the right place. i noticed the power output was being screwy, and all over the place. that's when i noticed the smoke coming out of my wattmeter. evidently, the directional coupler in that thing, even though it was good up to 200 watts, couldn't take 200 watts of carrier for more than 3 minutes. took the meter out of line, everything was good. last night keyed it up and left it sit for about 5 minutes, the output was steady.

[KB5MD]

That smoke will do it everytime.

[K5UJ]

I have had similar experiences.  For AM, every little hammy hambone POS in the RF path that was okay for slopbucket has to be tossed and replaced with something AMish and buzzardly.  If you have a little Cantenna type dummy load, you may as well start keeping an eye out for one of those Bird DLs with the fins.   they look like this:
http://www.ebay.com/itm/Bird-8251D-Termaline-1000W-Coaxial-RF-Dummy-Load-Tuned-Digital-Broadcast-/270836566875
you can find real old ones at hamfests from the 1950s that work FB and are not much money.

You can also buy resistors like these:  http://www.ebay.com/itm/CARBORUNDUM-185-OHM-NON-INDUCTIVE-RESISTOR-/400180772108
and combine them in such a way (look for 200 ohms) so as to make a 50 ohm load (or whatever Z you need) with mounting hardware and feedline connection then either put a fan on it or submerge it in a mineral oil bath.

[kb3ouk]

i got one of those big cantennas in the gallon paint can. i think it's good for like 1500 watts, so a 200 watt carrier don't heat it up too much. it's filled with mineral oil. but my little MFJ tuner that i just put after the amp handles 150-200 watts of carrier easily, with just a little bit of heat being dissipated by the coil. and it's only rated for 300 watts PEP.

[flintstone mop]

Glad you found the culprit Shelby.

But be careful with even the big cantenna.....1500W PEP for a very short time.
I have one of those military dummys that can handle 600W carrier all day long. I have Rotron fans mounted on top to cool it for extended AM testing at legal limit.
Usually the military stuff is underrated.....on purpose for the bozo effect.
Like the T368.........you can beat on it all day long and it keeps making power and you'll never over modulate it in AM.........in its stock condition.

[ke7trp]

I use a Bird 500watt dummy.  Of course, thats 500 watts modulated AM with no fans. 1000, with fans.  So it will take a BC rig for testing and tuning up no problem. It really takes alot to get that thing warm. 

Funny can tenna story.  We got an amp built by someone in CA.  It had a 4PR1000 in it, can caps. very very nicely built.  The PS was very heavy and sat on the floor.  6900 volts of PW PS on that tube.  I ran an FT101EE into it to test the amp before sale.  I said 1 sentance into the mic and we smelled something burning and heard a hissing noise. I had to run the cantenna outside to the front porch. I thought it was going to burn the house down. LOL

C