More Amp Questions

[NH7FL]

So I've got a few more questions from the QRO noob.  I've followed the procedures in the manual but can't seem to get very much power out of this thing... about 400W max on most the bands save 80 meters with a drive of about 65 watts.  I inspected the inside and found the ceramic base on one of the tube holders to be cracked... which makes me wonder what else might be wrong.  I've seen this amp in operation before and it didn't behave like this one.  Suggestions?

[W1QJ]

What model amp?

[KA5N]

Since he just asked about a AL811H yesterday, I suppose that today's question is about the
same amp.
Do you mean tube socket when you say "tube holder"?  If a tube socket is cracked or broken
it should be replaced.  I don't recall whether this is a new amp or a used amp.  The first thing
I would do with a used amp is take the covers off and give it a through visual inspection.
Buying a used amp is like buying a used car.  With a used car have a good mechanic give it
a good check out before you buy it.  With an amp a good check by someone who knows amps
is in order.  If that is impossible then a good check out BEFORE you start to use it.
Low output may just be bad tubes, but could be lots of other things.  Posting a series of
problems on a forum is a poor way to get an education on amps.  A local ham who can
serve as an elmer is much better.
Good Luck
Allen KA5N

[W8JX]

So I've got a few more questions from the QRO noob.  I've followed the procedures in the manual but can't seem to get very much power out of this thing... about 400W max on most the bands save 80 meters with a drive of about 65 watts.  I inspected the inside and found the ceramic base on one of the tube holders to be cracked... which makes me wonder what else might be wrong.  I've seen this amp in operation before and it didn't behave like this one.  Suggestions?

Sounds like soft/weak tubes. I suspect you got amp used. 811 tubes are not that rugged and pretty easy to damage with improper tune up at higher power levels.  With healthy tubes you should be able to get 550 to 600 watts out with same or less drive. While some will push them harder, 150 watts out per tube is a safe level for long tube life.

[WX7G]

A matched set of four new 811A tubes from RF parts is about $90. I think it would be worth a set of new tubes to see if that cures the low poer.

[N2RJ]

If this is the 811H it might be just that the glue has loosened from the anode caps of the tubes themselves. The anode will also have a cold solder joint. This is a fairly common problem.

I wouldn't bother with 811As. I would get 572Bs. The chinese ones are like ruggedized 811As.

[NH7FL]

I should have been clearer before I asked. This is the 811h purchased brand new from HRO. One of the white ceramic tube sockets is cracked and I've noticed that on that tube one of the filaments glows very brightly during transmit.  I can't get more then 350 watts out of the amp (dummy load). Here is another thing that's a bit scary. On 20 meters when I excite with about 30w or so my power supply running to my radio either shorts out or something? and the rig shuts off. (astron 35a) there is nothing else running off that power supply other then my 590S. Only happens on 20M. Puzzled. When I switch it off then back on everything comes back to life  Anyhow amp is going back to HRO for replacement. 

[W8JX]

One or more tubes could have been damaged in shipping. You could remove and replace tube one at a time and and check effect on output. (after safely bleeding off HV on tubes)

[KA5N]

It is unclear from your description whether the power supply cuts off, or the  590 cuts off.
The power supply has a crowbar circuit that cuts the power on the supply if the output voltage goes above a certain level.  The 590 has a foldback circuit that reduces the power output if the SWR goes too high.  The efffect is probably due the latter as it only does it on 20 meters.  
The problem you have is whether to send the amp in for repairs or to try to fix it on your own.
You should consider consulting Ameritron for advice (although many report difficulty getting
good results with this undertaking).  That one tube lights up brighter than the others worries
me particularly it is the one on the broken socket.  Swapping tubes around should only be
considered AFTER replacing the damaged socket and correcting any other (presently uncovered)damage.
Allen KA5N

[W4AMP]

Both the AL811 and AL811h design are grounded grid. The great thing about grounded grid is if you lose a tube and do not have a replacement you just remove the tube and keep going. The dual 3-500z grounded grid amps are the same way.

[WX7G]

With your additional data it's clear that there are two issues:

1) The amp is broken and it's going back to be replaced.
2) On 20 meters RF is getting into the Astron power supply, making it go overvoltage, and it's shutting off. While the amp is away you might order A couple/few MFJ-700B4 snap-on ferrite core kits from HRO ($14 each). Route both the (+) and (-) power supply wires thru the same core(s).

[NH7FL]

Thanks everyone for the help.  I re-checked everything and still have the poor power output.  As for the power supply/radio cutting out on 20... I have had no issues with input or output SWR being high... it's only slightly elevated on 17 meters (1.5)  Also, I've noticed that the power supply cuts out on the SSB portion of 20 with very little drive... and only cuts out on the lower portion of 20 meters if I run it at quite a bit more drive.  Also, a bit more info.  I can get quite a bit more power on the lower bands (about 500W) vs only about 250 tops 17 and up.... Very frustrating.  I did hook it up to the antenna system and had so SWR issues or any serious RFI in the shack. (touchlamps go off but they do that barefoot at 10W) ....

[W8JX]

  I did hook it up to the antenna system and had so SWR issues or any serious RFI in the shack. (touchlamps go off but they do that barefoot at 10W) ....

If touch lamps go off at 10 watts you have RFI issues. As suggested earlier I think you have RFI getting into power supply and tripping it. 20m seems to be good at that sometimes.

[N2RJ]

Both the AL811 and AL811h design are grounded grid. The great thing about grounded grid is if you lose a tube and do not have a replacement you just remove the tube and keep going. The dual 3-500z grounded grid amps are the same way.

That is something I've never tried!  But what do you do with the anode cap though, how do you prevent it from arcing? I once had a tube explode and shatter inside of my amp. I wasn't even driving it hard. Fuse immediately blew and took out the grid resistors, and there was a bright flash inside the amp. It was just a faulty chinese tube. I almost had a heart attack!

[W8JX]

  But what do you do with the anode cap though, how do you prevent it from arcing?

The voltage is not that high that it will arc some distance to ground. It will not arc unless it is very close/nearly touching chassis.