worrisome symptoms from recently repaired Ameritron

[radiozed]

A few months back I purchased a new Ameritron AL-811 600 watt amplifier.  Life was good for a couple of months until one evening it just "popped."  "Popped" meaning I seemingly blew it up.     The lights in the house flickered, there was a loud "pop" from inside the amp, and then the strong smell of electronic circuity burning as it died.  Not good.  Fortunately it was still under warranty so I sent it back to the good folks at Ameritron for some work.  They fixed it up good as new (still not sure all that they did, but I know they replaced the tubes) and sent it back.  I asked for their advice on how to keep this from happening in the future and they replied with a sort of generalization of "never use AM, make sure it's tuned up properly, and don't overdrive it."  (a slight off topic rant...I don't use AM, but why do they like to advertise that the 811 can handle "400 watts AM"?)

So, I've had the unit back for awhile and things have been going well.  However, the last two broadcasts I've noticed some strange symptoms from the Ameritron that came on quite suddenly.  The 811 is humming/buzzing loudly, in perfect oscillation with the broadcast.  When I listen to my broadcast on a remote SDR monitor, I can notice a faint "warble" to the broadcast that coincides with when the 811 is humming/buzzing the loudest.  Also, it's heating up much more rapidly than it used to.  Before the blow out, I could key down for 45 minutes before it started feeling what I'd call "really" warm to the touch.  Now it's getting pretty hot after 20 minutes or so.  I never run more than 450-500 watts, I'm tuned up as good as I can, and I'm showing an SWR of just barely over 1:1.  The 811's fan seems to be working perfectly. 

Right now I'm tempted to keep it shut down until I figure out what's causing these symptoms.  The last thing I want to do is blow it up again (yes, warranty is still in effect, but I still have to pay the $70 shipping to get it there).  Anyone have some advice?  Am I simply overdriving it?

[EliteData]

is this the AL-811H or just the AL-811 ?
what brand, part number and how many tubes are being used and are they rated CCS or ICAS ?
are you using 120VAC or 240VAC ?
what is your plate and grid current when you are using the amp ?
the next time you operate the amp, take a baking thermometer (the one with the point on the end) and place it at the vent exhaust for about 5 minutes and post back the temperature it reads.
additionally, after you are finished using the amp and its turned off, unplug the power and let it discharge for 60 seconds and take the cover off and touch the metal and wire wound sides of the HV xfmer and see if its blazing hot to the touch, if it is too hot to hold your finger on it, then you will need to reduce the power output of the amp.
the additional heat may be coming from the HV xfmer.

[radiozed]

is this the AL-811H or just the AL-811 ?
what brand, part number and how many tubes are being used and are they rated CCS or ICAS ?
are you using 120VAC or 240VAC ?
what is your plate and grid current when you are using the amp ?
the next time you operate the amp, take a baking thermometer (the one with the point on the end) and place it at the vent exhaust for about 5 minutes and post back the temperature it reads.
additionally, after you are finished using the amp and its turned off, unplug the power and let it discharge for 60 seconds and take the cover off and touch the metal and wire wound sides of the HV xfmer and see if its blazing hot to the touch, if it is too hot to hold your finger on it, then you will need to reduce the power output of the amp.
the additional heat may be coming from the HV xfmer.

Some of this I'll have to update later when I have a chance to fire it up again.  The few things I can tell you now:

- It's AL-811 with three 811A tubes running off 120 volts

- Only thing I can find on CCS vs ICAS is this from a posting on another board:  "The RCA rating of the 811A was 45 watts plate dissipation in CCS and 65 watts plate dissipation in ICAS"

In the meantime, however, I think I'm seeing what is the most likely problem in reading some other forums.  One post said you shouldn't attempt/expect to get more than 425 watts SSB out the of 811.  I read on another forum that the 811 is only rated for 400 watt key down for 1/2 hour.  So initially it would seem I'm trying to get way too much out of it at 450-500 watts.  Sounds like a maximum of 400 watts for no more than 30 minutes is the best I can expect if I want to keep it from blowing up again. 

[EliteData]

the important thing is to consider the maximum HV plate current the HV transformer can supply.
i think for this model it is 550mA at its max, and that is for intermittent use of less than 1/2 hour.
if you wanted to do continuous use @ 250mA, you would need to do some modifications to keep the HV transformer cool.
there are a few ways you can keep the HVT cool.
1) placing the HVT in a large can (with heat dissipation plates) of mineral oil.
2) modifying and improving the air circulation by three-fold on the amp with the amp operating in a temperature controlled environment set at 70F (keeps the tanks and tubes cool).
3) not operating at more than 250mA plate current continuous.
ive got a good idea the first time it "blew up", the HVT shorted from excessive heat and possibly destroyed the filament on one or more of the tubes.
generally, the first component to fail on these "compact" triode amps is the HVT, especially when the amp is operating beyond its tolerant duty cycle.

[redhat]

These rigs were never designed for lengthy use.  They skimped out on cooling, transformer capacity and tubes.  I will cite this as one of the reasons this manufacturer has attained the ominous moniker of 'Ameritrash'.  My suggestion is cut your PEP way back or get a better amp.  In my SSB days I ran a mil spec Harris tube linear with a 3CX800 in it and could routinely get 700W PEP out of it.  I suspect your amp was not built to the same standards.

+-RH

[radiozed]

These rigs were never designed for lengthy use.  They skimped out on cooling, transformer capacity and tubes.  I will cite this as one of the reasons this manufacturer has attained the ominous moniker of 'Ameritrash'.  My suggestion is cut your PEP way back or get a better amp.  In my SSB days I ran a mil spec Harris tube linear with a 3CX800 in it and could routinely get 700W PEP out of it.  I suspect your amp was not built to the same standards.

+-RH

Appreciate the advice.  Cut it back I shall. 

[Josh]

Cut the drive so the output's down a hundred watts, no one will notice but the amp and your power company.

[redhat]

That is sound advice.  For longevity, I would set it around 250W PEP to allow for some slop in your metering.  Your tubes and amp will last longer.  Remember, going to half power is half an S unit, and I agree, most won't notice.

+-RH

[ka1iic]

well...

It depends on what you are using this amp for...  If it is for continuous operation say RTTY or continuous AM operation never go above the CCS specs... and I mean ever!

RCA says 45 watts... well RCA tends to over rate 811's.  use 35-40 watts as the absolute maximum.  There is a much better version of the 811...  but I don't remember the numbers <sigh>

This Ameritron version has 3 811's so... (hopefully 811A's) 100 watts at best.  Personally I would NEVER go over 100 watts with this amp in the continuous AM mode.  Keep the drive low in the AM mode and bring it up to 100 watts.  And make damned sure your SWR is as close to 1.1 as possible... altho SWR really don't mean a crap but do it anyway to keep the tubes happy.

Remember this REALITY!!!  Doubling 100 watts is 200 watts but that double increase in power only realizes a small 3 db increase on the receive side!  As far as the listener is concerned 3db is hardly noticeable to them.  It's no big deal!  Receiver S-meters are the biggest lairs in the radio world.

The important point is this:  The only thing is to be heard and to hell with S-meter readings because they are pure BS.

Be kool and run it Cool... OK?

vgw

[radiozed]

Thanks, ka1iic!  For what it's worth, I only use SSB, so I don't know if that makes a lot of difference in what I can expect to get out of it or not.  It's amazing to me that they advertise these rigs as being able to handle "400 watts AM" when it doesn't sound like they're capable of anything close to that. 

[ka1iic]

OH! only SSB hmmmm...

Sounds like you might be over driving the unit...  something is wrong here...
Are you sure your SWR meter is correct?
I'll down load the manual and give it a look see.

If the tubes are heating up use a much larger external fan if possible.  People laugh at me because I have a 6 inch fan in back of this laptop.  What can I say...?  It's a damned HP Quad core and they are well known for over heating.

Later after I give the instructions a good look... may take awhile got a doctors appointment today so that kills this day. <sigh>