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[KM1H]

Lack of 160m is not a deal killer and its input is "better" because it is easy to tune???  And as far as built in SWR meter in amp that is not a big deal either as I watch power and SWR externally when tuning and if you have reasonable resonant antennas you do not even have to watch SWR much. To suggest a AL811 is a more rugged amp than a SB200 would be foolish. If they had been built as cheaply as AL811 amps are they would have all gone to junk pile long ago and would not still be a viable amp today.

Your missing the point again due to poor reading comprehension, try it again.....slowly

[VK3BL]

Wow, this thread really has become a free for all! 

[W8JX]

Lack of 160m is not a deal killer and its input is "better" because it is easy to tune???  And as far as built in SWR meter in amp that is not a big deal either as I watch power and SWR externally when tuning and if you have reasonable resonant antennas you do not even have to watch SWR much. To suggest a AL811 is a more rugged amp than a SB200 would be foolish. If they had been built as cheaply as AL811 amps are they would have all gone to junk pile long ago and would not still be a viable amp today.

Your missing the point again due to poor reading comprehension, try it again.....slowly

Nothing was missed....

[K9AXN]

Might I suggest that before pontificating too far, you all read the RCA Ham Tips for October 1939 (Vol2 nr 5) where they describe how they maltreated an 812 (not an 812A) until holes were melted in the plate and it subsequently provided full output?

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0205.pdf

Now I am more than ready to accept that the results from an original RCA tube and those from a Chinese or Russian tube bearing the same number may very well have an extremely tenuous relationship. My experience - amateur and professional - has been that for the same part number, US manufactured tubes always have the edge.....(Tin hat, head down, dodge the bullets!)

It was about an 811 and the hole was caused by excessive heat alloying the Zirconium plating with the plate metal.  Question --- are the plates in the OEM tubes Zirconium plated and if not what is used for the getter?  Do I care --- no just curious.  

[G3RZP]

Zirconium metal was over $22,000 a ton in 2007, but that would coat one heck of a lot of 811 plates! I wouldn't be at all surprised if current manufacture 811As don't have any zirconium in them.....

[W8JX]

Zirconium metal was over $22,000 a ton in 2007, but that would coat one heck of a lot of 811 plates! I wouldn't be at all surprised if current manufacture 811As don't have any zirconium in them.....

Given that even at that price that is only about 70 cents a ounce and given that one ounce could easily do hundreds of tubes, it would have basically no real impact of tube production cost. 

[VK3BL]

Zirconium metal was over $22,000 a ton in 2007, but that would coat one heck of a lot of 811 plates! I wouldn't be at all surprised if current manufacture 811As don't have any zirconium in them.....

I do not believe they do; I have a few old Chinese Taylor 811As with silver spots floating around (they came with my 811A), but I never managed to produce any silver spots on the 811As I ran hard and used up all of the filament emission.

Of course, whilst I ran them hard in terms of output, I was always meticulous in making sure I didn't ask anything crazy from the tubes.  Even the 1969 RCA tubes I ran down to 70% output still look NOS without any sign of silver spotting.

That said, one new production Chinese 811A did suffer severe overheating when I was operating remotely and my antenna developed a fault - I managed to darken the glass (plate evaporation), but still couldn't elicit any silver spots.

Take all of this with a grain of salt.  The process used to coat the plates with zirconium, as well as the cleanliness / purity of the underlying metal (molybdenum I believe) probably makes all the difference regarding whether or not the classic 'silver spot' zirconium/molybdenum alloy can form.  And lets face it, Shu Guang is hardly a clean room by any means.

 

[W1QJ]

I recently had Ten-Tecs answer to the Ameritrons Al-811 amplifier in here for some repair and upgrade work.  We don't hear much about this amp on here.  It is a Centaur, runs 3 811a tubes, just like the Al-811.  It is rated at 500w CW and 600w PEP SSB.  Sound familiar???  ? Anyhow I was told he was running this thing to 700 watts.  Oh Yeah, for sure.  The tubes I removed are black.  You can't even see the insides of the tube.  It looks like it was sprayed painted black.  How do you upload a photo on here?

[W8JX]

  How do you upload a photo on here?

You have to upload it somewhere else and provide a link to it.

[VK3BL]

I recently had Ten-Tecs answer to the Ameritrons Al-811 amplifier in here for some repair and upgrade work.  We don't hear much about this amp on here.  It is a Centaur, runs 3 811a tubes, just like the Al-811.  It is rated at 500w CW and 600w PEP SSB.  Sound familiar???  ? Anyhow I was told he was running this thing to 700 watts.  Oh Yeah, for sure.  The tubes I removed are black.  You can't even see the insides of the tube.  It looks like it was sprayed painted black.  How do you upload a photo on here?

Unfortunately thats an all to common story, and has much less to do with the amplifier brand or the designer than it does with the operators inability to properly tune their amplifier.

The best advice I can give you 811A amp owners is to tune up for peak power as quickly as possible, and then look thru the vent window whilst transmitting to check plate glow.  Anything more than a red and you can guarentee damage is likely sooner or later.

The other big culprit is running AM, where carrier efficiency is around half of peak output efficiency, and 811As are already on the edge for dissipation as it is.

I don't really think any 811A amp should be run in AM.  Most of them (the AL-811 for example) struggle to even tune up at 400 watts output due to lack of load C on that band.  Imagine how bad things get when you then start running a 100 watt carrier though an already inefficient pi network.

I'd find out whether or not your customer knows how to tune properly.  If they do (and you're certain of it), I'd ask them next whether or not they run AM.  I bet they do.

Also while you are repairing it, remove the grid resistors and grid bypass capacitors (if present).  An end user who melts plates is in big trouble when using Chinese / Russian 811As, which will sag inwards to the point they will eventually arc to grid when abused.  The arc will often take out the grid resistors, making problems even worse in the future (the now floating grid will encourage a plate to cathode short).  Better to trade off slightly better tube balance for much better exciter / amplifier protection in the event of an arc (which is inevitable in those who abuse chinese/russian 811As).

[G3RZP]

RCA rate one 811A as 160 watts PEP out, so for three that should be a maximum output of 480 watts. In Class C, 200 watts out per tube. In Class B telephony service (AM linear) three tubes are rated for 95 watts carrier out maximum, or 65mA of plate current per tube. As many AM transmissions are quite lengthy for some reason, probably better to drop to about 50mA per tube!

[K9AXN]

VK3, I have to wonder what your smoking as well.
A red plate at 'idle' shows you there is no plate dissipation left for head room  for when the tube is RF driven.
What don't you understand about that?

Its not ideal, we all know that.  But its a cheap amp with cheap tubes, and 1 of the 3/4 doing more work for a while isn't the end of the world.

As I have repeatedly stated: If you are using more than 2 tubes, and biasing them at 2/3rds of rated dissipation under zero signal, you should not be surprised if one of them draws more current than the others and develops a glow. 

Thats why RCA explicitly states that an attempt should be made to balance the tubes.  W8JI did that in the original design, but the shitty tube quality meant they kept arcing and taking out the tube balancing components (grid resistor and grid bypass cap), resulting in the grids floating, and possible exciter damage. 

At some stage, Ameritron / W8JI decided the customer would be better off with an amp that had less tube balance, but a lower risk of exciter / amp damage under fault (arc) conditions. 

After all, its a cheap ICAS amplifier, with cheap tubes that the manufacturer states will glow in ICAS.

Are there any owners out there who would like to chime in?  Just key the amp up in sideband, and watch the plates.  In under a minute, I guarantee one of the tubes will be glowing.

How did the grid resistor and bypass cap balance the tubes at idle?

[VK3BL]

How did the grid resistor and bypass cap balance the tubes at idle?

Firstly, I'll point out I was reporting the words of the designer.

That said, I believe it works as follows:

The cathode circuit (not referenced to DC ground) is where the DC operating bias is applied (pulled down from ~40 volts when keyed).  R19 to R22 (the grid resistors) function to normalise the DC load resistance across the tubes by providing a DC reference point to ground for each tube.

As such, a single tube is prevented from drawing more than its fair share of zero signal plate current.

Someone with an engineer's (rather than a technician's) understanding of cathode driven triode amplifiers could surely provide a better explanation.

[W8JX]

Something I should mention is that the center tube will always run hotter regardless of balance. Reason for this is that anode looses heat by radiation but it is a two way street. Center tube is receiving radiation on two sides from other two tubes and therefore has more net heat to shed and will get hotter. There is no magic wall that prevents heat from one tube from effecting another. Boosting airflow will increase heat dissipation some because by lowering temperature around tube it can "loose" heat more effectively.

[W1BR]

I think there should be a special forum for "peanut whistle" amps.  Leave this forum open for full power amps, OK, 1200 watts and above.  1200 watts and less goes to the peanut whistle forum.  Too much bandwidth being spent on amps that can't get out of their own way.

Nothing wrong with 572 or 813 amps.  But 811 and 807 based amps could be moved.  LOL