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Author Topic: SB-200 How Low will it go?  (Read 25500 times)
W1BR
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« Reply #15 on: October 27, 2014, 09:08:37 AM »

Regarding reducing power: use the drive control, and not the mic gain to do so.  That will preserve the carrier rejection levels at reduced power.  Using the mic gain will not.

Pete
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K3VAT
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« Reply #16 on: October 27, 2014, 02:17:42 PM »


<snip> ... But doing that would be a waste of energy and tube life. <snip>

Mike, regarding your note above: "waste ... of tube life" is there some technical reference that I can confirm this?  I'm confused on how running the amplifier at low drive levels would reduce tube life.

73, Rich, K3VAT
 

It's not that low drive per se damages the tubes (although the tubes might very well dissipate less power at a certain drive level when the plate current moves above idle a certain amount, since the efficiency is zero with no input). It's mainly that tubes have a finite life, and every time you turn it on, you have the Miller-Larson effect shortening the filament life. See http://www.w0btu.com/miller-larson_effect.html.

The OP didn't say what he was going to drive it with. Since most transceivers put out 100 watts, that's why I said what I did in my first reply. He might as well just run the rig and leave the amp off unless he needs it. Does that make sense?

Yes, thank you.  But the description of the Miller-Larson effect says nothing about the post topic which is low power drive.
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W0BTU
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« Reply #17 on: October 27, 2014, 03:06:28 PM »

Yes, thank you.  But the description of the Miller-Larson effect says nothing about the post topic which is low power drive.

Agreed, but we weren't certain what the OP was intending at first. :-)
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W1BR
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« Reply #18 on: October 29, 2014, 07:30:02 AM »

One other consideration, those tubes need to be run with red plates on occasion to keep them properly gettered.
 
Operating at 100 watt or less power output won't get the plates hot enough for that activity.

Pete
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K3VAT
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« Reply #19 on: October 29, 2014, 08:42:25 AM »

One other consideration, those tubes need to be run with red plates on occasion to keep them properly gettered.
 
Operating at 100 watt or less power output won't get the plates hot enough for that activity.

Pete

The above seems to contradict this posting: http://www.eham.net/ehamforum/smf/index.php/topic,99585.msg795181.html#msg795181.

Can someone please clarify (too hot or too cold?).

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W1BR
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« Reply #20 on: October 29, 2014, 09:10:58 AM »

One other consideration, those tubes need to be run with red plates on occasion to keep them properly gettered.
 
Operating at 100 watt or less power output won't get the plates hot enough for that activity.

Pete

The above seems to contradict this posting: http://www.eham.net/ehamforum/smf/index.php/topic,99585.msg795181.html#msg795181.

Can someone please clarify (too hot or too cold?).



Whatever...

http://www.w8ji.com/gettering_tubes_al572b_al80b.htm
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K3VAT
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« Reply #21 on: October 29, 2014, 09:26:13 AM »

One other consideration, those tubes need to be run with red plates on occasion to keep them properly gettered.
 
Operating at 100 watt or less power output won't get the plates hot enough for that activity.

Pete

The above seems to contradict this posting: http://www.eham.net/ehamforum/smf/index.php/topic,99585.msg795181.html#msg795181.

Can someone please clarify (too hot or too cold?).



Whatever...

http://www.w8ji.com/gettering_tubes_al572b_al80b.htm

Thanks for the link.  However, this procedure is for new tubes or as Tom says in #7, "Install the tube(s) you want to getter or burn it.  It doesn't say anything about this question that was raised about operating temperature of a vacuum tube when [constantly] run with low drive.

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W8JX
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« Reply #22 on: October 29, 2014, 10:32:06 AM »

One other consideration, those tubes need to be run with red plates on occasion to keep them properly gettered.
 
Operating at 100 watt or less power output won't get the plates hot enough for that activity.

Pete

The above seems to contradict this posting: http://www.eham.net/ehamforum/smf/index.php/topic,99585.msg795181.html#msg795181.

Can someone please clarify (too hot or too cold?).



Tube like 811a have metal anodes/plates and when they glow they can melt. Because of this they have a lower temperature getter coating on anode. The 572 has a graphic Anode that can safely run at a much higher temperature and has a different getter coating on it for higher temperatures. Because of the way a graphic anode is made some air is trapped in it that can leach out over time. This is most noticeable in 572's a few years old or newer. Because of this it is important to heat anodes red at times to activate bettering. A AL 811a amp cannot properly heat 572's so they tend to have a shorter life because as air leaches out of graphite anode it is never gettered out. A SB 200 will properly heat anodes at about 350 watts or more out. This is not to say you cannot run lower power sometime but you need to give it a days work at 400+ watts every month or so at very least. When tube is used the heated and cooling cycles tend to cause trapped air to leach out. Understand?
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K3VAT
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« Reply #23 on: October 29, 2014, 10:42:42 AM »


...<snip>...

but you need to give it a days work at 400+ watts every month or so at very least. When tube is used the heated and cooling cycles tend to cause trapped air to leach out. Understand?

Yes, thanks for the informative reply - it's kinda like your sports car: it will cruise around town just fine, but yearns for the open highway [to clean out the combustion chambers and valves]!

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W0BTU
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« Reply #24 on: October 29, 2014, 11:32:39 AM »

Because of the way a graphite anode is made some air is trapped in it that can leach out over time. This is most noticeable in 572's a few years old or newer. Because of this it is important to heat anodes red at times to activate gettering. ... This is not to say you cannot run lower power sometime but you need to give it a days work at 400+ watts every month or so at very least.

This is partially true and worth noting! Another source of air is the glass-to-metal pin and cap seals. They slowly leak air over time, eventually causing sudden tube failure.. The 3-500Z is notorious for this. (I don't think the 572B is as bad as the 3-500Z.)

Search for "getter" at http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm
« Last Edit: October 29, 2014, 11:34:46 AM by W0BTU » Logged

W8JX
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« Reply #25 on: October 29, 2014, 03:46:16 PM »

Because of the way a graphite anode is made some air is trapped in it that can leach out over time. This is most noticeable in 572's a few years old or newer. Because of this it is important to heat anodes red at times to activate gettering. ... This is not to say you cannot run lower power sometime but you need to give it a days work at 400+ watts every month or so at very least.

This is partially true and worth noting! Another source of air is the glass-to-metal pin and cap seals. They slowly leak air over time, eventually causing sudden tube failure.. The 3-500Z is notorious for this. (I don't think the 572B is as bad as the 3-500Z.)

Search for "getter" at http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

As the 572 gets older most of gas trapped in anode has escaped and gettering is less critical. The first couple years you need to heat them up. Not aware of seal problems with 572. When 572's are made they supposedly heat anodes and hold a vacuum on them for a day or more to remove most of air trapped in anode. I do not think they do this properly with recent Chinese tubes.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KM3F
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« Reply #26 on: October 29, 2014, 04:36:37 PM »

The question still has not been answered as to why the amplifier is to be run at 100 watts?
Is this average, PEP, on AM, SSB, or what?
For example, if run at 100 watt on AM your running this amplifier pretty hard. Don't do it.
Tuning for 100 watts with a CW tune signal will likely result is output greater than 100 watts on peaks.
Until this is answered all the rest of the replies are fine but is info overload without a place in the throught pattern until the conditions are specified.
.
Normally one matches system level compatibility rather than greatly mis match power levels like this unless the driver is  QRP rig. Even then is it not good engineering practice to do it. Sound familiar?
There is an easy test to observe non linear operation on any tube amplifier.
How; tune full out and note the final plate and load settings.
Reduce the drive to half or less and retune for max. You will see the plate and Load settings are quite different. The differences are due to a number of factors all present at the same time.
Having said this, when tuned for max power, the varying drive from the exciter in SSB mode still causes  non linear operation for the same reasons because the drive is changing in power just the same as the tuning test would show with different drive levels.
It's something you can't get away from, but don't aggravate the situation by running a 600 watt amplifier at 100 watts.
If your driver is capable of 90 to 100 watts you gain absolutely nothing by using the amplifier at 100 watts.
.
Many ops think running an amplifier at half power saves the tube. It's myth. Your into a number of issues doing this that you would otherwise not have or at least be minimized.
Tubes are ruined by excess tune time and being driven with excess power far more than running them at full rated power on a proper tune up..
Good luck.
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W8JX
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« Reply #27 on: October 29, 2014, 05:09:30 PM »

Many ops think running an amplifier at half power saves the tube. It's myth.

Not really as it depend on tube and how it is used vs its ratings and type of transmission.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KM3F
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« Reply #28 on: October 29, 2014, 05:18:23 PM »

Yes and know. I hear this all the time on SSB to which I was referring to.
Good luck.
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NO2A
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« Reply #29 on: October 30, 2014, 08:29:04 PM »

One possible problem with driving an amp to only 100 watts is if it`s loaded at that level output. For example if your rig has a power "spike"issue. Driving it with 10 watts gives you 100 watts for example. If the rig spikes to say 100 watts,that could cause the amp to arc if it`s loaded for 100 watts output only. Of course only some rigs have that problem. But an arc could cause damage in some cases. We still don`t know why the op wants to do this. Using a qrp rig maybe.
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