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Author Topic: AL80B, grid and plate current  (Read 16650 times)

Posts: 2483

« Reply #15 on: April 18, 2013, 06:19:39 AM »

I will admit that compared to the real experts on this site, my knowledge would not amount to a drop of water in a swimming pool.  Again...I never said an AL-80 would not put out 1000 watts PEP on some bands - it will.  I never said an AL-80 amp was not a good amp - it is.  I never said an AL-80 amp was not a well designed amp - it is.  I never said the 3-500 tube in an AL-80 amp was being driven so that its plate dissipation was being exceeded - it is not.  Actually I think the AL-80 offers (by far) the best bang for the buck on the market today.

When I studied EE in the late 1960's every amplifying device (tubes, transistors, and otherwise) came with a set of characteristic curves that showed what could be expected of the device under different operating conditions. Most of these curves were never a straight line for their entire length.  Typically the curves tapered off, even "wiggled" at some points along their length, but especially toward the high end of the curve which equates to maximum plate or collector current.  Depending on the application sometimes that mattered and sometimes it did not matter. I assume that still applies.

In order for a device to be used as a LINEAR amp I was taught that the device must be operated within the linear (straight line) part of the device's characteristic curve.  When the device is driven to the point where it is being operated out of the linear part of the curve, which is most often high plate or collector current, the device is no longer considered to be a LINEAR amp.  That is why when IN MY OPINION any 3-500Z is used in a LINEAR amp it should be operated within the linear portion of the curve.  In EIMAC tubes that used to be up to around 400 mA of plate current.  Driving an EIMAC 3-500Z so the plate current exceeds the linear portion of the curve results in an amp that is no longer linear.  Whether or not that causes problems or a "dirty" signal is an issue that I cannot address.

Dick  AD4U
« Last Edit: April 18, 2013, 06:25:23 AM by AD4U » Logged

Posts: 12626

« Reply #16 on: April 18, 2013, 06:28:15 AM »

What some seem to fail to realize is is not just about dissipation, it is also about cathode current and excessive plate/cathode current can in theory strip minute parts of filament/cathode coating away leading to tube loosing emissivity prematurely and going soft. You can exceed plate dissipation without exceeding plate current ratting with proper voltage just like you could exceed cathode/anode current rating without exceeding dissipation rating too. How much it will shorten its life is hard to say but it is kinda like using a 1/2 ton pick to carry a 3000 lbs load, it will work for a while maybe but it will fail sooner than if it was carrying its rated load.   Good debate though.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 177

« Reply #17 on: April 28, 2013, 06:34:26 PM »

Kind of on the same subject. Once the amp is up and running, can I just use the plate and load knob while looking at the output watt meter and tune them for "peak" on each? If I start out on plate, tune for peak wattage (even if I have it set on low input power) then hit the load, then go back and touch up the plate one more time for peak wattage on each, would this be an acceptable method to tune the amplifier? Then I can crank up the input drive as needed. Maybe a light re-tune for peak if needed.

ARRL Life Member|QRZ Life Member

Posts: 2747

« Reply #18 on: April 28, 2013, 07:03:01 PM »

always do a final peaking at the final drive level. Don't load up for peak and add drive without repeaking or else the amp will be underloaded
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