Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 3 Next   Go Down
  Print  
Author Topic: AL 80A How to tune an amp.  (Read 23988 times)
NB8I
Member

Posts: 44


WWW

Ignore
« on: January 17, 2012, 12:00:26 PM »

If you look at the Ameritron manuals for tuning they list a a cycle of bringing up a cw carrier at small
interveals to a peak of 200ma and tuning the plate and load controls to max output.

A number of hams I've run into say to use this method instead......

For your review:
_________________________________________________________________

The Importance ~f Amplifier Tuning

A grounded grid linear amplifier should be tuned so that most of the electrons emitted by

the cathode reach the anode circuit. Electrons that fail to reach the anode are lost to grid

current. This condition occurs when the amplifier is loaded too lightly -" the result of

setting the loading capacitor for too much capacitance during tune-up. As grid current

rises and fewer electrons reach the anode, distortion increases and output power decreases.

Thus, a triode amplifier can be tuned up with fair accuracy by simply applying maximum

drive power and quickly adjusting the" amplifier's TUNE and LOAD controls for maximum

power output. After the amplifier has been tuned for maximum output, a slight increase in

linearity can usually be secured by increasing the amplifier loading - that is, by decreasing

the loading capacitance - until output power decreases by a few percent Note, however,

that this small improvement in linearity is seldom detectable ~ practice because the IMD

performanceof tube amplifiersis usually significantly better than the IMD performanceof

the bipolar-transistor-output transceivers commonly used nowadays as exciters.

Electrons are negatively charged. Positive charges attract them. The more positive the

charge, the stronger the attraction. The 0-V potential of a grounded control grid is more

positive than the negative charge of an electron leaving the cathode of a vacuum tube.

This causes excessive current to flow in the control-grid circuit unless the tube anode is

substantially more positive than the control grid throughout the anode-voltage cycle.

The output of a grounded-gridRF amplifier tube "appearsat the tube anode as RF AC

superimposed on the DC anode supply. As a rule of thumb, successful attraction of most

of the cathode's electrons requires that the instantaneous anode voltage not fall below about

+200 to +300 V (relative to the grounded grid) during the lowest point of its downward

voltage swing (that is, during the anode-current peak). Loading the amplifier too lightly -

that is, adjusting the amplifier LOAD control for too much loading capacitance - causes

the minimum anode voltage to fall below this level. The result is a dramatic increase in

grid current and distortion, and a reduction in peak power output. Loading the amplifier

too heavily - insufficient loading capacitance - also causes the output power to decrease

because the amplifier output network is not adjusted to the impedance-transformation ratio

necessary for maximum power transfer between the amplifier and its load.

Reprinted from March, 1989 QST - Richard L. Measures, AG6K
Logged

K4RVN
Member

Posts: 261




Ignore
« Reply #1 on: January 17, 2012, 12:30:54 PM »

Here is how I tune my AL80A for ssb which has served me well for over 20 years.

Use suggested manual presets for load and plate on amp.
Set mode to CW on radio transceiver.
Set radio drive to about 20 watts or less.
Key amp and adjust plate and load for max out on watt meter quickly.
Turn drive up to 55 or 60, key amp, touch up plate and load for max watts out around 850 to 900 max.
unkey amp.
Advance load just very slightly less than one line graduation.
Switch to ssb, and operate as desired.
Check meters for grid and plate current to make sure they are within limits.

In lieu of this read W8JI site for the correct way to tune an amp.

Frank
Logged
AD4U
Member

Posts: 2538




Ignore
« Reply #2 on: January 17, 2012, 01:04:29 PM »

Frank K4RVN hit the nail on the head.  I have followed this exact same amp tuning procedure for the last 45 years.  My Drake L4B and my Heathkit SB-200 are still running the original tubes after over 40 years of use.

There is no reason to make this complicated.  Do what Frank said. No more.  No less.

Dick  AD4U
« Last Edit: January 17, 2012, 04:13:38 PM by AD4U » Logged
NB8I
Member

Posts: 44


WWW

Ignore
« Reply #3 on: January 17, 2012, 03:31:58 PM »

Thanks for the replies gentlemen..
Logged

W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #4 on: January 18, 2012, 02:21:37 PM »

If you filter all the needless (and occasionally questionable) wordiness out of those tuning instructions, what the author is saying is this:

"Apply maximum drive and tune for maximum possible power."

This is an age old method many people use, and it often works when the person has some idea where to start at with control settings,a nd if the exciter an amplifier are a close match.

While it works in many cases, but there are certain precautions that should be applied. While it usually works, it is also possible to seriously damage an amplifier by just following those instructions.

Quote
If you look at the Ameritron manuals for tuning they list a a cycle of bringing up a cw carrier at small
interveals to a peak of 200ma and tuning the plate and load controls to max output.

No, that isn't true. Almost every manual is different, but they basically tell you:

1.) Preset the amplifier to a typical setting for the band

2.) Verify a few quick things

3.) Adjust to rated power while watching grid current and plate current for maximum tremporary limits

You are welcome to follow those instructions, and not watch grid current, but be aware tuning for maximum output with full drive, without regard for grid current or anything else except maximum output power, can cause some serious damage pretty fast.

It is ALWAYS better to watch grid current, and do a quick test at reduced drive, than to simply apply maximum power and adjust blindly for maximum output power.

I did look at the AL80B instructions, and I do see some things that should be changed.

73 Tom
Logged
K2XT
Member

Posts: 39




Ignore
« Reply #5 on: January 19, 2012, 04:09:39 AM »

It needs to be mentioned that all the above discussion relates to tubes that can take large amounts of grid current, like 3-500s.  Anyone who tries throwing full drive into an 8877 or 3CX800-based amplifiers is going to instantaneously trip the grid protection, if the amp has one.  On my homebrew 8877 amp it requires at least 3-4 iterations of tuning, increasing drive, tuning again, etc to get to full output without drawing high grid current.
Logged
AD4U
Member

Posts: 2538




Ignore
« Reply #6 on: January 19, 2012, 05:51:03 AM »

There have been several posts cautioning people who follow K4RVN's simple amp tuning instructions to make sure grid current is within specs.  

In all fairness to K4RVN who posted the first reply to this question, the very last "step" in his simple amp tuning instructions states:

"Check the grid and plate current meters to be sure they are within limits"

Dick  AD4U
Logged
K4RVN
Member

Posts: 261




Ignore
« Reply #7 on: January 19, 2012, 01:23:07 PM »

Dick,
Thanks for the comments, but I believe the  comments in post you mentioned were in reply to the AG6K method of amp tuning. At least I hope so, because I use two steps to tune my amp and check the meters. I don't load by the meters as I only use a few seconds.
He, Richard,  I believe, said to tune initially at max power while I use 20 watts or less, then bring it up to my operating max that I want. Of course I also check the meters on the initial SSB operation. In fact I just put my method out there and did not encourage anyone to use it. You, I, and others who have a lot of experience and a good track record with tubes and equipment know this is one way to load an amp without abusing it. I also load my AL811 the same way, still on the original 811A tubes after over two years so I don't plan to change. After reading the AG6K tuning comments, I felt he made it too complicated for the average amateur with a first amp. I still don't know why he had to add all the theory, I suppose because he understands all he knows , which I don't LOL. I put a disclaimer in my comments to check with W8JI
who knows about the AL80A. No sense in making mountains out of mole hills which is in effect what you said. It's just not that much trouble to tune an amp.

Frank
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #8 on: January 19, 2012, 01:50:57 PM »

After reading the AG6K tuning comments, I felt he made it too complicated for the average amateur with a first amp. I still don't know why he had to add all the theory, I suppose because he understands all he knows , which I don't LOL.

The theory has little or no meaning for anyone trying to tune, and also only partly or marginally applies to grounded grid. 

The notion of just driving with full drive and tuning for maximum output can work in some cases, but can also cause problems like blown band switches, damaged grids, and other problems.

More than anything, grid current indicates danger of instantaneous damage. That is why grid current really should be watched in any grounded grid amp, or screen current in a grid driven tetrode.

73 Tom
Logged
AD4U
Member

Posts: 2538




Ignore
« Reply #9 on: January 19, 2012, 01:56:45 PM »

Frank - maybe so but I don't see where AG6K commented on this post.  All I was trying to say to those who commented about possible excessive grid current by using the method in your post, was that in the very last step you specifically said to check the meters for excessive current.

Regardless, you and I apparently know how to tune an amp.  I just chimed in and said tuning an amp does not have to be complicated and there is really no reason to make it so.  If it is, then I have been doing it "your way" and improperly for over 45 years.

Dick  AD4U
Logged
K4RVN
Member

Posts: 261




Ignore
« Reply #10 on: January 19, 2012, 06:35:08 PM »

Dick,
NB8I posted that in lieu of the manual that a number of hams had advised him to use the method he posted which is a reprint of a 1989 QST article by Richard AG6K. As I understand the article, AG6k stated to the effect that a triode amp could be tuned with max drive and max watts out. It is a little hard to dig the bottom line out which states that. I posted my method to maybe prevent some of the newer amp owners from smoking something due to their often longer than normal times for tuning due to inexperience. The credit to AG6k is down at the bottom of the first post by NB8I. You, I, and others could most likely do it without damage.

Frank
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #11 on: January 20, 2012, 09:32:06 AM »

Dick,
NB8I posted that in lieu of the manual that a number of hams had advised him to use the method he posted which is a reprint of a 1989 QST article by Richard AG6K. As I understand the article, AG6k stated to the effect that a triode amp could be tuned with max drive and max watts out. It is a little hard to dig the bottom line out which states that. I posted my method to maybe prevent some of the newer amp owners from smoking something due to their often longer than normal times for tuning due to inexperience. The credit to AG6k is down at the bottom of the first post by NB8I. You, I, and others could most likely do it without damage.

Frank

If I am being unfair or not being objective or factual, please let me know. My viewpoint is this:

1.) Much of the article posted is totally unnecessary for tuning, and other key areas are omitted.

2.) When I read all of that text, paraphrasing slightly to simplify, the only real tuning instruction offered is this:

"Apply maximum drive and tune for maximum output". Somehow, that took a whole lot of words to say.

3.) Much of the technical explaination in that article is inaccurate or incomplete.

For example, it blames plate voltage for the increase in grid current as the loading is too light, but in a grounded grid amplifier light loading greatly reduces negative feedback. This causes an increase in drive power available for the grid, and more exciter power is diverted to the grid. Certainly SOME of the increase is caused by reduced plate voltage during positive grid to cathode voltage swings, but the increase is a complex interaction of multiple things.

Distortion is caused by the transfer function becoming very non-linear as peaks clip, not just the increase in grid current, so the distortion explaination is very misleading or incomplete, also.

It's just my opinion, but throwing all the incomplete or incorrect unnecessary technical stuff in there really is not useful at all.

4.) Grid current is a good warning of approaching instananeous failure from tank circuit arcing or excessive grid heating. I think it is an almost fatal error to not warn people to watch grid current carefully while tuning.

Does this all make sense, or am I misreading what it says?

Wouldn't it be more effective to say:

1.) Watch grid current (follow factory recomended limits), and apply full drive to stay within those limits, while tuning quickly for maximum possible output

2.) As grid current falls back to safe ranges after peaking output, increase drive and tune for maximum output gain

3.) Stop increasing drive when the plate current limit or output power limit is reached.

??


It seems pretty foolish to not watch drive levels or grid current while tuning!

73 Tom
Logged
AD4U
Member

Posts: 2538




Ignore
« Reply #12 on: January 20, 2012, 12:08:29 PM »

Tom I ALWAYS read what you have to say and 99.99% of the time I agree with you.  This time you are "right-on".  After 45 years building amps (triode and tetrode) I think I can properly tune any of them.  Unfortunately not every HAM can make that statement.

All I was trying to convey is that tuning an amp DOES NOT have to be an all-day exercise in knob twisting and meter watching.  Properly tuning ANY properly designed amp is not that complicated and there is no reason to try to make it so.

Assuming "one" knows the proper "pre-sets" of the tuning cap and the loading cap, PROPER amp tuning can be done in 5 seconds or less.

Dick  AD4U
Logged
KC9TNH
Member

Posts: 304




Ignore
« Reply #13 on: January 20, 2012, 01:09:55 PM »

Assuming "one" knows the proper "pre-sets" of the tuning cap and the loading cap, PROPER amp tuning can be done in 5 seconds or less.

Dick  AD4U
As someone about to get their first amp I happen to agree but it took alot of reading and discerning what was actually meant - not all a bad thing as it fostered a better understanding of the relationships (and what's critical to watch). Still, the WRITING of the instructions in several of the Ameritron manuals could stand a serious re-write done by someone who really knows how to format instructions that are supposed to follow a progression (e.g., some of the things in the way the AL-80B instructions are formatted might be better written with a bent toward "if you're running ALC start here, if you're not running ALC skip to page whatever" rather than mixing them and even including an instruction that has no bearing if the ALC isn't connected). Has nothing to do with the good information provided, rather, more to do with how humans embarking on something like that for the first time - and naturally not wanting to destroy their new stuff - LEARN, versus a tech or engineer who may be assuming more knowledge at the customer end than is really there.
 Just a suggestion. Smiley

And I also find Tom's simplicity consistent. My AL-80B & I will likely be partners for awhile

73 all
Logged

73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
NO2A
Member

Posts: 1400




Ignore
« Reply #14 on: January 20, 2012, 03:20:04 PM »

Another manuel problem is the plate current max of 400ma. Provided the amp is tuned properly,you will never get max power out with only 400ma. More likely it would be 450-550ma. The point is this is misleading to new owners of this amp. The most important thing is grid current,and enough hv.
Logged
Pages: [1] 2 3 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!