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Author Topic: Heathkit SB100 (Ameritron A80)  (Read 11944 times)
WB6WSQ
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« on: September 06, 2013, 11:07:06 AM »

I have an old Heathkit SB1000 or the equivilent of the Ameritron AL80.  It has been stored for about 18 years in various climates and conditions.  I have cleaned it up, redone some solder joints and had to replace the Zener Diode as well as the plate choke that was broken.  The amplifier boots up, loads up to all specs in the manual and will load up to the 1000 watts.  The problem is that even driving it with minimum power of 10 watts (200 out from the amp) or maximum input, after about 30 characters of cw, the plates start glowing red from the bottom up and when they glow up to about half, the tube arcs and the fuse blows.  I have heard about "gettering" and other things but can never get it to evidently run long enough without blowing.  Again, everything works perfect for transmission and loading but cannot get past about 20 seconds of transmission.  Is my suspicion of the tube being shot correct?  One final note is that when I first turned it on to warm up, there was a blue hue in the base of the tube that is not there anymore that I can see.  HELP PLEASE!
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KG6YV
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« Reply #1 on: September 06, 2013, 11:19:04 AM »

The blue glow indicates that the tube is gassy which isn't surprising since it has been stored for a while.  I'm not sure if it is still salvageable but getter'ing could be tried.  W8JI and others can comment as I am no expert.

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KA5N
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« Reply #2 on: September 06, 2013, 11:21:20 AM »

Sounds like it is not tuned properly  (off resonance) or the grid of the tube is shorted to
the cathode (filament)  something could be loose inside the tube so the short only happens
after the tube warms up.    The blue glow is a common effect that causes no problem.
With a rig that has sat that long, who knows what might be going on.
Good Luck

Allen KA5N
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W7VO
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« Reply #3 on: September 06, 2013, 02:26:22 PM »

Sounds like a gassy tube that is arcing over. Maybe you can buy the extra tube that KA8WEP has leftover from his recent e-Ham post!  Wink

http://www.eham.net/ehamforum/smf/index.php/topic,92081.0.html

73;

Mike, W7VO



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W1QJ
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« Reply #4 on: September 06, 2013, 03:19:28 PM »

First off just for the record, the SB-1000 is a clone of the AL-80A not the AL-80.  Running the amp at 10 watts input with 200 watts output is worse than running it a full bore as the plate design impedance is way off and the plate dissipation is even worse in the long run.  Though that might not be the problem the tube could be arcing over at high currents if you load the amp to 1000 watts output.  Another consideration is that the line current could be too high for the fuses if the amp is being run on a 120v line.  You did not mention what the line voltage was set for.  Blowing fuses could be caused by a number of things especially after a period of time goes by.  A severly shorted tube would normally blow a fuse instantly.  Other conditions of malady make take a longer time.  More precise information is needed to pinpoint the problem.
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WB6WSQ
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« Reply #5 on: September 06, 2013, 05:59:30 PM »

I am running the rig, (the linear) on 120V.  Again, at the intitial test and load per the manual, All checked out perfect.  I even bypassed my tuner to run the linear direct to the antenna with only a 1:2 to 1 with no difference.  I cannot do the "gettering" because the tube will not get initially hot enouugh. I just do not want to buy a new tube to only find the same problem. Again, it loads perfect to all the manual specs for tuning and is oerfect until I start sending some CW for about 20 seconds then blows the fuse.  With the tuner I get a 1 to 1.   
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WB2WIK
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« Reply #6 on: September 06, 2013, 06:24:06 PM »

What is the ZSAC (plate current when you're transmitting but not actually driving them amp)?

Your description makes me think it's way too high.  Should be 60-80mA, somewhere around there.

Key your rig and the amp without actually generating a signal (no drive) and let us know what that plate current is, recorded by the AL-80 meter.
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WB6WSQ
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« Reply #7 on: September 07, 2013, 09:41:48 AM »

The plate current with no drive to the amp is 50 milliamp on the amplifier meter.  Grid current is 40 milliamp on the meter.  With 10 watts minimum of drive (my ICOM 730) my meter shows 175 millamp of plate and 70 milliamp of Grid current.  According to the manual, this is acceptable range with the amplifier tuned.
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KF7CG
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« Reply #8 on: September 10, 2013, 11:46:15 AM »

Something is peculiar as the grid current should be very close to zero with no drive. How is the nuetalization on the amplifier, it could be that there is a parasitic in the amplifier.

With no drive, but with the amplifier keyed, try tuning the amp. There should be minimal change in either the plate or the gred current. If there any significant change you have a parasitic in the amp. It could be oscillating in the high HF or even lower VHF without drive or when triggered by drive.
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WB6WSQ
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« Reply #9 on: September 11, 2013, 08:42:29 AM »

In regard to KF7CG coment, I followed the instructions in the Heathkit manual for initial testing of the amp by shorting the relay to key the amp with no antenna or drive and it says it should have a plate current of around 50 milliamp and 40 milliamp of grid current at load.  In regard to a parasitic in the amp, I must plead ignorance in my old age and do not know what the "parasitic" is.  Sounds like a parasite lol.  I have also tried running the amp direct to the antenna without the tuner on 20 meters since I have a 1 to 1 match without the tuner.  This was done to see if the old Heath antenna tuner was shorting out causing the problem.  Still, same results.  Loads to full power all within current ratings, sounds great, but only last for about 30 CW characters before the tube starts glowing red from the bottom up and then blowing the fuse at about half way up the plates.  Again, I hate to pop for new tube if in fact that is not the problem.  I have now cleaned the local Radio Shack out of 15amp ceramic fast blowing fuses!  I expect a Christmas card from them this year now!  Looking forward to anymore suggestions and definition as to the "parasitic".
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WB2WIK
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« Reply #10 on: September 11, 2013, 09:25:36 AM »

I've used AL-80s lots of times and currently own an AL-80B, which is very updated but still uses the same tube.

Anode of the 3-500Z never gets red within 30 seconds of using it on CW at any power level, including full power.  It takes probably 3 minutes of transmitting a lot of CW before the plate starts turning red, and even then it's not very bright.  A very long CW transmission (maybe 10 minutes) will get it red, as will about 1 minute of RTTY at 700W output or 1 minute of AM at 200W output (carrier power).

Something weird is going on with your amp.
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WB6WSQ
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« Reply #11 on: September 11, 2013, 10:24:47 AM »

WB2WIK, thank you for the post.  That info definitely helps that during normal CW, the plates should not glow red. Some additional information is that when I first dug the amp out of storage and plugged it in, it was a very impressive light show for a second til the fuse blew.  A lot of arcing within the tube. I found a wire grounded that had broken off in the HV circuit and corrected it plus replaced the Zener Diode that had also shorted.  The RFC choke also had some broken coils not allowing the volatage to the tube.  I replaced that with a new one from Ameritron.  That is when I thought I was home free until the current problem started.  Again, the tube does some arcing before the fuse blows.  Maybe the amp is now possessed!
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G3RZP
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« Reply #12 on: September 11, 2013, 02:28:47 PM »

Run it into a 50 ohm dummy load - not an antenna. Grid current  with no drive should be zero. If not and there are no bad components, change the tube - but check the circuit diagram to make sure you aren't going to be misled.. To check for parasitics etc, put a 50 ohm load on the output and a 50 ohm load on the input. Vary plate tuning and load through all possible settings, especially on 10 an 15 metres. if the plate current doesn't change, it suggests things are OK. If that's the case, you need to try another tube. If it does , you probably have parasitics and then check the suppressors.
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KF7CG
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« Reply #13 on: September 12, 2013, 11:42:13 AM »

To explain a parasitic: A parasitic is an oscillation that occurs within an amplifier. They can be of the type that only occur when there is drive on the amplifier of the type that occur whenever the tube has voltage and operating bias or be of the type that must be triggered by drive but will continue as long as the tube has power and operating bias.

I am not directly familiar with your amplifier, so I can only work on tube amplifier theory and experience with other amplifiers to help. If you amplifier, like my Ameritron 811H maintains power and bias as long as the transmit relay is energized it could be that the tube is drawing full power all the time even between code elements. This would contribue significantly to heating.

If you keep the transmit relay energized and only send one dit, does the amplifier still overheat?

KF7CG
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WB6WSQ
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« Reply #14 on: September 21, 2013, 02:18:57 PM »

Thanks to all of you for your comments and advise.  Checked for the parasitic problem and appears there is none.  The final test I did was with no drive and triggering the transmit relay and adjusting the plate and grid controls through spectrum with no change in current.  With the transmit relay keyed, the tube starts glowing red instantly and after a few seconds will blow the fuse.  From all of your comments on the subject, it leads me to believe that the tube is in fact shorted out and needs replaced ---ugh---
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