Ameritron AL-80B - Tube Cherry Red and power dropped out,, What happened?

I was operating my amplifier a couple of days ago, when all of the sudden it stopped transmitting. I had loaded it up fully (using CW carrier) and it was looking good, power out was about 900W and grid current was about 400 milliamps, and plate current was looking normal. SWR was a very good, maybe 1.5, 1.6 to 1. Everything looked ok.

Then all of the sudden while transmitting SSB during a QSO, I looked down and there was no power output. And my signal had dropped off the air. Grid current was showing about maybe 125 milliamps constant, with no input from the exciter with the transmit light off (Amp not keyed). I then looked at the 3-500Z power tube and it was glowing cherry red!

I then switched off the amplifier and turned it off for the day. The following day I turned the amplifier back on and loaded it into a dummy load. It seemed to load up and work just fine. I transmitted into the dummy load for a while and nothing seemed to be a problem.

What happened?

 

Page 8 of the Manual:

"Incorrect operation is most likely to damage the tube's control grid or anode. Applying full drive power for several seconds with excessively "light" loading can cause control grid damage. Several seconds of high drive power operation with improper tuning or excessively "heavy" loading can melt or damage the anode. These conditions can be avoided by following the tuning procedures in this manual. In the AL-80B, a grid current of 250 mA (at 100 watts of drive) produces the rated grid dissipation of 20 watts. For maximum tube life, the grid current and drive power should always be kept below these values. WARNING: NEVER drive this amplifier with more than 100 watts of short term average envelope power. NEVER allow the grid current to exceed 250 mA under any operating condition. Maximum power output normally will occur with approximately 175 mA of grid current on CW, or approximately 45 mA of maximum grid current (175 mA instantaneous peak) on SSB (two-tone test). The grid dissipation with 175 mA of grid current and 70 watts of drive is approximately ten watts."

Page 9: "The continuous commercial plate current rating of the 3-500Z is 400 mA. For maximum tube life, plate current should be maintained below 400 mA on CW and other "steady" carrier modes. Brief periods of plate current exceeding 400 mA will not cause loss of emission or shorten the life of the tube in this amplifier."

Pheel

 

Well I had it backwards, Plate current was at 400ma Max, Grid current was around 150ma as I recall. I was driving with 60w from the exciter and the ALC was connected and was set at around 18 on the Amp's meter. I was driving with an Icom 7300, mic gain at 50%. The plate current is what was "frozen" at maybe 125ma even when I had turned the amp back to stby.

 

If you now see abut 100 ma +/- plate current at turn on without excitation, the auto bias circuit has failed.
Usually a transistor monnted to the back wall just off the board.
It could be the small driver transistor driving it or both.
It detects grid drive signal and brings the tube out of cut-off. Works very fast and does not normally cut off voice modulation.
If this is the case, it has not hurt anything but a loss of cut-off bias to save a little heat/power between voice pauses.
.
There could be other reasons but you will have to troubleshoot and see.

 

You said the plate current was reading over 100 ma with the amp not keyed into transmit. If that is true then the tube is bad. The amp should not draw any plate current when it’s just on and in standby. Sometimes you can key the amp
Into transmit and it will draw idle current. That amp has electronic auto bias so the tube is not biased on until a small amount of RF is flowing in. So if the tube draws current on standby the tube is bad. It it draws current when keyed up and no drive then the bias transistor is shorted.

 

I was wondering if the tube might have gone into a failure mode during the transmission I was making. When I turned the amp on the following day, there was no steady plate current showing in the unexcited/un-keyed condition. As I noted, I then loaded it up into a dummy load and modulated for awhile to simulate a QSO and it did not do it again, but I'll put it back on in real world use and see if it recurs.

I will say that determining the output power "peak" during the LOAD knob adjustment, can be a bit vague... It isn't as sure as finding the peak with the PLATE control. I wonder if that had caused me to have too much loading at that particular tune. If so, would that have caused the tube to run very hot?

However,,, the Plate and Grid currents were under the max allowed, using a closed CW key carrier, as I went on the air.

This is the tuning procedure I used from W8JI's instructions:

"Easiest Possible Tuning Steps using a closed CW key, FM (push to talk), or AM (push to talk) Tuning"

1. Set the transceiver’s power to about 10-20 watts.
2. Preset the PLATE, LOAD and BAND according to the presets listed in the manual.
3. Transmit and quickly adjust the PLATE control for maximum RF output meter power. (DO NOT adjust the LOAD control.) Limit to 5 seconds on, and 15 second no-transmit cool-off on standby.
4. On standby (so amplifier is bypassed), increase radio power to 60 watts.
5. Transmit through the amplifier and quickly adjust the LOAD control for maximum RF output power. (Never exceed 5 seconds of transmission time).
6. After 15 seconds of cool-off, transmit and quickly turn the PLATE control back and forth a small amount to make sure the PLATE is at peak output power. Take no longer than five seconds to do this without 15 seconds cool-off time.
7. After 15 seconds cool-off, transmit a steady carrier and quickly make sure the plate and grid current meters never go beyond the maximum rated values listed here. This paper supersedes all other maximum current ratings.
8. If grid meter and plate meter currents are below maximum allowable values, increase the transceiver’s power until the grid meter reaches the maximum level for the tubes. If plate or grid current is too high, stop here and reduce transceiver power to bring the meter in safe range.
9. In operation or tuning, never exceed or go above the unit ratings.
10. After 15 seconds cool-off, if you have not reached maximum allowed plate or grid current, transmit and re-peak the LOAD control for maximum output. Do not transmit for more the 5 seconds without a suitable 15 second cool-down.
11. As a final adjustment, moving the LOAD control very slightly clockwise of maximum produces a cleaner SSB signal. Move the LOAD no more than ½ of a number position clockwise. It also helps to reduce the transceiver’s power a few watts from tune power.

 

I didn't just look, but those "tune-up" instructions sound like they're for an AL-811 or AL-811H (because of the 5-second limit and cool-down warning).

 

One Fits All... +- a few.

 

Amps using tubes like 3-500Zs don't need brief tune-up periods with cool-down periods between.

Neither to any of the ceramic-metal external anode tubes, if you just watch grid current (and that's all you really need to watch!).

811As need the brief tuning and cool-downs because in amateur amps they're used at power dissipation levels that greatly exceed their CCS ratings. The bigger transmitting tubes aren't.

 

I had something identical happen a couple of years ago with my AL80B. Turned out to be a grid to filament short. Seeing the 3-500Z glowing orange yellow was startling to say the least!

Frank, W1NK