[Amps] TL-922 question.

[Georgens, Tom]

Hello -

I am looking for some help on a TL922 problem.  At my contest station in Barbados I had a TL922 fail in February.  Looking at it, I saw that the doorknob blocking cap had split and was burned inside.  I also noticed that the HV fuse resistors I had installed had been blown.  I reasoned that the cap shorted, the  output RFC took the HV to ground and the fuse resistors blew protecting the power supply and plate choke.

Only this past weekend have I been back to work on it.  I took down a spare doorknob cap and output RFC, if the prior one had been damaged.  I had looked at the plate choke before I left and it showed no sign of damage.

The first issue was that the TL922 has soldered over all of the mechanical connections and I did not have a sufficient heat source to take them apart.  Nonetheless I did find a place to mount the new cap.  The output RFC still looked good.

When I turned it on, the amp made power but the output was only about 500W.  I looked in the cage and both filaments were definitely on.  I power it all down to take a look inside to see if some connection is missing.  The connections looked fine but I could feel that one tube was still scorching hot and the other was merely warm.  It seemed that only one tube was making power.  It is possible some tube failure was the precipitating event and not the blocking cap so I replaced the tube with a spare I tested about a year ago.  The result was the same.  Once again it seemed like only one tube was active.  I was out of time here and I should have swapped the tubes but I did not.

What I did do is bring back the original tube to test at home.  Assuming that it works, I am open to ideas on what may be wrong.  I am assuming that the plate voltage is connected since my experience with RF drive and no HV is high grid current.  This was not the case.  All readings were normal, for one tube.  About 100 ma Grid and 450 mA plate.  One possibility is that RF is not getting to one of the tubes.  As it turns out, the rf is coupled through a cap to the first tube (the one that is working) and then another cap couples to the other tube (the one that is not).  It could be a simple as the cap failing.  I will bring a replacement next time but I want to be prepared in case it is not that easy.

My question is whether there could be another reason for the symptoms I am seeing.  My only other theory is the biasing circuit could be damaged and the tubes are indeed both on but not at the right bias point and they are not sharing the load equally.  I did not measure the zener voltage but the part was not obviously damaged.

Thanks in advance for any suggestions.

Fortunately I had three other amps that work fine

Tom W2SC 8P7A