[Amps] TL922 AMP PROBLEMS

[Brian Miller]

Hi all

The ZL6QH team is using a TL922 amplifier which has developed some problems over the past 9 months.

Here is a chronological summary of the events. Note that I have not personally observed all of the
events - the following is a combination of what I have been told and what I have seen.

Jan 2002 - New RF Parts tubes installed and then one of the grid chokes burnt out on start-up. The
choke was repaired and the amplifier appeared to work normally again.

Sep 2002 - One of the new tubes failed with an open circuit filament half way through a contest. An
old "used" tube was installed as a temporary replacement.

Oct 2002 - The second new tube failed, again with an open filament. This time it failed on start-up,
along with several "popping" noises. At this stage I also noted that the enamel wire on the L3
filament choke was discoloured - indicating that it may have been overheating.

Oct 2002 - Installed two good "used" Eimac tubes. The amplifier now appears to work normally in the
CW 2 KV position but is not stable in the SSB 3 KV "Operate" position. After switching to "Operate",
the IP current (indicated on the front panel meter) gradually increases and then suddenly starts to
intermittently jump to full scale - along with "popping"noises.

It is likely that parasitic oscillations are contributing to the above situation. We will have
careful look at the relevant components in the anode, grid and bias circuits.

I understand that the amplifier does NOT have a history of instability - the above problems are a
recent development. So something has changed!

Two questions

1. Have other owners of TL922 amplifiers observed a similar sequence of events. If so, then what was
the root cause and how was it fixed?

2. Any thoughts on why the filaments in both tubes should go open circuit? I did notice that the
filament voltage was running around 5% high. We have fixed this by changing the line voltage setting
from 220V to 240V and reducing the external line voltage to the amplifier.

Note. I HAVE read the earlier threads about parasitic suppression - including Richard's web pages.
No
need to repeat this information please.

Thanks and 73

Brian Miller ZL1AZE
Registered Electrical Engineer
67 Hornsey Rd
Melrose
Wellington
New Zealand

Tel 027 4446614
Email: brianmiller@xtra.co.nz