it finally happend…. After a decade of reliable service, my Alpha91b amplifier broke. Most likely on it’s way to Spain one of the tubes broke. During the comissioning at ED1R contest station, fire & smoke shot out of a chimney. The first shock was big, but a few measurements confirmed that just the tubes were broken. In this blog post I will share the knowledge I gained during during debugging, comissioning and breathing new life into the amplifier.
Disclaimer & WARNING
WARNING! Do not perform ANY work on your amplifier if you are not exactly sure what you are doing. The amplifier is using high voltages up to 3000V which is LETHAL. Touching the wrong part inside the amp can KILL YOU. Again – if you are not absolutely sure, contact RF-concepts or your local dealer. Perform everything which is written here under your own responsibility. It has worked for me, it might work for you, but it might also not work for you. This article claims no professional advice.
AGAIN: With the slightest doubts, please don’t experiment with high voltages. Look for professional help from your next dealer.
The broken amplifier
While I couldn’t figure out the root cause of the tube’s failure, the symptoms where quite obvious.
The most likely theory is that one of the tubes was damaged during the shipment. The broken tube then caused the remaining good tube to overheat and finally burn out during the commissioning tests.
During the comissioning, I noted three strange behaviors of the Alpha91b (across all bands):
- The tuning peak became extremely sharp; slight readjustments of tune/load would cause the amp to go into error-modus
- The tuning meter wasn’t really useable; Tuning had to be performed with the max-output indicator; Once max output was reached, the tuning LED would always remain 3-4 LEDs right to the center.
- Max output was limited to approx. 1000 Watts
- Both tubes removed from the Alpha91b
- High Voltage connector physically disconnected from the power supply board
- Voltage of Grid 1 at the tube sockets
- Voltage of Grid 2 at the tube sockets
- Voltage of Grid 1 with PTT pressed
- Grid 1: -125 V
- Grid 2: +350V
- Grid 1 (with PTT pressed): -77V
- Place the visually undamaged GU74b into the first socket and then repeat the test with the same tube in the other socket
- Apply high voltage
- Connect Dummyload
- Apply 5-10 Watts
- Output power of the Alpha91
Ordering a matched pair of GU74b / 4CX800
Gettering / Conditioning GU74b / 4CX800 tubes
- Remove the high voltage connector from the Power Supply board
- Remove J1 and J3 from the Power Supply board and connect them as indicated on the picture.