Review on High-Precision Nobsound 8-Pin Dual Bias Current Probes Tester for EL34 KT88 6L6 🧪 6V6 6550 Vacuum Tube Amp Amplifier - 2 Meter + 2CT1-C - Cathode Current Measurement by Heather Rheault

I have had a pair of wired octal connectors to set lamp bias for many years. They disappeared during a recent move and I replaced them with this gauge. WOW! Next thing I remember, a customer walked in with a 4/40w Blackstar switchable amp that had failed and the output tubes were smoking. I soon discovered that all maintenance records for these amps were closely guarded at the Blackstar facility. So we started troubleshooting and traced the circuits to see what went wrong. The amps use a FET to switch the B+ voltage to the output tubes from around 225VDC to 450VDC. The crazy thing about this circuit is that the amp has a higher cathode current at the low power setting! The shield voltage also changes, producing the same output tone at 4W as at 40W. It was a crazy intermittent problem that ended up having to do with the switching FET. It was pushed to its limits (like everything nowadays!), it was replaced by a 950V device. It was quite an enlightening experience fixing this device and I think this gauge really helped.