I have an old diytube getsetgo single-ended amplifier. It uses a 6.3V valve rectifier that shares a heater with one of the output valves (6A3). The rectifier valve came to some cataclysmic grief in terms of a cracked envelope. The design uses a cathode bias for the output stage. The bias resistors on one channel then burned up and the bypass capacitor got a hefty bulge. I've replaced the resistors and cap on that channel and there is still no sound. What else do you think got worked when the 'event' happened? Hopefully not an output transformer!
Thanks for any thoughts.
Thanks for any thoughts.
Not impossible after such a dramatic event. Check for continuity with a DMM.
Wait, the recifier used the same heater winding as one of the DHT output tubes? That's brave, to say it politely.
Exactly so!
The filament of the 6A3 is liklely to be ruptured, since the heater→cathode short in the rectifier will have routed high current through the filament, on its way through the cathode bias resistor + cap.
Please don't power it on again until the 6A3 has a private filament supply. Don't share with the other's channel's filament either!
It's best to run rectifier heaters with one side connected to the rectifier cathode (the DC out terminal) This way, the H→K stress is zero, and faults like this won't happen again. Naturally, this means no sharing with any other heater; but that's a good thing.
We can speculate or you can do a systematic investigation, step by step:
- disconnect from mains
- pull all tubes from the dead channel and test their filaments for continuity with your ohm meter - primary suspects
- with tubes still out discharge all power supply caps, confirm they are empty with volt meter, check components for discoloration, loose solder joints
- tubes still out, test all passive components in dead channel with ohm meter, compare readings with working channel, caps should read open, resistors close to their nominal or at least close to the working (there are not many in a SE amp)
- output transformer primary can also be tested for continuity, compare to good channel
- if all is good, insert tubes, connect black wire of volt meter to a ground terminal with a clip
- power up and with one hand measure voltages on cathode and plate of tubes, compare to working channel, keep other hand away from chassis
- if cathode reads close to zero, you know there is no current flowing ...
- disconnect from mains
- pull all tubes from the dead channel and test their filaments for continuity with your ohm meter - primary suspects
- with tubes still out discharge all power supply caps, confirm they are empty with volt meter, check components for discoloration, loose solder joints
- tubes still out, test all passive components in dead channel with ohm meter, compare readings with working channel, caps should read open, resistors close to their nominal or at least close to the working (there are not many in a SE amp)
- output transformer primary can also be tested for continuity, compare to good channel
- if all is good, insert tubes, connect black wire of volt meter to a ground terminal with a clip
- power up and with one hand measure voltages on cathode and plate of tubes, compare to working channel, keep other hand away from chassis
- if cathode reads close to zero, you know there is no current flowing ...