This is on my own, 2-channel LA-2A-style PCB layout, with a few key differences:
- Cinemag 1:1 in AND out transformers (so no huge step-up on the input, no step-down on the output)
- Power and audio are separate PCBs, each with its own separate path to star ground point
- Heater supply is 12VDC regulated
- 6AQ5 replaced with 12AQ5
- "variable capacitors" on both channels have been replaced with fixed 470pF
- neons replaced with zener diode
- All switches have been replaced with relays controlled via the front panel (separate, regulated 9v supply for relays and meter LEDs)
Circumstances under which oscillation (always right around 16kHz) occurs:
- output and gain reduction turned all the way up
- changing pre-emphasis sidechain filter setting has no effect on oscillation
I've tried much of the "standard" tube stuff to eliminate the oscillation:
- Tried 1K grid stopper resistors in many places:
- both sections of 12BH7A
- input of sidechain triode (second section already has one as per original LA-2A schematic)
- input of sidechain pentode
- Tried anode bypass caps (100pF - 220pF) everywhere but the 12BH7A
- Swapped T4s just in case, to no avail.
- Re-sanded star ground point to maximize chassis contact, to no avail.
The weird/Voodoo Stuff:
- It doesn't happen on channel 1, only on channel 2.
- Probing with 'scope on sidechain triode anodes makes it go bye-bye
- A combination of grid stoppers and a 220pF anode bypass cap on the sidechain triode solved the problem in channel 1; same bypass cap doesn't help with channel 2.
Finally...[/b]
- DISABLING the sidechain (a la an 1176 bypass) makes the problem disappear, which tells me the problem is cropping up in the sidechain somewhere.
- Further, the oscillation is measurable on the input of the pentode; it goes away when the GR control is reduced OR the sidechain is disabled. This tells me the problem is coming from the sidechain triode.
- BUT, removing ANY tube, including the output amp tubes eliminates the oscillation....
- Oscillation also seems to be load-dependent. When the output is terminated into a 10k load, the HF oscillation goes away and it just starts motorboating. Again, this ONLY channel 2!
This is DRIVING ME INSANE. I've wasted two full days trying to chase this damn thing down.
What the hell have I missed?!
- Cinemag 1:1 in AND out transformers (so no huge step-up on the input, no step-down on the output)
- Power and audio are separate PCBs, each with its own separate path to star ground point
- Heater supply is 12VDC regulated
- 6AQ5 replaced with 12AQ5
- "variable capacitors" on both channels have been replaced with fixed 470pF
- neons replaced with zener diode
- All switches have been replaced with relays controlled via the front panel (separate, regulated 9v supply for relays and meter LEDs)
Circumstances under which oscillation (always right around 16kHz) occurs:
- output and gain reduction turned all the way up
- changing pre-emphasis sidechain filter setting has no effect on oscillation
I've tried much of the "standard" tube stuff to eliminate the oscillation:
- Tried 1K grid stopper resistors in many places:
- both sections of 12BH7A
- input of sidechain triode (second section already has one as per original LA-2A schematic)
- input of sidechain pentode
- Tried anode bypass caps (100pF - 220pF) everywhere but the 12BH7A
- Swapped T4s just in case, to no avail.
- Re-sanded star ground point to maximize chassis contact, to no avail.
The weird/Voodoo Stuff:
- It doesn't happen on channel 1, only on channel 2.
- Probing with 'scope on sidechain triode anodes makes it go bye-bye
- A combination of grid stoppers and a 220pF anode bypass cap on the sidechain triode solved the problem in channel 1; same bypass cap doesn't help with channel 2.
Finally...[/b]
- DISABLING the sidechain (a la an 1176 bypass) makes the problem disappear, which tells me the problem is cropping up in the sidechain somewhere.
- Further, the oscillation is measurable on the input of the pentode; it goes away when the GR control is reduced OR the sidechain is disabled. This tells me the problem is coming from the sidechain triode.
- BUT, removing ANY tube, including the output amp tubes eliminates the oscillation....
- Oscillation also seems to be load-dependent. When the output is terminated into a 10k load, the HF oscillation goes away and it just starts motorboating. Again, this ONLY channel 2!
This is DRIVING ME INSANE. I've wasted two full days trying to chase this damn thing down.
What the hell have I missed?!