Fender Concert 6g12-A?

[Chuck H]

I seem to recall there was talk that larger bypass caps may have been used to help with heater hum?

I couldn't say about heater hum. Though it makes sense. As for myself I've adopted using a fully bypassed first stage cathode circuit for emission noise. Some exerimenting years ago revealed that most new tubes and a lot of older ones do have an emission noise floor associated with the cathode that is largely mitigated by fully bypassing the earliest gain stage. I don't remember any filament hum interfering with testing. I do design relatively high gain circuits and most contemporary designs for this type of amp use partially bypassed cathodes in the first stage. Many of these designers have moved to DC filament circuits. Likely for heater or emission noise and for the same reason I'm using a fully bypassed first stage in new designs. No noise problem yet, but I haven't designed anything near as high gain as the modern metal machines out there. So DC filaments remain an option but for now sticking with the fully bypassed first stage has been good.

EDIT: and going with a higher uF value than the ubiquitous 25/25 cap would be an option for me if I though it would help. As it is most of the emission noise I've registered has been managed with the 22uf standard value so far.

[Chuck H]

Rethinking the emission noise issue and I wanted to add that I don't see how DC filaments could help with that. Which makes me wonder why modern high gain amp designs haven't adopted full cathode bypass on early gain stages since noise floor is always a big consideration in these designs. Unless I'm wrong and DC filaments can help. I don't see how but I'm open to learn something.