5E3 help/advice needed

[Bruce / Mission Amps]

I reckon I might be pushing my luck running the 6V6s at 13w each, so I might up the cathode resistor to take it down a bit more.

I've worked and refurbished lots of vintage 6V6 amps where the two power tubes have been running 15-17 watts each decades.... I don't think I'd sweat them running at 13 watts for more then a moment.

[Steve Conner]

Thanks - I hadn't quite understood what you meant by 'referencing the filament to the cathode resistor' - didn't realise it meant grounding it.

It doesn't. There are two ways of referencing the filament circuit: either the transformer winding has a centre tap, or you make a virtual one using two 100 ohm resistors. For the purposes of this discussion, they both work the same.

Normally you connect this centre tap to ground, but you can get a worthwhile reduction in hum by connecting it to a positive DC voltage, to discourage the filaments from emitting electrons. This is sometimes called "elevating the heaters". In a cathode biased amp, the power tube cathode voltage is just about right for the job. So instead of connecting your filament circuit centre tap to ground, just connect it to the power tube cathodes.

You might be concerned about signal feeding back into the preamp tubes through the filament wiring, but the power tubes' cathode bypass capacitor seems to do a good job of squishing it.

[Jazz P Bass]

I had an Ampeg Gemini 12 in for repair.
I noticed the center tap on the heaters had a 0.1uf capacitor connection to the chassis.
No resistors.
Any thoughts on this concept?

[stephenhartley]

Maybe just smoothing out the ground? A lot of circuits just have a filament ground without either resistors or caps.

Thanks for the reassurance about running the output tubes at 13w each - I can't believe I've managed to build a compact little amp designed to be about 15 to 20w that is running at 26w - much louder than my 1st 5e3 build - great sound and plenty loud enough to match any drum kit.

[soundguruman]

I had an Ampeg Gemini 12 in for repair.
I noticed the center tap on the heaters had a 0.1uf capacitor connection to the chassis.
No resistors.
Any thoughts on this concept?

It was pretty common to do that when there was no electrical chassis safety ground. This prevented current from being on the chassis...
Just as one leg of the electrical power was also tied to the chassis with a capacitor (death cap).
In the old old days, the AC power, one leg was DIRECTLY tied to the chassis. It was supposed to be the neutral leg, but turn the plug around and...the chassis is HOT.
That's how people died.