Blackface Princeton Reverb clone, el84 reverb driver mod

[sluckey]

I just noticed that with the reverb pot on 0 there's still some wet signal passing through :\

Bad pot, bad ground on pot, or wiring error. Changing to el84 driver should have nothing to do with that, unless you inadvertently boogered something in the process.

[martin manning]

However, with idle current at 9.5 mA the driver transformer is running about twice the level it would see using a 12AT7, and it may not stand that.

It doesn't make sense to run that tube into such a high impedance.

Could you please roughly explain me these? about the transformer and the impedance of the tube? Does the impedance of the tube change with its' bias?

Single ended output transformers are limited in continuous current to avoid damage due to overheating. I don't see any specs for that limit on the typical Fender reverb driver transformer, but running 2-3x the normal current seems like a risk to me.

The "correct" load impedance for a given tube is a function of its basic design (including it's maximum plate dissipation), and the plate and screen voltages at which it operates. You need to understand plate characteristic curves and load lines to fully appreciate this, so I recommend a read through this: http://www.valvewizard.co.uk/se.html

[jjman]

The 135watt Twin shows 9ma thru the reverb driver on the schematic, doing the math. Same tranny sku as when under lower current from older generations. 12at7 can idle much more current than 12ax7. I also could not find a spec on the max dc current of that tranny when I was looking a few years ago.