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.
Blackface Princeton Reverb clone, el84 reverb driver mod
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- martin manning
- Posts: 13208
- Joined: Sun Jul 06, 2008 12:43 am
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Re: Blackface Princeton Reverb clone, el84 reverb driver mod
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.varaso wrote: ↑Wed Dec 19, 2018 6:04 pmmartin manning wrote: ↑Wed Dec 19, 2018 3:37 pm 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.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?martin manning wrote: ↑Wed Dec 19, 2018 3:37 pm It doesn't make sense to run that tube into such a high impedance.
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
Re: Blackface Princeton Reverb clone, el84 reverb driver mod
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.
If it says "Vintage" on it, -it isn't.