I've been butting heads with the Fender PJ bias circuit schematic.
I have one apart with tubes pulled, which let's the unloaded B+ go higher than I expected...367 V, IIRC, the unaltered circuit bias was -10.5 VDC. I cannot calculate how this can be based on the schematic. I see R28 as the resistive leg of a voltage divider consisting of C12 and R28....BUT...R29 is in parallel with R28 when the bias rectifier is conducting on negative halves of sine wave), reducing the divider ratio...to somewhere in the range of 11-13 % (don't have access to my notes). I remember 0.047 uF being -j 56.5k ohms at 60 Hz, but the voltage with 50% C-R divider is higher than the bias electrolytics voltage ratings...so I realized R29 wasn't part of the 'output' divider with the 'floating' R30 33k resistor.
R29 being a different value for export seems to bring the C12-R28||R29 divider to the nearly the same 'effective' ratio as non-export. I incorrectly assumed voltage was higher for Europe (duh, they use a 240 pri. PT instead of 120!), but C12 is -j67.8K, skewing the ratio. (Using j instead of s shows some of you my education level).
I can not see what R30 is conducting thru to have a role in voltage dropping. Maybe I'm naively assuming there is 0 bias current. There isn't an obvious path thru the EL84's, to ignorant me.
I like the 2 or 3 resistor bias divider concept. 2 seems to be enough, but 3 I inadvertently figured out allows shifting the range of adjustment. I used Excel and assumption that half-time conduction of CR5 plays no duty-cycle role in the voltage across the first bias electrolytics...assuming there is negligible bias current thru the EL84 path.
I didn't simulate because I haven't used PSpice since mid-80's (rusty). Just started trying LTSpice recently, following Richard Kuehnel's Simulation book.
Re: reliability of C12. I used to work in the electronic fluorescent ballast industry and spend a lot of time looking at failed components. We expected inverter operation in the 15-45 kHz range to beat up polyester film caps & used the highest voltage metallized polypropylene accounting would let us. Failure cost usually won over argument they couldn't raise the cost to build.
But what really stunned me was analyzing a local company's primitive fluorescent 'dimmer' that used a metallized polyester capacitor as an impedance to drop voltage across. They had potted the capacitor in a plastic photo film canister, and it was severely bloated by the overheated capacitor...at 60Hz. They, too, objected to to recommendation to use MPP...cost & size, but failure was more disappointing.
So I been leery about polyester/Mylar for large voltage AC ever since...and AC vs. DC voltage rating for capacitors us a whole different thing.
Products have been built for decades before fancy power film caps refinement came about, but I always wonder what actually makes old coupling caps go bad (as in failure mode rather than just slanderous assumptions).
I am going to wrap up the Pro Jr. bias mod for a friend so he can have his amp back.
Topic: Re: Hoffman Pro Jr Turret board (Read 1750 times)