Cathode bias and one or more resistors ?

[Healer amps]

Hi !
I have a pro junior on my desk and newer Pro and blues junior are now cathode bias .
there is no schematic on pro (have contacted Fender 2 times no answer ) but there is on Blues :
https://www.guitarscanada.com/cdn-cgi/i ... ng.564096/

And as u see it have 2X 270 ohm resistors and MEGA caps 22/450V .
So my question is , what is the benefit of having 2 resistors ?

Historical most amp have only one .

And so you know one of the 270 ohm was burnt .

Best regard
OJ

[maxkracht]

Two resistors allows for a bit more tube mismatch. Also, if there were one resistor, that would be burned and both tubes could die instead of just the one. No idea why they used such a high voltage cap. Maybe for appearance, reliability, derating for heat, use for their stash of unreliable caps that couldn't handle 450v?

Edit: Just wondering if your amp actually has 450v caps or if it's just in the schematic? Typo makes more sense to me than any of the above reasons. I could buy doing it for lower BOM count, but you would think it would be cheaper to use a slightly higher voltage cap somewhere else in the circuit and make the cathode caps 50 or 100v.

[Stevem]

Those big voltage cathode bypass caps are not a good idea.
A very excess voltage rating can limit the cap from forming right and reaching its stated uf rating.

For how cheap 22uf @ 50 volt caps are I would change them out.

Also yes, two separate resistors make the output stage more stable.

[Healer amps]

Thanks for the info !
And yes I will change out the 2watt resistors and the big caps

Best regards OJ

[sluckey]

The 22µF cathode cap for the bottom power tube is drawn backwards on the schematic. Cap positive should connect to the tube cathode.

Here's the schematic and PCB layout...

[B Ingram]

...
there is no schematic on pro (have contacted Fender 2 times no answer ) but there is on Blues :
https://www.guitarscanada.com/cdn-cgi/i ... ng.564096/

... MEGA caps 22/450V ...

The 22µF cathode cap for the bottom power tube is drawn backwards on the schematic. Cap positive should connect to the tube cathode.

Unfortunately, it looks like a homebrew schematic made from Cut/Paste of other real Fender schematics.

[Stevem]

Good catch on that backwards cap!

[Healer amps]

Hi !

As I mention before I ask Fender to send me a schematic.
Well, after sending 2 mails and 2 weeks later I got A answer :As this is still a current product, the schematics are currently classified.

Like a simple amp like that is a BIG secret !?

Okay, I just make a schematic, no big deal , but I was hoping Fender was more services minded .

Best regards OJ