Diodes parallel to 1R bias test resistor?

[elipsey]

I see this in several commercial amps (like the Fender Super Sonic 22, D35 and D36, schematic attached, see pg. 2). What is the use of including this part?

EDIT: Fixed attachment.
Super-Sonic22_schematic.pdf

[R.G.]

It lets them use a cheaper resistor. If the output tubes lose their bias, they conduct a lot of current until something else blows. Could be the tubes, could be the OT, could be the AC mains fuse. However, in earlier amps with a 1R current sense resistor, they found that the resistor was often burned out too. Since the useful range on the resistors is down in the sub-half-volt range, a 1A diode can "protect" the resistors.

Whether this is a good idea or not, as it forces the current stresses back onto the OT (at least), is arguable. But they can make do with a smaller, cheaper resistor than they'd otherwise have to use.

[elipsey]

Thanks.

I figured Fender didn't get to be Fender by condoning the profligate waste of 3 cents. So the diode is cheaper than a better resister, or a bigger one might not fit with machine loaded PCBs? If the resistor did fail without the diode, would it be unsafe for the OT primary to be ungrounded?

[Steve A.]

I see this in several commercial amps (like the Fender Super Sonic 22, D35 and D36, schematic attached, see pg. 2). What is the use of including this part?

EDIT: Fixed attachment.
[ATTACH]39442[/ATTACH]

Do you have a better schematic? Perhaps it is just me but the drawings I downloaded were very blurred.

Thanks!

Steve A.

[Gregg]

This is Ampeg's field modification guide from 1992 where adding a diode across the 1 Ohm resistor is recommended together with an explanation why:

[loudthud]

The issue is not that the diode saves the resistor, it's that the diode limits the Voltage at the test point that is accessible outside the chassis.

[Randall]

What is meant by Fp1 on those resistors?

[nsubulysses]

What is meant by Fp1 on those resistors?

I believe it means Flame proof 1 ohm

[tedmich]

Hey I remember SGM, now there was a flame proof resistor!

[g1]

A historical anecdote, at least for the Fender. The '94 "The Twin" was the first Fender amp with user adjustable bias on the rear panel, with test points. When the 1R resistors burnt, they were burning holes in the little boards and melting the test point jacks.
There was a service bulletin to add the diodes across the resistors whenever doing any warranty work. The boards were revised and the diodes became part of production. And Fp resistors.

[jmaf]

Same system as Hotrod Deluxe. Also note how Fender had started using the 3000 volts flyback protection diodes.

[mhuss]

I've had a few small 1R cathode resistors fail open after a moderate surge, with no obvious physical damage. The diode might help with this phenomenon as well. I just use bigger 1R resistors now.

[Gregg]

I always use 1R 2W metal film resistors and I have yet to see one burnt (at least in my amps).