Anybody know what the plate-to-catode diodes do?

[Merlinb]

Seems like any meter that a tech has would be auto ranging, and not much worry if something measures 1 volt or 500.
But anyway, regardless of that.....Do companies really have to think in those terms? The phrase "Idiot Proof" comes to mind......

I don't think it would be intended to protect the meter, rather that if the reason the resistor failed is because a direct short from B+ to cathode (for whatever reason) then someone messing with the DMM or poking things in the sockets could come in direct contact with the B+. The diodes, however, will pass amps of current, so the B+ fuse blows reliably.

[trem]

I don't think it would be intended to protect the meter, rather that if the reason the resistor failed is because a direct short from B+ to cathode (for whatever reason) then someone messing with the DMM or poking things in the sockets could come in direct contact with the B+. The diodes, however, will pass amps of current, so the B+ fuse blows reliably.

OK, I see what you guys are saying.
Maybe I should know this, but I really have not been working on amps very long......
The fuse for the B+, would that be relatively small? Maybe 500mA or something like that?
Thank You

[Old Tele man]

...they're called "fly-back" diodes; their purpose is to 'clamp' any inductive "fly-back" voltages created in the OT primary and possibly protect the tubes and OT primary. "Possibly" because they often don't work as well as theory says they should.

[R.G.]

The last few notes were talking about the forward biased diode parallel with a cathode resistor, I think, not the catch diodes.

[Morris Slutsky]

I don't think it would be intended to protect the meter, rather that if the reason the resistor failed is because a direct short from B+ to cathode (for whatever reason) then someone messing with the DMM or poking things in the sockets could come in direct contact with the B+. The diodes, however, will pass amps of current, so the B+ fuse blows reliably.

All good. Except that, according to that spec, there IS no B+ fuse! Yeah, there's a 3A mains fuse, that might keep the amp from actually catching fire, but it won't protect the output transformer or the power transformer from becoming a smoldering expensive lump.

[Merlinb]

[QUOTE=trem;174425]
Maybe I should know this, but I really have not been working on amps very long......
The fuse for the B+, would that be relatively small? Maybe 500mA or something like that?
/QUOTE]
Yes something like that. But fuses are somewhat vague devices, and it might be possible for a little 1/4W resistor to burn open faster than a fuse. Presumably they thought 1 penny of extra diodes was worth more than a million dollar law suit...