Do carbon film resistors dry up?

[beasleybodyshop]

Is this bad? most of the carbon films in this 1976 ampeg VT40 look like the outer covering is flaking off, almost like the amp dried out or something. are these safe? Or will i have to replace them?

[Invertiguy]

No, resistors don't dry out over time, although they can drift in value over time, especially older carbon composition resistors. That is an example of heat damage. Either that resistor was severely underrated for it's given duty or something downstream of it is drawing way too much current. What does the resistor connect to in the circuit?

[Stevem]

Carbon type can wick up moisture and than make noise!

[Leo_Gnardo]

I have seen the surface coat flake away on similar resistors, but the one in the photo looks more like it's been thru some trauma. I'd replace it without another thought, oops maybe yes another thought - if it has to dissipate near its wattage rating in normal use, bump the wattage up a notch on the replacement.

Another feature of those 70's Ampegs is paint flaking off the "tropical fish" caps they sometimes used. I take that as an invitation to replace. Cracking surface = a way for dampness to get in and probably already has. Swap 'em out if they look flakey.

[beasleybodyshop]

I have seen the surface coat flake away on similar resistors, but the one in the photo looks more like it's been thru some trauma. I'd replace it without another thought, oops maybe yes another thought - if it has to dissipate near its wattage rating in normal use, bump the wattage up a notch on the replacement.

Another feature of those 70's Ampegs is paint flaking off the "tropical fish" caps they sometimes used. I take that as an invitation to replace. Cracking surface = a way for dampness to get in and probably already has. Swap 'em out if they look flakey.

Leo, you are correct as usual. I have highlighted all the questionable resistors on the schematic - they all are either plate resistors, dropping string or cathode resistors. My guess is that this amp was rode hard and put up wet for many years, and a failure in the power supply caused the plate resistors to give up the ghost...

Does that sound, er, sound?

[Leo_Gnardo]

My guess is that this amp was rode hard and put up wet for many years, and a failure in the power supply caused the plate resistors to give up the ghost...

Does that sound, er, sound?

Well maybe but I'd just chalk it up to age & heat cycling mostly on the rough looking resistors. And tube or cap failures for the ones that are super baked. In many cases I mount new resistors on the foil side of those Ampegs circus boards because A) I'm lazy and 2) if I ever need to change 'em again I know I'll still be lazy. Last thing I wanna do is flip those heavy chassis over and over again.

[beasleybodyshop]

Amen to that. It is one heavy bastard to move around.

[crbowman]

I had an old VT-40 in the 70's. Great sounding amp. Ampeg mounted their preamp tube sockets on the PC board (obvious). I hate that design. With the tubes-down design everything on that board is subjected to way too much heat, and the heat up-cool down cycle plays havoc with everything. Too bad, cause I really like these amps otherwise.
And yes, it is a HEAVY bitch.

[beasleybodyshop]

I had an old VT-40 in the 70's. Great sounding amp. Ampeg mounted their preamp tube sockets on the PC board (obvious). I hate that design. With the tubes-down design everything on that board is subjected to way too much heat, and the heat up-cool down cycle plays havoc with everything. Too bad, cause I really like these amps otherwise.
And yes, it is a HEAVY bitch.

Your spot on! The PCB board has shown some heat damage around the tube sockets and plate resistors/power supply. I will take Leo's advice and mount everything else on top of the PCB for future sanity.....and laziness