Amps that are fan cooled questions

[DavidEdenAria]

Sure, everybody has an opinion. Some opinions might be better backed up with design experience AND decades of tracking these reliability metrics. I can't tell you how many "repairmen" who say they know more than the "dumb-ass engineers" that designed the amp (not knowing it was me they were talking to) I have tried to walk through troubleshooting a circuit while they are insisting that they know exactly what the problem is... well then if you know so much, why are you calling me for help? That usually leaves them babbling in their own drool.

I can't confirm other engineer's designs, but any design that I have been responsible for has a REAL WORLD capacitor life of over 20 years, and in most cases will be good for 2x that. I don't care what your "repairman might think, he is not an engineer and he doesn't know as much about this particular subject as I do, nor does he have YEARS of reliability data that make me 100% positive of my comments.

Sorry for trying to help, no good deed goes unpunished I guess. Go ahead and do what makes you feel good. Your ideas do not relate very well to the real world as I see it.

You've been very helpful to me, as have others here at Talkbass!

 

[DavidEdenAria]

It's a legitimate test, it's just not an impedance test. Best if you know the value of R for the driver or at least have another to compare it to. The value of R can be found in the T/S specs. If measuring a cab, you would have to know what the resistance measurement was when it was new or at least known good. Individual drivers would have to be disconnected from wiring to get a useful reading.

I've horded old alnico USA guitar speakers for years....many times they are a recone, so you never know for sure what impedance it is unless you measure the resistance.

Trust me, an old original alnico Chicago Jensen is everything to a tweed Fender amp collector, and many are recons...so I always check resistance.

 

[BadExample]

I bet those are getting hard to find in good condition.

 

[MrCash]

Sure, everybody has an opinion. Some opinions might be better backed up with design experience AND decades of tracking these reliability metrics. I can't tell you how many "repairmen" who say they know more than the "dumb-ass engineers" that designed the amp (not knowing it was me they were talking to) I have tried to walk through troubleshooting a circuit while they are insisting that they know exactly what the problem is... well then if you know so much, why are you calling me for help? That usually leaves them babbling in their own drool.

I can't confirm other engineer's designs, but any design that I have been responsible for has a REAL WORLD capacitor life of over 20 years, and in most cases will be good for 2x that. I don't care what your "repairman might think, he is not an engineer and he doesn't know as much about this particular subject as I do, nor does he have YEARS of reliability data that make me 100% positive of my comments.

Sorry for trying to help, no good deed goes unpunished I guess. Go ahead and do what makes you feel good. Your ideas do not relate very well to the real world as I see it.

I agree with agedhorse about caps. Basically old vintage caps from (50,60,70s and probably the 80s) even if still working should be replaced as they did have a certain service life for several reasons, they may look good and still perform in spec for certain measurements, but measure there esr rating too and they might not look as good.

More modern caps as well as not having as many hours on them have differences in the design and can last much longer, there all round more durable. Really 12 years old for a modern cap is no big deal if they were 25 years old and older and you were going out gigging I would say change them.

If there is no noise and visually they look alright I'm willing to bet they will be perfectly reliable.
As agedhorse kinda said its unnecessary work at this point and it might disturb something.
I have learned this myself!

 

[agedhorse]

Without knowing the design ESR, measuring the ESR won't reveal anything valuable. ESR is a variable that's part of the design parameters, and within the same series will vary by value. ESR values have gotten lower via design improvements, but that may not be a particularly "desirable" improvement in some vintage amp designs, though I think the focus on ESR is quite over-rated. ESR is important for calculating ripple losses and ripple current power dissipation, is important for bulk discharge time constants in order to account for available fault energy under say a short circuit condition. For example, a crowbar circuit will need to account for stored energy and short circuit current when clamping a DC fault to ground from a shorted output transistor.

 

[MrCash]

Without knowing the design ESR, measuring the ESR won't reveal anything valuable. ESR is a variable that's part of the design parameters, and within the same series will vary by value. ESR values have gotten lower via design improvements, but that may not be a particularly "desirable" improvement in some vintage amp designs, though I think the focus on ESR is quite over-rated. ESR is important for calculating ripple losses and ripple current power dissipation, is important for bulk discharge time constants in order to account for available fault energy under say a short circuit condition. For example, a crowbar circuit will need to account for stored energy and short circuit current when clamping a DC fault to ground from a shorted output transistor.

I agree kinda, I think in general as part of a series of tests rather than focus on it, it can be a useful tool. I only mentioned it as most people would not have a dedicated meter.
There is ways of knowing in general what a good esr rating should be, sometimes I guess it would be a hack and not to exact spec, but common sense suggests If there is a unusually higher reading then I would be suspicious of that cap.
And yes of course low esr ratings are not always desirable in some vintage amps.

 

[agedhorse]

If the ESR is say double the typical spec, that would suggest a possible issue, but the tolerances are quite wide for this. Could be as much as +/-50% from typical. It really depends on how the spec is written, and I recall that it varies some with different applied voltages.