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Discussion in 'Amp Tech Center' started by Jason McG, May 8, 2020.
are these the usual suspects for replacing on a cap job? anything else while I'm in there?
Generally its the electrolytics that need replacing... Bypass and filters.
OMMV, but I am not a fan of the carbon comp resistors for the 6v6 bias and HT filter dropping resistors.
I would increase the wattage rating for the 6v6 470R bias resistor while I was at it.
schematics show this cap should be 25/25, but it is 100uf/25V, and looks to be factory installed.
Should I replace with the same, or go back to schematic? Maybe Fender was out of 25/25s that day?
I'm not convinced that's a factory cap.
Is it an IC?
Someone has been in there and that may be a replacement.
Someone who lives inside these things will know better.
Max low value is ~25uF. Max high value is ~100uF. I shoot for something in the middle.
25 volt is minimum. Go with 50 or 63 volt if you can find one in that range.
I doubt it was factory installed.
so a 50/50 would fit the bill?
Both... I would pick up a 25uf/50V and a 100uf/50V. Go with the original value and see if that creates any "oddities" elsewhere in the circuit.
Are you replacing the cap can while you're at it?
yes, will be.
should the other 25/25s and 10/25 also be upgraded to 50volt?
Nah. They're cathode bypass caps, so they are seeing 1.5-3 Volts in circuit.
this is the resistor tied to what will be the new 50u/50V cap correct? and is a 2W 470 ohm metal oxide the right one to use?
thanks very much for the help guys!
2W should be enough but most people suggest some overkill at 5W. The cement block kind is popular and cheap. No problem with metal oxide.
I would check everything. It's a pretty simple circuit, I think most resistors can be checked without having to disconnect anything as long as you remove the tubes (but work with a schematic).
The caps can also be drained and checked using the capacitance meter from a multimeter, and in some cases an ESR meter.
That looks like a modern Nichicon, I suppose your amp could have been worked on.
The modern orange drop and nichi bypass cap suggest it's been worked on.
Are you having any symptoms of bad filter caps?
Replacing the vanilla tootsie roll white mallory bypass caps wouldn't hurt. But it might not help anything.
I'm a compulsive tinkerer, but I might just play that amp until it tells me something needs replaced.
Edit: OK, I'd replace teh tootsie rolls and the cathode resistor on the 6V6 with a 5W. Then I'd be tempted to leave it be.
The white Mallory caps. Just test them in circuit. If they are within spec, no need to replace. If it ain't broke...
Never had much luck with old electrolytics, I'd do them all, cap can, white Mallorys and that fairly new one on the power tube.
Testing them for capacitance with a multimeter is fun, but 9 volts into an 400 plus voltage cap doesn't tell you much. Had many filter caps bulging, leaking or just giving the bass farts that test pretty good at 9 volts. Maybe an ESR meter does more but I'm pretty skeptical of anything low voltage.
Ecaps are like car batteries, time does them no favors. They might test fine at 70 above but at -40 you're dealing with stiff jumper cables.