Fender Bassman 100 Head Silverface 1974 - Blows Fuse

[misterc57]

Regarding the diode type, I looked in the Twin Amp service manual to see what they call out. In the parts list, they called out 1N4006 1A/800V rectifier diodes, though in the schematic, it shows 1N5062TR diodes, also 800V but 2A rating. I'd go with those....Mouser has stock. The TR suffix is Tape & Reel (instead of loose).

Is the current handling added when they are in series? So (3) at 2 amp are now rated at 6 amps?

[Helmholtz]

Is the current handling added when they are in series? So (3) at 2 amp are now rated at 6 amps?

No, only voltage handling.

[Enzo]

Agree^^^

NO, current is the same through a series circuit.

[tubeswell]

Is the current handling added when they are in series? So (3) at 2 amp are now rated at 6 amps?

The attachment shows (by calculation) that, for resistance in series, the current through the load is the same for each resistor. The same current has to travel through all the resistances to get from the source to the return, no matter what resistances they are. (The thing that does change is the amount of voltage drop across each resistance. But the current through each resistor is the same)

[misterc57]

Getting my parts list together. I will be replacing the 6L6 resistors and the bias and filter circuit resistors. Does it matter if they are carbon film versus metal film? Is there a preference? I would think that all the original are carbon film.

Thank you

[Helmholtz]

I will be replacing the 6L6 resistors and the bias and filter circuit resistors.

Why?
It is easy to check resistors.
While it may be a good idea to get some spare parts (especially tubes), I am not in favour of blindly replacing working components. Such action always bears the risk of unconsciously/accidentally producing new problems.

I have some Fender amps from the 60s and 70s that work just fine with original caps and resistors.

Original resistors are carbon compound not carbon film.

[misterc57]

Why?
It is easy to check resistors.
While it may be a good idea to get some spare parts (especially tubes), I am not in favour of blindly replacing working components. Such action bears the risk of unconsciously producing new problems.

I have some Fender amps from the 60s and 70s that work just fine with original caps and resistors.

Original resistors are carbon compound not carbon film.

Well that is food for thought. The only thing bad that I know of are those rectifier diodes. All the filter caps tested good but are original 44 year old caps.

[Helmholtz]

Well that is food for thought. The only thing bad that I know of are those rectifier diodes. All the filter caps tested good but are original 44 year old caps.

I suggest to do it step by step. First replace to diodes and then look for signs of bad caps. Cap life depends on operating hours and temperature.

[tubeswell]

If they are 44 year-old filter caps, I vote for replacing them. It's better to replace vintage filter caps than finding yourself having to replace those vintage transformers

[Helmholtz]

It's better to replace vintage filter caps than finding yourself having to replace those vintage transformers

Such scenario would be highly improbable. I never found a shorted e-cap.
If the amp shows correct voltages and doesn't hum excessively after repair there is no need to replace caps.

[Leo_Gnardo]

If they are 44 year-old filter caps, I vote for replacing them. It's better to replace vintage filter caps than finding yourself having to replace those vintage transformers

And then you would have to replace those old filter caps too anyway.

I never found a shorted e-cap.

I have. Reservoir cap in almost new gear, less than 2 years since manufactured, dead short. In a Mesa JP-2 no less, an expensive "prestige" amp. In other amps, electrolytic caps that charge up to a certain voltage, then start conducting. Those heat up by themselves. You hope you catch 'em before they go kaboom. Others that have lost most of their capacitance. Yet others that have gobs of goo leaking out, BUT still appear to be operating normally somehow. With caps that are beyond 30 years old, consider them with deep suspicion. Past 40? Swap 'em out without a second thought.

[g1]

I agree with Helmholtz and Leo_Gnardo.
I've never found a shorted vintage e-cap. But I have seen shorts in newer manufactures.
I won't weigh in on whether you should just replace them or not, (that's an argument that goes for pages every time it crops up ), but I do agree you should always fix the problem first, perform upgrades later.

[tubeswell]

I find shorted e-caps quite often. Especially on the older amps

[tubeswell]

I found a several shorted caps on a Tonemaster last year - 'it keeps on blowing fuses' the guy said. And after they were replaced the thing worked again.

I had a 67 Super reverb come in a couple of months ago with a blown OT and a shorted bias supply cap. 2 of the filter caps were bulging as well, so I changed them. Customer was happy to have his amp working again

I had a 73 Vibrasonic Reverb humming like crazy last month with a dead smoothing cap. Another cap job. I replaced the old white mallory bias supply cap just in case. (Only took a couple of minutes) Amp sounded all better afterwards.

[Helmholtz]

I wasn't aware of e-caps eventully shorting. Might be a good reason for adding a B+ fuse as the failure doesn't seem to be predictable. If even new caps may fail after only 2 years I prefer to stay with the old ones as long as they measure O.K. and show no signs of physical leakage - at least in my own amps.

In my former company we continuously tested hundreds of e-caps of several brands for over 50,000 operating hours at 85°C. They were measured before and after testing. Never heard of a shorted one. But we only used SMPS grade 105°C types.