As I recall, back in the 70s and 80s, high voltage ecaps might typically have a tolerance to value spec as wide as +100 to -50%. So a 80uF cap from then may measure higher than a 100uF +/-20%.
The only real effect on performance in circuit is probably in regard of the time it takes for the bias supply voltage charge up after power on. As long as adequate bias voltage is available by the time the output valves have warmed up, eg 30 seconds, all should be fine.
70's fender bassman 100 head recap?
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Re: 70's fender bassman 100 head recap?
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BonesMachine
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Re: 70's fender bassman 100 head recap?
I'm living on the edge! Ok thanks for all the help. I'm gonna do the filter and bias caps. The screen resistors. Put in 2 tubes. Make sure the OT is good. Plug it into a bulb limiter and see if I can't get this bad boy working again...
Re: 70's fender bassman 100 head recap?
The schematic posted is missing a .0047 cap on the Bass Channel as I recall? I’m not near a schematic but I believe it was a treble roll-off bridging cap that existed between the volume pot and treble pot. This needs to be removed for guitar work, be advised.
As to replacing the stock electrolytic caps I would NOT do this unless there was obvious blowout or leakage. It’s my opinion that these early Mallory caps were/are superior to anything being made today. I have two HPTTs that I’ve built that are using these Mallory caps from older Fender amps, one a ‘65 BFBM, the other a ‘70 SFSR. In either amp there was a cap or two that displayed some stress on the cap ‘blow hole’, and these were tossed. But the others I Variac’d back to spec and they are still in use today. Your light bulb limiter would duplicate the Variac result.
I know this seems contrary to the advice you might read in books (or in posts here), but you have to remember that these early caps were built using diff tech and ingredients. To be clear, I’m currently using Mallory 500/525 VDC electrolytic capacitors that were made 52-57 years ago. Yep.
As to replacing the stock electrolytic caps I would NOT do this unless there was obvious blowout or leakage. It’s my opinion that these early Mallory caps were/are superior to anything being made today. I have two HPTTs that I’ve built that are using these Mallory caps from older Fender amps, one a ‘65 BFBM, the other a ‘70 SFSR. In either amp there was a cap or two that displayed some stress on the cap ‘blow hole’, and these were tossed. But the others I Variac’d back to spec and they are still in use today. Your light bulb limiter would duplicate the Variac result.
I know this seems contrary to the advice you might read in books (or in posts here), but you have to remember that these early caps were built using diff tech and ingredients. To be clear, I’m currently using Mallory 500/525 VDC electrolytic capacitors that were made 52-57 years ago. Yep.
Most people stall out when fixing a mistake that they've made. Why?
Re: 70's fender bassman 100 head recap?
The one that I posted? It's there. Look again.rooster wrote: ↑Thu Feb 24, 2022 8:48 pm The schematic posted is missing a .0047 cap on the Bass Channel as I recall? I’m not near a schematic but I believe it was a treble roll-off bridging cap that existed between the volume pot and treble pot. This needs to be removed for guitar work, be advised.
Re: 70's fender bassman 100 head recap?
Ah, it didn’t look like a Fender ‘OEM’ schematic so I ignored it. Sorry, Ten Over. I see so many ‘non-OEM’ produced schematics these days that I just instinctively question their accuracy.
Most people stall out when fixing a mistake that they've made. Why?