Good to know since the 22000's are nearly 50% more C than the design 15000's.
I mentioned the inrush because it looks like there's a thermal fuse link in the X'FMR. I know from experience with other amps that these can fatigue over time/inrush power cycles and 50 + years and 50% more C can be a lot of inrush ...
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I appreciate the concern and it is a valid concern, but you would think at the minimum, some SWAG math would have been done before spending time in redesigning this circuit
The following are thoughts specific to B1 and this use case. With other amps, like Sony(s), IMMV.
First of all,
the primary winding THERMAL fuse would be triggered likely at temperatures above 100c..or more. Let's not mix things up in here.
You are much more likely to see that fuse go bad for keeping that B-1 at idle in a shelve cabinet with increased unnecessary thermal exposure due to poor ventilation for a long time (sustained baking of those transformers).
But
YES, The primary winding of the transformer (not the thermal fuse) would be a valid concern if it was undersized, or if the increase of inrush current would be substantially greater. "Greater" in my book would be few times that of the original design, and here's why I am not concerned at all with this use case:
The original main PSU capacitor bank per channel consists of 2 × 15,000 µF electrolytic capacitors. Due to part availability, these are replaced with 2 × 22,000 µF capacitors, increasing the bulk capacitance by ~46%.
While the 46% increase in capacitance theoretically increases inrush current magnitude and duration, the thermal fuse:
- Is not a current-limiting component
- Does not react to millisecond-scale transients
- Is designed to withstand power-on surges that are part of normal transformer operation
Additionally, older electrolytic capacitors often had wide positive tolerance bands (e.g., –10% to +50%, sometimes up to +100%), meaning the original 15,000 µF parts may have operated effectively near or above 20,000 µF when new — narrowing the practical delta compared to the modern replacements.
Therefore,
the primary stress point from increased inrush is borne by the external mains fuse, not the thermal fuse. As long as the external slow-blow fuse remains in place and no faults exist in the rectifier circuit, the transformer thermal fuse is
highly unlikely to be affected under normal operating conditions.
A separate concern would be stressing the original rectifier, but that is not applicable here.
If you intend to upgrade your amp and can only find 22,000uf caps and still have hesitations, I think an NTC inrush current limiter would actually work with the B-1.
