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SA-300 inrush limiter question

tutor_turtle

tutor_turtle

AK Subscriber
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I have a heavily modified SA-300 from a top notch tech in Canada (Sams Audio).
The rework is some of the best I've seen. All caps, rectifiers, sockets.. the works all neatly laid out.
There issue is the GZ34 rectifier tubes are intermittently sparking on startup. New tubes did not solve the issue.
I suspect the inrush current is the problem, as bringing it up slowly on a variac, always works.
The sound quality is outstanding, the customer states "it comes up clean most of the time, but sometimes the rectifier tubes start and blows the line fuse"
I have some Themistors (NTC type CL-50) good for 5 amps. Should I try these? Or a different value? Or none at all?
Any thoughts?
 
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Is the first cap larger than stock? Thats what smokes rectifiers. I have my doubts that a thermistor on the primary will change it.

There are also a couple of resistors between the output of the rectifier and the first cap, if those were removed it adds a lot of stress too.
 
C1 which should be 60uf, is 70 uf. That shouldn't be too much added inrush. The rest are the correct values.
I'll trace the lines off the rectifiers to see if the resistors were deleted or had their values changed.
Judging from the neatness of the component/wiring layout and the quality of soldering technique, this tech knows his stuff.
I have heard some scuttlebutt that the new GZ34's are not as robust as the NOS tubes.
I don't know either way, I do know that the Russians make some of the best new tubes available (which these are).
 
New rectifiers not as tough for sure but an old Mullard should hold up. You may get some joy out of the "yellow sheet mod" that adds a pair of silicon diodes in series with the plate to increase PIV rating but that won't help failure from excess current

Agree 70 should be ok if the resistors are in place. Its within original tolerance. Sometimes people mod stuff by replacing caps with stuff that is multiples of the original value or other things that might squeeze a couple of watts but it causes failures of other parts.
 
The voltages change, and you have to make sure the caps are all rated for enough voltage since it will be instant-on and a fair bit higher than it would be with the rectifier. Probably can fix the running voltage by increasing the values of those resistors between the rectifier and the first cap but the turn-on spike is still going to be there.


@dcgillespie has actual experience with the SA-300, he may be able to provide better info on this.
 
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